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Peaks and nulls

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In Reply to: RE: How to find the area that causes a peak??? posted by Jolida on June 28, 2009 at 23:36:16

When sound travels in a room it is reflected from the room boundaries. Because of these reflections two phenomena occur: standing waves and comb filtering.

Standing waves are generated when two sound waves travelling in opposite directions, i.e. a sound wave and its reflection, are superposed. In a room, they only occur between parallel and opposite walls (to keep it simple). At the wall a phase change of 180 degrees occurs so when the incident and reflected waves (pink and blue) are superposed amplitude cancellation and enhancement occurs:

The physical location in the room of the nulls and peaks of the black wave does not change (see red dots), that's why the (black) wave is called a standing wave.

When a room dimension is equal to multiples of half the wavelength, we have a resonance which means that at both walls pressure maxima are present so that the sound pressure of this particular standing wave or room mode is the highest possible and higher than the sound pressure of an ordinary standing wave.

Standing waves and room modes exist at all frequencies throughout the audio range. The number of room modes per third octave, for instance, increases with frequency and above a threshold frequeny (commonly called Schroeder frequency) is that high that the individual mode is no longer perceivable as such.

So at a particular location in a room you will measure a pressure peak of the 800 Hz standing wave (I suppose you do mean 800 and not 80, but for the 80 Hz wave the same applies). The physical distance between peaks and nulls is one quarter of the wavelength, so at 800 Hz that is 10.7 cm (4.22 inch). It's possible that you can distinguish peaks and nulls at this frequency when walking through the room but I doubt it.

When a sound wave travels toward a wall at an angle, say the side wall, the reflection from that wall arrives at measuring position some time after the initial wave. The two waves (initial wave + reflection) are superposed and because of the time delay, again, cancellations and enhancement occurs, a structure resembling a comb is generated:

The frequencies at which the peaks and nulls of this structure occur depend of the time delay T (in milliseconds). Peaks occur at 1000 x n/T, nulls occur at 1000 x (0.5n/T). The amplitude of the peaks and nulls depends on the level of the reflection. Comb filters may cause audible coloration, but not necessarily so: thresholds of audibility exist so the mere fact of comb filter structures appearing in a measurement does not say anything about their audibility. The more reflections there are and the less regularly they are distributed on the time scale, the less audible is the coloration.

Unless there is a solid correlation between what can be measured and what is actually perceived, I would not worry too much about peaks and valleys in graphs, as long as the music sounds fine to your ears. If on the other hand you do perceive a problem when listening to music, measurements will help you to identify the problem area.

Klaus

Follow Ups:

RE: Peaks and nulls - Jolida 22:24:37 06/30/09 (42)
Hi Klaus,
Thanks very much for the explanation. I really appreciate.
As i said, i was compelled to take my room measurements using the test-cd JUST because i have a peak in many parts of EVERY song. For eg. when the singer raises his voice in a particular passage, it just comes at me, audibly hurting. For eg. if u are familiar with Dianna Krall's album "Live in Paris". When i listen to the song "The Look", there are many passages where her voice hurts while listening. Even on music which has a Piano, some notes hurt while being played. I have tried to eliminate & rule out my system entirely by changing the Cd player, Interconnects, Amp, Speakers, Isolations, but still the problem remains. When i measure using the Rives test-cd, it shows a boost of 7db at 630hz & 800hz. I have also tried moving my listening chair back& forth, but with no positive results. Im frustrated with this weird problem. How do i go about resolving this??????? Please enlighten.
- RE: Peaks and nulls - Ethan Winer 12:32:48 07/01/09 (40)
  > if u are familiar with Dianna Krall's album "Live in Paris".
  
  I have that concert on DVD and the sound quality is excellent. If you have no absorption at the side-wall and ceiling reflection points, that's the first thing you should look into. Absorbers there will reduce comb filtering, which is your most likely problem.
  
  --Ethan
  - RE: Peaks and nulls - Jolida 21:50:31 07/01/09 (39)
    Hi Ethan,
    I have absorption on the entire front wall & on the side-wall till the 1st reflection point. But i have'nt yet tried on the ceiling. I am attaching a picture of the room for a general idea....
    - RE: Peaks and nulls - Ethan Winer 12:46:55 08/04/09 (2)
      LOL, after I posted yesterday I realized this is an old settled thread! :-> )
      
      --Ethan
      - RE: Peaks and nulls - Jolida 23:22:51 08/04/09 (1)
        
        In Reply to: RE: Peaks and nulls posted by Ethan Winer on August 04, 2009 at 12:46:55
        
        Hi Ethan,
        As Joamonte has replied in this thread regarding Broadband High Frequency Diffusors, what exactly does he mean by that? I have made Diffusors following the measurements from the AcoustiCalc Software for the 600Hz to 2.9Khz region. How do i calculate the measurements for the Broadband Diffusor? The diffusor i had made neither performs well at the front-wall nor the wall behind my head. It tends to alter the original tone & develops a wee bit strain in the mids. Have i gone wrong in anyway???
        
        RE: Peaks and nulls - Ethan Winer 12:58:28 08/05/09 (0)
        
        In Reply to: RE: Peaks and nulls posted by Jolida on August 04, 2009 at 23:22:51
        
        Any diffusor that works over that range is considered broadband. So you probably made it correctly. Diffusors are good on the rear wall behind you, but many people prefer them farther away. If the wall behind you is very close, that might account for what you heard and didn't like.
        
        --Ethan
    - RE: Peaks and nulls - David Aiken 15:28:49 07/02/09 (11)
      Is that an exercise machine on the right wall in the photograph?
      
      Try removing that and see if your peaks disappear. There's moving parts and springs in such a machine, and a lot to resonate at some frequency or other. I'd guess from what I can see in the photo that that may be a prime candidate for the cause of your problem since it's the one thing that looks out of place and most rooms used for audio don't have your problem or such an item in the room.
      
      It's easy to test.
      
      David Aiken
      - RE: Peaks and nulls - Jolida 12:23:24 07/03/09 (9)
        
        In Reply to: RE: Peaks and nulls posted by David Aiken on July 02, 2009 at 15:28:49
        
        Hi David.
        I removed the treadmill from its place today. Its made quite a big difference in the performance. I must say the problem has been solved upto 70%. Its also more pleasing to hear the music now, & the imaging has also improved. I was wondering how a Treadmill next to the speaker could skew the response to such a degree. Do u think its the removal of the treadmill that's made the difference or placing the absorbers at the same height as the left 1st reflection point has contributed??? Coz when the treadmill was there, i had to keep them on it which elevated the height to 1ft higher than the other side.
        
        Whatever may be, it has benefited a lot. Thanks for the suggestion.
        Ethan, i just saw ur new video. Its very impressive to virtually see & hear, than just reading about what a product could do. Since i live in India & its hard to import your diffusors, i tried to DIY the same following the AcoustiCalc software. I made it using 11 wells, operating in the range of 600hz to 2.9khz. As u have seen the picture of my set-up, i had placed it on the table thats behind & between the speakers. The result was more impact, much more retrieval of detail & cleaner mid-bass. But i did not like it for 2 reasons. It tend to change the vocals a bit by adding sibilance & slight rough-edges. Is it placement critical? It was place just 2.5 ft away from the speakers, so i guess it was a little too close. The music also had become a tad louder. Did i go wrong in anyway????
        I had taken a picture when i had kept them last week...
        
        RE: Peaks and nulls - Ethan Winer 13:57:01 07/07/09 (0)
        
        In Reply to: RE: Peaks and nulls posted by Jolida on July 03, 2009 at 12:23:24
        
        If those are the only two diffusors you have I'd try them elsewhere, like behind you.
        
        --Ethan
        
        RE: Peaks and nulls - joamonte 22:05:58 07/04/09 (0)
        
        In Reply to: RE: Peaks and nulls posted by Jolida on July 03, 2009 at 12:23:24
        
        You heard the vocals added sibilance & slight rough-edges because you DIY diffuser only work at mid range....
        
        Within its working range it able to reduce the reflection and spread the sound 180 degree in horizon back into the room, but above 2.9Khz it act like a bare reflective surface, it reflect the sound directionally to you....worse part is those curtain around the diffuser make the reflective high frequency even more obvious...you open out all curtain the sibilance will reduced, but the best solution is to make sound diffuser that able to diffuser at least to 12Khz....
        
        In my personal experience, High Frequency diffusion is much more important then mid and low frequency.The reason is because for 99.999.....% of loudspeaker, the higher the frequency the more directional it is , with High frequency diffuser at five surface (2 side wall,front and back wall, ceiling), we able to create a illusion of diffused sound field like in live musical event....Say given a choice between 2 diffuser, one working range is 250Hz to 3.4Khz and the other one of 1Kz to 16Khz, I would choose the latter any time in any room.
        
        RE: Peaks and nulls - David Aiken 13:26:14 07/03/09 (6)
        
        In Reply to: RE: Peaks and nulls posted by Jolida on July 03, 2009 at 12:23:24
        
        Well, the treadmill would have had a number of freely moving parts which would have resonated in response to any sound in your room. I'm not surprised removing it had a beneficial effect.
        
        Another source of resonance may be the hinged glass doors in your equipment rack and removing them may help if that's possible. The ceramic tiled floor will certainly be adding some brightness to the room but I would expect that to be broad band rather than creating a peak at a particular frequency.
        
        I can't tell the exact proportions of your rug from the photos but I suspect that it's longer than it's broad and that you have the narrow side facing you. You could try turning the rug 90 degrees so the long side runs across the room and locating the rug with its centre halfway between you and the plane of the speakers. That may or may not help a little but it only takes seconds to try and things like that are always worth the experiment in my view.
        
        David Aiken
        
        RE: Peaks and nulls - Jolida 22:02:57 07/03/09 (4)
        
        In Reply to: RE: Peaks and nulls posted by David Aiken on July 03, 2009 at 13:26:14
        
        And one more thing....
        As the flooring of my room is entirely ceramic, will carpeting it till the 1st half make a considerable ( worthwhile) difference???
        I do not want to make the whole room wall-to-wall carpeted as it gets rather annoying to maintain....
        
        RE: Peaks and nulls - David Aiken 01:16:01 07/04/09 (2)
        
        In Reply to: RE: Peaks and nulls posted by Jolida on July 03, 2009 at 22:02:57
        
        Klaus' citing of absorption coefficients really begs the question. Ceramic tiles are very reflective at high frequencies, just as glass is. Carpet will absorb high frequencies but is not very absorbent at low frequencies—too thin, often not dense enough, and in direct contact with the floor rather than spaced off it by 3 or 4", for effective bass absorption. Carpeting, or a rug covering the floor reflection points, won't change the sound of things at low to mid frequencies but it certainly can help somewhat with the brightness that the high frequency reflectivity of ceramic tiles can produce and that can be a useful thing in many rooms.
        
        Numbers like absorption coefficients are fine, and useful in a number of ways, but they often don't tell you much about what the effect on the actual sound in the room will be. In this case you don't need to worry about the figures when it comes to your rug. The amount it will absorb won't change because the rug isn't changing in any way, but it's being placed in a slightly different position and that means that it's going to be absorbing in some locations where previously it wasn't and not absorbing in some where it previously was. The reflections from those areas where things are changing don't all reach you at the same point in time so you are actually making a small change in the characteristics of the reflected sound as it decays. It's not going to be a big change and it may not be an audible change. On the other hand, something may be audible and you may prefer the sound with the rug in the new position.
        
        The thing to remember about all acoustic treatment is that it's all about changing something in the reflected sound reaching your ears. The direct sound, the sound travelling in a straight line from the speaker to you, is always unchanged unless you actually put something physically between you and the speaker at ear and speaker driver height. What you put in the room and where you put it is what makes the differences you hear, and placing the same thing in different locations can result in audible differences in sound. The easiest way to prove that is simply to move your corner bass traps to somewhere else in the room, leaving the corners untreated. You'll still have the same amount of treatment in the room but things will sound different. Moving the rug won't make as big a difference as moving the corner bass traps, it may not make an audible difference, but it just might make a small but useful difference.
        
        What counts in the end is always the way things sound to you, not what the numbers say. Measurements can help us to get things sounding better but our aim isn't to get the system measuring well, our aim is to improve our enjoyment of the music we listen to. The measurements are worthless if our enjoyment doesn't improve.
        
        Personally I'd prefer not to have a ceramic tiled floor. My music room is carpeted wall to wall but at least maintenance isn't too onerous since I live alone and tend not to get it dirty or messy. I vacuum clean it weekly while vacuuming the house and that is basically all I need to do. It would be different if I had kids or if my music room got a lot of foot traffic through it.
        
        The open plan area where my TV system is located does have ceramic tiles, and I often find them a bit more of a pain to keep clean since that area gets a lot more foot traffic from me than the music room does. I walk through that open plan area all of the time to get from one part of the house to another. I don't walk through the music room to get anywhere, it's an "end point" in the house so it's easier to keep clean even if carpet can trap dirt better than ceramic tiles in some ways. Since I don't have carpet or a rug in the area with the TV system, I rely on the Audyssey room EQ function in the AV receiver to tame the brightness caused by the reflectivity of the tiles and the sound certainly benefits from the equalisation. While that does a quite adequate job for movie and TV soundtracks, I don't enjoy listening to music on that system as much as I do on my audio system which has no EQ functions. Overall I think balanced absorption does a better job in many ways than EQ.
        
        I think you may find that changing the rug's position as I suggested may help a bit. It may not, and a lot depends on just how much placing the rug in the other orientation affects the absorption of your floor reflection. You're probably getting some absorption of that already and the carpet's absorption characteristics aren't going to change. The only change occurring is where you're placing the carpet and that's a small change so I wouldn't expect a big change but you may get a noticeable change and you may find it preferable. As I said, it's free and quick to try.
        
        It's worth playing around with simple changes like that in my view. They cost you nothing apart from a little time and they can make worthwhile differences, even if they aren't big differences. Still, several small differences from a number of minor changes can add up to a much more noticeable improvement overall than any single change makes and that's not to be sneezed at, especially when the changes are free.
        
        Finally, working with your room and what you've got is always worthwhile in my view. The difference between good and very good results can easily be nothing more, or less, than simply paying attention to the little details. When all it's costing you is a little time, there's no real reason not to try experimenting with the small things and it's often surprising what you can learn in the process. What you learn is always going to be valuable, and it will help you a lot if you ever move house, swap rooms, or even simply change speakers and simply need to start fine tuning again to get the most out of the new speakers.
        
        David Aiken
        
        RE: Peaks and nulls - Jolida 05:10:26 07/04/09 (1)
        
        In Reply to: RE: Peaks and nulls posted by David Aiken on July 04, 2009 at 01:16:01
        
        David,
        I am in total agreement with what you said. I've been observing since your first reply to my post, that ur suggestions are very practical. Since i, or for that matter many people do not have much knowledge about physics or psycho-acoustics, practical suggestions sound as golden-words as opposed to theory.
        
        My music room, is not just an Audio-Only room, & i have a small child who spends most of his time there, so i would refrain from full wall-to-wall carpeting. I would try to carpet the floor till my listening chair, coz i guess it would sound considerably better, as opposed to bare ceramic floor. I will try to orient the rug in the other way today & post a feedback. Also i will take-off the glass doors from the table & see if performance differs. I appreciate the time you have taken to resolve my issue.
        
        RE: Peaks and nulls - David Aiken 14:50:17 07/04/09 (0)
        
        In Reply to: RE: Peaks and nulls posted by Jolida on July 04, 2009 at 05:10:26
        
        Different circumstances can't help but steer us in different directions.
        
        I don't think you "need" to carpet your room up to the listening chair but I do think there will be a "compromise" point that you're prepared to live with in order to balance use with sound and I do think that there will be a minimum carpet/rug size which will achieve that for you provided it's placed in the right position. Don't think only in terms of increasing carpet/rug area and do think also on where you place it between the speakers and the chair. Experiment a bit as I suggested. You may have enough area already to give you that compromise if it's placed slightly differently. If not, you may be able to achieve the result you want by placing something else like a felt backing beneath the rug you've got in order to increase its absorption characteristics. You may have enough area if you increase effectiveness and placing felt or a similar backing under the rug isn't going to make things any more difficult to take care of.
        
        Another "freebie" you could try is moving the speakers a little closer together so the carpet, as it currently is located, is in front of the speakers.
        
        Playing with what you've got is always a good starting point before you consider changing what you've got. Spend some time playing with what's in the room right now, moving things around or removing some things rather than changing or adding things. You might get some surprises, both good and bad, but I think you can end up with things sounding better without replacing or adding anything. If you do, those benefits will probably still work in your favour when you do replace or add something so they won't be wasted.
        
        David Aiken
        
        Carpets - KlausR. 23:31:24 07/03/09 (0)
        
        In Reply to: RE: Peaks and nulls posted by Jolida on July 03, 2009 at 22:02:57
        
        The absorption coefficient of carpet on concrete are
        
        125 Hz - 0.02
        250 Hz - 0.06
        500 Hz - 0.14
        1000 Hz - 0.37
        2000 Hz - 0.60
        4000 Hz - 0.65
        
        Data from http://www.sengpielaudio.com/calculator-RT60Coeff.htm
        
        Using more carpet will lower reverberation time, but it will have little effect on frequencies below 1000 Hz.
        
        Klaus
        
        RE: Peaks and nulls - Jolida 21:54:59 07/03/09 (0)
        
        In Reply to: RE: Peaks and nulls posted by David Aiken on July 03, 2009 at 13:26:14
        
        I surely will try to rotate the rug & experiment this evening. The glass doors in the equipment table can also be removed. i will try to implement that as well. Now it crosses my mind that when i place my amp & cd player in the rack, it sounds inferior that when its kept on top of the table. But i dint pay attention to this, as it would defeat the purpose of making that rack as a whole. Even thought i have isolated the cd player on air suspension & the amp on rollerberings, they do sound better when kept on top of the table rather than inside.
        Thanks for enlightening me. You are right, its easy to experiment though.....
      - RE: Peaks and nulls - Jolida 02:37:31 07/03/09 (0)
        
        In Reply to: RE: Peaks and nulls posted by David Aiken on July 02, 2009 at 15:28:49
        
        Hi David,
        Yes, thats a Treadmill. I do not use it ever. I surely will try to eliminate & see if it does make the difference.
        And Ethan, the absorbers are till the 1st reflection points on the side-walls as u can see in the picture. The chair was moved backward when i took the picture, so at that point, it looks as if the panels or traps are not covering the 1st reflections.
        I will try & remove the excercise machine this evening & post a feedback...
    - RE: Peaks and nulls - Ethan Winer 13:10:51 07/02/09 (0)
      > on the side-wall till the 1st reflection point
      
      Does that mean the reflection points are not covered? It looks that way from the photo.
      
      --Ethan
    - Comb filters - KlausR. 02:05:51 07/02/09 (22)
      Comb filter induced coloration may or may not be audible. Thresholds of perception are known, but as long as you don't correlate a measurement with these thresholds you don't know whether or not the comb filter structures showing up in that measurement are audible as coloration. You certainly cannot state that they are audible just because they show up in a measurement.
      
      A peak at 630 or 800 Hz would correspond to the first peak of the floor reflection, or a ceiling reflection if the ceiling was low.
      
      In the present case there is already absorption at side walls and front wall and, to some extent on the floor. As a general rule, the more reflections are absorbed, the more the remaining reflections may become audibly disturbing, so in this case there are 2 options: remove all absorbers currently in the room and 2. use additional broad-band absorbers on floor and ceiling.
      
      Klaus
      - RE: Comb filters - Jolida 06:46:09 07/02/09 (21)
        
        In Reply to: RE: Comb filters posted by KlausR. on July 02, 2009 at 02:05:51
        
        Hi Klaus,
        Thanks for the reply. I appreciate your effort in resolving my issue. As per the picture of my room which i posted in this thread, the carpet thats seen is of a very minimal thickness & would least act in the upper or lower midrange. As you insist, i will try with better absorption at this point. As for the ceiling, it is 9.5 ft high & flat ( cemented ). Its quite painful to put an absorbent at that point, but i guess i cant do without it as well. The rear wall ( not in the picture), is 10 ft behind my listening chair, with just one 13" tubetrap in the centre. Im looking forward to make more tubetraps as well. Is the front-wall, as critical as the side-wall for absorption??? Coz i put a QRD diffusor there which i DIY according to the AcoustiCalc software, & it brought about sibilance in the singers voice with rough edges. So i dont use it, though it was quite labour-intensive to make one.
        
        RE: Comb filters - KlausR. 10:48:58 07/02/09 (20)
        
        In Reply to: RE: Comb filters posted by Jolida on July 02, 2009 at 06:46:09
        
        The frequency range of porous absorbers, that's what your carpet is, is determined by their thickness. When a quarter of wavelength is equal to the thickness of the carpet, the carpet will be effective and this frequency will be the lowest to be absorbed. So when your rug is 1 inch thick, it will absorb down to 3375 Hz.
        
        Floyd Toole recommends to use absorbers that work down to at least 200 Hz, which in the case of porous absorbers means 17 inch thickness.
        
        An important point may be that when all but the ceiling reflection have been absorbed, that remaining ceiling reflection may be rather disturbing in terms of confused source localisation:
        
        Guski, “Auditory lateralisation: effects of reflecting surfaces”, Perception 1990, vol. 19, p.819
        
        Reflections from the front wall are well masked by the direct sound, but they have high interaural cross-correlation meaning that the sounds arriving at right and left ears are very similar. Listeners prefer low correlation, or dissimilar right/left ear signals so treatment might be beneficial.
        
        In any case, narrow-band high-frequency absorption will have the same effect as lowering the level of the tweeter. And since you are changing the spectrum of the reflection with regard to the spectrum of the direct sound, it may affect source localisation (keyword: precedence effect).
        
        Klaus
        
        RE: Comb filters - Ethan Winer 10:04:02 07/03/09 (1)
        
        In Reply to: RE: Comb filters posted by KlausR. on July 02, 2009 at 10:48:58
        
        > Floyd Toole recommends to use absorbers that work down to at least 200 Hz, which in the case of porous absorbers means 17 inch thickness. <
        
        David already straightened out that myth, and I can do one better. I recently made a video showing 6-inch thick bass traps making quite a large improvement to below 40 Hz:
        
        Hearing is Believing
        
        Also, we have to be clear in what is being improved and by how much. Nulls are often the biggest problem, especially in very small rooms. It does not take 100 percent absorption to make a very large improvement in a null! Even 25 percent absorption will reduce a null that's infinitely deep to only 18 dB deep, and 50 percent absorption reduces a null to only 11 dB deep.
        
        --Ethan
        
        RE: Comb filters - joamonte 23:44:16 07/04/09 (0)
        
        In Reply to: RE: Comb filters posted by Ethan Winer on July 03, 2009 at 10:04:02
        
        Nice video...
        
        As mention before, I find its more enjoyable to listen music in a diffuser room than a absorbing room, the micro dynamic is enhanced by the diffuser ,and the loudspeaker/room also seem disappear better...
        
        ...for those who have proper(diffuser that able to diffuser above 12Khz at least) diffuser,can try this,put some diffuser on ceiling,side wall ,listener front and back wall (reflection point or not is not important), use little loudspeaker(less than 15 degree) toe in , you will get very beautiful sound from the setup~~ imagine high frequency reflection is weaken but sustain longer in time domain and those "airyness" will spread evenly and occupied every inch of room,the loudspeaker now sound like it have a much wider and better High frequency dispersion pattern; it's definitely a kind of coloration , and I think it's not exactly what Mixing engineer want in their control room ..... but, it's music enjoyment!
        
        "Floyd Toole recommends to use absorbers that work down to at least 200 Hz…" - David Aiken 15:25:37 07/02/09 (17)
        
        In Reply to: RE: Comb filters posted by KlausR. on July 02, 2009 at 10:48:58
        
        "Floyd Toole recommends to use absorbers that work down to at least 200 Hz, which in the case of porous absorbers means 17 inch thickness"
        
        Well, one quarter of the wavelength of 200 Hz = 17" and you don't need something that thick to absorb sound at 200 Hz.
        
        Just look at the absorption plots for any of the commercial grade acoustic absorption products made by firms like ASC, RPG, RealTraps and others and you'll see good absorption figures at 200 Hz and lower for products a lot thinner than 17".
        
        Still, it's a shame to let the measured facts get in the way of a good theory and story.
        
        Somehow I can't imagine Floyd Toole having penned that recommendation if it were going to commit people to having absorption products 17" thick or thicker in their rooms. Place them on 2 opposing walls, as he recommends with front and back walls, and the room loses 3' in length. Do the same on side walls and you lose the same there. A hell of a lot of the average small to medium size room which most people use for audio or HT purposes would simply be unusable if that were the case. There's nothing in Toole's book to suggest that his treatment recommendations lead to that kind of result and there's no doubt he would have mentioned it if that were the case since that outcome goes completely against his stated intentions for the book.
        
        I'd say that suggesting you need absorption as thick as the quarter wavelength of the lowest frequency you want to absorb is about as reliable an audio statement as the one that states that rooms can't support sound at frequencies with a wavelength longer than the length of the room's longest dimension.
        
        David Aiken
        
        Porous absorbers - KlausR. 23:54:48 07/02/09 (16)
        
        In Reply to: RE: "Floyd Toole recommends to use absorbers that work down to at least 200 Hz…" posted by David Aiken on July 02, 2009 at 15:25:37
        
        Porous absorbers work best where the velocity of the air particles is maximum. This is the case at 1/4 wavelength distance from room boundaries. Of course, these absorbers will absorb some energy at 1/2 wavelength, but less. A 2 inch carpet or a concrete wall, too, will absorb at 200 Hz, but look at the absorption coefficients!
        
        For maximum absorption of any material, the 1/4 wavelength rule applies. If less than 100% is suffucient, fine. It all depends how much you want to absorb.
        
        Klaus
        
        RE: Porous absorbers - David Aiken 00:21:31 07/03/09 (15)
        
        In Reply to: RE: Porous absorbers posted by KlausR. on July 02, 2009 at 23:54:48
        
        There's a difference between saying that "Porous absorbers work best where the velocity of the air particles is maximum. This is the case at 1/4 wavelength distance from room boundaries" and saying that the absorber has to be a quarter of a wavelength thick.
        
        And saying "If less than 100% is suffucient, fine." as if less than 100% is insufficient or inadequate certainly gives a wrong idea. The only 100% efficient absorber I know of has no thickness at all—it's an open window or door to the outside. When it comes to getting practical benefits, a lot less than 100% efficiency can still be very effective. If it wasn't, acoustic treatment with standard commercial products simply wouldn't give the benefits it does.
        
        Your acoustic ceiling is effective enough for you according to your previous comments and the absorption material in it isn't 17" thick or thicker yet it obviously has quite beneficial effects at frequencies of 200 Hz and lower according to your previous comments, and that's for a product which I bet doesn't claim 100% absorption over a lot of its operating range.
        
        David Aiken
        
        My acoustic ceiling... - KlausR. 02:08:44 07/03/09 (14)
        
        In Reply to: RE: Porous absorbers posted by David Aiken on July 03, 2009 at 00:21:31
        
        is not a porous absorber!
        
        Being synthetic fabric under tension, it's rather of the membrane type with 26 cm air space behind and partially filled with rock wool to improve the broad-band effect.
        
        alpha values for 20 cm air space + rock wool are
        
        125 Hz - 0.72
        250 Hz - 0.65
        500 Hz - 0.71
        1000 Hz - 0.75
        2000 Hz - 0.84
        4000 hz - 0.94
        
        You said: "When it comes to getting practical benefits, a lot less than 100% efficiency can still be very effective."
        
        Recommendations, such as
        
        http://www.tonmeister.de/foren/surround/texte/SSF_01_1_E_2002_v2a.PDF
        
        require the level of first reflections to be 10 or 15 dB below the level of the direct sound. Are commercial absorbers capable of doing that and what thickness do you need to address 200 Hz? Note that the figure 15 dB is based on speech, music needs between 18 and 25 dB!
        
        Klaus
        
        Hmmmmmm... - David Aiken 13:15:55 07/03/09 (13)
        
        In Reply to: RE: My acoustic ceiling... posted by KlausR. on July 03, 2009 at 02:08:44
        
        So, at roughly 200 Hz, ie 250 Hz, your ceiling is only 65% effective yet you said previously "If less than 100% is suffucient, fine. It all depends how much you want to absorb." Seems like a fair bit less than 100% is quite sufficient which was exactly my point.
        
        As for the recommendations in your link, the recommendation in table 1 is simply for <-10 dB. I skimmed it quickly and found no reference to a -15 dB level nor any mention of that figure being based on speech, nor an 18-25 dB reduction being necessary for music. The recommendation I've previously seen is the graph from Olive and Toole's research which suggests a -10 dB level for the early reflections and you've previously been rather scathing about my use of that reference.
        
        As for commercial products, the RealTrap Mondo Traps I use have NRCs of 1.52 at 200 Hz when mounted on the wall and 1.39 at 200 Hz when corner mounted. The higher than 1.0 specifications result from the fact that the traps are double sided and absorb sound from both sides. Those figures are for a 4" thick trap. You can find data at other frequencies and for a number of other commercial products at the link below.
        
        Commercial products are certainly capable of meeting the recommendations you refer to and are used in studio situations regularly for that purpose. What would be the point of publishing a standard that no one could meet?
        
        David Aiken
        Absorption data (Open in New Window)
        
        RE: Hmmmmmm... - KlausR. 23:19:52 07/03/09 (12)
        
        In Reply to: RE: Hmmmmmm... posted by David Aiken on July 03, 2009 at 13:15:55
        
        The intended purpose of that ceiling was to decrease reverberation time, not to absorb the ceiling reflection! Having read what I've read about room acoustics and related psychoacoustics, I note that there is no evidence that early reflections are detrimental and need to be treated, so I will not advise treatment to anyone asking here or elsewhere. One researcher has started a series of experiments investigating the effects of early reflections in real listening environments, but so far the two-channel case has not been investigated:
        
        Naqvi et al., “The active listening room- a novel approach to early reflection manipulation in critical listening rooms”, J. of the Audio Eng. Soc. 2005, p.385
        
        SSF-01 2002 recommends 10 dB, Walker for his Controlled Image Design recommends 15 dB. If you look at Olive's data, at 10 ms perception threshold for a single lateral reflection is at -15 dB (anechoic), -11 dB (IEC room), -12 dB (treated IEC room).
        
        The treshold data for music are from
        
        Schubert (1966), “Detectability of single reflections for music” (Untersuchungen über die Wahrnehmbarkeit von Einzelrückwürfen bei Musik), Technische Mitteilungen RFZ, vol. 10, no. 3, p.124
        
        which, btw. is listed in SSF-01
        
        Note that these data are all absolute thresholds for the single loudspeaker/single reflection case meaning that the mere fact of perceiving this reflection automatically leads to undesired effects so that it has to be eliminated.
        
        The question now is, why use absolute thresholds of a single speaker/single reflection scenario and speech when all of us use at least two loudspeakers, have multiple reflections and listen to music???
        
        I further note, that neither Naqvi nor Schubert are mentioned in Toole's book (of which I have a copy by now).
        
        Klaus
        
        RE: Hmmmmmm... - joamonte 02:32:52 07/05/09 (3)
        
        In Reply to: RE: Hmmmmmm... posted by KlausR. on July 03, 2009 at 23:19:52
        
        Can share with us your ETC of the front 50ms of your room?
        
        I am curious about the ETC of a room without early reflection control will look like, but I agree with you if the room is big enough and the speaker is small enough, early reflection control is not a must.....
        
        Above picture is mine ETC graph in my very small 4.5 meter L X 2.9 meter W X 2.6 meter H listening room....
        
        I am using broadband diffuser at side and front wall , and custom make BAD panel at ceiling and back wall ,no absorption is used at early reflection point, given the small size of the room and the big floor stander speaker I used (Marten Coltrane) , without all the diffuser at early reflection point, the ETC look really bad, as all the wall is pretty close to the speaker.
        
        ETC - KlausR. 13:32:06 07/05/09 (2)
        
        In Reply to: RE: Hmmmmmm... posted by joamonte on July 05, 2009 at 02:32:52
        
        I don't have any measurements of my room, and I don't think that measurements that are not correlated to human perception are of much use. In his paper
        
        "The detection of reflections in typical rooms", JAES 1989, p.539
        
        the authors (Olive & Toole) present ETC of the IEC room some of the threshold experiments were conducted in, in both treated and untreated conditions. The difference is clear, but since early reflections are merged with the direct sound within a time window, the size of which depends on the signal type (50 ms for speech, 80 ms for slow music) they do not represent any danger, if you will.
        
        Lipshitz and Vanderkooy have written a paper about ETC
        
        http://www.aes.org/e-lib/browse.cfm?elib=5837
        
        which is maybe interesting reading (haven't yet read it myself).
        
        It appears that when Dick Heyser introduced his TDS technique in the late 60's, for the first time it was possible to "see" early reflections, which initiated the different concepts of a reflection free zone in studio control rooms. No psychoacoustic research, no listening tests were made to connect the measurement to human perception:
        
        Voetmann, "50 years of control room design", AES paper 7140.
        
        So personally, I do not read too much into a measurement, ETC or other, when human perception is not taken into account.
        
        My room is 4.9 x 8.6 x 2.5 m, speakers are 75 x 40 x 45 cm (h x w x d).
        
        Klaus
        
        RE: ETC - joamonte 14:32:37 07/08/09 (1)
        
        In Reply to: RE: ETC posted by KlausR. on July 05, 2009 at 13:32:06
        
        Well....I am disagree with you on the part you mention room measurement is not important ,to me used only "human perception" to judge a room most of the time could be too subjective; I mean ,how do you know what is ACTUALLY happen in the first 6 ms or 30ms of early reflection, by using ear only if you hear something is affecting the sound quality ? human brain are too slow to response to what the ear have received...
        
        But I sure for those believe absorption on early reflection point is best solution in any room acoustic treatment should read below pdf....."Broadband reflection"~~ hmmmm...It do make a lot of sense to me...LOL.;-D
        http://www.linkwitzlab.com/AES'07/AES123-final2.pdf (Open in New Window)
        
        RE: ETC - KlausR. 01:40:57 07/09/09 (0)
        
        In Reply to: RE: ETC posted by joamonte on July 08, 2009 at 14:32:37
        
        To give you one example: some people say that comb filters are a problem and the proof is that they appear in measurements (e.g. Fig.1 on
        http://www.realtraps.com/rfz.htm)
        BUT
        How do you know that what you measure is indeed audible?
        You don't unless you go to the lab and run psychoacoustic experiments with comb filters. This has been done and subsequently thresholds of audibility do exist but simple measurements not taking such thresholds into acount are preety much meaningless:
        Salomons (1995), “Coloration and binaural decoloration of sound due to reflections”, Thesis, Delft University
        http://www.darenet.nl/en/page/repository.item/show?saharaIdentifier=tuddare:oai:tudelft.nl:200755
        The same goes for "what happens in the first 6 or 30 ms of early reflections". The precedence effect has been studied very extensively so perceptional scientists have a good idea of what is going on:
        Litovsky et al. (1999), “The precedence effect”, J. of Acoust. Soc. of America, vol. 106, p.1633
        I'm not saying "do not measure but listen instead", I'm saying" measure but correlate what you've been measuring to how humans hear.
        Klaus
        
        Edits: 07/09/09
        
        RE: Hmmmmmm... - David Aiken 01:25:34 07/04/09 (7)
        
        In Reply to: RE: Hmmmmmm... posted by KlausR. on July 03, 2009 at 23:19:52
        
        My main point was that absorption which is less than 100% efficient can most certainly be beneficial, as you find with your ceiling, and that there are porous absorbers which are just as effective as membrane absorbers like your ceiling.
        
        Whether or not you're unwilling to recommend treatment to anyone, the fact remains that you've chosen it in your room and quite extensive treatment at that. As I've said previously, if I covered my ceiling area with absorption as effective as that in your ceiling, the amount of absorption in my room would be quite a bit greater than what I've got in the treatments I have installed. Whether or not you're prepared to accept or admit the fact, your room contains quite significant acoustic treatment and the treatment most people here are talking about or adopt is often significantly less than you're using. That being the case, perhaps you might consider being a little less critical of them for considering/using treatment since they're getting similar benefits in kind to what you're getting and you adopted the approach you did quite deliberately in order to achieve at least some of those benefits.
        
        David Aiken
        
        RE: Hmmmmmm... - KlausR. 06:35:34 07/04/09 (6)
        
        In Reply to: RE: Hmmmmmm... posted by David Aiken on July 04, 2009 at 01:25:34
        
        > Whether or not you're prepared to accept or admit the fact, your room contains quite significant acoustic treatment and the treatment most people here are talking about or adopt is often significantly less than you're using. <
        
        The room being built using bricks, covered with plaster and wallpaper, ceramic tiles wall to wall, would have been a disaster without any reverberation treatment. As I said, the one and only reason for having that ceiling installed was to lower reverberation time. In that particular sense I deliberately did what I did. As a side effect, it decreases the Q of room modes. Note that the modes are still excited and perceivable when playing sine tones. Apart from that ceiling I don't have any further treatment.
        
        I was quite surprised to read the conclusions Toole draws in his 2006 AES paper, so I want and read the background literature myself. I won't stop people from doing what they want to do, all I am saying is that there is no evidence that early reflections are detrimental so there's no need to treat them. If you, or anyone else, prefers reflections treated, be my guest. I do not argue about preferences.
        
        Klaus
        
        RE: Hmmmmmm... - David Aiken 14:36:05 07/04/09 (5)
        
        In Reply to: RE: Hmmmmmm... posted by KlausR. on July 04, 2009 at 06:35:34
        
        I quite appreciate your point about reducing reverberation time and actually think I get more benefit from that than from the smoothing of room response I get from my bass traps. Given the non-rectangular shape of my room which results in 1 more axial mode than I would get in a rectangular room, plus some additional oblique and tangential modes as well, plus the 2 open archway entrances which open the room to other spaces, variation in level of room modes isn't as extreme in my room as it would be in a closed rectangular room. The gain in bass clarity from reduction of reverb time, especially at modal frequencies, is subjectively a stronger gain for me in my room.
        
        But there's no way of getting only a single benefit from any absorption treatment. You can't reduce reverb time at bass frequencies without smoothing room response, and you can't reduce reverb time at any frequency without reducing the overall level of reflections relative to the level of the direct sound. Any change we make brings other effects besides the one or ones we're particularly chasing. It's quite possible that a person may have a preference for untreated first reflections but on balance prefer acoustic treatment at a first reflection point simply because gains elsewhere are more important to theme and outweigh what they might perceive as a loss from the reduction in early reflections.
        
        While lab experiments can be designed to isolate specific effects, and the studies on the effect of treating reflections weren't done by treating reflections but by simulating them with a secondary sound source whose level was separately controllable to the level of the "direct" source, we can't isolate and treat individual effects once we start actually treating a room. Put in a given treatment and you have to accept all of the effects produced by that treatment and all you can do is to assess the overall result of that treatment compared to the original untreated sound and decide which you prefer. It may be hard to tell which effect is most responsible for someone's preference for one over the other.
        
        In my case my preference is definitely for treatment at side first reflection points but those reflections have a big effect on the L/R symmetry of the sound you get and for the last almost 30 years now I've had my system in 2 asymmetric rooms, each with a different form of asymmetry. Treating the side wall first reflections in each case has resulted in significant improvements in the symmetry of the sound field and I respond strongly in favour of that outcome. I've also been an amateur musician and instruments and voices sound more "natural" to me when the direct sound is stronger than that of the reflected sound, but that's the way voices and instruments sound when you're standing next to them or playing them and the walls are some distance away. Musicians can't avoid listening in the near field and what sounds "natural" is the sound you're used to so it's not surprising that Toole comments that musicians tend to be more sensitive to side wall first reflections than others and prefer the level of those reflections much lower.
        
        When I listen at someone else's place, in a symmetrical and untreated room, I don't have a problem with side reflections. The sound is simply closer in some ways to what I hear when I sit in a hall to listen to a live performance and the reflected sound is stronger relative to that of the direct sound than when I play myself or stand close to a performer, even in a normal sized living room. I don't find anything wrong with that but it doesn't have the same "feel" as music does when I'm the player or I'm in close proximity to a friend who's playing and those experiences are the ones which really inform what I regard as the "sound" of music, what seems "natural" to me. I suspect that if I ever manage to find myself in the position of having a rectangular, symmetrical room, I'd opt to treat the side wall early reflections just to get that same strong direct/weak reflected sound combination for the sort of music I mostly listen to which is small group and solo performers.
        
        On the other hand, I do have a preference for untreated early reflections with symphonic and big band music, music which I've only heard from a position in the audience in a concert venue but that kind of music is music I don't listen to regularly. If it were my main fare in recorded music I'd be adopting a different approach to room treatment than I have, even in an asymmetric room.
        
        So, my main point here is simply that I don't think it's possible to simply say that there's no need to treat early reflections because there's no evidence that they're detrimental. You can't treat early reflections without affecting some other things, including reverberation time at frequencies over the absorption range of the treatment, and there's no way in practice to restrict the effects of any treatment to the ones you specifically want. It doesn't make sense to me to say "don't treat early reflections because there's no evidence they're harmful". It makes sense to me to point out what the effects of treating those reflections is going to be on different aspects of the sound and to say to people that if you're chasing one or more of those effects, then that sort of treatment can help you get what you're looking for but if those effects aren't what you want then don't go that way which is the way I try to deal with these questions.
        
        Too many of the people asking questions here, and even some of the respondents, seem to think that there's really only 1 way to treat a room and that any room treatment is always going to result in the same sort of audible outcome. That simply isn't the case and it is possible to steer the outcome in different directions by treating a room differently. Whatever we do, however, we end up with a "package" of outcomes that can't be individually treated with no effect on other outcomes. What counts in the end is whether a given "package" suits you better than another package.
        
        You can certainly enjoy the reduction in reverberation time in your room from your ceiling treatment but it comes with a cost that includes significant bass trapping and a smoothing of the room's modal response which you can't avoid. There's no treatment that will reduce reverb time at low frequencies without also providing the effects of bass traps, just as there's no way to smooth a room's modal response without reducing reverb time. You have no idea of how things would sound in your room if you could reduce the reverberation time to its current level at each frequency while not affecting smoothness of modal response or changing the level of reflections you would get from the untreated ceiling. You can't achieve that reverb time result without those other effects and your preference for the results you're getting may be due as much to the benefits of the unavoidable bass trapping and reduction in ceiling reflections as it is to reduction in reverb time. You simply have no way of knowing because you can't get one without the other, just as I have no way of knowing how much my preference for treatment at lateral first reflections is due to their benefit in reducing L/R asymmetry in the soundfield in my room and how much is due to the change in balance of direct and reflected sound in my room because I, likewise, can't get one without the other.
        
        I think your reading has taught you a lot about the trees but when we get to our rooms and what we can and can't do in them we're dealing with forests and a forest is something different to simply a given number of trees in close proximity to each other. The behaviour of forests is not simply a magnification of the behaviour of a single tree. You can't consider room treatment in terms of what its effect on one or more specific features will be in isolation from each other or ignore what other effects are going to occur and the way the different effects will combine depends a lot on the nature of the specific room. How we perceive those effects depends a lot on what our experience of live music is and what aspects of recorded music we find most critical to our enjoyment of it. Theory is most certainly a useful guide but it's certainly not a reliable predictor of overall results for a specific user of a specific room. It would be a better predictor if the relative strengths of different issues were the same across all rooms but they're not, and if we could control a single parameter within a room without affecting other parameters but we can't and neither of those things is going to change so theory is always going to have to be tempered with an appreciation that any given treatment won't produce the same effect in every room and that we can't change one characteristic without changing others at the same time.
        
        David Aiken
        
        RE: Hmmmmmm... - joamonte 22:54:57 07/04/09 (4)
        
        In Reply to: RE: Hmmmmmm... posted by David Aiken on July 04, 2009 at 14:36:05
        
        I have totally loss the focus of what you guys arguing about after reading David's long reply...LOL!
        hmmm....I though we all are working hard trying to understand more about the forests by study the individual tree behavior, isn't we??If we didn't study each individual tree? how can we understand the forest??...
        
        Edits: 07/04/09
        
        RE: Hmmmmmm... - David Aiken 16:11:53 07/05/09 (3)
        
        In Reply to: RE: Hmmmmmm... posted by joamonte on July 04, 2009 at 22:54:57
        
        For my part I think the argument is about how to respond to requests for advice.
        
        The average request indicates that the poster has a problem, whether it be uneven bass response or something else. Klaus's approach seems to me to assume that there is only that problem and everything else is sweet. He tends to question whether the stated issue is a problem, often cites studies that indicate that the stated issue isn't a problem, and often says don't do anything.
        
        I tend not to think that there is only one problem though I do tend to accept the poster's view that he does have problems with his stated issue. Having said that, I sometimes wonder whether the stated problem is the real issue for the poster or whether they're simply seizing on something they've read or heard and assuming that's the cause of their problem because there's certainly an element of that which goes on from time to time. Finally, I also tend to think that if someone is considering acoustic treatment then they basically do want to change the sound they're currently getting in some way so, even if they're unsure about what they want to achieve or even wrong about what their real issue is. There's no reason to consider acoustic treatment if you don't want to change the sound you're getting for some reason, even if you don't know what the reason is, so I'm more inclined to make some recommendations and to try to present those recommendations in a way that addresses what kind of sound presentation the poster might really be chasing. I also tend not to think of something like reducing reverberation time in isolation from other things like the effects of bass trapping or reducing reflections because there's no way to do one without the other.
        
        It would be really nice if we could address each aspect of sound behaviour in a room individually, to tailor modal response without affecting anything else, to control reverberation time without changing tonal balance and other characteristics, and so on but when it comes down to how we can actually treat a room that's currently impossible and, as far as I can tell, will always be impossible. I think Klaus' approach would be fine if we could treat things in isolation but we can't, and I also think it ignores the fact that there has to be some desire to change the sound in some way if someone is going to be serious enough to start thinking and asking about acoustic treatment so it's worth while to start presenting options for them and indicating what the advantages and disadvantages of each option are.
        
        Sure, we do study individual tree behaviour to learn how trees work and how trees work is vital to how forests work but how forests work is more than just the sum of how the individual trees work. Assessing whether or not there is a problem or how severe a problem is on the basis of one parameter only deals with one tree but ignores the forest. Assuming that changing one problem parameter for the better will produce a satisfying outcome without considering the effect of that change on other parameters and the possibility that those other changes may not suit the person and may make things worse ignores the forest. Assuming that acoustical issues in a home listening room can be addressed without worrying about the user's preferences for one sort of sound over another ignores the forest. We need to study the individual trees but we can never understand a forest simply by studying individual trees in isolation and we can't really assess the state of the forest by assessing each tree in isolation.
        
        Finally, if what we're really on about when we play around with the acoustics of our rooms is improving our musical enjoyment, which is definitely what I'm concerned about with my room, the things we can measure easily at home, or even that we could get a professional to measure, will never be a reliable guide to what will make us happy. Why do some people prefer a front row seat to a mid hall seat in a particular hall? Does everyone who prefers a similar location in that hall do so for the same reasons. Why do some people prefer one hall and others a different hall? Dealing with the scientific aspects don't address those kind of questions and measurements alone can't answer those questions. Similar kinds of audible differences in sound which result in those differences also affect how we each experience sound at home and how much we enjoy it. We can't deal with those issues and help people to get a more enjoyable result simply by dealing with the trees, the things we can measure. Most rooms produce acceptable measurable results, often even very good results, just as most concert halls do but that doesn't mean that everyone will be happy with the sound any given room provides. In fact we can guarantee that not everyone will be happy with any given room even though some people will be. Saying there's no reason to do anything because the room measures fine ignores the human element in what we're doing. Analysing the measurable issues, pronouncing judgements on them, and then saying "anything else is preference and I don't deal in preferences" ignores why people consider room treatment in the first place and that is simply to get a sound that makes them happier. If we aren't prepared to deal with preferences and happiness there's no reason to worry about acoustics. Any room is likely to do just fine.
        
        Unlike Klaus, I'm more interested in the preferences and happiness and how different treatment strategies affect how I and others feel about the music we listen to than how things measure. The science side of acoustics certainly helps when it comes to working out what to do but it doesn't answer everything. Preferences are messy and often confusing but they can be dealt with and I think they should be addressed as a matter of course, not dismissed as inconsequential.
        
        David Aiken
        
        RE: Hmmmmmm... - Jolida 22:41:34 07/05/09 (2)
        
        In Reply to: RE: Hmmmmmm... posted by David Aiken on July 05, 2009 at 16:11:53
        
        Hi David,
        The issue has been 100% resolved now. Thanks a million to you. I finally managed to get rid of the nuasense completely, that had been bothering me for a very long time. As u had suggested, i wanted to remove the glasses from that table last evening. But i was'nt sure if i should be doing it coz it may spoil the look of it by exposing the components & the wiring behind them. Reluctantly, i just threw a thick blanket on top of the table, AND WAS STUNNED as to what happened. The whole damn problem just disappeared as if it never exhisted. I dont know if its the glass or the top of the table being close behind the speaker that's causing it. I was delighted at what happened, & i just can't thank you enough. I was listening to all the cd's i had major issues with & all sound absolutely clean without a hint of colouration, unlike earlier. I had made that table less than a year ago, but at that time i had disconnected my system for quite sumtime. After i connected the system after about 2 months, i had bought new speakers & cd player & amp. When the problem started popping out, i thought it might be either of those newly added components that were contributing to it.
        Im in such a relief now.... Thanks so much
        
        RE: Hmmmmmm... - David Aiken 23:57:15 07/05/09 (1)
        
        In Reply to: RE: Hmmmmmm... posted by Jolida on July 05, 2009 at 22:41:34
        
        Jolida,
        
        I'm glad you've solved the problem. Objects resonating in the room can certainly be a problem and there's always going to be a tension between living with the problem, removing the offending object from the room, or simply doing something like "throwing a blanket over it".
        
        I suspect the problem is the glass which is facing forward. Reflections off the top are going to go up and get reflected at least once more before reaching you and they're going to lose some high frequency energy in that process. Glass can be more reflective than wood at high frequencies and the glass doors are probably free to move a little and resonate also which the top isn't so the glass doors would definitely be my choice for the cause.
        
        And removing colourations from the sound really does make a difference.
        
        Enjoy.
        
        David
        David Aiken
        
        RE: Hmmmmmm... - Jolida 03:08:32 07/06/09 (0)
        
        In Reply to: RE: Hmmmmmm... posted by David Aiken on July 05, 2009 at 23:57:15
        
        Yes very true... It feels like i am hearing strain-free music after ages. As throwing the blanket is not practical for everyday listening, i will remove the glasses this evening to see what actually is happening.
        Also i have another question, which may be inappropriate to ask in this forum, as this a room Acoustic forum, so im posting a query for u in the "cable" asylum. Once again, thanks a real lot for helping me in solving the problem. i really appreciate.....
        
        Here is the link...
        
        http://www.audioasylum.com/cgi/vt.mpl?f=cables&m=142190
- RE: Peaks and nulls - KlausR. 00:17:39 07/01/09 (0)
  Hi,
  
  your hearing may be rather sensitive to the frequency range in question, but then there's the boost in the measurements. What about other listeners, do they consider the passages you are mentioning also as "audibly hurting" ? Maybe that the boost is benign and you are more sensitive to it than others?
  
  If that boost appears regardless of measuring location in the room, I would consider it not an issue related to room acoustics (standing waves or comb filters). Does that boost also appear when using different speakers/amp etc.? What happens when you move the system to another room?
  
  The most likely explanation is, as David already said, a resonance somewhere.
  
  Klaus

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