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Hmmmmmm...

124.187.7.33

David Aiken

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)

Follow Ups:

RE: Hmmmmmm... - KlausR. 23:19:52 07/03/09 (12)
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 �ber die Wahrnehmbarkeit von Einzelr�ckw�rfen 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)
  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)
    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)
      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)
  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)
    > 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)
      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

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