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In Reply to: RE: Early reflections: 5 ms time window for sound source localisation? posted by KlausR. on July 05, 2010 at 22:20:52
Hi
I don’t have any links handy, but I can say that close reflections are universally accepted as being “bad” sonically in the loudspeaker design end of things. These reflections cause a series of late arrivals, partial duplicates of the actual signal but delayed according to the difference in distances they have traveled.
In the recording studio, since the 80’s it has become normal to make the front half of the room absorptive and the rear scattering reflections.
A real Stereo image depends on the “mono phantom” or image that floats between the two speakers.
That image is one your brain derives when the ears are presented with two identical signals with no time delays or other aural clues that originate at / near the loudspeakers. What one DOESN’T want is anything that wasn’t part of the original signal, everything else acts to make the brains job more difficult by adding “clues” as to the speakers location as well as the reflected sounds arriving from different physical locations.
When one can reduce everything but the original signal, a weird thing also happens. Playing one speaker with a voice signal, one finds that with your eyes closed, it is easy to hear what direction the sound is coming from BUT the more like one source it is, the harder it is to hear how far away it is.
In other words, most speakers, even well away from the walls or floor, still radiate addition delayed sounds, sounds which have other origins and you ears can identify those discontinuities and use that to localize the physical depth of the speaker. The stronger these “extra” signals are, the more easily the depth is audible AND the less able the speaker is to produce a strong mono phantom or stereo image. This is also why the ability to locate depth is stronger in a typical multi-way speaker over a single radiator element, the multi-way system is not coherent, in addition to “where the sound goes” by having separate sources, does not have a single source or acoustic origin.
This all might sound rather odd, that disconnected or reflected sound is “bad” but it is true so far as hearing the original signal. Also, the larger the room dimensions, the worse the problem of reflected sound becomes (because the volume of the room and surface area do not track linearly) and the more important it is to project the sound only where the people are.
It is summer in the northern half, one can demonstrate what I mean pretty easily and have a nice time.
Set a spell and listen to a couple of your “best” recordings, then, move your stereo outside. Set it up at roughly the same distance as it was indoors, with the speakers at the same height BUT no walls behind or to the sides of the speakers. You system will be bass shy because of the lack of LF room gain etc so ignore that. Now play the same recordings, sit back and listen. If your speakers are pretty coherent, you will hear a much stronger stereo image, you are hearing more of the recording and less of the stuff which was arriving after any signal impulse. Fwiw, this is a great excuse to have a BBQ also.
Now, when you move back in your room the difference will be more noticeable. The up side is absorbing close reflections will help a great deal. Keeping the speakers away from the walls is also a strategy which reduces the strength of the reflections BUT makes the image / sweet spot proportionally narrower. If you can put actual sound absorbing foam on the side walls near the speakers, you make a very noticeable improvement / allow a wider speaker position. I had a narrow listening room once where this was a REAL problem and by putting absorbing foam UNDER a couple of my wife’s weavings, I had a very good solution with SAF.
Best,
Tom
Follow Ups:
Hi Tom,
> I don’t have any links handy, but I can say that close reflections are universally accepted as being “bad” sonically in the loudspeaker design end of things. <
Not universally! Floyd Toole doesn’t, and I don’t. I had a look at the relevant scientific literature (and prepared a 23 pages write-up, if interested, drop me a mail) and there is no thorough, systematic research that shows that first reflections are bad as a matter of principle. There are indications that different directivities have an effect on imaging. In some individual cases (strong right-left acoustical asymmetry, bad off-axis behaviour of the speakers) it may be beneficial to treat, but even for that there is no evidence.
I strongly suspect the reason for considering reflections as bad is the simple fact that there are thresholds of perception. In order to perceive a reflection some cue must be present and detected, change in loudness, change in timbre, spatial changes. If you detect a reflection by a change in timbre then yes, the reflections might be considered as having a negative effect. However, all threshold research I know of is using a single speaker as direct sound source and a single reflection. To the best of my knowledge no research exists that examines the 2-channel/multiple reflections case.
> That image is one your brain derives when the ears are presented with two identical signals with no time delays or other aural clues that originate at / near the loudspeakers. <
Summing localisation. Reflections arriving within this very short time window ( < 1 ms) may or may not have an effect, but also for this there is no research. One can always play safe and avoid these reflections, i.e. no cabinet edge diffraction etc.
> Set a spell and listen to a couple of your “best” recordings, then, move your stereo outside. <
That’ll be quite an act with my speakers being 70 kg each! But I tried this once with my Genelec desktop monitors, using particularly adapted tracks from EBU SQAM and Chesky JD37. The ceiling of our living room is highly absorptive, so the imaging the speakers produced indoors and outside was the same to me.
I consider the message that there is no principle need for reflection treatment as really good news, since such treatment in domestic living rooms have extremely low WAF and will therefore in most cases inevitably lead to problems.
Klaus
Hi
“Not universally! Floyd Toole doesn’t, and I don’t. I had a look at the relevant scientific literature”
I am not sure Floyd actually designs speakers anymore and add to that how strongly home audio is driven by popular belief promoted by manufacturers and one has “how things are” in the home. I was speaking in terms of acoustics and among loudspeaker and room designers I know and deal with.
“There are indications that different directivities have an effect on imaging.”
Obviously AND if one measures one finds that the more directional the system is the more like the anechoic response the response at the listening position looks.
This is because there is progressively less reflected / late sound as the directivity is increased. In more familiar terms, a speaker in a room has a near-field AND a far field. In the near field, the direct from the driver sound is stronger than the reflected sound, equality or +10dB is often cited as where this begins. In the far field, the reflected / reverberant sound is greater in level than the direct sound. The reverberant field does reflect the acoustic power vs frequency of the total speakers output pitted against the absorption vs frequency of the room.
The greater the direct sound level relative to reverberant / reflected level, the more like the in room listening position curve looks like an anechoic response curve. Out doors, there are ideally no significant radiations other than those associated with the speaker itself and so if the speaker has constant directivity, it’s response curve stays the same shape as you increase the listening distance.
Conversely, if the speaker does not have constant directivity, then the response curve changes as you change the distance. If one can create a coherent single point source over a wide bandwidth, then with the exception of hf air absorption, the speaker sounds the same (except for loudness) at 10 feet or 100 feet but 20dB quieter.
One thing Floyd Toole did confirm in testing is that the average listener prefers the reverberant sound field have roughly the same spectral balance as the direct sound and this points to Constant directivity over a wide bandwidth.
This goal is (like an absence of close reflections) also pretty much the norm in larger listening spaces where all of the room acoustic problems are worse and the listening area larger.
Another poster mentions edge diffraction; this also falls in the category of “where the sound goes after leaving the source”. Sound is re-radiated from each diffraction point and arrives as a delayed partial signal like a reflection. Like reflections, reducing these improves the stereo image / reduces the speakers source signature or ability to hear it’s physical depth when ones eyes are closed, listening to one speaker.
Ideally, what you want to arrive at your ears is the signal the voice coil produced with everything that followed (via reflection / diffraction / resonance) held to a minimum or ideally not existent if it wasn’t part of the electrical signal.
A Huge unrealized problem is the “free sound” speakers radiate which interferes with the actual signal your trying to “reproduce”. Take a short broad band impulsive signal like the classical 20-20k impulse, feed that to a loudspeaker and examine what comes out and then what arrives at the listening position, how much “extra” there is, how much spread out in time it is.
Any wonder most loudspeakers don’t sound “real”?.
In my work, Horns are the only way to produce the intensity one needs in a larger scale system like say a state of the art movie theater or media room however the need to use separate units for different frequency ranges within one channel, causes them to interfere with each other in the crossover range when two sources are radiating.
Unfortunately the acoustic dimensions required to make multiple sources add coherently conflicts with what is needed to make a proper horn. While time delay can fix these problems in depth, they cannot fix source origin spatial errors in X and Y and any “fix” is not universal and produces different results if you move left or right, up or down and is not constant directivity.
Compared to cones and domes however, larger speakers like horns and large sources like ess speakers which have directivity are more to my liking in most rooms.
I know you mentioned your speakers are heavy, so are mine but I have no choice but to move them around occasionally. If you can, if others are wondering, DO set them up outside and listen critically. Ignoring source origin problems, the less directivity your system has, the greater the improvement in stereo imaging one hears without a room. To the degree one hears an improvement, one would benefit from treatment of the side walls where the strongest specular reflection originates.
Place a mirror flat against the side wall, have a helper move it around until you can see the tweeter from your listening position and treat that spot with absorption several feet square (more for aesthetic purposes).
Outdoors, with one speaker at head height producing a voice, the less self interference the speaker has (which includes refraction, resonances, interference lobes and nulls etc), the less able you are to tell the speakers actual physical depth when your eyes are closed.
The more strongly the source ques are radiated, the easier to hear exactly how far away it is. The things which identify to ones ears the sources actual physical depth, detract from its ability to make a mono phantom image.
I know it is a pain to set up outside but one can hear or even learn a great deal about what a given set of speakers do outdoors.
Best,
Tom
Hi,
> Ideally, what you want to arrive at your ears is the signal the voice coil produced with everything that followed (via reflection / diffraction / resonance) held to a minimum or ideally not existent if it wasn’t part of the electrical signal. <
Ideally yes, but is it really necessary? As it is the case for cabinet edge diffraction, one can always play safe and reduce the level of early reflections to below perception thresholds, which, at -18 to -25 dB re: direct sound, are quite low, so getting there seems to be quite difficult.
I haven't seen research that confirms the claim that early reflections are bad as a matter of principle. I made the outdoor experiment with my small Genelecs. I made an indoor experiment where I placed the Genelecs on our dining table, and placed room doors against the table to generate lateral reflections. Listening nearfield I could not hear a difference in imaging (same tracks from EBU SQAM, Chesky JD37). If there was coloration, it was not strong, I didn't notice anything unusual. According to Toole reflection treatment may be beneficial if the speakers have odd-looking off axis response. Seems as if I'm lucky and my speakers behave well in that respect, both (main and desktop) are designed to avoid cabinet edge diffraction.
Outdoor vs indoor listening: indoor you have a reverberation time, which may, or may not, have an effect. What is known is that reveberation has an effect on absolute detection levels of reflections, but apart from that?
> Any wonder most loudspeakers don’t sound “real”?. <
I think that the main reason for this is the, compared to loudspeakers, very different radiation patterns of acoustical instruments, in combination with the direcionality of human hearing.
Klaus
“Ideally yes, but is it really necessary? As it is the case for cabinet edge diffraction, one can always play safe and reduce the level of early reflections to below perception thresholds, which, at -18 to -25 dB re: direct sound, are quite low, so getting there seems to be quite difficult.”
What is necessary, is a cost benefit vs judgment I think and somewhat outside of the acoustic / hearing part of the discussion.
Also, while reflections are mentioned specifically, I would stress that Time is the issue and not the specific source of the late radiation. Those late signals can come from a number of sources, close reflections being just one.
In the time view, if one considers Heyser’s “determination of loudspeaker arrival time” and other work on measuring time / acoustic phase, one finds that even one full range driver can appear to be at different physical positions depending on frequency.
Using Heyser’s Time Delay Spectrometry, one finds that a typical loudspeaker does spread a single impulsive signal out in time even measured anechoically, many have hundreds or thousands of degrees of phase rotation high Frequencies relative to the lows.
Sound travels about 1 foot in .883 ms, so consider a thought experiment.
To say short time delays do not matter, requires comparing two cases, one with and one without.
What kind of loudspeaker could be used for such a test which suppresses secondary radiations out to several or even 5 ms In time? What kind of loudspeaker could be used for this test which by itself appeared to be one source in time and space over a wide frequency range?
“According to Toole reflection treatment may be beneficial if the speakers have odd-looking off axis response.” Floyde did study the preferred spectrum of the reverberant field, the conclusion was listeners prefer the reverberant field to have roughly the same spectrum as the direct field has.
Because the reverberant field is related to the total acoustic power the speaker radiates, there is a built in desirability of having constant directivty. In order to have the largest nearfield possible,
You mention you have Genelec’s. You will notice they take some pains to minimize the re-radiations from the baffle board region, consider what one has if the baffle and treatment were MUCH larger so to accommodate control of mid and lower frequencies and made so the at the high frequency radiation angle was approximately the same for the mid and lower frequencies too. I don’t recall if they publish polar plots but these show the radiation balloon vs frequency in the two planes Vertical and Horizontal. Imagine narrowing the pattern over a broad band down to say 40 to 60 degrees so that there simply aren’t any near reflections.
“I think that the main reason for this is the, compared to loudspeakers, very different radiation patterns of acoustical instruments, in combination with the direcionality of human hearing.”
I should have made a distinction about stereo, capturing a real stereo image is hard to do live, most (by a large degree) of modern recordings have an entirely contrived stereo image. I would agree, except I would add that relative to the electronic end, loudspeakers by comparison appear to be broken, are usually the weakest link and directivity does play a very strong part in what you hear at the listening position.
It would seem you’re a keen listener. I have been working on a different way to capture a stereo image for live sounds, I would be curious to get your reactions to it. If you’re interested in an alternative recording technique (a work in progress) , down load the Harley or other sound files at the bottom of this page. Try these with headphones first, DO NOT convert them to mp-3 files, that kills them as there is no compression in the recording.
http://www.danleysoundlabs.com/technical%20downloads.html
Best,
Tom
> In the time view, if one considers Heyser’s “determination of loudspeaker arrival time” and other work on measuring time / acoustic phase, one finds that even one full range driver can appear to be at different physical positions depending on frequency. <
Localisation blur is quite high anyway, when compared to localisation precision for sight, so any imprecision is possibly "masked" by the blur.
> To say short time delays do not matter, requires comparing two cases, one with and one without.
What kind of loudspeaker could be used for such a test which suppresses secondary radiations out to several or even 5 ms In time? What kind of loudspeaker could be used for this test which by itself appeared to be one source in time and space over a wide frequency range? <
If loudspeakers themselves are not capable of operating without any secondary radiation, then the question of baffle re-radiation becomes possibly moot.
> You mention you have Genelec’s. You will notice they take some pains to minimize the re-radiations from the baffle board region... <
The current line is called Minimum Diffraction Enclosure. I've also got a single 1029A of the previous line, the successor 8020 is a clear improvement.
> It would seem you’re a keen listener. <
I'd say I'm more a keen reader than a keen listener. Listening is just for pleasure, I don't spend hours in "critical listening mode". I did some homework before buying my current speakers, discovered the work of Toole and Olive and selected the speakers accordingly. I tried a couple of things, such as generating early reflections, listening outdoors, but that's it.
We are leaving for holiday tomorrow, so I'm afraid that listening to that sound file will have to wait until mid-August. Thanks for the link.
Klaus
Did Floyd ever design loudspeakers? To my knowledge, he conducted research tests and wrote papers.
As for reflections and diffraction, my friend who has been researching the studies says that reflections below 1ms are not perceptually distinguishable as a separate sound event and only affect the perceived tonal balance.
Donald North
"says that reflections below 1ms are not perceptually distinguishable as a separate sound event and only affect the perceived tonal balance."
Quite true, the time span must be much larger in order to hear secondary radiations as a separate event. Earl Geddes work identified small changes effect the stability of the mono image which is consistent with my listening as commented in an earlier post.
When you remove these "extra" radiations in time, it becomes harder to hear the physical depth when one speaker is playing. When the speaker's "clues" are small enough, one can't easily hear it's distance with your ears.
If you can, set your stereo up outside, away from walls etc, have a BBQ and enjoy what it sounds like and the stereo image with no room effects.
I did exactly this 20 years ago. To me the hi-fi sounded excellent, notably clearer, while listening outdoors.
Donald North
"I consider the message that there is no principle need for reflection treatment as really good news, since such treatment in domestic living rooms have extremely low WAF and will therefore in most cases inevitably lead to problems."
The room is the most important component in an audio playback system. One can not have a high end system without a suitable room, which means either: (1) a dedicated room that is optimized for sound reproduction, (2) a shared room that is optimized for sound reproduction and the corresponding absence of a "partner" who prevents this optimization. Looking at the associated economics, one sees that the room is likely to be the most costly component, unless one happens to be single.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
“The room is the most important component in an audio playback system. One can not have a high end system without a suitable room, which means either: (1) a dedicated room that is optimized for sound reproduction, (2) a shared room that is optimized for sound reproduction and the corresponding absence of a "partner" who prevents this optimization. Looking at the associated economics, one sees that the room is likely to be the most costly component, unless one happens to be single.”
This is all true with a hidden assumption.
Obviously, one can listen outside where there are no walls and no ceiling, no room gain and no reverberant field.
A suggestion I made is to actually try that to see what difference the room makes with a given set of speakers. Normally with hifi speakers it makes a big difference.
Your assumption is on the speaker’s directivity or lack of.
To the degree the sound radiation could be confined to avoid any close reflections, then one is approaching the condition one hears outdoors where the direct sound is much louder than the first or following reflection sound. This is the proverbial “nearfield” condition.
If one measured two speaker systems that had an identical amplitude response (and all other things equal) BUT one was a wide dispersion speaker and the other narrow, they would measure identically outdoors. Once in the room however, the response curve taken at the listening position (a place that actually matters more than at one meter) will be much worse for the wide dispersion speaker as it contains much more room sound. With a narrow speaker, in a living room, one can measure a variance of say + -3dB greater than the one meter curve while a dome /cone system in the same location is more like + - 10-20dB.
Directivity at the source is doing what massive room absorption does for a non-directive system.
Unfortunately for home audio, for the most part the importance of directivity is not widely recognized and more importantly, directivity is only achieved with physical size commensurate to the wavelength being controlled and that does not lead to pretty shoe boxes, making constant directivity more difficult yet.
The larger the acoustic space, the more important all of this consideration of where the sound goes becomes.
To properly measure ‘where the sound goes” takes hundreds of measurements taken over the surface of a sphere surrounding the speaker.
This is normal practice in the technical end of commercial sound as some people design sound installations based on what the speakers actually do.
For the speaker I design at work, we use the CLF data format with the measurements taken by an independent lab (the spherical measurements are too much work).
http://www.clfgroup.org/index.htm
So your room could do anything from destroy any imaging in the recording to being slightly worse than outdoors depending on the directivity and the consistency of it.
Best,
Tom
This is, of course, what high-end aficionados think. Having read the relevant scientific/technical literature, I think that there are many, too many, myths around room acoustics, myths because there's no supporting evidence, the myth of principally bad early reflections being just one of them. Read Floyd Toole's paper "Loudspeakers and Rooms for Sound Reproduction--A Scientific Review", read the literature quoted therein, read the literature listed here , then conclude.
Klaus
Are you disagreeing with my statement that the room is the most important component or are you just commenting on details?
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
> Are you disagreeing with my statement that the room is the most important component? <
Yes, I do!
You will obtain very different results in different locations of the speakers within the same room:
Olive et al. (1994), “The effects of loudspeaker placement on listener preference ratings”, J. of Audio Eng. Soc., p.651
Bech (1994), “Perception of timbre of reproduced sound in small rooms: influence of room and loudspeaker position”, J. of Audio Eng. Soc., p.999
You will obtain very different results in different locations of the listening chair within the same room:
Salava (2004), “Imperfections at low frequencies – how much are they audible or annoying?”, Audio Eng. Soc. Preprint 6144
The room doesn’t change, but the result does. Behaviour and perception of bass depend on placement of speakers and listening chair. Effects of early reflections depend on placement of speakers and listening chair and on the speakers’ off-axis behaviour. Since there is interaction of room and speakers you cannot separate the two and say the room is most important, placement is most important, the speakers are most important. You can put the best speakers in the world in a room built to all the audiophile standards you know, if you put speakers and yourself in the wrong place, the result may be disappointing.
In the same room, badly designed speakers will cause more problems than well designed speakers. Most people don’t know or don’t care, yet think that high-end is a label for quality.
Klaus
I don't believe that we are really disagreeing. There is no doubt that speakers interact with rooms and that location is very important. As you say, the bass varies according to location, as does early reflections. In addition, even if the room does not need treatment the WAF still affects the location of speakers and listening position, since the layout of all furniture in a room is likely to be affected by the location of speakers and listening chair/couch.
While we are at it, I would expand the list of interacting components to include the power amplifier(s) as well as the speakers and room.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
> I don't believe that we are really disagreeing. <
If “room” includes the interaction between room and speakers, hence placement and the speakers’ off-axis behaviour, then we indeed agree.
> In addition, even if the room does not need treatment the WAF still affects the location of speakers and listening position, since the layout of all furniture in a room is likely to be affected by the location of speakers and listening chair/couch. <
When living rooms are used for music listening, WAF inevitably is a parameter, unless you’re single. That’s why I said that the fact that there is no principle need for reflection treatment is actually good news. No treatment always has the highest possible WAF. You have to select the loudspeakers accordingly though. In this respect the discussion on whatsbest is interesting. Bass boom and room mode issues can be addressed electronically, again with highest possible WAF.
> While we are at it, I would expand the list of interacting components to include the power amplifier(s) as well as the speakers and room. <
In particular cases certain amps may intact audibly differently with certain speakers
E. Brad Meyer, “The amp/speaker interface”, Stereo Review June 1991, p.53
In such cases it may be worth paying attention. This problem, however, is elegantly solved in active speakers . No more complex loads for the amps, each amp is optimized for the driver he is powering.
Klaus
"One can always play safe and avoid these reflections, i.e. no cabinet edge diffraction etc."
I think that is good advice indeed. Not being a speaker guy I've been amazed at how much I can improve the sound of many of them with a little rubbery weather stripping at the edges in the tweeter area. My current ones have addressed the problem by design but for the rest it's worth considering.
It's similar to a mild pain, even if one isn't conscious of it as a problem there is still great relief when it it's gone.
Rick
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