|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
24.13.232.33
In Reply to: RE: Early reflections: 5 ms time window for sound source localisation? posted by KlausR. on July 08, 2010 at 01:30:12
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
Follow Ups:
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
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: