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RE: Early reflections: 5 ms time window for sound source localisation?

Posted by tomservo (M) on July 8, 2010 at 09:20:02

In Reply to: RE: Early reflections: 5 ms time window for sound source localisation? posted by KlausR. on July 8, 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

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