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Diffusing 1st reflections of speakers that measure great on and off-axis - instead of absorbing

March Audio

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In other words, I DO NOT THINK we need to wait for somebody to produce data before we use words like "large" and "small" when referring to acoustic spaces. Nothing against acoustic data, but we don't ALWAYS need it to have a conversation.
I disagree. Without data you are fumbling around in the dark. You need to know what is perceptually significant and then compare that to what you have measured. Otherwise we are just talking generalisations - or as we have seen, some extreme interpretations.

Ironic point is that the vast majority of listeners cant do a damn thing about any of this. Their rooms are their rooms and usually constricted by reasons of domestic harmony ;)
 

Duke

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LEDE requirements for the termination (sharply delineating the effectively anechoic ISD-gap) are "not be below -12dB from Ld (direct signal)". this can actually be quite difficult to do in small rooms as diffuse returns that high in magnitude will require sufficiently broadband diffusers - that then in themselves have greater minimum distance requirements which must be aligned with the chosen ISD length. this is usually accomplished with nested diffusers (a large array constructed of smaller/individual diffusers that has a much lower design frequency than the individuals). there could also be a haas kicker, which induces a high-gain specular reflection from a large (wrt wavelength) flat/planar panel installed at the rear side-walls and angled towards the listening position - arriving at the termination, or slightly later (nested within the diffuse returns).
Very interesting - THANK YOU!!

As a possible alternative technique for meeting the requirement that the termination of the ISD-gap "not be below -12 dB from the direct signal", what do you think of this approach:

Use fairly directional dipole speakers and pull them 10-12 feet out in front of the wall (or whatever distance gives the desired ISD-gap), and toe them in aggressively? Seems to me a lot room treatment effort would be needed to meet that requirement.
 

Duke

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I disagree. Without data you are fumbling around in the dark. You need to know what is perceptually significant and then compare that to what you have measured. Otherwise we are just talking generalisations - or as we have seen, some extreme interpretations.
So let me ask you a very simple question:

Is the acoustic space you listen in at home "large" or "small"?

And if you cannot answer that question, what data do you need in order to not be "fumbling around in the dark"?
 

March Audio

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So let me ask you a very simple question:

Is the acoustic space you listen in at home "large" or "small"?

And if you cannot answer that question, what data do you need in order to not be "fumbling around in the dark"?
Its irrelevant. It is what it is.

You need a suite of measurements and make a judgement based on the overall information. REW will capture what you need.
FR, impulse, RT(x) etc

1596422277836.png
 
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Very interesting - THANK YOU!!

As a possible alternative technique for meeting the requirement that the termination of the ISD-gap "not be below -12 dB from the direct signal", what do you think of this approach:

Use fairly directional dipole speakers and pull them 10-12 feet out in front of the wall (or whatever distance gives the desired ISD-gap), and toe them in aggressively? Seems to me a lot room treatment effort would be needed to meet that requirement.
the potential issue with active returns (reproduced direct signal + digital reverb effects/decay, if desirable) is that you still have a single point of radiation of that energy. where-as diffusers physically modify the indirect sound-field and create many reflections spatially and temporally dispersed, which will have different psycho-acoustic effects and wildly different IACC. a loudspeaker providing this "effect" will not sound or induce the same superposed response at the listening position. however it would be akin to the effect of that of the haas trigger (flat panel to induce high-gain specular reflection).

it is fairly easy to do these perceptual tests with a 2nd set of identical loudspeakers - and relatively easy to move them about (thus changing the direction and/or distance/time-arrival vs the direct signal). you can also add your own "Reverb IR" or reverb FX which can be quite interesting to experiment with, and then quickly switching off the rear set of loudspeakers and seeing how your brain behaves as it "focuses" on the direct signal. ie, do you notice whether it is easier to "lock-on" to the direct signal with the presence of that trigger (terminating the ISD) or without.
 

Duke

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It [whether an acoustic space is "large" or "small"] is irrelevant. It is what it is.

You need a suite of measurements and make a judgement based on the overall information.
I think we can talk about acoustic spaces using adjectives instead of measurements to a certain extent, and I guess I'll stop there.
 

March Audio

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I think we can talk about acoustic spaces using adjectives instead of measurements to a certain extent, and I guess I'll stop there.
You can, but its too generalised. What answers are you looking for and for what situation (room)?
 

Duke

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the potential issue with active returns (reproduced direct signal + digital reverb effects/decay, if desirable) is that you still have a single point of radiation of that energy. where-as diffusers physically modify the indirect sound-field and create many reflections spatially and temporally dispersed, which will have different psycho-acoustic effects and wildly different IACC. a loudspeaker providing this "effect" will not sound or induce the same superposed response at the listening position. however it would be akin to the effect of that of the haas trigger (flat panel to induce high-gain specular reflection).
I see your point, using the backwave of a dipole to terminate the ISD gap isn't ideal because it normally isn't sufficiently diffuse. Ime diffusion of the dipole's backwave can be done at the frontwall reflection zone. I'll refrain from anecdotes because they have no credibility here if not supported by data.

it is fairly easy to do these perceptual tests with a 2nd set of identical loudspeakers - and relatively easy to move them about (thus changing the direction and/or distance/time-arrival vs the direct signal). you can also add your own "Reverb IR" or reverb FX which can be quite interesting to experiment with, and then quickly switching off the rear set of loudspeakers and seeing how your brain behaves as it "focuses" on the direct signal. ie, do you notice whether it is easier to "lock-on" to the direct signal with the presence of that trigger (terminating the ISD) or without.
I have done this extensively, and it seems to me that clarity is degraded if the "backwave" is either too loud or arrives too early. This is anecdotal, so consider it to be an opinion rather than a claim.

If you don't mind me asking, what are some of the texts you have found particularly useful?
 

Duke

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You can, but its too generalised. What answers are you looking for and for what situation (room)?
Bjorn and localhost128 are providing the kinds of answers I'm looking for, for home audio ("small") rooms.
 
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March Audio

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Bjorn and localhost 1128 are providing the kinds of answers I'm looking for, for home audio ("small") rooms.
Which isnt a simple binary delineation to "large rooms" as is impled.

Have you made measurements?
 
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Duke

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Which isn't a simple binary delineation to "large rooms" as is implied.
My understanding is that the terms "large" and "small" have specific acoustic implications, but of course there is a continuum. And on that continuum, home audio rooms are "small", to an adequate degree of precision that I've been able to have conversations with both Bjorn and localhost128 without, as far as I can tell, there being any ambiguity about what sort of rooms we're talking about.

Have you made measurements?
Yes, but measurements are not the same thing as psychoacoustic principles (and their application). The latter are what I'm interested in.
 
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j_j

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Maybe you should measure something in XYZW, and do so at several points, say 15cm or so, apart, with the same exact stimulii.
 

Duke

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Maybe you should measure something in XYZW, and do so at several points, say 15cm or so, apart, with the same exact stimulii.
I'm not familiar with the term "XYZW". Are you referring to positioning the microphone at a specific location (XYZ) and then rotating the loudspeaker?

If so, I do that (with time-gating to exclude reflections) but my measurement system is not as sophisticated as Amir's so it typically takes me several days to develop a suite of driver measurements suitable for crossover design work. I space each set of measurements by angle rather than by distance, but I suppose the spacing between measurement locations could be translated into a distance instead.

Anyway that's probably not what you have in mind because that's a speaker design application and this thread is about room acoustics.

What do you have in mind, and what would be the point? I'm interested in learning useful information about acoustics and psychoacoustics, and would rather not spend time making measurements without knowing the purpose.
 

Bjorn

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I have a question about something in that paper:

"Since in small rooms, there is no Dc, no well mixed sound field, hence, no reverberation but merely a series of early reflected energy, the measurement of RT60 becomes meaningless in such environments."

My understanding apparently has gaps and/or misconceptions. Can you explain to me why there is no Dc ("critical distance", I presume) in a small room? (I think I understand the rest of that statement, and that I have been using the term "reverberation" in a way which is not technically correct since small rooms always have discrete reflections.)

Thanks!
Dc is where the sound level of the sound source and reverberation/reflective sound is equal. Imagine a speaker in a large room. Up close the level from the speaker will be higher vs the combined reflective sound. Move further away and at a point the level is approximately the same. If the room is naked with hard surfaces this will happen closer to the speaker because the level of the reflections will be high and higher compared to a treated room.

A small room is specular and modal, meaning the level varies with frequencies. The reason is the combination of close boundaries and the complex impedance of surfaces and furnitures. So while you may have a Dc at some spesific frequencies at a certain point in the room, it will not be so for a wider frequency area.

RTxx is the measurement of the decay time of a well mixed reverberant sound field well beyond Dc. And a well mixed reverberant sound field isn't something we have in a small room unless is anechoich. It implies that the sound field is uniform and stochastic (randomely distributed) with no particular identifiable direction. Specular and modal is the opposite of this.

The result of using the Sabine equations to small room goes badly when treating it accordingly. The random distribution of absorptive materials, like Harman has done to a great deal, leads to a very poor result compared to controlling specific energy return with surgical placed treatment. How low in frequency the treatment is effective will also be completely different in these two rooms and the random incidence measurements of acoustic products are very misleading when used in small rooms.

Termination of ISD gap
The intensity of the ISD termination has a significant impact on the sense of liveliness of the space, while the laterally arriving semi-reverberant soundfield add to the sense of space of the room. Using reflective hard panels and the old "Haas trigger" will give a higher termination and precendence effect but also at the cost of accuracy compared to diffusers. In LEDE room accuracy was the goal and the Haas kicker was abandoned when diffusers were developed. The goal of the LEDE room was an environment that would reveal flaws and inaccuracies in micing techniques, recordings, and mixing choices. In that type of room there are indications that the energy of behind should be at something like 10-15dB lower than the direct signal.
However, that doesn't mean you can't choose something else in a listening room. Playing with Haas kicker is fun and can actually make it sound better than original.

Another goal of the LEDE room was resemble the best concert halls and that's why it's important that the diffuse tail energies arrives laterally. Schroeder wrote the following:
"..research, based upon a subjective evaluation of the acoustics of 20 major European concert halls, has shown that many modern halls have poor acoustics because their ceilings are low relative to their widths. Such halls do not provide the listener with enough laterally traveling sound waves - as opposed to sound traveling in front/back direction and arriving at the listener's head in his "median" plane (the symmetry plane through his head). Such median plan sound, of course gives rise to two very similar acoustic signals at the listener's ears, and it is thought that the resulting excessive "binaural similarity" is responsible for the poor acoustical quality."
The easiest way to achieve a strong termination of the ISD gap is to use speaker with controlled/narrow directivity, focusing more of the energy towards the diffusers rather than spraying the sound in other directions. Similar to adjusting a water hose to a narrower beam. That's one of the reasons why I personally like large horn speakers with a uniform directivity. As a side note. Most horns are not constant though, and I find discussion of horns at forums very difficult because so few have heard good horn designs and refer them as colored, resonant, etc.

Take note that using active surrounds to achieve a high level of return is something completely else. More sources leads to more comb filtering and polar lobing. A much more chaotic response and a step away from clarity, intelligibility, localization and correct tonality. Not my cup of tea.
 

Bjorn

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Below is example of an ETC showing a control room with the use of a Haas kicker. This was used quite a bit before well designed diffusers came along. It also had the effect of removing the detrimental effect of later arriving high gain specular reflections (if there were any) because the brain is locked to the first strong reflection.

LEDE 1 ETC.png


What a lot of people don't know, is that the LEDE design developed over many years and ended up being very differently than how it started. The name, live and dead end, isn't very accurate of how eventually became.

Measurements that wasn't available at the start changed the concept of simply having loads of absorptive materal in the front and a reflective rear wall. When "Pudgy" Rogers developed the ability to measure specular reflections, one quickly moved to surgical treatment of specular energy. What you read about LEDE today at many places, and which is also the understanding of many, is quite misleading.
 

j_j

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Sorry, I mean a soundfield mike. Capture both all 3 volume velocities and the pressure at two points about 15-20 CM apart, on a line parallel to the line between the speakers.

Remember that in any point in the atmosphere, there are 4 variables, 3 volume velocities (which can be represented in two ways, but velocity is easier to interpret than polar for what I'd care about) and the pressure. The pressure at one point (that's what an omni does) is only 1 of 4 variables at that point.

With all four variables you learn a great deal about what reflects where, what room modes look like, what various time delays are, and so on.
 

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