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

[localhost127]

If one can measure it, it exists. I can measure it, as a function of frequency, even. Q.E.D. Such a thing therefore exists. Therefore, your imagination is wrong. Furthermore thin absorbers do not work, therefore your paradigm is mistaken. Q.E.D.

Are you possibly not realizing that reverberation time is a function of frequency?

reverberation time isn't applicable in small acoustical spaces. it's a Large Acoustical Space measurement (calculation). there are pre-requisites required to be met in order for RT60 to be considered valid. otherwise, garbage in = garbage out, regardless of what the software spits out. it's a case where the operator needs to be smarter than the analysis tool.

reverberation does not exist in small acoustical spaces. it's exactly the reason why the subject of acoustics is separated into Large and Small rooms.

 

[localhost127]

For those of you have an interest, you can read a short write up about reverberation time here:
https://www.lydogakustikk.no/sma-rom-og-etterklangstid/

If true reverberation time existed in small rooms, that would imply that thin absorption would work very well. It would imply that two types of very different random placement of treatment and without treating specifically specular reflections should both yield pretty much the same and good result. Anyone who has experience with various acoustic treatment, know however that this isn't the case.

And as side note but also examplifiies how specular reflections effects the sound in a small room is how a speaker with almost the same uniform directivity but different directivty index sounds very different in a small room. There may be no better example here than B&O Beolab 50 or 90; where you can change the directivity from 120° to 180° by a pressing a button and experience how completely different they sound. An RTx measurement of the speaker in the room would tell they measure very much the same.

precisely.

if there existed an exponentially-rising, exponentially-decaying reverberant sound-field in small acoustical spaces, we would be applying absorption statistically (% coverage) to bring down RT to a given requirement. fact is there exists no critical-distance (Dc) and thus no statistically random-incidence reverberant sound-field where the energy flows are equal, probable and from any/all directions simultaneously. instead, we have modal resonances (literally a contradiction in terms of the acoustical definition of reverberation/reverberant sound-field), and focused sparse specular reflections who (by time-domain analysis) can be isolated and traced back to their incident boundary and treated surgically/directly (vs statistically). reflections whose time-delay, direction/vector, gain, spectral content, etc can all be resolved is literally the opposite/contradictory of reverberation.

there exist no reverberant sound field in small acoustical spaces that can mask the early reflections as with large auditoriums, concert halls, etc. it's bewildering that such erroneous commentary continues to propagate. it's literally the physical difference in sound field that constitutes the separation of acoustics into Large and Small sub-catagories.

 

[Thomas savage]

Hi @amirm (also @j_j @mitchco )

Putting aside room bass issues (<500 Hz), if one has really good on and off-axis measurements, like you mentioned from these JBL's and Genelec 8341A and others:



Question: Non-linear absorbers could change the tone of 1st reflections of great speakers like these - but are there any downsides to diffusing these 1st reflections, especially linear diffusers like:

View attachment 68986

I do this at point if first reflection, you can look into the science but you may or may not like it in your room .

Make some , or just rig some bought ones up and see if you like it.

 

[localhost127]

In a large room with true reverberation you can rely on random incidence measurements of acoustic treatment. If true reverberation existed in small rooms, you could do the same there. But that's not the case. Absorbents function very differently in small rooms compared to large rooms. Something that we see when measuring the rooms as well. The reason is because reverberation time doesn't exist in small rooms.

The small acoustical space is dominated by focused specular reflections. The days of shotgun application of treatment which leads to dead rooms with no control of specular reflections should have been over a long time ago. Which is what you get by following RTx measurements. We have better tools.

There's no misunderstanding here. Fundamental point: modal decay rates are not reverberation. Reverberation is “the time in seconds that it takes a diffuse sound field, well beyond a real critical distance, to lower in level by 60 dB when the sound source is silenced.” Modal decay rates are dB-per-second (dB/s) rate of decay for a specific modal frequency.

+1

there is a reason why the context of absorption coefficient measurement data needs to be understood. far too often people imply an absorption coefficient measured via reverberation chamber method (random-incidence) is somehow applicable to a "% absorbed" or that if they use that particular absorber/material in their small residential sized room that it implies the same amount of absorption. even Bob Gold's absorption coefficient page that so many reference mentions this right at the top - but it seems no one takes the time to read or understand it. why would i look at data generated within an environment that has no bearing to that of my small residential room? in a home listening setup, the angle of incidence into the absorber is known; there is no "random-incidence, diffuse sound-field" imposed on the absorber from all incident angles simultaneously.

in small rooms, discrete (focused) specular reflections are isolated and traced back to their incident boundary via time-domain analysis. the sound-field is not "random" and thus absorption isn't applied statistically throughout the room. it is all LOCALIZED sound-field dependent upon source and receiver positions, which is why the response changes as you move about 3space vs in a reverberant sound-field where it becomes consistent as you move about (because reverberation in effect becomes the exponentially rising and decaying noise floor).

RTxx is not applicable in small acoustical spaces for any accurate measurement and especially not for placement of treatments to meet a room's particular design requirements (time and frequency domain). RT60 REQUIRES the existing of a statistically, random-incidence reverberant sound-field to be present in order for the calculation to be considered valid. and the meuasrement must be done so with an omni-direction source and taken *well-past* Dc (critical-distance). that is because RT60 is a measurement/property of the bounded acoustical space - NOT a measurement of the speaker-room transfer function as so commonly implied.

such simple nuances that are continuously being erroneously confused even by "professionals".

 

[localhost127]

Please read the post that I wrote that invalidates all of this. Dc does exist. And by definition, anything past transition frequencies is random (enough).

you didn't invalidate anything except your own understanding of basic acoustical terms.

critical-distance (Dc) does not exist in small acoustical spaces. it's a Large Acoustical Space property (same with reverberation, RT, etc).

no where in a small residential-sized room (including if bare) does the indirect sound-field become EQUAL and then extending HIGHER in gain than the direct signal as you continue to move away from the source.

 

[localhost127]

The physics surrounding the absorber are exactly the same. .

but the indirect sound field properties/characteristics imposed on an absorber differ between Large and Small rooms.

in a small room, the angle of incidence into the absorber can be defined, and thus the "absorption coefficient" can and will be different vs that of a reverberation chamber method where random-incidence soundfield implies energy flow impeding the absorber from any/all directions simultaneously.

it's the reason we have impedance/normal-incidence tube measurements vs reverberation chamber, and why i cannot take a "reverberation chamber" ISO absorption coefficient value and imply that is how the absorber will "perform" when placed in my residential-sized room where the loudspeaker/absorber orientation implies a 45* angle of incident. you then also need to factor in effects of edge diffraction and sample size wrt wavelength and method.

 

[j_j]

A gather of data doesn't prove anything. You need to consider the conditions. From the paper you posted the graph:

They are overlooking the preconditions.

So, actual measurements don't mean anything to you? Sorry, actual scientists use actual data.

 

[j_j]

critical-distance (Dc) does not exist in small acoustical spaces. it's a Large Acoustical Space property (same with reverberation, RT, etc).

no where in a small residential-sized room (including if bare) does the indirect sound-field become EQUAL and then extending HIGHER in gain than the direct signal as you continue to move away from the source.

First, the word "gain" does not even belong in that sentence. Second, you've seen measurements, but you don't want to accept the actual evidence.

Actual evidence is what matters, not your preconceptions. You do understand things like energy envelope, yes? You appear to be claiming that small rooms can not store energy. That would be a great shock to all those 1940's reverb chambers people built in basements in studios.

 

[j_j]

but the indirect sound field properties/characteristics imposed on an absorber differ between Large and Small rooms.

So, mister 127.0.0.1, you really deny the physics. Unless you're talking about a membrane absorber (and you haven't said anything about one) the volume velocity through the material is what matters. Have you ever actually used this stuff? Seriously!

Angle of incidence is, of course, relevant, but only your straw-man nonsense raises the issue it's not. Again, did you ever actually use this stuff, and do you understand that your issue about angle of incidence actually runs counter to your other claim, that is, if your other claim is what it seems to be, and isn't some disingenuous attempt at a whipsaw.

 

[j_j]

RTxx is not applicable in small acoustical spaces for any accurate measurement and especially not for placement of treatments to meet a room's particular design requirements (time and frequency domain). RT60 REQUIRES the existing of a statistically, random-incidence reverberant sound-field to be present in order for the calculation to be considered valid.

You, as well as your friend here, seem to be playing semantics and nothing more. You demand an old, oversimplified take on RT that simply does not describe the acoustics anywhere very well.

Have you even addressed how stationary your arguments about random incidence are? Heat sources exist in small rooms, you know, even tiny control rooms, where in fact they often create audible differences in a "near-field" setting, even.

Have you even measured this or things like it?

 

[localhost127]

First, the word "gain" does not even belong in that sentence. Second, you've seen measurements, but you don't want to accept the actual evidence.

Actual evidence is what matters, not your preconceptions. You do understand things like energy envelope, yes? You appear to be claiming that small rooms can not store energy. That would be a great shock to all those 1940's reverb chambers people built in basements in studios.

you can "appear" to infer whatever you wish, or simply respond to what was written.

where were measurements presented that illustrate the existence of critical-distance (Dc) in a small, home-residential-sized room and thus the continuation of the indirect sound-field becoming higher in magnitude than the direct signal (Ld) as one moves further "into the reverberation". what "actual evidence"?

 

[j_j]

you can "appear" to infer whatever you wish, or simply respond to what was written.

where were measurements presented that illustrate the existence of critical-distance (Dc) in a small, home-residential-sized room and thus the continuation of the indirect sound-field becoming higher in magnitude than the direct signal (Ld) as one moves further "into the reverberation". what "actual evidence"?

Your complaint, of course, flies in the face of one set of actual data you've been shown, and quite a few you haven't. Get your thinking out of the 1960's and understand there is no hard "edge" to "small" and "large", and that the distinction only comes about because of your over simplified model.

You make extraordinary claims when presented with a set of measurements (I could provide quite a few more, but I don't work for free, sorry), but then complain that something is wrong with actual measurements when you're confronted with those actual measurements.

You're also playing games with the term "reverberation" and ignoring the difference between spatial decorrelation, stationary vs. nonstationary impulse response, and data that looks a particular way just because of the time window of the measurement.

Does the following mean anything to you? df * dt > c

 

[localhost127]

So, mister 127.0.0.1, you really deny the physics. Unless you're talking about a membrane absorber (and you haven't said anything about one) the volume velocity through the material is what matters. Have you ever actually used this stuff? Seriously!

Angle of incidence is, of course, relevant, but only your straw-man nonsense raises the issue it's not. Again, did you ever actually use this stuff, and do you understand that your issue about angle of incidence actually runs counter to your other claim, that is, if your other claim is what it seems to be, and isn't some disingenuous attempt at a whipsaw.

it's not denying physics, it's accepting physics and knowing that measured data from one test environment is not directly applicable to another.

fact is physics dictates that the fundamental physical properties of the indirect sound-field differ wildly in that from Large to Small bounded acoustical spaces - dictated by volume, mean free path, etc.

small room indirect sound-fields involve modal resonances (localized behavior via that of nodes and anti-nodes), focused sparse specular indirect reflections (again, also locaized behavior as the reflection's time-delay, vector/ingress direction, spectral content, gain, etc are all dictated by source/receiver positions with respect to the room's boundaries), and specular decay until the last of the specular energy is damped below the ambient noise floor. local behavior is fundamentally contradictory to that of Large Acoustical Space reverberant (diffuse, random-incidence) soundfield's above Manfred Schroeder's F sub L equation.

you cannot conclude that an absorber's "performance" (absorption coefficient) is relatable from one environment to the next. absorbers tested to reverberation-chamber method are done so with a indirect sound-field that (optimally) is fully diffuse across a given bandwidth, which implies equal energy flows impeding the absorber from ALL DIRECTIONS SIMULTANEOUSLY. this is useful to test and understand the performance across multiple DUTs/absorber samplings. however you cannot imply that such data is directly relatable to the performance in a home, residential-sized room because in a small acoustical space, the sound-field is localized (NOT statistical/random-incidence), and thus the signal takes a known flight path with known angle of incidence into the absorber.

reverberation chamber method absorption coefficients are not "percent absorbed", and one must also account for edge diffraction effects. contrast this with impedance tube measurements (or empirical models) that factor in the appropriate angle of incidence to understand the performance data for that particular application/use-case.

the sound-fields are different, and thus that must be factored into consideration. data taken from a Large Acoustical Space environment (reverb-chamber method, random-incidence) is NOT directly applicable to the performance if you placed that absorber in your room at a sidewall with an ingress vector at 45*.

So, mister 127.0.0.1, you really deny the physics. Unless you're talking about a membrane absorber (and you haven't said anything about one) the volume velocity through the material is what matters. Have you ever actually used this stuff? Seriously!

Angle of incidence is, of course, relevant, but only your straw-man nonsense raises the issue it's not. Again, did you ever actually use this stuff, and do you understand that your issue about angle of incidence actually runs counter to your other claim, that is, if your other claim is what it seems to be, and isn't some disingenuous attempt at a whipsaw.

your post literally has zero substance and your tone and sarcasm is unprofessional. what exactly are you asking or refuting here? please attempt to do so without all of this deflection antics.

 

[j_j]

you cannot conclude that an absorber's "performance" (absorption coefficient) is relatable from one environment to the next.

I have no idea why you even bother to say this. Exactly nobody has claimed otherwise.

Modes have already been addressed, clearly and specifically, yet you ignore that (again).

You have the overt (*(* to build a whole army of straw men, burn them down, and then try to claim others asserted them.

Your name "localhost" is of course a pathetic admission you are a troll, as well. You don't have the guts OR the honesty to actually stand behind your false accusations. VPN much, there, bud?

 

[localhost127]

You, as well as your friend here, seem to be playing semantics and nothing more. You demand an old, oversimplified take on RT that simply does not describe the acoustics anywhere very well.

Have you even addressed how stationary your arguments about random incidence are? Heat sources exist in small rooms, you know, even tiny control rooms, where in fact they often create audible differences in a "near-field" setting, even.

Have you even measured this or things like it?

how unprofessional to refer to someone that shares the same vocabulary and understanding of the actual physical soundfields of Large vs Small acoustical spaces as "your friend". what are you implying by this statement? could you at least reply solely focused on the subject matter and not attempt to deflect with these antics?

it's not semantics. RT is an inherent, fundamental property of the bounded acoustical space - NOT a transfer function measurement of a given source (loudspeaker with particular dispersion characteristics) and receiver placement with respect to the room.

and thus for RT to be a property of the bounded acoustical space alone, you need to test with an (optimally) true omni-directional source such that the room is energized evenly across a given bandwidth, and also that the measurement is taken with the mic well past critical-distance (Dc) such that one is "in the reverberation" of the room.

if no statistically random-incidence reveberent sound-field exists, and the measurement is not taken with an omni-directional source, and the mic is not placed "well past Dc", then you have a classic case of garbage in = garbage out. just because your measurement suite spits out a value does not mean that value is accurate! the operator must always be smarter than the tool and understand the limitation and subsequently the actual pre-requirements for the calculation to be considered valid.

this is akin to someone performing supersonic modeling in subsonic airflow.

Have you even addressed how stationary your arguments about random incidence are? Heat sources exist in small rooms, you know, even tiny control rooms, where in fact they often create audible differences in a "near-field" setting, even.

i'm confused what you are attempting to insinuate here. are you implying that heat sources are going to magically modify the measured indirect sound-field where time-delay and direction data of indirect specular reflections is suddenly lost, and the sound field suddenly becomes random-incidence and energy flows incident from all directions simultaneously?

Have you even measured this or things like it?

i've performed measurements and have not been able to find any contradiction to any established acoustical publications that some are insinuating here: primarily that i have NOT been able to find a critical-distance where the indirect sound-field becomes higher in gain than the direct signal anywhere in my room, and a point where-after where as i continue moving away from my loudspeaker, the indirect "reverberant sound field" becomes many times higher in gain than the direct signal. no such point exists in Small Acoustical Space.

 

[localhost127]

Your complaint, of course, flies in the face of one set of actual data you've been shown, and quite a few you haven't. Get your thinking out of the 1960's and understand there is no hard "edge" to "small" and "large", and that the distinction only comes about because of your over simplified model.

You make extraordinary claims when presented with a set of measurements (I could provide quite a few more, but I don't work for free, sorry), but then complain that something is wrong with actual measurements when you're confronted with those actual measurements.

You're also playing games with the term "reverberation" and ignoring the difference between spatial decorrelation, stationary vs. nonstationary impulse response, and data that looks a particular way just because of the time window of the measurement.

Does the following mean anything to you? df * dt > c

the "extraordinary claim" is someone attempting to imply critical-distance (Dc) and a reverberant, random-incidence sound-field exists in small, residential-sized rooms! it does not!! the sound-field is entirely localized (which is a direct contradiction to reverberant sound-field which implies statistical). modal issues are localized. focused, sparse indirect specular reflections are localized. specular room decay is in effect localized.

an exponentially rising, exponentially decaying reverberant sound-field, implying a diffuse, or random-incidence sound-field, does not develop at any appreciable frequency above the ambient noise floor in small acoustical spaces such as home, residential-sized rooms.

 

[localhost127]

You have the overt (*(* to build a whole army of straw men, burn them down, and then try to claim others asserted them.

Your name "localhost" is of course a pathetic admission you are a troll, as well. You don't have the guts OR the honesty to actually stand behind your false accusations. VPN much, there, bud?

incredibly unprofessional behavior. localhost127 is in reference to 127.0.0.1 in IP networking, not used as a "troll" (how does a networking term imply troll behavior??). what does this have to do with vpn?

mods can you please address this incredibly childish behavior that is not on-topic and a poor attempt at ad hominem and deflection.

 

[j_j]

the "extraordinary claim" is someone attempting to imply critical-distance (Dc) and a reverberant, random-incidence sound-field exists in small, residential-sized rooms! it does not!! the sound-field is entirely localized (which is a direct contradiction to reverberant sound-field which implies statistical). modal issues are localized. focused, sparse indirect specular reflections are localized. specular room decay is in effect localized.

And yet you still hold on to your 1960's definitions. You haven't answered any more than your twin exactly what the difference between perceptually diffuse and analytically diffuse is.

You argue "the sound field is entirely localized". You haven't made a good enough measurement, I can tell just from that statement alone.

Or why that matters.

Or how a simple modern measurement can trivially distinguish modes, direct vs. reflections vs. stationary vs. nonstationary impulse response without having to have these imaginary large/small distinctions with hard "edges" that arrive because of oversimplifications.

Enough. You're not willing to even say who you are. I think you're a VPN griefer.

You were provided with data. You choose to reject data because it does not support your belief system. I see no reason to offer you any further data.

Do you have any solution for the question asked in the OP, or do you not? Or did you just come here to make serious professional accusations from an anonymous net-pseudonym and spew disinformation?

 

[localhost127]

Your complaint, of course, flies in the face of one set of actual data you've been shown, and quite a few you haven't. Get your thinking out of the 1960's and understand there is no hard "edge" to "small" and "large", and that the distinction only comes about because of your over simplified model.

in case you are not aware, there exists a "transition" region in Small Rooms from Modal to Specular - whereby boundary is Small with respect to wavelength and transitioning to that (and especially as not all room dimensions are assumed equal), where boundary is Large with respect to wavelength and specular nature (indirect, focused reflections) develop. likewise in Large Acoustical Space where there is also a "transition" region from Specular to Reverberant/Diffuse (as explored through the work of Mandfred Schroeder and his F sub L equation for minimum volume to be able to consider reverberant field).

so to make the insinuation that it is being inferred that there is a "hard edge" and "simplified model" is misleading and false. another wild distraction and strawman claim. fact is there is a transition region inherent to the physical sound field characteristics - and which exists in both Large and Small rooms.

 

[j_j]

incredibly unprofessional behavior. localhost127 is in reference to 127.0.0.1 in IP networking, not used as a "troll" (how does a networking term imply troll behavior??). what does this have to do with vpn?

Precisely, it's a ridiculous attempt at anonymity that anyone can see through. You did notice, I presume, way up there, that I addressed you as "mr 127.0.0.1"? Oh, no, you didn't notice that.

You're a troll operating under a pseudonym.