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

Duke

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Bjorn

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Ah, Duke and Brian Waslo use a different approach than you touched on in your post. You didn't actually address this approach, which is why I brought it up. The delayed signal comes from two channels placed very close to the main left and right speakers, which in both Duke's and Brian Waslo's approaches involved highly controlled directivity main channels. THESE ARE NOT ACTIVE SURROUNDS. I would let @Duke address this further, rather than myself.

Thanks,

Young-Ho
I understand. You're talking about something like ambience tweeters or in the case of "bwaslo" the sound from the sources are also delayed with 10-20 ms. I think it's important to understand this is not diffused energy and you'll increase high gain specular reflections (depending on location) as well as lobing and combing between drivers. Obviously the audibility of this much lower when used in only high frequencies and the signal is also delayed. However, the energy is never diffused. And that's where I think the main difference is. Also worth mentioning here that if you're going to diffuse the highest frequencies, you need a diffuser with fractals (small wells).

With diffusion the energy get's tigthly packed and avoids frequencies from sticking out. The energy is also attenuated some (but not as much as with absorption). With temporal diffusion the energy is spread out in time and this increases the sense of space. So while these approaches have a few similarities and mainly in regards to a late return, they are also quite different. Adding another signal, while may be preferably to some and especially if the setup is rather dry sounding (related to speaker dispersion and acoustics), is a step away from accuracy and the goal of LEDE. Late arriving diffusion will be closer to the recorded signal.
 

Duke

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You're talking about something like ambience tweeters... I think it's important to understand this is not diffused energy...
Thanks for replying to Young-Ho's question, Bjorn. No it doesn't start out at diffused energy, but it could be diffused. So it's not necessarily either/or.

With diffusion the energy get's tigthly packed and avoids frequencies from sticking out. The energy is also attenuated some (but not as much as with absorption).
May I ask you a general question about diffusion? (I'm still awaiting delivery of D'Antonio & Cox's book.) Does the attenuation you mention tend to be fairly uniform across the spectrum, or does it also tend to alter the spectrum somewhat? If the latter, are the shorter wavelengths generally attenuated more than the longer ones?

Adding another signal, while may be preferably to some and especially if the setup is rather dry sounding (related to speaker dispersion and acoustics), is a step away from accuracy...
I'd rather not go into much detail about what I do at this point, and for now prefer to make neither claims nor concessions regarding "accuracy". That day may come, but is not here yet.

In my opinion I'm not really "adding another signal"... I'm delivering the original signal differently.
 
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dasdoing

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I don't think it's that trivial: https://www.audiosciencereview.com/...f-axis-instead-of-absorbing.14096/post-471238. It's not like all reflections would be uniformly attenuated -20dB but rather certain parts of their frequency spectra would be more attenuated than others, also depending on the angle of incidence. Look at the graphs, then come back about "-20 dB" for the highs.

Young-Ho
that is not what is happening in the real world. as I said, it's easy to keep ETC below -20dB (at least in the 3 rooms I treated), and that is the meassurement we have to look at when treating reflections

On the other hand, a truly dead room with very well matched speakers, can quite seriously give you the "central image in your head" effect, too.
can you explain what you actualy mean by this? speakers in an anechoic room should have a very defined localization at the plane they are put at, the opposite of "in your head". what is true is that the phantom center is more "present" since in a reflective room it has dubble the amount of reflections as the hard panned sound
 

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what is true is that the phantom center is more "present" since in a reflective room it has dubble the amount of reflections as the hard panned sound
It seems to me that, while the number of reflections is doubled for a phantom center sound, the intensity of the individual reflections is halved (-6 dB), relative to a hard-panned sound of the same SPL. Am I missing something?
 
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j_j

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It seems to me that, while the number of reflections is doubled for a phantom center sound, the intensity of the individual reflections is halved (-6 dB), relative to a hard-panned sound of the same SPL. Am I missing something?
So - that depends a great deal on the source and spectrum of the reflections. (linear independence is an interesting thing here, because you can find independence in many (literally a near-infinite number) of ways. So it could be 3dB, 6dB or something else in between, at the very least.
 

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I seams that only in perfect rooms both channels sum to +6dB https://en.wikipedia.org/wiki/Pan_law
I actualy struggle to understand how more reflections result in less loudness in the sum, but this seams to be what is happening
First, we are talking about SPL. SPL is not loudness. Loudness is sensory intensity, how loud it SOUNDS. If you do frequency shaping, loudness and intensity do not track.

But if two signals are direct inverses they will cancel in a perfect sum. If they are identical they will add to 2x the intensity. If they are orthogonal, they will add to 1.414 times the intensity at the position they are orthogonal. So, even understanding the sum requires knowing the transfer function from each speaker to the microphone (and is that a pressure sum, a 4-dimensional sum (including volume velocities, or what).

But please use "loudness" only when you mean the sensory intensity. Intensity by itself is a cognate of SPL (although depending on how you measure the intensity SPL may be missing some components).

http://www.aes-media.org/sections/pnw/pnwrecaps/2014/jj_jan2014/ is a partial discussion on loudness vs. intensity. The later hearing tutorial will also explain some aspects of loudness.
 
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dasdoing

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First, we are talking about SPL. SPL is not loudness. Loudness is sensory intensity, how loud it SOUNDS. If you do frequency shaping, loudness and intensity do not track.

But if two signals are direct inverses they will cancel in a perfect sum. If they are identical they will add to 2x the intensity. If they are orthogonal, they will add to 1.414 times the intensity at the position they are orthogonal. So, even understanding the sum requires knowing the transfer function from each speaker to the microphone (and is that a pressure sum, a 4-dimensional sum (including volume velocities, or what).

But please use "loudness" only when you mean the sensory intensity. Intensity by itself is a cognate of SPL (although depending on how you measure the intensity SPL may be missing some components).

http://www.aes-media.org/sections/pnw/pnwrecaps/2014/jj_jan2014/ is a partial discussion on loudness vs. intensity. The later hearing tutorial will also explain some aspects of loudness.
though I undertstand that I used the wrong tecnical term, SPL and loudness are still realated, and yes you mentioned that.

So the answer is that phantom center in a reflective room is SPL attunated because of cancelation? that sure makes sense


----------------

on the subject: this video indicates that at least in mono angled reflections close to a difuser can actualy sound good:
the generaly notion of difusion being destructive at close distance is probably more related to the more direct incidance on the pannel in the (most commom) backwall case.
 

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though I undertstand that I used the wrong tecnical term, SPL and loudness are still realated, and yes you mentioned that.

So the answer is that phantom center in a reflective room is SPL attunated because of cancelation? that sure makes sense
It's more complicated than that. The PRESSURE at the center for phantom center can be less than twice due to the transfer function of the paths to the microphone. Ditto the intensity.

Now, if we're talking about a head, there will be additional modifications due to two wavefronts (one from each speaker, with different delays) adding at each ear, that result in frequency shaping for a phantom center.

But oversimplifying is very easy here. Using the term "cancellation" is not what I'd do, rather I'd look at this as a vector sum and see what happens when you add two vectors, i.e. what is their crosscorrelation. Furthermore, that will vary with frequency, so the signal you're using to measure comes into play.

Measuring overall level is a great way to get results that have little to do with what you hear.
 
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The Laukkanen paper has a chapter called "Background", which presents terms and definitions and what is accepted as commonly known. Helpful to read, to gain a common understanding and avoid discussing what acousticians already have studied and agreed upon.
concur. it's only in home audio forums where these well-established terms and definitions seem not well known or understood and thus have to be unfortunately "debated" - not so in acoustics or engineering.

quoting Ted Schultz (formerly of Bolt, Beranek, and Newman):

In a large room, if one has a sound source whose power output is known, one can determine the total amount of absorption in the room by measuring the average pressure throughout the room. This total absorption can then be used to calculate the reverberation-time from Sabine formula. This method fails badly in a small room, however, where a large part of the spectrum of interest lies in a frequency range where resonant modes of the room do not overlap but may be isolated....In this case the microphone, instead of responding to a random sound field (as required for the validity of the theory on which these methods depend), will delineate a transfer function of the room...It does not provide a valid measurement of the reverberation time in the room."

quoting SSE:

What is often overlooked in the attempted measurement of RT60 in small rooms it that the definition of RT60 has two parts. The first part is unfortunately commonly overlooked.

1. RT60 is the measurement of the decay time of a well-mixed reverberant sound field well beyond Dc.
2. RT60 is the time in seconds for the reverberant sound field to decay 60 dB after the sound source is shut off.


In small control rooms there is no Dc, no well-mixed sound field, and, hence, no reverberation. There is merely a series of early reflected energy. Consequently, the measurement of RT60 becomes meaningless in such environments.

fact is RT60 is an inherent property of the bounded acoustical space itself, and is NOT the transfer function measurement of a given loudspeaker (with a given polar response) measured at a given position in 3space within the room - dictated by room geometry. hence the requirements for a well-mixed sound field, the source to be as omni-directional as possible across a wide bandwidth (vs a typical loudspeaker that will energize the room as frequency-dependent), and that the measurement be taken "well past critical-distance Dc" such that one is "in the reverberation" and the direct sound field does not dominate.

there is no such region as "well past Dc" in small/home residential-sized rooms such as what many deal with here. it does not exist.

p.s., RIP to the late/great Ted Schultz.
 
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And I'm not a supporter of LEDE. That's both a question of physics and perception. That's also where an absolutist separation of "small" vs. "large" falls flat on its face.
are you willing to exemplify your objections to LEDE as a 2ch stereo reproduction model - either the physics or perception of the response?

is it a matter of subjective preference of the time-domain response and not preferring a dense, reflection-rich later-arriving/lateral-arriving exponentially decaying semi-diffuse sound-field (seeing as you state you subjectively prefer "dead" room) - or is it an issue of perception of accuracy with respect to the direct signal in terms of meeting a "neutral reproduction space" design requirement?
 

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I will note that "transfer function" does not become meaningless in a large room, and that such "hard definitions" simply do not define or describe the actual response of a room.

Remaining in the mode where absolute definitions that are not supported by the mathematics accomplishes nothing useful, in my view. What's more, the modern interpretation of T60, for instance, as a function of frequency, fully eliminates the attempted objections to use of the term.

But it's pointless to discuss. The mathematics is resoundingly obvious and incontrovertible.
 
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But it's pointless to discuss. The mathematics is resoundingly obvious and incontrovertible.
i was honestly hoping for a more thorough contradiction to the published work of Dr Manfred Schroeder and others in the acoustics field to learn from - especially after user March Audio continually deferred to you as this being your area of expertise. it is exceedingly difficult for me to parse what you are actually trying to state or refute here (if anything)? hand-waving away the conversation by stating "the mathematics is resoundingly obvious" does not assist me to understand whether there are fundamental issues with the quotes presented above of which should be corrected such that such misinformation does not continue to be spread.

even just a simple response on the questioning of "physics and perception" on why you wouldn't support the LEDE response would be greatly beneficial to learn from. what aspects of "physics" in a room built to LEDE standards do you not support? is it an issue with the way broadband absorption (or splayed walls) to attenuate the first order reflections - or perhaps the "physics" of the way Reflection Phase Grating diffusers work to break-apart sparse, specular reflections into many reflections of lower gain both spatially and temporally dispersed to emulate a Large Room diffuse sound field?
 

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i was honestly hoping for a more thorough contradiction to the published work of Dr Manfred Schroeder and others in the acoustics field to learn from -
Well, I wish Manfred was still with us. I'd let him answer that.

Let's start here, what do you think the "Schroeder Frequency" is?
 
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Let's start here, what do you think the "Schroeder Frequency" is?
i'm confused why the need to go on a fishing expedition to try and glean my knowledge on a particular subject matter in what feels like a form of deflection. user March Audio insisted in this thread that this is your area of expertise and he deferred to you numerous times here, so why would you feel the need to ask me these questions vs simply explaining them to the community such that we can all benefit?

i'm mostly interested in clarification of your comments above, as i don't seem to be able to parse what it is exactly you are attempting to communicate or refute. it is simply not clear to me - but that is likely a fundamental failure on my part.

for example, you made the simple claim that "And I'm not a supporter of LEDE. That's both a question of physics and perception" - but a simple follow-up with what the precise issue in terms of physics and perception goes unanswered.

if this is indeed your area of expertise i think we would all benefit from such input to further gain an understanding of the subject matter.
 

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i'm confused why the need to go on a fishing expedition to try and glean my knowledge on a particular subject matter in what feels like a form of deflection. user March Audio insisted in this thread that this is your area of expertise and he deferred to you numerous times here, so why would you feel the need to ask me these questions vs simply explaining them to the community such that we can all benefit?
Because that is where the technical discussion must start. That is where the problem of LTI systems, to start with, must be introduced. Furthermore, there are questions of observation method and time that start there.

Why do you avoid this?
 

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[quoting j_j]: "And I'm not a supporter of LEDE. That's both a question of physics and perception"
I too am interested in j_j's thinking about this, especially the perceptual side, which presumably is related to why he prefers acoustically dead rooms.

Even if he has to dumb it down a bit for people like me who have to google terms like "LTI".
 
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Because that is where the technical discussion must start. That is where the problem of LTI systems, to start with, must be introduced. Furthermore, there are questions of observation method and time that start there.

Why do you avoid this?
if you'd like to refute Schroeder, Ted Schultz, or any other well-understood acoustical concept that is accepted within the community, by all means please simply state that such that we can learn and understand since it was implied by another in this thread that this is your area of expertise.

the onus is not on me to answer this for you. i am not contesting what has been quoted above.

i am starting to think the problem is with me because after your replies i am simply unable to actually parse what you are trying to say, or what specifically you are attempting to refute (if anything)? each reply leaves me more and more confused on your position since there isn't sufficient follow-up to better explain the cryptic responses prior.

i'd like to gain an understanding of what you mean when you state you don't support LEDE (physics & perception) - surely it should be straight-forward to clarify your statement so we can all as a community gain a better understanding of the subject matter and any legitimate points of contention.
 
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