Above the Schroeder Frequency. Diffraction.

[Seamus TM]

So, I've had a lot of time to kill while I'm waiting for my Roxul truck to get here from Canada. Sometimes that's a good thing, sometimes a bad thing.

Anyway, can anyone explain to me what the "Diffraction and Diffusion Dominate" portion of this room analysis means, and how I would go about addressing that range? As it so happens, I have a few nasty dips in the 100Hz-400Hz area.

Frequency Regions:
- No modal boost: 1hz to 27hz
- Room Modes dominate: 27hz to 101hz
- Diffraction and Diffusion dominate: 101hz to 404hz
- Specular reflections and ray accoustics prevail: 404hz to 20000hz

This is from Bob Gold's site, by the way.

Thanks,
Seamus

[Seamus TM]

Here's some good info, in case anyone ever wonders this again.

Acoustics Forum • View topic - Schroeder Frequency

[jhbrandt]

This region is known as the transition region where the wave model changes to the ray model.

As you know, in an enclosure, sound behaves like waves from VLF up to a point that is dictated by the volume of the enclosure and then proceeds to change to the ray model which begins approximately 4 times the upper frequency of the wave region. Is that confusing enough? Maybe SAC could chime in here and completely baffle the rest. (I understand you SAC...)

All joking aside, this region is best treated with broadband absorption and diffusion. Absorption because in this transition region are usually many near incidences (same frequencies) of axial, tangential, and oblique modes or standing wave reflections - even in the best of rooms. Diffusion because these frequencies can also take on characteristics of the ray region and can cause specular distortion in the form of comb filtering and the Precedence Effect or Haas Effect and can mess up the imaging.

[Seamus TM]

Yeah, right on.
I must be in some real psychological trouble because I feel like I'm really starting to get a handle on some of this stuff. I am getting way more into it than I ever intended to be.

It sounds dumb, but it really is looking at the whole room as a "system" and not trying to pick on specific points.

To be honest, it was "SAC" that got me thinking about this a bit more when he mentioned something about the difference between modal problems in the low end and specular problems in the low end.

I'm still not even 100% sure what that means, but it knocked something loose in my brain... in a good way.

On the one hand, I guess it doesn't really matter if all you want to do is fix everything the best you can with one big blanket, but what I am beginning to find out is that I have a need to know why.

Thanks,
Seamus

Originally Posted by jhbrandt

This region is known as the transition region where the wave model changes to the ray model.

As you know, in an enclosure, sound behaves like waves from VLF up to a point that is dictated by the volume of the enclosure and then proceeds to change to the ray model which begins approximately 4 times the upper frequency of the wave region. Is that confusing enough? Maybe SAC could chime in here and completely baffle the rest. (I understand you SAC...)

All joking aside, this region is best treated with broadband absorption and diffusion. Absorption because in this transition region are usually many near incidences (same frequencies) of axial, tangential, and oblique modes or standing wave reflections - even in the best of rooms. Diffusion because these frequencies can also take on characteristics of the ray region and can cause specular distortion in the form of comb filtering and the Precedence Effect or Haas Effect and can mess up the imaging.

[PaulP]

Originally Posted by Mumblesound

I must be in some real psychological trouble because I feel like I'm really starting to get a handle on some of this stuff. I am getting way more into it than I ever intended to be.

And you're only half way. One thing we never mention, but is the source of
all our problems, is the speakers (at least for control and listening rooms).
I've come to the conclusion, as you have, that the whole room is a system.
But the system doesn't stop on the outside of the speaker enclosure. It goes
right on in.

For example, if your monitors are designed with a particular positioning in
mind and you put them somewhere else, you'll never get your room to sound
properly. I've seen the frequency response of the monitors mentioned a
couple of times but never the idea that you might be able to do something
about it.

The way forums are set up works against this integrated view. We have
forums for gear, forums for acoustics, forums for speaker and electronics
building. There isn't much mention of speakers in an acoustic forum, just
like there isn't much mention of acoustics in a speaker building forum. And
sometimes if the subject is brought up the poster is urged to take the
subject to the proper forum. So as not to clutter every forum we'd need
a forum dedicated to the interaction and intergration of all the pieces.

Paul P

---------------
PS...

I had a similar aha moment yesterday. I was thinking about the large doorless
opening that's between my music room and our dining room. Up until now
I've been thinking of ways to close off the music room so it wouldn't interfere
with the function the dining room. All of a sudden I saw my entire music
room as a sound source of music for the dining room. Instead of having
a door to close the room off, the opening would be like the big end of a
horn.

The fascinating part that came after this enlightenment is that now I can
treat the dining room so that the sound coming from the music room will
sound its best. The dining room is now part of the system and I don't see
a problem in justifying acoustic treatment in it when the purpose is to
have fantastic music (and very clear conversation) while seated at the
dining room table. Whereas before I was looking at using the dining room
to make my music room sound better I can now see the dining room using
the music room as a music source, like a stage.

[Seamus TM]

Check this out, if you haven't already:
John Sayers' Recording Studio Design Forum • View forum - Speaking of Speakers
Half of the threads are about soffit mounting, but there is a lot of good info.

Sayer's site is fantastic.
I am absorbing (ha) information from there, here and Studio Tips.
All three seem to generally focus on different areas.
This may be Forum Profiling, but I go to Sayers for construction and tuned trap info.
I am here for porous absorber info.
I go to Studio Tips for straight up theory and math.
On all three sites, there are great, generous people with amazing knowledge.

All three seem to love superchunks.
I find that interesting, so I'm going to build them.

I keep peeling away layers of the acoustic onion.
Everything might not be immediately applicable to my current building, but I won't be there forever.

That's a great way to think about your dining room.
It seems that it would add a quality of life aspect to your home when you are actually living in it, as opposed to working in it.

Thanks,
Seamus

[SAC]

This refers to the classic diagram originally sourced from Bolt, Beranek and Newman portraying the 'controllers of steady state room acoustic response' for LARGE acoustical spaces.

This describes the behavior about the critical frequency, fc , as well as the large room frequency as defined by Manfred Schroeder, FsubL. The critical frequency, fc, is synonymous with FsubL, and you will see both notations used. While fc refers to the characteristic as it exists in any acoustical space, the FsubL nomenclature refers to when a space effectively becomes “large” in its acoustical behavior.

For wide range frequency response to 30 Hz, this corresponds to a space greater than ~250,000 cubic feet.

Since FsubL is dependent upon the low frequency response one desired it to accommodate (meaning the lower the effective LF reproduction cutoff, the larger the room), it is possible to divide the audio spectrum into 3 decades on a linear frequency scale - with the first decade as a characterized by modes, while approaching the upper two decades consisted of diffusion, and also of absorption of specular reflections.

FsubL = K {Sqrt (RT60/V)}

where
FsubL = large room frequency
K = 2000 in SI and 11,885 in US metrics (Bolt, Beranek & Newman have used a value of 1,893 in SI and 11,250 for US)
V = volume of room

Note, that a small acoustical space lacks a substantial reverberant sound field that rises above the ambient noise floor. Hence this equation in this form describes the large acoustical space where a statistically reverberant space exists.


The LEDE standard (what some have called the ‘Davis frequency’ after Don Davis) has used the following which yields a closely corresponding value:

FsubL = {3(Velocity of Sound)}/( Room’s smallest dimension)


Note: FsubL moves to ~250-500Hz for a small acoustical space.

FsubL is a transitional area. Where the behavior shifts from that of a small acoustical space to a large acoustical space (per Schroeder). This critical frequency, fc, varies as the signal wavelengths progressively become equal to, and shorter than the various room dimensions and as their behavior shifts from the pressure model that characterizes the modal region into the ray/particle model characterized by the dominance of specular reflections.

What is most important is to recognize the shift in behavior and methods used to characterize and also to measure said responses based upon the predominant determinant behavior in each region.


Controllers of steady state room acoustic response

[jhbrandt]

See?

told ya...

[Seamus TM]

Outstanding.
After a few more cups of coffee.....

Thanks,
Seamus

[Seamus TM]

Originally Posted by SAC

This refers to the classic diagram originally sourced from Bolt, Beranek and Newman portraying the 'controllers of steady state room acoustic response' for LARGE acoustical spaces.

This describes the behavior about the critical frequency, fc , as well as the large room frequency as defined by Manfred Schroeder, FsubL. The critical frequency, fc, is synonymous with FsubL, and you will see both notations used. While fc refers to the characteristic as it exists in any acoustical space, the FsubL nomenclature refers to when a space effectively becomes “large” in its acoustical behavior.

I already, mostly, understand what the Schroeder Frequency represents and how it relates to RT60 and reverberant vs direct sound.
I did not fully realize that it is not meant for smaller rooms because of this:

Originally Posted by SAC

Note, that a small acoustical space lacks a substantial reverberant sound field that rises above the ambient noise floor. Hence this equation in this form describes the large acoustical space where a statistically reverberant space exists.

Originally Posted by SAC

This refers to the classic diagram originally sourced from Bolt, Beranek and Newman portraying the 'controllers of steady state room acoustic response' for LARGE acoustical spaces.

This describes the behavior about the critical frequency, fc , as well as the large room frequency as defined by Manfred Schroeder, FsubL. The critical frequency, fc, is synonymous with FsubL, and you will see both notations used. While fc refers to the characteristic as it exists in any acoustical space, the FsubL nomenclature refers to when a space effectively becomes “large” in its acoustical behavior.

I already, mostly, understand what the Schroeder Frequency represents and how it relates to RT60 and reverberant vs direct sound.
I did not fully realize that it is not meant for smaller rooms because of this:

Originally Posted by SAC

FsubL is a transitional area. Where the behavior shifts from that of a small acoustical space to a large acoustical space (per Schroeder). This critical frequency, fc, varies as the signal wavelengths progressively become equal to, and shorter than the various room dimensions and as their behavior shifts from the pressure model that characterizes the modal region into the ray/particle model characterized by the dominance of specular reflections.

I love it.
Totally understood.

Originally Posted by SAC

What is most important is to recognize the shift in behavior and methods used to characterize and also to measure said responses based upon the predominant determinant behavior in each region.

You lost me.

So, my room is only about 2,800 cubic feet.
I should be looking into the Davis?




Thanks,
Seamus

[Jeffrey Hedback]

this may help you further relate these terms to your room:
- the pressure zone begins at with a frequency who's 1/2 wavelength is greater than your room's largest dimension...this doesn't mean your room can't "produce" that frequency...but that it's produced only when driven (no resonance).
- The modal zone extends up to the freq who's 1/2 wavelength is equal to your room's (speaking of length, width and height) shortest dimension...these are the issues talked about regarding bass traps and shown nicely with waterfall plots.
- above that is a little tougher to visualize...I think it's easier to begin at the higher freq's and think about a specular reflection (one who's angle of reflection is = to angle of incidence)...those are identifiable with ETC (glorious huh SAC!)...mostly predictable in a Control Room with mirror trick to identify reflections...and treated to suit with absorbers/diffusors
- the grey area is where the wavelengths do not create modal resonance but also don't "reflect" predictably...these midrange frequencies diffract (bend around) room features, can create resonant pockets with racks and surfaces, can disrupt the spatial balance of a room (how the balance of lo/mid/hi's are perceived).

So this all brings into play the room dimensions/ratios, reflection control, LF control, speaker position, listener position...the whole game!

[Seamus TM]

... which for my room (20.4'L x 13'W x 10.75'H) would be around 300Hz.

right?

[Seamus TM]

Originally Posted by Jeffrey Hedback

this may help you further relate these terms to your room:
- the pressure zone begins at with a frequency who's 1/2 wavelength is greater than your room's largest dimension...this doesn't mean your room can't "produce" that frequency...but that it's produced only when driven (no resonance).
- The modal zone extends up to the freq who's 1/2 wavelength is equal to your room's (speaking of length, width and height) shortest dimension...these are the issues talked about regarding bass traps and shown nicely with waterfall plots.
- above that is a little tougher to visualize...I think it's easier to begin at the higher freq's and think about a specular reflection (one who's angle of reflection is = to angle of incidence)...those are identifiable with ETC (glorious huh SAC!)...mostly predictable in a Control Room with mirror trick to identify reflections...and treated to suit with absorbers/diffusors
- the grey area is where the wavelengths do not create modal resonance but also don't "reflect" predictably...these midrange frequencies diffract (bend around) room features, can create resonant pockets with racks and surfaces, can disrupt the spatial balance of a room (how the balance of lo/mid/hi's are perceived).

So this all brings into play the room dimensions/ratios, reflection control, LF control, speaker position, listener position...the whole game!

Hi Jeff,
You snuck that in there while I was doing math.

I totally get the modal vs specular.
It's that grey area that is baffling (ba doom ba).

Although I do have obvious problems in the 100Hz - 300Hz range, according to Davis, they are modal in nature and according to Schroeder, they are in the grey area.
BUT, the Schroeder theory doesn't really apply to my room size, so I should listen to Davis.

...or maybe it's just SBIR.

[Jeffrey Hedback]

right...back to original post of nasty dips 100Hz to 400Hz.

I know you have a great deal of data posted on your space, but can you present data in this area (or link to it if already posted)?

Could be floor dip (can often be in 100Hz to 200Hz zone).

Thing is that often narrow dips aren't a major problem as our ears can deal with them...when they are neighbored by sharp peaks...then you can have big issues.

Have you "trapped" all corners including wall/ceiling junctures? If so, you're likely not having a modal issue but rather a boundary interference...and it's not out of the question that room furnishings are a factor.

I know you're manically trying to solve...hope this helps and sorry if too vague...comes a time when you need ears in the room to solve pesky singular issues.

[Seamus TM]

Maniacally is the perfect way to describe this.
I'm driving everyone around me crazy.

Anyway,
No, I don't have any corner trapping at the moment.
I'm going to do the super chunks as soon as the Roxul truck arrives.
I'm going to start by doing the front verticals and the wall/ceiling in the front and the back.
After that, I'll probably do the side wall/ceilings.
I have structural obstacles in both back verticals.

Warning:
Information Diarrhea Forthcoming.

I messed around with putting 4" 703 panels in vertical corners, but I couldn't get them to have any effect.
I had made soffits that were 8'H x 2'W x 18"D out of R30 in the front vertical corners.
They had little or no effect.

Right now, the only absorption in the room are as follows:
Front wall completely covered with 12" R38 kraft paper out.
Some 2" and 4" 703 panels that start at the front wall and go back 6' on the side walls that are 4' high positioned for RFZ.
A 4'x8'x2" ceiling cloud position for RFZ.
A 8'H x 6'W x 18"D section on the back wall of R38, that I don't think is doing much of anything.
There is carpet over the all of the floor except the front 3' or so... where the speakers are.

No matter where I move the speakers I have one or both of two problems.
Problem 1: Null at 60Hz
Problem 2: Nulls at approximately 115Hz, 150Hz and 300-350Hz (it moves).
I do have a console that is about 6.5' long and 3' deep.

With the speakers as far away from the front and side walls as possible the low end response is VERY flat down to 80Hz. At that point the 60Hz null is at it's worst and pulls the response down to -30 @60Hz.

With the speakers almost all the way into the corners, the 60Hz null is almost gone, but the three amigos (115, 150 and 300) are at their worst.

Right now, I have the speakers in the middle of these two places so that both problems are about even and the lowest dip is about -12dB @ 60Hz and around
-10dB for the three amigos.

I am playing around with the idea to use two sets of speakers instead on one pair. One set on the meter bridge (as "nearfields") to get the flat down to 80Hz response, and then a set in the corners (as "mains") to get the thump of 80Hz down.

I know that I need more treatment, but I am skeptical that the 60Hz problem will ever be solved enough.
From what I can tell, it may be a 1,0,1 mode.
The dimensions are 20.4'L x 13'W x 10.75'H.

I have not done any RT60 analysis as of yet.

Thanks,
Seamus

[Seamus TM]

I have other information posted here:
Changing Control Room Ratios (plus a parallel wall question)

and here:
John Sayers' Recording Studio Design Forum • View topic - Quick Slot Absorber Questions?

[Jeffrey Hedback]

Stickler point...you actually will not do any RT60 analysis because there is no reverb in a space of your size...I know people talk of decay time and that's important to look at in terms of modal resonance...but there is no reverb.

Sorry to go "SAC" on ya...important point to clean up though

I did some analysis that allows me to shows not just modes, but how speaker/ear locations factor in what is heard...and by pushing the speakers wider I could duplicate the smoothing of the 60Hz area and create big push around 175Hz...so alot of what you mave may indeed be modal.

The ceiling: that's the weak link in your plan is given...a simple general suggestion is to drop your cloud about 16" below ceiling and add R-30 batt insulation above the 2" fiberglass. If your cloud is 4' wide, you may need to go twice that.

Now...I don't want to send you on goose chase...think of this after you have superchunk and rear wall/ceiling done. As the 1,0,1 is an issue...the rear wall/ceiling should help.

The worry in all this is losing the war of a balanced room to win the battle against one mode...simply put a totally dry/dead room is not good (sure you're aware).

[Seamus TM]

Originally Posted by Jeffrey Hedback

Stickler point...you actually will not do any RT60 analysis because there is no reverb in a space of your size...I know people talk of decay time and that's important to look at in terms of modal resonance...but there is no reverb.

Sorry to go "SAC" on ya...important point to clean up though

By all means, SAC away.
Decay time is what I was after with that statement.

Originally Posted by Jeffrey Hedback

I did some analysis that allows me to shows not just modes, but how speaker/ear locations factor in what is heard...and by pushing the speakers wider I could duplicate the smoothing of the 60Hz area and create big push around 175Hz...so alot of what you mave may indeed be modal.

Very cool.
If you don't mind me asking, how is that analysis done?

Originally Posted by Jeffrey Hedback

The ceiling: that's the weak link in your plan is given...a simple general suggestion is to drop your cloud about 16" below ceiling and add R-30 batt insulation above the 2" fiberglass. If your cloud is 4' wide, you may need to go twice that.

The cloud is actually running the other way (8'W and 4'L), but that makes sense to me.

Originally Posted by Jeffrey Hedback

Now...I don't want to send you on goose chase...think of this after you have superchunk and rear wall/ceiling done. As the 1,0,1 is an issue...the rear wall/ceiling should help.

That sounds good and something that I can focus my understanding on. Thank you for that.
As the 1,0,1 is concerned, I was thinking of doing the superchunks along with some sort of acoustical hanger baffling. I'll measure with just the superchunks first, and then we'll see where I'm at.

I am also considering pulling the R30 out of that back wall section and doing something else with the space.
Maybe trying hangers there, as well.

As a side bar, what do you think about the 2 sets of monitors idea and the theory behind it?

Thanks so much, Jeff.

[Jeffrey Hedback]

two sets of monitors is obviously the norm...but monitor placement is almost always a subtle compromise between LF response and imaging...if proper placement of a monitor for imaging puts speaker in modal null...somthing got to give...so hopefully the trapping you're strategically addressing will allow good monitor placement...there are so many things involved about a specific speaker on a specific meter bridge that I wouldn't ever go with the thought that this set is accurate to 80Hz. I would do everything possible to have a fullrange/hi-resolution set of "mains" that have very few console reflections...and then use the meter bridge set for mid range balance reference.

[Seamus TM]

Thanks, Jeff.

[SAC]

Maybe I can help clear up a bit of the confusion regarding a few things

Don't get hung up on Schroeder versus Davis. they are essentially the same!

Manfred Schroeder (an incredible mathematician) did a comprehensive study of what makes the best large concert halls so good. In the process he determined just how large a hall must be to support a reverberant sound field.

But it’s a little more than that, as you see, the lower the frequencies you want to produce, the larger the space must be. Common reference points for this are 80 Hz for speech and 30 Hz for music. Thus, if the halls should be primarily for speech, then its volume should be ~35,000 ft^3 or larger, and if it for music, then it should be ~250,000 ft^3 or larger.

But don't worry about this! The only thing you need to know is that you are NOT dealing with a Large Acoustical Apace! And the significance of this is that the acoustical behavior in the room is DIFFERENT from Large room acoustical behavior.

You are dealing with a Small Acoustical Space. And as such a space does not support statistically reverberant sound fields, the room is dominated by modal standing waves in the low frequencies, and by focused specular reflections in the mid and high frequencies.

And Don Davis, along with Dick Heyser and Dr. Eugene Patronis and a notable group of fellow researchers (including Dr. Schroeder) did substantial research into the behavior of sound with the advent of Dick Heyser’s revolutionary TEF analyzer. And out of this can such acoustical room models as the LEDE and RFZ concepts.

OK...

To deal with the gray area that seems to confuse... Don't let it!
That just means that, depending upon the room dimensions, that each dimension supports different modes based upon their length. And as the sound wavelengths reach the point where they are shorter than the room dimension, they cease to act as a pressure wave and begin to be reflected as a specular reflection.

Remember, wavelengths longer than an object is large, flow around the object like water does a boulder in a stream. Wavelengths smaller than an object are reflected and blocked. Hence why some support standing pressure waves, and others support specular reflections.

So, for instance, the height of say 8 foot will only support standing waves for frequencies with wavelengths of 8 ft or more (~141 Hz and below). A room with a length of, say, 20 feet, will only support standing wave modes of frequencies with wavelengths longer than 20 feet (56 Hz and below).

You see, as the dimensions support differing modal ranges, and as the wavelengths become shorter, modes are supported less and less by different surfaces, until the wavelengths are smaller than all of the room surfaces and you then have a region above which all behavior is specular in nature.

So you have a transition region where one dimension will support modes, but the others will support specular reflections. It is not necessarily one fixed frequency at which a magic change occurs.

The important thing is to understand that this occurs. And that you will have a LF region where you deal with room modes.
And these are most easily identified with the frequency response and the cumulative spectral decay or waterfall plot. (they are not exactly the same, hence listing both, but for all practical purposes, they will look the same to you and you interpret them the same!)

In a Small Acoustical Space (SAS) which small rooms are, you have a finite amount of acoustical energy that drives the room. And it decays quickly.

And where you have thought of decay times and reverb, you will have ALL of that info in the CSD/waterfall in terms of each frequency’s persistence. These are resonances that persist in time. And they will coincide with the reinforced modal frequencies. So, essentially by identifying and treating the modes, you deal with the decay issues.

So, for the most part on the forum, we have the modal frequencies and frequency responses and waterfalls down.

But above that point, all that has been 'known' is that the frequency response displays comb filtering. And comb filtering is the nature response to the combination of signals (think direct signals and reflected signals) that are from spaced sources. And spaced sources vary in time and distance. And this last statement is important!

As time and distance are simply two ways to describe the same event! For instance, if a signal traveling at a fixed velocity travels for 5 seconds more than another signal; we know that the later arriving signal travels a distance of 5 seconds X the rate of travel.

So, if sound travels at 1130 feet per second, it travels at 1.13 feet per ms.
And if a signal arrives, say, 5 ms after the arrival of the direct signal, we can accurately say that the signal traveled (5ms X 1.13 feet per second) or 5.65 feet further than the direct signal used as a reference.

So....all we need now is a way to determine with precision, the arrival times of each reflection, and to determine the exact path the focused (specular) reflection has traversed.

And the tool that provides us with exactly the arrival time and the ability to determine this for each reflection, is the Envelope (Energy) Time Curve first introduced by Dick Heyser and his TEF/TDS measurement system.

Once we have that info, depending upon the sophistication of the measurement system you are using, we can rather quickly determine the exact path of the reflection.

This can be done in a variety of ways, ranging from very basic to quite elegant.

Let me start with a few basics...

(NOTE! I am dealing here with only the mechanics of the process. I am NOT addressing the available acoustical room models by which you might want to tune the room response. This is important, as in practice you need to know where you are going! You need to know what and why you are treating reflections in a certain manner, as you are wasting your time if you simply start 'treating ' reflections without a defined goal! But that is another important discussion as it gets into the psycho-acoustics of how we hear and how we want to adjust the room response to make maximal use of that understanding!)

If we know the location of the speaker used to generate the source signal, and we know the location of the mic capsule that received the signals, we know the distance and time of the direct signal simply by looking at the ETC. It is the first spike in the ETC {generally! But I won't bore you here with the occurrence of acasual or 'signals arriving before they left'. And yes, you can occasionally encounter this due to say a speaker sitting on something and being tightly coupled where the rat e of signal transmission is faster through that medium than it is through air! So the signal arrival via the alternative medium arrives before the direct signal through air!! ;-))

But weirdness such as that aside, - oh, and the ETC and Heyser spiral can indeed tell you about those too! - but let’s move on.

In some systems you must calculate the propagation delay of the equipment and correct for that by subtracting it from the signal arrival times. After all, the real travel time is simply from when the signal leaves the speaker and when it arrives at the mic. Some gear lets you simply correct for this by moving the reference point in the display to correspond to the direct signal arrival time. We will assume this. (If it does not, you simply have an additional correction to make for each signal time.)

So, assuming that we have the arrival times for each reflection relative to the direct signal, we know how much further the reflection traveled to reach the mic.

Now, how to determine the path…
The most basic method….

Say we have a string, and we mark the string with the distance that corresponds to the time of travel of the reflection.

We know the start and ending points of the travel, right? The speaker acoustic center and the mic capsule. So....if we fix the ends of the string at those points, we will have some amount of loose string left. Now, if we extend the loose string by holding it loosely at one point and see where the loop might touch any boundary surface, where it touched with the string being taut, IS the reflection point of incidence. This will be the spot many refer to when they use the mirror trick. But it will be much more precise, as we will not simply be guessing and saying that some reflections will go that way or that way; instead we have identified the specific path that THIS particular reflection has gone. And we can do this for essentially all of the reflections - including those that may touch on multiple surfaces. (Oh, and if we encounter a symmetrical instance where two reflective paths have the have the same length and coincide with the arrival times...no problem, we can easily identify both in the same manner and we will know this as their gain will reflect this (sorry for the pun) accordingly!)

A more practical method is, after identifying a reflection of interest, to repeat the measurement manually or to let the platform automatically repeat it at intervals, and to use a small piece of absorption to block a path to the side of the microphone. And as you move the blocking piece around the mic, you will eventually identify a position where that particular reflection is 'gone' - or blocked. And here you know that you have intercepted it in 'flight'. From this, you can extrapolate the path from the mic capsule through the blocking location to identify the general location of the reflection incident point on a boundary. Further iterations of this process can further refine the location.

After doing this a 'few' times, pattern recognition will quickly set in, and as you will know the relative distances of the various boundaries in a room (walls, ceiling, floor, etc., and you will be able to quickly identify the source of the first, second and third, etc. reflections. And this will help you to more quickly focus in on the particular spot of reflection incidence.

'Larger' measurement and analysis platforms enable one to more quickly do this. One even is able to generate a 3D ETC that when you move the cursor to the particular point of the reflection, will generate the X,Y, and Z coordinates such that you can replace the mic with a laser pointer mounted in a mount similar to a surveyor's transit and to simply dial in the X,Y, and Z coordinate and the laser will point to the exact spot on the boundary surface!

What makes this even more significant is that some reflections will be of much higher strength (gain) then others. And if they arrive too hot, or at the 'wrong' time to cause problems in the imaging or intelligibility of the signal, we can identify them and correct for the particular issue specifically. And that is the key difference that a 'professionally' tuned room offers differently from a 'non-professional' room. The reflection energy is used to literally tune the room. It is not simply seen as an enemy to be eliminated! In fact, a key component in many designs is to take exactly that excess focused energy and to both reduce it to a desired level (not eliminate it!) and to spread it out in time (temporal dispersion) and to disperse it in space (spatial dispersion) such that we create a more well-behaved semi-diffuse sound field.

OK, I'll stop here for now...

A sample ETC so that you have something to look at....

[Seamus TM]

Oh yeah.
That's the stuff.
Great information, SAC.
Thank you very much.

There some points in that post that really bridged some gaps in my understanding of acoustics in a listening room as a system.

I'll read this thread quite a few times over the coming weeks, I think.
While I'm waiting for my Roxul truck.

[Brainchild]

Great. That was a great post, SAC. Thanks for conscientiously tailoring your response to the audience! I am getting a lot out of your recent posts.

[johndykstra]

+3

Bravo

This should be required reading

[dykesh]

Thanks so much for the explanation. So, if I'm reading the graph example you gave us; the initial sound was around 80 msec. Then we have spikes approximately 120, 185, 205, 222, 250 and 400 msec. So if we subtract the 80 msec, that gives us 40, 105, etc... These are the reflections in msec that we should look for and treat in out room, yes?

I thought about this some more. It seems like going out 400 msec is way to far if we're talking about first and second reflections. My rooms only 14Wx18.5Lx10H so I should probably be concerned with the first 40 msec or so? thanks

[PaulP]

Originally Posted by dykesh

I thought about this some more. It seems like going out 400 msec is way to far if we're talking about first and second reflections. My rooms only 14Wx18.5Lx10H so I should probably be concerned with the first 40 msec or so? thanks

This is where SAC meets the rest of the gurus around here. If you collapse
all his rhetoric down to your small room you'll end up with the same bass traps
for the modes and panels on the walls and ceiling for the early reflections
that Ethan, Glenn et al have been suggesting all along.

Paul P

[DanDan]

A more practical method is, after identifying a reflection of interest, to repeat the measurement manually or to let the platform automatically repeat it at intervals, and to use a small piece of absorption to block a path to the side of the microphone. And as you move the blocking piece around the mic, you will eventually identify a position where that particular reflection is 'gone' - or blocked. And here you know that you have intercepted it in 'flight'. From this, you can extrapolate the path from the mic capsule through the blocking location to identify the general location of the reflection incident point on a boundary. Further iterations of this process can further refine the location.

There is a faster more accurate way to do this. Use a kinetic, psycho-acoustically servo controlled unit to generate stimuli which trigger the reflections of interest.
(Move about the room, clapping your hands. Stop at each spot where the slap or flutter is worst.)
Now introduce another kinetic, servo and voice controlled unit, capable of positioning a reflection stopping device in any position or plane.
( A friend! with a panel of 703, wrapped in fabric)
The interaction of the two units plus their combined central processing units results in speedy and certain identification of the best locations for the reflection stopping devices, obviating iteration.
(Friend moves panel to spot, flutter stops, hang the panel there)

:-)

DD

[SAC]

Faster? Really?
Servo controlled??? LOL! That's the ticket for so many who cannot even justify procuring an appropriate mic for anything but LF room modes! Plus your analogy fails as the servo controlled unit would require some sort of brain. (Oh, and it should be noted that a mic appropriate for measurement is appropriate for this process! We are no longer in the real of LF modes. A smaller capsule becomes of use here.)


In fact, the process described takes about 2-3 sweeps (~10-15 secs real time) and pattern recognition will reduce this to I-2 sweeps to fine tune the location.

Done. And with no need for anyone else running about carrying a large panel!

But as was stated, the process was a VERY basic description of process to explain a concept for someone not yet familiar with the process. It was NOT designed to be an elegant mechanism that presumes their familiarity!

Besides, part of the purpose of this procedure is to identify placement for more surgical treatment - and please note that I did NOT say "absorption"! - not simply an excuse and rationalization to cover yet another wall with large panels of absorption!

And if one is truly serious about this, they can use the Polar ETC program and have the program auto-generate the 3Space co-ordinates from ONE measurement. And all the work that is required is the operator rotaining and aligning a laser pointer to the co-ordinates supplied!

But how it is done is secondary and not of real consequence.

There is an even more fundamental concept that has apparently been missed.

As was stated previously, this process is merely a procedure for identifying the reflective points of incidence. It is NOT simply a prescription for putting up absorption!

This is a critical point of understanding!

Once the points of incidence are identified, one must then evaluate the response relative to the acoustical model they seek to design in the small acoustical space.

Only by then establishing a correspondence between the ETC response and the acoustical model does one then know what to address and evaluate the various choices by which to accomplish the desired goal. And thus begins the iterative process of treating the points of incidence in order to achieve the desired response sufficient to contribute to the larger room response.

And this is but the beginning of establishing a framework of the well-controlled response.

As one will discover if they are serious in pursuing the process further, there is more techniques and principles one needs to understand to appropriately treat the room. As we are going to need to establish psycho-acoustic triggers by which to exploit the various responses our ear-brain require for proper imaging and localization.

There is MORE to tuning a room than simply slapping absorptive panels up on the walls - even with precise placement.

And while not overly complex, a basic awareness and understanding of these principles are important and necessary. As is an understanding of the various acoustical room models that make the achievement of said goals possible.

Unlike with room modes where the goal is to simply place as many traps and absorbers as is possible, we have entered a 'region' where the process is a bit more moderated, judicious and selective - and where awareness leads one to consider multiple choices by which the same result may be accomplished.

[DanDan]

Jeez, I can't believe he took the bait. Must have been the collection of ridiculously long words.

Yes really faster, and totally certain. The technique is just as useful in identifying spots for diffusion or other types of treatment. That, of course, was not at all the point of the post.
Irony, look it up!
Mark, aka SAC, aka foxfyr, aka Mac, I will also ask once again, the same question which is never answered. How does that haircut (ETC) do a better job than a mirror, handclaps, a friend and a panel (diffusing or absorbing)? Furthermore what is the purpose of all this technobabble? I have never seen it actually help anyone. Many of us have ears, brains, books, academic qualifications, and for that matter, expensive DPA and Bruel and Kjaer microphones. Peacock-like flashing of such assets is hardly good form or even manners in the context of this forum. Or anywhere for that matter.

DD

[SAC]

Originally Posted by PaulP

This is where SAC meets the rest of the gurus around here. If you collapse
all his rhetoric down to your small room you'll end up with the same bass traps
for the modes and panels on the walls and ceiling for the early reflections
that Ethan, Glenn et al have been suggesting all along.

Paul P

Sorry, but that is incorrect. But you KEEP repeating this over and over.

There are quite ingenious ways to create an ISD in a room whose natural decay seems to preclude this, and this has been done for over 30 years now.

But its always refreshing to read the comments of one who does not understand this, and who mistakenly keeps saying that Davis maintains that small acoustical spaces begin at 2000 ft^3, provide a erroneous critique.

What's even funnier, if you were aware of what your comments imply, is that you are saying the work of not only Heyser, Davis Patronis, and MANY more Very qualified folks are incorrect, but that other developers of the concept whose names you might recognize, such as Berger and D'Antonnio, are wrong too.

Originally Posted by DanDan

I will also ask once again, the same question which is never answered. How does that haircut (ETC) do a better job than a mirror, handclaps, a friend and a panel (diffusing or absorbing)? Furthermore what is the purpose of this technobabble? Many of us have ears, brains, books, academic qualifications, and for that matter, expensive DPA and Bruel and Kjaer microphones. Peacock-like flashing of such resources is hardly good form or even manners in the context of this forum.

DD

Its amazing, but not surprising, that one who is not only not familiar with the meaning of this 'technobable' and process is also unaware of why and how it is used.

Peacock flashing????? LMAO!

Let's see, TEF was not only the first, but it has been around now for over 30 years...subsequently joined by several other industry norms such as EASERA. This technique has been mature for almost as long - begun in the late 1970's and mature at least since 1984 when QRD diffusion was introduced.


But by all means keep clapping. It is always appropriate for one to function at a level where you are able to understand the tools which one employs. And in the same spirit, one might also question and wonder as to what the true necessity of a handkerchief is.

If anyone else is seriously interested and PM's me, I can provide source material from a few VERY prominent practitioners that will dispel the ignorant nonsense of a few that this process is not valid and is in fact a de facto norm in professional analysis. The supreme irony is that this knowledge goes back to at least 1984. And I can even provide documentation that shows Alton Everest and his wife in attendance at one of the very workshops.