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SBIR
Old 25th February 2011
  #61
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Quote:
Originally Posted by Jeffrey Hedback View Post
http://alexandria.tue.nl/extra2/afst...unster2003.pdf

this gem seems to have the topic summarized, short of the actual Syn-Aud-Con paper from Russ.

Page 58 covers SBIR
So, virtual images it is since this paper also refers to Peter D'Antonio. The 1/4 wavelength illustration of SBIR is only valid in the situations described above and this representation has lead to many misunderstanding of the concept, right?

If so, how about my question regarding limit between SBIR and modal behaviour?
Old 25th February 2011
  #62
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The SBIR graph in that link to the munster2003 paper would not exihibit such a reinforcement in dip if we were to think about it in paths to the listener. Unless in a very special case.
Usually the combfiltering would reach us at different paths even though the speaker were equally close to 3 surfaces.
And that would mean different frequencys and different SPL.

As an exampel, in a equilateral setup the front wall, sidewall and floor (all of them 4' from speaker) would then create the first out of phase frequencies at 77Hz, 98 HZ and 117 Hz. This is what I would call combfiltering.
The 70-80Hz dip is something else and does indeed seem closer to quarter wavelength (you can see it get deeper and closer to 70Hz more surfaces are added).

I find it all very interesting.
Old 25th February 2011
  #63
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Quote:
Originally Posted by DanDan View Post
Rod, I would have edited my 'interrogation' (too much watching The Practice and Boston Legal!). Thanks for your perseverance.
Dan,

One of the good things about a conversation like this is that it forces me to critically examine my thoughts on a subject - which is always a good thing.

I think that one of the problems is that this term is used to cover a wide variety of effects...

I now see modal activity creeping into the thread......

Let's try to break this down a bit...... every thing that takes place inside of an enclosed space is cause by room boundaries.

When we refer to "early reflections" it is clearly understood by everyone that we are speaking of reflections from either walls, the ceiling or floor - perhaps the desk - that are combining at the listening position.

When we refer to 'flutter echo" it is just as clearly understood that we are speaking of in-line reflections from 2 opposing walls.

Room Modes are clearly defined as either Axial, Tangential or Oblique.

And yet we have this confusion about SBIR - which seems to be a bag that a little of everything seems to get thrown into, along with this added effect of what happens with a a low frequency reflection off the surface that couples with the original signal prior to the listening position.

From my way of thinking - it makes no sense to describe an effect that is totally related to the distance of the sound source being 1/4 wavelength away from a surface (or multiple surfaces) that will cause a dip in the signal (due to coupling prior to the listening position) by using a term that covers a multitude of other effects.....

To say "the problem is caused by SBIR - and let me clarify by explaining exactly which of the many SBIR effects I am referring to" - it seem to me much easier to always use the term to describe this particular phenomenon.

Quote:
I am well prepared to be wrong but I also reckon the SBIR null is present throughout the room, from the speaker forward.
Is this a harbinger of a Minimum Phase phenomenon?
With this I agree...... meaning that once coupled - the resultant signal is present form that point forward.... although this is not to suggest (just saying this so no one misunderstands my meaning) that there could not be other forces (not related to SBIR) that affect the (now altered) signal in some manner to either constructively/destructively alter it.

I would point out that minimum phase phenomenon is also caused by room modes........ and thus would require something more to clarify the initial description. The term (in and of itself) could (again) be describing an outcome caused by more than one effect.......

Heck - when you think of it - this was the very reason there are multiple names for the modal activity - one term could not simply cover all of the possible causes........ this even though the effect was always the same.....

Rod
Old 25th February 2011
  #64
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Quote:
Originally Posted by Jens Eklund View Post
If playing a single steady frequency, provided in phase (and in free field setup), a 6 dB (SPL) increases is observed if the amount of speakers are doubled (if same amplitude).
Thanks for catching that.......

For a coherent wave a summing of the waves would result in a 6dB increase in amplitude......

For incoherent waves a doubling of power (setting 2 speakers side by each with the same amplitude) would result in a net gain of 3dB. It would take 4 speakers to achieve 6dB.

Thus a steady signal frequency will produce a 6dB increase in amplitude - whereas white noise would produce a 3dB increase (given a true doubling in power - not taking into account anything that might alter the signal, such as temperature, humidity, etc.)

I appreciate the catch..........

Rod

Rod
Old 25th February 2011
  #65
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Lightbulb

Quote:
Originally Posted by cborg View Post
Usually the combfiltering would reach us at different paths even though the speaker were equally close to 3 surfaces. And that would mean different frequencys and different SPL.
Yes! That's why I said earlier "But in small rooms it becomes a big jumble that's much more difficult to predict."

--Ethan
Old 25th February 2011
  #66
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Quote:
Originally Posted by Jeffrey Hedback View Post
http://alexandria.tue.nl/extra2/afst...unster2003.pdf

this gem seems to have the topic summarized, short of the actual Syn-Aud-Con paper from Russ.

Page 58 covers SBIR


Now way off topic of this thread, but highly on topic to much current discussion: lateral termination of the ITD, call me a "Wrightsonian" (pg60).
Thanks for this Jeff
Old 25th February 2011
  #67
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Quote:
Originally Posted by Jeffrey Hedback View Post
Cborg,

That is a total summary of SBIR (at least to me).

If you happened to listen/measure between the speaker and the boundary, you would not have SBIR because the interference doesn't occur until the wavelength specific to the distance involved arrives back at the location of origin. After that point, the event is non-localized.

Rod, I like the analogy to active noise cancellation.

I suggest that the whole point of the term SBIR as defined by Syn-Aud-Con ~30 years ago was to separate it as a singular event worthy of addressing apart from the general constructive and destructive combination of reflected and direct energy.

I'm not sure that SAC was participating in those workshops, but he had to be there not long after. Maybe he will offer his direct knowledge of those original studies by the Davis's, Berger and others.
Jeff,

Great link......... one comment i would make - the resultant signals indicated in Figure 53 are not what I would describe as comb-filtering..... although there are (in 2 of the cases) a sharp dip that occurs after the initial spike - beyond that everything just sort of "fizzles" out..........

Thanks,

Rod
Old 25th February 2011
  #68
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Quote:
Originally Posted by Jeffrey Hedback View Post
http://alexandria.tue.nl/extra2/afst...unster2003.pdf

this gem seems to have the topic summarized, short of the actual Syn-Aud-Con paper from Russ.
A marvelous document. Thank you Jeff. thumbsup

Andre
Old 27th February 2011
  #69
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Hi again guys!

This have been a very good debate, with great material and opinions beeing put forth.

I am still not completely satisfied with (or still confused about) the different use of this terminology, but i can now see were Mr Gervais was coming from.

Camp Nr1:

Some of us are describing the SBIR effect as a sort of speaker boundary related modal activity (deep nulls that are not related to the room dimensions) - like the right illustration in the OP and in Mr Hedback's link.

Camp Nr2:

Some of us are also including the, what Mr Gervais refer to as, LF early reflections into the term SBIR - Like in the left illustration in the OP and Mr Eklund's first virtual image drawing and MHOA link.

I am interpreting The following article by Dr. Peter D'Antonio to be more in terms of 'Camp Nr2', but i could be wrong. Mr Lachot however seems to have interpreted it in a simular fashion. (this article by the way seems to be a mix between Mr Hedback's link and Mr Eklund's MHOA link)

Dr. Peter D'Antonio Link:ROOM REFLECTIONS CAUSE ACOUSTIC DISTORTION

Some essential quotes from the article:

"Speaker-Boundary Interference - the coherent interaction between the direct sound and the omni-directional early reflections from the room’s adjacent boundaries."

"This type of acoustic distortion is due to the coherent interference between the direct sound from a loudspeaker and the reflections from the room, in particular the corners immediately surrounding it."

"This distortion occurs across the entire frequency spectrum, but is more significant at low frequencies."

Mr Lachot's interpretation (at the point of this discussion at least): Non-modal peaks and nulls

Mr Lachots conclution:

"What I am saying in a nutshell is that speaker-boundary interference and comb filtering are symmetrical, mirror image embodiments of the exact same phenomenon. The two transducers, the speaker and the ear, are not all that different if you see it as a symmetry problem."

Breaking it down

Sören would like to associate the following effects to their proper terminology:

1. The shelving boost that occurs when a speaker is placed close to a boundary (the proximity effect?? modal coupling??)

2. The 1/4 the wavelength boundary effect that causes wave cancellations (not comb filtering per se) and a sort of non-modal frequency null. (well this one i think we all agree on is SBIR)

3. The 'asymmetrical'/omnidirectional LF early reflections that bounce around everywhere in the room, causing comb filtering at varies frequencies and positions around the room (SBIR? LF early reflection? Acoustic interference? LBIR?)

4. Specular early reflections that cause comb filtering. (early reflections? LBIR? or simply comb filtering?)

5. The LF problem that occurs when the speakers (or the listening position) are placed in a bad modal position. (modal coupling? A bad modal position? heh)

What is LBIR?:

Mr Hedback seems to call the Nr3 LBIR (am i right?)... Mr Gervais seems to call Nr3 'early reflections'... Mr Winer along with Mr Lachot and Dr. D'Antonio seems to call Nr3 SBIR. Mr Lachot seems to call early specular reflection LBIR, while Dr. D'Antonio simply calls early specular reflections 'comb filtering' (in the article that is).


Help Please!!

Last edited by SörenHjalmarsson; 27th February 2011 at 05:06 PM.. Reason: Addad point Nr 5
Old 27th February 2011
  #70
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Lightbulb

LBIR = Listener Boundary Interference Response. It's just like SBIR, but the distances are related to your ears (or measuring microphone) and the reflecting boundaries. I don't think LBIR is an official term. I seem to recall someone making that up in a forum a few years ago. But it's appropriate!

--Ethan
Old 27th February 2011
  #71
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Hi Soren,

I actually think there is less confusion than it seems on the topic.

Situation #2 from DD's OP is SBIR. This thread began with most (or all) in agreement on that. It is separate from modal and other behaviors, but intertwined with the response at any measured position.

Echoing part of Ethan's earlier point that "in small rooms it becomes a big jumble"...absolutely. The worse the modal control and the more excessive the decay time (for a given application), the harder it is to identify SBIR in measurements (I've found). BUT, it is always predictable: 180 deg phase cancellation due to distance from source>boundary>source. Again the 1/4 wavelength...

LBIR is Listener Boundary Interference Response (and I do agree with Ethan that it is a forum evolved term):a phase cancellation null but related to the listener/client position to the rear wall behind. This can be measured by distance and equated to the 1/4 wavelength to identify the cancellation null just like SBIR.

Wes is very sensitive to this (LBIR) and PURPOSELY designs his rear wall systems to place the client's ears in the ~250Hz null range. He also finds issues with certain other rear wall designs of greater depth as they place the client in a null at a lower more critical freq (say between 70Hz and 150Hz as an example). These are purpose built rooms with exceptional full spectrum acoustical control. Which is supportive of my statement that the better the modal control, the more apparent the Interference Response nulls will be.

As a source moves the SBIR moves accordingly. As the listener moves, the LBIR moves accordingly. Why is it important to look at the Interference Response separate from general modal response and specific modal response due to source/listener locations...because these events can work together toward a better overall response or can really cause harm if they coincide in the same range (meaning the SBIR coincides with a modal null or LBIR).

It's complex because we are looking closely at complex events. I suspect that is what drove Russ Berger to identify it as a unique factor.
Old 27th February 2011
  #72
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Thank you guys!

I thought that LBIR could be like in the first virtual image drawing by jens (when the virtual images are pointing towards the listening position) or like in the first illustration in the OP...

LBIR = The pathway/distance to/from rear wall to listener, Check!

Quote:
Originally Posted by Ethan Winer View Post
Now consider what happens when you add more walls. If there's a wall behind the speaker (and microphone), opposite the first wall, some frequencies will be reinforced as they bounce repeatedly. You still have the basic comb filtering off the first wall, and with no side walls or floor or ceiling the resonance is not strong because much of the sound escapes.

Then add the side walls, floor, and ceiling. Now sound can bounce around and travel many paths to arrive and collide in and out of phase at the microphone. We still have the basic comb filtering we started with! But now you'll measure additional peaks and nulls of various frequencies and strengths at various locations. This is due to all the added paths and their varying path lengths.

So I maintain that the core effect is comb filtering, regardless of the frequency. And this is why all small rooms need broadband absorption, because the frequencies that need absorbing are not necessarily related to the room dimensions.
Quote:
Originally Posted by Jeffrey Hedback View Post
Situation #2 from DD's OP is SBIR. This thread began with most (or all) in agreement on that. It is separate from modal and other behaviors, but intertwined with the response at any measured position.

Echoing part of Ethan's earlier point that "in small rooms it becomes a big jumble"...absolutely. The worse the modal control and the more excessive the decay time (for a given application), the harder it is to identify SBIR in measurements (I've found). BUT, it is always predictable: 180 deg phase cancellation due to distance from source>boundary>source. Again the 1/4 wavelength...
Breaking It Down Again

Ok so what you guys are saying is, that SBIR starts out as a wave cancellation at a low frequency (Mr Winer refers to this 'phase problem' as comb filtering aswell, even though it might look more like a mode on the measurement result). This cancellation (null) will remain fixed throughout the room, but as those waves travel around the room (and mingle with the other SBIR reflections, 'LF early reflections' and room modes etc.,), you'll measure additional peaks and nulls of various frequencies and strengths at various locations in the room.

If i'm right... this quote would make sence (from Gullfos link to linkwitz lab):

"The wall reflects the rearwards radiated sound which is 180 degrees out-of-phase with the forward radiated sound. The interference between the two leads to progressive cancellation at low frequencies and comb filtering at higher frequencies."

Question Nr3
What i'm still curious about is what you would call the first illustration in the OP?? LF early reflections??

For those of you who wonder why i included the last two question (Nr4 and Nr5)... I was just trying to be clear and really separate all the different problems and give them all different terms heh.

Nr4
I included because the quote below suggests that even specular early reflections should be included in the term SBIR (well, it is a sort of speaker boundary interference aswell, but that is not what we mean when we say SBIR)

"This distortion occurs across the entire frequency spectrum, but is more significant at low frequencies."

Nr5
I just included to see if there might be a good way to separate this from the SBIR/LBIR. kind of like... "This or that position would not be a good place for your speakers/listening position due to SBIR/LBIR and..." "This or that position would not be a good place for your speakers/listening position due to 'modal origo loss'."


Thanks Again!
/Sören
Old 28th February 2011
  #73
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Quote:
Originally Posted by Ethan Winer View Post
LBIR = Listener Boundary Interference Response. It's just like SBIR, but the distances are related to your ears (or measuring microphone) and the reflecting boundaries. I don't think LBIR is an official term. I seem to recall someone making that up in a forum a few years ago. But it's appropriate!
I coined the phrase/acronym. I am not just somebody, I am a well known somebody! As far is being "official" whatever that means, the is a post in this thread where one of owners of Realtraps uses it!

With humour,
Andre
Old 28th February 2011
  #74
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Quote:
Originally Posted by avare View Post
I coined the phrase/acronym. I am not just somebody, I am a well known somebody! As far is being "official" whatever that means, the is a post in this thread where one of owners of Realtraps uses it!

With humour,
Andre
Andre,

You are a very well known somebody....... I remember you both well and fondly......

Said without humor,

Rod
Old 28th February 2011
  #75
ok so SBIR and 1/4 wavelenght......calculator
Wavelength
in my case it's clear the nulls around 70Hz are caused by the floor and the ceiling.....nice...now what?
70 Hz are not easy to treat and the floor right under the monitor is basically impossible to treat.....
I'm planning to make a new stand filled with sand could it help?
In my case, anyway, big Bass trap on the front wall did help smoothing the LF.
Old 28th February 2011
  #76
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So, lets break it down to the simplest form; one boundary (solid and big compared to wavelength) in a free field setup (focusing only on SBIR) with one source (and one receiver):

SBIR-sbir-2.gif
All items at same height.

Can someone from the ”¼ wavelength camp” explain how the first cancellation frequency can be the same for all positions? It will naturally be the same for position A & B since the path difference between the direct sound and the reflection will be the same. In position C however (unless a lucky coincidence), the path difference is not equal to the A and B scenario and the cancellation frequency must differ depending on position.
Old 28th February 2011
  #77
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Quote:
Originally Posted by Jens Eklund View Post
... the path difference is not equal to the A and B scenario and the cancellation frequency must differ depending on position.
the source impacting effect (SBIR) would (should) remain the same since the source hasn't moved... the listener may experience a difference because they have shifted (LBIR). any difference in the SBIR would seem to be minimal even with a change in incident angle. this could be tested easily on an outdoor handball court where the large single concrete wall has limited boundaries - the 1 wall and the floor... the LBIR would seem like a static (slow motion) doppler effect where the change would occur by listener position changes but the SBIR would (should) only change when you move the source.
Old 28th February 2011
  #78
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Quote:
Originally Posted by gullfo View Post
the source impacting effect (SBIR) would (should) remain the same since the source hasn't moved... the listener may experience a difference because they have shifted (LBIR). any difference in the SBIR would seem to be minimal even with a change in incident angle. this could be tested easily on an outdoor handball court where the large single concrete wall has limited boundaries - the 1 wall and the floor... the LBIR would seem like a static (slow motion) doppler effect where the change would occur by listener position changes but the SBIR would (should) only change when you move the source.
I once again fail to understand why one would be interested in the resultant sound field at any other location then the point you actually measure? So, at position C, the effect will be different. If the term “SBIR” cannot be applied to a real scenario (the sum of phasors at the receiver), but only at the source, what is the use of it?

Again;

“In addition to modal pressure variations, the interaction of the direct sound from the loudspeakers with reflections from the walls can result in dips and peaks in the spectra due to interference effects. We refer to this as the speaker-boundary interference response. This issue has been examined by Allison [9,10], Waterhouse [11,12] and Waterhouse and Cook [13].”

As found in:
http://www.rpginc.com/news/library/ropt_wp.pdf
Old 28th February 2011
  #79
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where is the "measuring point"? the impact of speaker placement relative to boundaries would impact the entire measuring space, versus placement of the measuring device relative to boundaries.
Old 28th February 2011
  #80
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Quote:
Originally Posted by gullfo View Post
where is the "measuring point"?

Ears / measuring microphone
Old 28th February 2011
  #81
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Quote:
Originally Posted by gullfo View Post
the impact of speaker placement relative to boundaries would impact the entire measuring space
but in different ways depending on the position of the receiver, says I (and Dr. Peter D’Antonio and Dr. Trevor J Cox amongst others).
Old 28th February 2011
  #82
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Quote:
Originally Posted by gullfo View Post
the source impacting effect (SBIR) would (should) remain the same since the source hasn't moved... the listener may experience a difference because they have shifted (LBIR).
But the LBIR would only change in position B in post #76, right? Since that's the only position in which the listening and rearwall relationship has changed... or is LBIR something else?

Regarding Post #78:

If i may draw your attention to figure 4 in the link provided, there are 4 different virtual images in that drawing...

1. The first two virtual images are situated behind the source and are aiming towards the listening position (like the left illustration in the OP). Would you call these SBIR (even though they are NOT pointing back at the speakers like they do in the right OP illustration)? Or would you call them LBIR when they are aiming towards the listening position like that? Or would you call them early reflections like Mr Gervais?

2.One virtual image is situated behind the listener - this one would be called LBIR according to Mr Hedback, right?

3. The last virtual image is reflecting against the wall on the opposite side of the room, it then proceeds directly to the listener (without passing by the speaker) i.e. the reflection occurs between the source and the listener. This would also be like in the left OP illustration - would you call this SBIR? Or LBIR? Or early reflections?


Cheers,
Old 28th February 2011
  #83
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Lightbulb

Quote:
Originally Posted by avare View Post
I am not just somebody, I am a well known somebody!
You certainly are!

Thanks for clarifying. I knew that term was coined by someone very cool.

--Ethan
Old 28th February 2011
  #84
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Quote:
Originally Posted by SörenHjalmarsson View Post
But the LBIR would only change in position B in post #76, right? Since that's the only position in which the listening and rearwall relationship has changed... or is LBIR something else?
unless there is a side wall involved... or rear wall... or floor or ceiling... i think Wes Lachot had summed it up pretty well on the recording.org discussion - if its a change between source and boundary its SBIR, if its a change between listener and boundary its LBE (listener boundary effect - now LBIR per Andre) hence the change in response as the measuring point moves versus a persistent alteration in the "expected" response. (and this generalization does not account for all the other room effects)
Old 28th February 2011
  #85
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Glenn,

I read through both of the 'Lachot links' that i provided again and i think (and hope) i understand what your saying.

In short:

1. The left OP illustration is... what Mr Gervais calls early reflections... what Mr Lachot calls LBE... what Dr. D'Antonio calls comb filtering (and what his chapter 'comb filtering' is all about)... what you (Glenn) here are refering to as LBIR. Even the front wall bounce?

2. The right OP illustration is... SBIR!

How does that sound?
Old 28th February 2011
  #86
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Sören,

The illustration you refer to is indicating 2 totally different phenomena - one being an effect that is constant as long as the listener is perpendicular to the center of the sound source and wall in question - the other which is one (of an infinite number of possible positions) other than perpendicular to the wall in question.

In the first case - as long as you remain on a perpendicular line - nothing will ever change (assuming perfect cancellation) - after all a null is a null.

As you walk out of line from that "zone" an additional effect takes place.......... what I refer to as "early reflections" - Andre's LBIR - which causes comb filtering in the listening position....... this does not change the null (or dip as the case may be) it simply adds another dimension to the sound being transmitted.

I do not see what the difficulty is here..........

Add side walls ceilings and floors into the picture and you can have a whole series of other reflections that can add effects of comb-filtering at the listening position - perhaps even at the same frequency (given just the right conditions) - however - the solution for the one is not the solution for the many.......

So the concept that we further define causes - creating new categories (to as clearly as possible) define the effects that sound has within spaces - makes perfect sense.

This is simply growth in the understanding of occurrences......

The labeling of completely different effects in order to have a meaningful discourse when people come to us saying "I am dealing with "A" - what do I do to solve my problems?" makes (to me at least) perfect sense.

Why leave ourselves in a position where we have to play 20 questions in order to determine which (of several) possible sources there are for the problem?

After all - if creating new categories in order to further understand this was not acceptable - there would not even be the term SBIR for us to discuss.........

Respectfully,

Rod
Old 28th February 2011
  #87
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Decay function in REW

Spurred on by some of Jens' great posts and links, there is a way (I'm suggesting) within REW to separate the Boundary Interference & Modal aspects of a response.

Given (or fact statement): the freq response (CR that is) is a function of speaker in room to listener.

From the RPG data highlighted by Jens, the window from ~30ms to 65 ms is the boundary interference (forget for now any specificity of SBIR, LBIR or not).

If you go to the decay function in REW and select 15ms for slice intervals, it seems apparent that if you select 30ms, 45ms & 60ms you are looking at the speaker/boundary portion of the response (more than not). This would include speaker-centric elements, SBIR, LBIR and boundary reinforcement (previously highlighted by Soren and worthy of it's own thread).

Next, if you "un-select" those three (retaining the direct response) and then select 75ms>120ms, you will see the modal portion of the response forming. As I've gone through a few room examples, it appears that the better the modal control the more the decay curves follow the direct response...and conversely, the more out of the control the room is the more you'll see these slices take their own shape (radically different than direct response).

Net gains???

30ms-65ms: If you want to change the direct response, you have to change the speaker and/or listener (testing) positions. You should be able to identify nulls using the 1/4 wavelength measurement and zero in on a positive/negative analysis. You could then possibly verify the benefits of a device intended to address SBIR (or LBIR)

75ms-120ms: you can verify how controlled the modal resonances are separated from boundary interference. Beyond that, I'm not sure it's more helpful than waterfall/spectrogram. There could be benefits though in progress situations if you're attacking a specific issue to see just the early stage of modal resonance (cause and effect).
Old 28th February 2011
  #88
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application note: "unfill" the slices in the Decay function control otherwise you can't see the layers properly
Old 1st March 2011
  #89
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I managed to create an illustration of what I previously tried to describe.
(you have time for that when you're home with a bad cold ...)

It's not an exact illustration but close enough to perhaps clarify a bit. And at the beginning you can see how the wave "follows" before it becomes a big jumble .
You might have to enlarge the video to see it but at the bottom you have the amplitudes and you can see the "measuring" spots in the beginning.

Heres the SBIR: YouTube - Interference - SBIR

And the comb: YouTube - Interference - Comb filter

Everything is interference (or superposition) but I would as well like make a distinction between modal, sbir and comb filtering as in this paper.
Perhaps the above videos also explains why I don't see SBIR as a path difference problem in the same way as combfiltering.

With that said, I have no worries now for linking my unfinished reflection calculation sheet (excel 2007 and up). There may be some errors and you all know the limitations by now. Only first reflection pathways in a rectangular room. Press Calculate when you want to update. Perhaps someone finds use for it.

Jeffrey, interesting method btw. I'll have to check that out later.
Old 1st March 2011
  #90
SAC
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Just a comment.
Comb filtering is not a wave behavior.

It is a category mistake to equate modal and SBIR interference patterns, which differ only in the focus upon global or local behaviors within the larger context of the interaction and distribution of bounded waveforms distribution, resulting in in a subsequent physical distribution with comb filtering which is simply a 'pattern' existing in a measurement display. Comb filtering has no physically existing component!

ALL interactions of wave behavior are superposition. In all cases the result is a spatial distortion pattern, be it polar lobing or modal (eigenvalue) distribution.

Comb filtering is an abstraction that describes how a physical spatially distributed energy patter is displayed when one measures the frequency response corresponding to the physical energy distribution of the soundfield. It does not exist independently of the measurement display! Comb filtering does not have a physical existence independent of the frequency measurement.

Thus one sent of descriptors have a physical correspondence, while comb filtering is an abstraction describing the display for a particular measurement perspective relative to the position within the physical energy field.

One source of confusion is that many have focused exclusively upon a measurement pattern without understanding and associating the 'pattern' to the physical corollary to which it is dependent. In other words, there is such a 'physical thing' as modal or SBIR energy distribution (dependent only on one's imposed frame of reference). But there is no 'physical thing' corresponding to comb filters.


(This probably won't help, but it is as if we measure Bob's height and discover that Bob is 73 inches tall. Bob has a physical existence. Bit it makes no sense to go and actually think that we can find the "inches" that serve as a descriptive element of Bob as if they exist independently of the measurement. So it is with 'comb filtering'. On the other hand, I did once have a comb filter, but it ended up dieing as I could not figure out what to feed the darned thing. Oh, and it was mean!)


Edit:

And rather than post this separately, I may as well add it as an addendum here. Questions were raised as to how and why SBIR came to be defined, complete with some very elegant equations for predicting and defining specific behaviors.

Its pretty simple after one notes that many can hold Neil Muncy responsible for almost drowning when he showed up with a set of wave tanks augmenting the experimental exploration of wave mechanics. And anyone who has played with such an assembly for any time knows that such behavior quickly devolves into a complex 'mess'...chaos.

Thus is rather natural that we attempt to impose a system of order - of well behaved relationships enabling our ability to define and predict relative behaviors in terms of common way points and references.

Superposition of energy waves results in a complex interference pattern that exists in 3space.

It is important to note that the common method of modeling and examining superposition as if it were a 2space representation as it appears on an oscilloscope is simply a fantasy. And it is a representation that causes as much or more misunderstanding than it solves.

Likewise, such a pattern has little ‘meaning’ unless we define the constraints upon the system. This frame of reference proves one an abstract organizing construct from which one is able to define orientations, and an assortment of relative relationships that may or may not correspond to any global metrics defined in yet another frame of reference.

I might also state that reality is an organic complex squiggle. And our system of math and logic functions by projecting an abstract organizing principle, like a piece of graph paper, onto this complex squiggle. And by then viewing the incorporated behavior in terms of locally defined reference points, the complex squiggle is reduced to neat, tightly defined behaviors specified in neat (typically) orthogonal relationships. Thus a squiggle becomes a series of straight lines defined in terms of 3space X, Y and Z coordinates with respect to time.

Now only a fool would actually think that the reality is actually and literally comprised of neat well–behaved straight lines, but if we impose this organizing principle on the organic ‘thing itself at a small enough scale, all behavior can be modeled in this manner. (The limits of this methodology is a topic for another time!)

And in this manner, we obtain a reasonably complete means by which to define relationships between various local and global behaviors with respect to one another.

Thus it is with phenomena such as modal behavior and SBIR within bounded small acoustical spaces.

This behavior is the result of superposition. This behavior necessarily exists in 3space.

But we are able with the imposed organizing principles to define the behavior in terms of its Euclidean 3 space coordinate relationships, be it as simple position or in terms of vectors.

So what is the relationship between these behaviors? It is defined by the abstract constraints defined by the point of view! There is no ‘absolute’ difference! It is strictly one of convenience! One can view a system globally, or one can focus one’s attention onto a local realm and define behaviors more atomistically in terms of the various locally defined relationships.

To look at the organizing mechanism still further. Does anyone actually believe that behaviors only function in a 3space X, Y and Z orientation? When you walk, do you consciously determine the X, Y and Z orientations and consciously or subconsciously perform a vector calculation sufficient to determine the various components required to achieve the resultant? If you do, I would suggest that you seek consoling soon! Instead, this is simply a convenient way to describe and convey in a uniform manner such behavior to others by use of a shared strictly defined organizing coordinate system with shared corresponding unit values. And from this, we are able to define relationships relative to the local desired point of view or interest.

It is purely an artificially imposed practical system designed for convenience!

Thus looking for the universal absolute significance is an act of futility. Even more, it illustrates a fundamental ignore-ance of the nature of the organizing principle itself. Just at it makes no sense for physicists and historians to send much time seeking to discover and identify the graph paper that is abstractly imposed on our physical world. Nor should anyone spend too much time trying to locate the physical grid that defines such constructs as the Mapsco grid in cities or to locate the physical line called the equator or the continental divide.

They do not exist independently as things in themselves. They assume meaning as constructs defined relatively with respect to other elements. And we distinguish between the various constructs as they serve our purposes, be it on a local or more global basis. And it is with respect to these constraints and relationships that they assume definition and meaning.

And so, as we have tried to put the 'meaning' of the systems and terms that seem to be causing many problems into perspective - let me quickly now mention just a few reasons such seemingly arbitrary systems can play a useful purpose specifically by being defined in terms of local relationships useful for addressing practical needs...

To use another poor metaphor, we can examine wave action in the Atlantic Ocean globally. But then the same behavior defined by superposition defines the behavior or local wave action as it encounters the hull of a ship. The difference is primarily one of scale, and it makes little sense to define the behavior of the incidence with the ships hull interns of the larger continental boundaries!

I will also note, that generally in acoustics most assume that incident signals are generally at or near the same gain levels. If ONLY such were the case. We are lucky that psycho-acoustically that there are functional relational gain levels that define how relative signals are perceived, but nevertheless, as someone begins to examine the significance of superposed signals, relative gain levels become very significant. thus one should begin to better appreciate the significance of both global and local behavioral modeling systems. So there are some very practical issues that routinely come to bear. A simple example would be to ask when the last time someone had reason to either deal with, or to create a masking system?Suddenly such aspects that some would tend to dismiss become very important!
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