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SBIR
Old 13th January 2012
  #211
Gear Guru
Game

The possibility of a singular null at the speaker source is intriguing. Some of us consider this as 'Classic' Speaker Boundary Interference Response.
If it exists, it would diminish the energy at source and this lack of energy, null, would feature throughout the room. We could use this to advantage in our treatment plan.
I do not know that this null is strong enough to be significant or not.
Rod's reported report says the cone is affected 'to some large extent'.
Is this strongly enough to cause a significant room wide null.
In the words of the Bard, THAT is the question.


DD

Last edited by DanDan; 15th March 2012 at 03:06 PM.. Reason: REVISED
Old 13th January 2012
  #212
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Jens Eklund's Avatar
If you don’t care to show me why you think I’m wrong, I cannot continue the argument ...

https://www.gearslutz.com/board/7435740-post199.html

Last edited by Jens Eklund; 3rd June 2012 at 11:54 AM.. Reason: Pardon my French …
Old 13th January 2012
  #213
Gear Guru
Moving On

This Singular Null at the speaker is on screen at the moment.
It came up due to an objection to the term LBIR. Not great parentage perhaps.
However, we are where we are, and I am now curious as to whether it is significant or not.

I am far more curious about other SBIR matters though.
Top of my list is that Ethan's tests seem to show Back Wall Bounce to be highly significant, while Front Wall ones don't seem to even register.

Given Omni LF and the high level with proximity to the FW, what is going on there?
This kinda ties in with the complete lack of change which we often when absorption is placed on the FW behind the speakers.

Shortly, probably tomorrow afternoon, I will do some tests.
I expect to get the same results as Ethan, and will welcome any theories as to the predominance of the Back Wall Bounce over the FW.
Later.

DD

Last edited by DanDan; 15th March 2012 at 03:07 PM.. Reason: REVISED
Old 13th January 2012
  #214
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
Jens once again, repeating, I never said you are wrong. I don't do wrong. You certainly cannot continue the argument because there isn't one.
Argue, Prove, Wrong, Incorrect, WTF. Oh Lordly, or is it Oh Roddy....

Last post today, off to see a great young band....
This Singular Null at the speaker is on screen at the moment.
FYI, I am curious as to whether it is significant, but open minded.
I am far more curious about other matters though.
Primarily, that Ethan's tests seem to show Back Wall Bounce to be highly significant, while Front Wall ones don't seem to even register.

Given Omni LF and the high level with proximity to the FW, what is going on there?
This kinda ties in with the complete lack of change which we often when absorption is placed on the FW behind the speakers.

Shortly, probably tomorrow afternoon, I will do some tests.
I expect to get the same results as Ethan, and will welcome any theories as to the predominance of the Back Wall Bounce over the FW.
Manana.

DD
Any test not made in an enclosure with very rigid boundaries will only demonstrate that lossy boundary conditions (normal domestic walls) will introduce the problem of complex impedance (as discussed previously). Further, any test made and analyzed, without taking all phasors into consideration will be hard to interpret since you simply cannot exclude boundaries in the real world unless you happened to have access to a anechoic chamber and the ability to construct large and solid walls to be able to test these things (or perhaps a hemi anechoic would be a better choice).

What I’m trying to say is simply: By all means; do tests, but be aware that they might not be able to prove anything due to the limitations. It will show what happens in this particular room in this particular setup but this does not necessarily mean that you can draw general conclusions from it.


Sorry if I sound pessimistic, but better sceptic than fooled …
Old 14th January 2012
  #215
DAH
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Hey, you clever acoustic guys (namely DanDan and Jens) stabbing each other, did you have any care to read my post\question\hypothesis and comment on it\rip it apart?
It should be quite EASY for you as acousticians to do the test, hell I would do it if it was not +1 Celcius outside.
Old 14th January 2012
  #216
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cborg's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
It sounds like you think this proves me wrong, how?
Well, actually I don't think you are wrong per se (which I've actually tried to make clear), I just disagree with your conclusion. I will make one more effort to explain it combined with what I've written previously.

Quote:
The centre frequency of cancellation will depend on position unless right in front of the source, perpendicular to the boundary.
So below is a screen shot of the wavetank simulation with some freehand scribblin but I think it will suffice as an example.

The marked area (and actually some more closer to the source) is where we have a lower level at this particular frequency. As you see it's pretty much all of the area.

So in any place of the marked area this frequency will pretty much show as a dip. You can compare the yellow and white levels (in the area) to the cyan and purple (right behind source and outside the area).

Now let's say we work with 170Hz so the source is 0,5 m from the front wall.
So all of the marked area has a dip at 170Hz! (which is the phenomena)

But if we use simple rays for the three points I've drawn we can see that they all have different first order cancellations due to different path length/differences.
And this does not add up with the description of the phenomena.
So using this kind of geometrics will not explain a case like this.
SBIR-sbirvsray.png
And as a remark, more logical than scientific maybe:
None of the cases has to invalidate the other imo.
Old 14th January 2012
  #217
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cborg's Avatar
 

Quote:
Originally Posted by DAH View Post
It should be quite EASY for you as acousticians to do the test, hell I would do it if it was not +1 Celcius outside.
Yeah, too hot, isn't it! heh
Old 14th January 2012
  #218
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Jens Eklund's Avatar
cborg:

SBIR-ripple-1.gif
SBIR-ripple-2.gif

Comment?
Old 14th January 2012
  #219
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Jens Eklund's Avatar
Quote:
Originally Posted by DAH View Post
did you have any care to read my post\question\hypothesis and comment on it\rip it apart?
See the post above.
Old 14th January 2012
  #220
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cborg's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
cborg:
Comment?
Yes, it would be better if you commented specifically on the example I've given instead. I think I've stated all the premises.
Old 14th January 2012
  #221
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Jens Eklund's Avatar
Quote:
Originally Posted by cborg View Post
Yes, it would be better if you commented specifically on the example I've given instead. I think I've stated all the premises.
Ok, some comments on your post then:


I’m struggling to understand what you mean when stating:

“So in any place of the marked area this frequency will pretty much show as a dip. You can compare the yellow and white levels (in the area) to the cyan and purple (right behind source and outside the area).”

And for:

Now let's say we work with 170Hz so the source is 0,5 m from the front wall. So all of the marked area has a dip at 170Hz! (which is the phenomena)!”, it completely contradicts what you can observe in the first illustration of post 218.
Old 14th January 2012
  #222
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cborg's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
Ok, some comments on your post then:
I’m struggling to understand what you mean when stating:

“So in any place of the marked area this frequency will pretty much show as a dip. You can compare the yellow and white levels (in the area) to the cyan and purple (right behind source and outside the area).”
Well any of the (about 20) points I measured in the marked area showed a considarable lower level compared to source or examples where frequency or distance was different.

Quote:
And for:
Now let's say we work with 170Hz so the source is 0,5 m from the front wall. So all of the marked area has a dip at 170Hz! (which is the phenomena)!”, it completely contradicts what you can observe in the first illustration of post 220.
First: You now compare with something else and out of the context I've given. I look at the phenomena within the context I've given.( perhaps you are saying that my example is false?).
Second: I'm actually guessing it doesn't contradict at all (I've said it before). It's just different contexts.
Old 14th January 2012
  #223
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cborg's Avatar
 

Quote:
Originally Posted by akebrake View Post
Exactly! SBIR and LBIR are two different animals. Some background.

My first encounter with SBIR-expression was in a Syn-Aud-Con workshop 1983 hosted by Russ Berger. He demonstrated his prediction program where the relative power response of the source (loudspeaker) could be estimated/ smoothed depending on chosen position close to, one or more of the boundaries surrounding it.

E.g It is only about the position/ directivity of the source. How it’s radiating e.g. Full, 1/2 space (2pi) or less at the lowest frequencies.

Note: This is my interpretation of Berger's article which was later presented (-84) in a ”Syn-Aud-Con Tech Topic” where he also called it Speaker-Boundary-Interferens power-Response in the text.
It's about sound power.
Thanks akebrake!
You don't know if there is any review or larger summary of that original paper available on the net?
Old 14th January 2012
  #224
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Ethan Winer's Avatar
 

Lightbulb

Quote:
Originally Posted by Jens Eklund View Post
No, they are not (created at the source). Please have a look ay my illustration in previous post.
Look at my illustration. It seems to me that for either listener position there will be a null at 100 Hz. If this is not the case, I'd love to know why.

--Ethan
Attached Thumbnails
SBIR-sbir.gif  
Old 14th January 2012
  #225
Gear Guru
Test

Tests Monday probably. I have a fairly 'clean' room available.
One corner has three solid concrete boundaries. The other boundaries are relatively far away and have a floating layer of plasterboard.
I have a speaker with decent LF there already but will have to run cables to the computer etc. etc. So let's make this worth while. Let's see if collaboration can replace whatever
The first test is to investigate SBIR. For this test I mean a null caused by nearby boundary reflection, which pervades the room with a constant frequency.
I want to know if it exists at all and if so, how does it compare to the LBIRs.

I wish to use the three boundaries to contribute equally to the null, to make it more visible.
The height of the woofer is fixed at 88cm. This is going to be a bit vague, there is a port.
So Jens, even though I think we both know what this test will show, let's pull together. What distance would you like the woofer from each of the corner walls? Which way would you point the woofer?

Proposed methodology-
Predict the null frequency, maybe Jens would do this for us please?
Using an analogue Sine Generator, sweep carefully around that frequency. Use an SLM and ears and eyes on the cone perhaps to detect nulling.
Slide or rotate the speaker about to get the fullest effect.
Take FR measurements throughout the room to see if the null frequency stays constant.

If anyone wants anything else done in this scenario, now's our chance.

The second test will be to find out if how and why back wall reflections are more significant than front. I will put that in the next post to keep things tidy.

DD
Old 14th January 2012
  #226
DAH
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Quote:
Originally Posted by DanDan View Post
Tests Monday probably. I have a fairly 'clean' room available.
One corner has three solid concrete boundaries. The other boundaries are relatively far away and have a floating layer of plasterboard.
I have a speaker with decent LF there already but will have to run cables to the computer etc. etc. So let's make this worth while. Let's see if collaboration can replace whatever
The first test is to investigate SBIR. For this test I mean a null caused by nearby boundary reflection, which pervades the room with a constant frequency.
I want to know if it exists at all and if so, how does it compare to the LBIRs.

I wish to use the three boundaries to contribute equally to the null, to make it more visible.
The height of the woofer is fixed at 88cm. This is going to be a bit vague, there is a port.
So Jens, even though I think we both know what this test will show, let's pull together. What distance would you like the woofer from each of the corner walls? Which way would you point the woofer?

Proposed methodology-
Predict the null frequency, maybe Jens would do this for us please?
Using an analogue Sine Generator, sweep carefully around that frequency. Use an SLM and ears and eyes on the cone perhaps to detect nulling.
Slide or rotate the speaker about to get the fullest effect.
Take FR measurements throughout the room to see if the null frequency stays constant.

If anyone wants anything else done in this scenario, now's our chance.

The second test will be to find out if how and why back wall reflections are more significant than front. I will put that in the next post to keep things tidy.

DD
My suggestions:
1. plug the port just for the test.
2. It will not null everywhere as much as at the speaker at some points because the modal behavior will influence the whole response. That is why I suggested testing for SBIR outdoors over a hard surface.
Old 14th January 2012
  #227
Gear Guru
Good

Thanks DAH. I will plug the port with a cork or such. I understand your outdoor suggestion of course, but it's wet and cold here.
I will identify the modal frequencies and do my best to avoid them.
cborg thanks for the contributions. Could you explain what we are seeing in those graphics please.

DD
Old 14th January 2012
  #228
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Ethan Winer's Avatar
 

Lightbulb

Quote:
Originally Posted by DanDan View Post
The first test is to investigate SBIR. For this test I mean a null caused by nearby boundary reflection, which pervades the room with a constant frequency.
Key is keeping the loudspeaker reasonably close to a single boundary in relation to the room dimensions, and also relatively far from other boundaries. Do you have a high ceiling and a minimalist platform for the speaker? Otherwise other room effects might override the "SBIR at the source" response.

--Ethan
Old 14th January 2012
  #229
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Rod Gervais's Avatar
 

Quote:
Originally Posted by Ethan Winer View Post
Look at my illustration. It seems to me that for either listener position there will be a null at 100 Hz. If this is not the case, I'd love to know why.

--Ethan
I would have to agree that there would be a dip - the effect takes place at the speaker itself - and thus any position within the room should receive the same effect.


For what it's worth - I have always assumed that the effect was caused by
the summing of the signals at the speaker, and although I have been curious about the possibility of room pressure affecting cone travel - I really do not have a strong feeling that this is the case with SBIR......

The paoer I was referring to earlier was not written relating to SBIR but rather sub placement - I was simply speaking to the fact that sound pressure within a space can overpower the driver on a speaker.

Rod
Old 14th January 2012
  #230
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Glenn Kuras's Avatar
Quote:
Originally Posted by DanDan View Post
Tests Monday probably. I have a fairly 'clean' room available.
One corner has three solid concrete boundaries. The other boundaries are relatively far away and have a floating layer of plasterboard.
I have a speaker with decent LF there already but will have to run cables to the computer etc. etc. So let's make this worth while. Let's see if collaboration can replace whatever
The first test is to investigate SBIR. For this test I mean a null caused by nearby boundary reflection, which pervades the room with a constant frequency.
I want to know if it exists at all and if so, how does it compare to the LBIRs.

I wish to use the three boundaries to contribute equally to the null, to make it more visible.
The height of the woofer is fixed at 88cm. This is going to be a bit vague, there is a port.
So Jens, even though I think we both know what this test will show, let's pull together. What distance would you like the woofer from each of the corner walls? Which way would you point the woofer?

Proposed methodology-
Predict the null frequency, maybe Jens would do this for us please?
Using an analogue Sine Generator, sweep carefully around that frequency. Use an SLM and ears and eyes on the cone perhaps to detect nulling.
Slide or rotate the speaker about to get the fullest effect.
Take FR measurements throughout the room to see if the null frequency stays constant.

If anyone wants anything else done in this scenario, now's our chance.

The second test will be to find out if how and why back wall reflections are more significant than front. I will put that in the next post to keep things tidy.

DD
Would you mind taking pictures of how you set up everything? I would like to do the same test in our lab/test room.
Old 14th January 2012
  #231
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Jens Eklund's Avatar
Quote:
Originally Posted by Ethan Winer View Post
Look at my illustration. It seems to me that for either listener position there will be a null at 100 Hz. If this is not the case, I'd love to know why.

--Ethan
The illustrations in post 218 show why.


EDIT:
Oh, and your choice of example positions for receiver positions in your illustration indicates that you know what I mean ...
Old 14th January 2012
  #232
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Jens Eklund's Avatar
Quote:
Originally Posted by cborg View Post
First: You now compare with something else and out of the context I've given.
What do you mean?
Old 14th January 2012
  #233
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
Tests Monday probably. I have a fairly 'clean' room available.
One corner has three solid concrete boundaries. The other boundaries are relatively far away and have a floating layer of plasterboard.
I have a speaker with decent LF there already but will have to run cables to the computer etc. etc. So let's make this worth while. Let's see if collaboration can replace whatever
The first test is to investigate SBIR. For this test I mean a null caused by nearby boundary reflection, which pervades the room with a constant frequency.
I want to know if it exists at all and if so, how does it compare to the LBIRs.

I wish to use the three boundaries to contribute equally to the null, to make it more visible.
The height of the woofer is fixed at 88cm. This is going to be a bit vague, there is a port.
So Jens, even though I think we both know what this test will show, let's pull together. What distance would you like the woofer from each of the corner walls? Which way would you point the woofer?

Proposed methodology-
Predict the null frequency, maybe Jens would do this for us please?
Using an analogue Sine Generator, sweep carefully around that frequency. Use an SLM and ears and eyes on the cone perhaps to detect nulling.
Slide or rotate the speaker about to get the fullest effect.
Take FR measurements throughout the room to see if the null frequency stays constant.

If anyone wants anything else done in this scenario, now's our chance.

The second test will be to find out if how and why back wall reflections are more significant than front. I will put that in the next post to keep things tidy.

DD
Did you see post 214?
Old 14th January 2012
  #234
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Rod Gervais's Avatar
 

Quote:
Originally Posted by shawn19 View Post
Hi all,
I have been reading this thread with interest, and would like to add my 2 cents. Reading over the posts, it seems that there are two positions: one that SBIR is only an effect between the speaker and the boundaries, and the other that SBIR is an effect between the speaker, the boundaries, and the listener. Jens seems to be kind of alone in his argument, but my understanding is that he is correct. There can be cancellation at the woofer cone, and still have sound at other places in the room.

The physical effect on the woofer cone is interesting, just as there are a multitude of other effects, but bringing them all in at once can cause confusion.

Assuming the simplest case, we would have a speaker that radiates spherically, and a boundary that is 100% reflective. If there is only one boundary, there will be the original source, and a phantom source caused by the reflective boundary. This is the same as having two sources in the free field. If they are producing the same frequency, then there will be an interference pattern of the waves, adding in some places and canceling in others, but it is not possible for one speaker to completely cancel the sound from another speaker in three dimensions.

If there are three boundaries, say the floor and two walls, then there will be more phantom sources, but still a finite number. Again, this would be the same as having a finite number of sources in a free field, and again it is not possible for these sources to completely cancel out everywhere in three dimensions. This happens because it is wave interference, not particle interference.
Shaun,

What we are discussing here is simply the effect that a near boundary has on a particular signal from a single speaker.....

When a speaker is set at a 1/4 wavelength of a low frequency from a near boundary (for the purpose of this discussion) it is a known fact that there will be a dip that takes place in the resultant signal ...... that 2 near boundaries set at that same 1/4 wavelength will increase the dip - that 3 will increase it even further.

In this case, regardless of the cause of the effect (be it the summing of the signals at the speaker or a pressure variance at the speaker which directly affects the movement of the speaker cone itself) there is a nulling effect that takes place.

Now it is entirely possible that this effect could be further aggravated or even mitigated by other factors within the space. But that is not what this discussion is about.

However - taking into consideration only the effect of the near source itself - in theory, once the signal is summed (at the speaker) or the cone itself is affected (possibly the combination of the 2)- the resultant signal would be constant throughout the room if no other constructive/destructive forces were to influence it.

Rod
Old 14th January 2012
  #235
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Jens Eklund's Avatar
Quote:
Originally Posted by Rod Gervais View Post
When a speaker is set at a 1/4 wavelength of a low frequency from a near boundary (for the purpose of this discussion) it is a known fact that there will be a dip that takes place in the resultant signal ...... that 2 near boundaries set at that same 1/4 wavelength will increase the dip - that 3 will increase it even further.
But this is where I think a lot of people are assuming that the centre frequency of this null will be related only to the first order of reflections. As the pervious example; a source located 1 meter away from each boundary of a corner will not produce a resultant centre frequency of cancellation at 85 Hz if you include all of the source images related to this corner (a total of seven, as discussed previously), and why would you exclude some of them if their contribution is significant?
Old 14th January 2012
  #236
Gear Guru
Morse Code

I did read post 216 and have done again just now. Sorry, I am not picking up a salient point.

Quote:
why would you exclude some of them if their contribution is significant?
Obviously one wouldn't. However, in the actual room in question, the three boundaries are concrete and very close to the woofer, and equally so. The next nearest boundary is 3 or 4 times the distance away and is a light plasterboard ceiling. The next nearest after that is 4-5 times the distance and has a plasterboard over insulation cladding and a couple of windows and a door. The last boundary is 7-8 times the distance.

I think it safe to say the reflection from that ceiling will be very very small compared to the combined three? Ditto the more distant boundaries.

I think that addresses Ethans points also.

Glenn, for your, I will do that.

I am fairly convinced that the source/speaker null does actually exist.
I am almost equally convinced that it won't show up in measurements because of nearby other information. I have a slight hope that the combination of three concrete reflections may persuade this shy fella to show but let's see what happens.

In any case, this is all just a waypoint. My bigger puzzlement is if how and why the back wall reflection does more damage than the front.
DD
Old 14th January 2012
  #237
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
I think it safe to say the reflection from that ceiling will be very very small compared to the combined three? Ditto the more distant boundaries.
Three?

I'm sorry Dan, but I still think you fail to see the complete picture here. Again, remember that you must take into consideration all phasors with high gain relative to direct sound and other early reflections. Ignoring some, or most of them is likely to result in an incorrect prediction of centre frequency of cancellation at any point. Please revisit post 196. You simply cannot choose to include only some of the early reflections when trying to estimate the combined effect of SBIR like centre frequency(ies) of cancellation in a given point.
Old 15th January 2012
  #238
Gear Guru
Repeat

Jens I absolutely understand.

The question throughout here is one of level. I can't see one layer of plasterboard 2 Metres away as 'High gain'

You are good with Arithmetic and such.

How about you calculate the combined level of the 3 reflections caused by equidistance from floor, wall, wall.

Then calculate the level of the next loudest reflection, from a wall 3 Metres away. Diminish it's reflectivity slightly due to the plasterboard and insulation layer.

Lets see how they stack up.

In Noise Control measurement, the guideline is to ignore any factor 10dB lower than the others as it will not contribute significantly to the log sum.

I have an open mind here. If you wish to convince me of the power of all these far away and/or weakly reflective boundaries, do it by numbers.

Without that we are both guessing aren't we?


DD
Old 15th January 2012
  #239
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Rod Gervais's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
But this is where I think a lot of people are assuming that the centre frequency of this null will be related only to the first order of reflections. As the pervious example; a source located 1 meter from away from each boundary of a corner will not produce a resultant centre frequency of cancellation at 85 Hz if you include all of the source images related to this corner (a total of seven, as discussed previously), and why would you exclude some of them if their contribution is significant?
Jens,

with all due respect - what other reflections may or may not produce is certainly relevant in real world applications that include the problem as a part of the equation when it comes to the sound levels within a space....... but - this is not the question here (as least it is not in my mind)

Let's suppose for a moment that interference from a near boundary were to cause a 6dB dip in that 85Hz you're using in your example - but at the listening position there was a 10dB spike in the level of that frequency due to other reflections within the space - perhaps modal activity within the room.......

That fact that there was an actual spike at the listening position would not negate the physical effect of the null itself - the difference is practical application within a space versus an understanding of a physical event that can take place.

No one is saying that nothing else within a room can affect the outcome at the listening position - nor is anyone saying that this should be focused on
to the exclusion of all else.

However - it is real - it can affect the outcome in a room - and therefor should be considered as a part of room examinations, especially in cases where other possible sources for a problem have been investigated and found not to be the issue.

It does not make the top of my list of things to look at - however it is not something that I would ignore as I work my way down the list for a client.

Sincerely,

Rod
Old 15th January 2012
  #240
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Jens Eklund's Avatar
Rod,

with all due respect – I seriously do not understand the point in trying to estimate the cancellation frequency for a certain situation (like the corner discussed previously) if you know that the result will be incorrect?

Again, there are 7 source images (in addition to the real source) related to a single corner in free space and thinking that you can estimate the centre frequency of cancellation using only three of these is ignorant.

Even if the gain of the 4 reaming reflections is slightly lower than the first three, their addition to the total when summing the phasors will have an impact on the fc of the dip. It will be lower than 85 Hz (at the source).
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