Bass Nulls - Mix Position - or - Room Dimesions or ?

[Ethan Winer]

Jeff,

As one how does plenty of grandstanding myself your comments are totally in line.

> I recently built Speaker Boundary Interference Response (SBIR - the technical term for the "peak/null problems" mentioned above) into a test version of an LF response spreadsheet <

I think it's important to distinguish SBIR and LBIR, where the "L" stands for Listener. These appear similar to be sure, but are not at all the same thing. Wes Lachot explained the key difference a while back over at RO. If anyone cares I can try to track down that thread.

> the modal response still dominates the net response. <

It depends on the frequency. As shown clearly and irrefutably in my "Room Modes Part Deux" threads elsewhere, below about 165 Hz (in my test room) the modal response does indeed dominate. Above that point LBIR dominates. Since others here may not have seen these posts, I reproduce the data again below.




Both graphs show the change in response as the measuring microphone is moved closer to the rear wall, with the graph lines becoming darker as the mike gets closer to that wall. The first graph shows the response below 200 Hz, and the second graph shows what happens above 200 Hz. Note how the peaks and nulls do not shift with the changing "listener" position at low frequencies, but they do shift at higher frequencies. In this room the shift occurs above about 165 Hz.

The reason I insist that modes are a subset of the more general case of acoustic interference is because acoustic interference came first. It occurs outdoors against a single boundary, so you don't even need a room at all. As you close off one wall, then two, and so forth, until the room is completely enclosed, that's when the interference patterns become stronger due to reinforcement within the enclosed space. But simple boundary interference is the basis of all of this - the parent, if you will - and the additional properties of room modes develop only in an enclosed room.

> Modal response and SBIR are mutually exclusive. <

I'd love to hear that explanation!

--Ethan

[nukmusic]

[lovecow]

I think it's important to distinguish SBIR and LBIR, where the "L" stands for Listener. These appear similar to be sure, but are not at all the same thing. Wes Lachot explained the key difference a while back over at RO. If anyone cares I can try to track down that thread.

By definition, SBIR includes the listener position. You need a source and a receiver to measure (or predict) any room response, be it from modal excitation or from SBI effects.

> the modal response still dominates the net response. <

It depends on the frequency.

It depends on a lot more than the frequency.

> Modal response and SBIR are mutually exclusive. <

I'd love to hear that explanation!

It should be fairly obvious if you understand the physics involved.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

[Ethan Winer]

Jeff,

> By definition, SBIR includes the listener position. <

No, SBIR and LBIR are two different and unrelated sets of peaks and nulls. SBIR creates one set of peaks and nulls whose frequencies depend on the distance between the loudspeaker and the room boundaries, and those frequencies remain constant no matter where the listener (or measuring microphone) is in the room. The other set of peaks and nulls change frequency as the listener moves around in the room. So in fact there are two sets of peaks and nulls, though they do interact and combine to yield a single response for any given speaker placement and listener position.

> It should be fairly obvious if you understand the physics involved. <

That contributes nothing to the discussion.

--Ethan

[lovecow]

No, SBIR and LBIR are two different and unrelated sets of peaks and nulls. SBIR creates one set of peaks and nulls whose frequencies depend on the distance between the loudspeaker and the room boundaries, and those frequencies remain constant no matter where the listener (or measuring microphone) is in the room. The other set of peaks and nulls change frequency as the listener moves around in the room. So in fact there are two sets of peaks and nulls, though they do interact and combine to yield a single response for any given speaker placement and listener position.

No, SBIR is defined by the position(s) of loudspeaker(s) and listener(s). What you've just described above is not SBIR. I guess I can reassure you and tell you that it is something, just not SBIR.

Regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

[lovecow]

Speaker-Boundary Interference Response
The next type of acoustic distortion is due to the coherent interference between the direct sound of a loudspeaker and the reflections from the room...It is called the speaker-boundary interference response, or SBIR. The room's boundaries surrounding the loudspeaker mirror the loudspeaker forming virutal images. When these virtual loudspeakers (reflections) combine with the direct sound, they can either enhance or cancel it to varying degrees depending on the amplitude and phase relationship between the reflection and the direct sound at the listening position.

Emphasis added by me.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

[barefoot]

SBIR creates one set of peaks and nulls whose frequencies depend on the distance between the loudspeaker and the room boundaries, and those frequencies remain constant no matter where the listener (or measuring microphone) is in the room. The other set of peaks and nulls change frequency as the listener moves around in the room. So in fact there are two sets of peaks and nulls, though they do interact and combine to yield a single response for any given speaker placement and listener position.

No, there is only one special case where the inference response is independent of the listening position. This is the case where one speaker is directly between the listener and a wall - as modeled in my 1-dimesional Wall Bounce Calculator. And considering that it's impossible maintain this ideal alignment with respect to two or more speakers in a room with 6 or more reflective surfaces, this whole idea of positionally independent interference patterns is useless.

My 1-D calculator is really only good for getting a feel for the magnitude of the problem, as well as seeing the bass power response step of freestanding speakers in proximity to a wall.
Thomas

[Ethan Winer]

Guys,

We've beaten this to death already, but I don't understand why you can't see that two independent sets of peaks and nulls are involved. Yes, the listener receives only one composite response, of course, and I acknowledged that earlier. So you could rightly say the response always changes based on the listener position. But there are still two sets of peaks and nulls at work - one based on the distance between the speaker and the room boundaries, and another based on the distance between the listener and the room boundaries. No?

Wes Lachot touched on the distinction between SBIR and LBIR briefly at RO a long time ago, fourth post on this page:

www.recording.org/postx17872-0-15.html

In fact, on page 3 of that thread Jeff said this, which I read as acknowledging the additional path:

As with many acronyms, "SBIR" doesn't really do the phenomenon justice. I would lean towards "L²BIR" for Listener/Loudspeaker-boundary Interference Response.

--Ethan

[lovecow]

We've beaten this to death already, but I don't understand why you can't see that two independent sets of peaks and nulls are involved. Yes, the listener receives only one composite response, of course, and I acknowledged that earlier. So you could rightly say the response always changes based on the listener position. But there are still two sets of peaks and nulls at work - one based on the distance between the speaker and the room boundaries, and another based on the distance between the listener and the room boundaries. No?

No. I believe Thomas and I have sufficiently covered it for you above.

In fact, on page 3 of that thread Jeff said this, which I read as acknowledging the additional path:

As with many acronyms, "SBIR" doesn't really do the phenomenon justice. I would lean towards "L²BIR" for Listener/Loudspeaker-boundary Interference Response.

Sounds about right. It's quoted here completely out of context. (Valiant effort!) But since the link is also given, I will trust people to judge accordingly...

Semantics aside, neither this discussion nor that one can change how SBIR is technically defined.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

[Ethan Winer]

Jeff,

Are you really saying you can't see that there are two paths, each contributing different distances and thus different delays?

> Semantics aside, neither this discussion nor that one can change how SBIR is technically defined. <

Some definitions are open for interpretation, as my recent poll over at alt.sci.physics.acoustics made clear. You insisted that standing waves and modes are the exact same thing, but at least one pro acoustician at that group said my distinction has merit. Dr. Matt Nobile at IBM also agrees with my distinction, as did every other respondant in that poll beside you and Eric. Now, I fully understand that science is not a popularity contest. But to not recognize multiple interference paths amazes me. Or maybe you do recognize it, but disagreeing with me is so entertaining you can't resist?

--Ethan

[lovecow]

Are you really saying you can't see that there are two paths, each contributing different distances and thus different delays?

Asked and answered. If you still don't get it, I cannot help you.

Some definitions are open for interpretation, as my recent poll over at alt.sci.physics.acoustics made clear. You insisted that standing waves and modes are the exact same thing, but at least one pro acoustician at that group said my distinction has merit. Dr. Matt Nobile at IBM also agrees with my distinction, as did every other respondant in that poll beside you and Eric.

Off topic.

But to not recognize multiple interference paths amazes me. Or maybe you do recognize it, but disagreeing with me is so entertaining you can't resist?

Don't flatter yourself.

Regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

[Eric_Desart]

Ethan,

Don't disgrace my name.

I assume you mean one of your traditional word games you referred to in the science group?
Where you started out saying nothing, vague as always, hardly knowing yourself what you were saying, then confirmed what others assumed you were saying, thereby contradicting what you always were saying?

Then further contradicted yourself, by confirming something else?

And then confirmed someone else without knowing what the plain wave is he referred too?
You mean that thread?
The thread were you took no clearly expressed point of view, just picked out what you can use as confirmation of what you didn't express?

I told you once before.
I'm always staggered how you can twist everything, even plain lie, and still believe yourself.

You pick the know-how of others, act as if it is yours, even when not understanding what you're saying, and start explaining the world via articles, as an expert with decades of experience.

There was NO reason to refer to me in this thread at all, so don't.

And your response now again is a smoke screen to hide that you don't understand what Jeff is saying (nor what you're saying yourself).

Eric

[lovecow]

Folks,

1. Ethan asserts that standing waves and modes are not the same thing. Here's a quote from the alt.sci.physics.acoustics referred to above:

To me, a standing wave is a pattern of nodes and anti-nodes caused by a wave bouncing off a room boundary and interfering with waves still emanating from the sound source, and with other reflected waves.

2. Ethan also asserts that modes are a subset of SBIR.

3. Take Ethan's definition of standing waves from the above quote and of SBIR from this thread and it should be fair to conclude that Ethan believes SBIR to be a phenomenon caused by standing wave behavior.

So: Standing waves are not modes, but SBIR is caused by standing waves and does actually include modes as a subset. Interesting.

Seriously, folks, if you want the skinny on modes and SBIR and the rest, I would recommend Everest's MHoA. There are other resources cited elsewhere, but this one should provide the clearest (read: least mathematical) explanations.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.