SBIR

[Rod Gervais]

Originally Posted by DanDan

If that is not the one Rod saw, it is just as good, as it involves how much resistance to radiation the speaker cone is experiencing, at particular frequencies.
If I may flip flop again, I now fully believe the null at the speaker is more than a hypothetical construct.

DanDan,

No that is not the paper....... and I really hope to find it again at some point in time - I believe I saved it somewhere - but can't for the life of me find it.....

But at least this helps to prove I'm not completely insane.....

Seriously - there is the theoretical as well as the practical side to all of this.....

From a purely scientific point of view (even forgetting the physical effect that might take place with the speaker cone itself) we know that signals sum and modify the amplitude of the original signal....

Of this there is no question. So, if summing takes place at the point of the speaker itself (as could result with reflections from a near surface) then the resultant signal would remain constant (allowing for the natural reduction in amplitude that takes place of course over distance) unless it were to interact with reflection from a different location.

From a purely practical point of view - especially as rooms become smaller - the combination of signals within the room become so complex that one is forced to deal with the whole rather than individual elements.

In this (meaning practical application) I think the point becomes moot.

I have designed more than a couple of studios - and I have never encountered a finished space (where I was involved in either design or construction) where we had to deal with LF issues due to near wall reflections. I did have one occurrence where I needed to install absorption at the rear of the speakers after the fact - however this was due to a reflection off a very large window - and it was not a low frequency issue.... we already touched base on this earlier in this thread.

So I think that although it's interesting to examine - and I especially find the physical effect on the speaker cone quite fascinating - I don't know that in the scheme of things it really becomes all that relevant........

Just my 2 cents worth,

Sincerely,

Rod

[Jens Eklund]

Originally Posted by Rod Gervais

From a purely practical point of view - especially as rooms become smaller - the combination of signals within the room become so complex that one is forced to deal with the whole rather than individual elements.

+1


This is what I´ve (and others) been trying to communicate over and over again:
SBIR
SBIR
SBIR
SBIR
SBIR

[Dange]

The radiation load at the loudspeaker is shown here:


From Glyn Adam's section of Loudspeaker and Headphone Handbook

From my post here - Modal nulls and reflection nulls

[DanDan]

Great looking studios too Rod. However I presume any use Soffit mount? And angled side walls? I can see why there would be little in terms of near wall effects. However, I, and Ethan, have singled out the rear wall.
The tests show it to be vastly more destructive than other walls and modes.
I think this, although well known, is worth pointing out now and again.

I absolutely agree and understand fully that everything affects everything else and small room behaviour is deceivingly complex.
The graphs also clearly show the predominant effects of individual walls.
A little time spent on the Editing you promised would be very welcome Jens. Rather than repetition.

DD

[Rod Gervais]

Originally Posted by DanDan

Great looking studios too Rod. However I presume any use Soffit mount? And angled side walls? I can see why there would be little in terms of near wall effects. However, I, and Ethan, have singled out the rear wall.
The tests show it to be vastly more destructive than other walls and modes.
I think this, although well known, is worth pointing out now and again.

I absolutely agree and understand fully that everything affects everything else and small room behaviour is deceivingly complex. The graphs also clearly show the predominant effects of individual walls.

DD

Dan,

I have never had a client express a preference for soffit mounted speakers, as such none of my designs have ever used them. I assure you the moment a client has that preference they will be a part of that design - which will make life even that much easier for me.

I do take a great amount of care in my room design - as long as space is allowing I always have splayed walls and cathedral ceilings........... and the back wall of the control room also gets a whole lot of my attention.....

However - I achieved the same results in my 7.1 mixing room in the Philippines, and that is a rectangular room with an 8'-4" ceiling that's parallel with the floor....... so it is not just about having enough space to allow the luxury of splayed walls and ceilings....

Rod

[DanDan]

I see Rod. I was kinda intrigued by your statement

I have never encountered a finished space (where I was involved in either design or construction) where we had to deal with LF issues due to near wall reflections.

But I obviously won't be asking you to reveal your techniques. So let's leave it there.

Do you concur that the graphs show reflections directly associated with single boundaries? Predominantly that is. The FWBIR did vanish pretty much entirely when a trap was inserted.


EDIT. A couple of BIR calculators. Like all of these Calcs, they assume idealised conditions so measured frequencies will be somewhat different.
http://tripp.com.au/avfcbm.htm

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=630

DD

[Skylinestar]

http://www.sendspace.com/file/47z983

My first attempt at SBIR with my Klispch Synergy B3 bookshelf speaker. Speakers are placed inward and closer to the MLP by 1 foot....but with this placement, the speakers are no longer at the 30 degree angle (ie. no longer equilateral with MLP). The rear ports are also plugged.

I'm hoping that adding bass traps can fix it with the speakers at the old position. Is it possible?

[DanDan]

A client searched for optimum speaker and listening positions recently.
It was like a matrix. He had incremental Listening Positions for each loudspeaker position. Each Speaker position included distance from front and side walls, and height!

Obviously impossible combinations were ruled out, but there was a lot of data to view. The big surprise was that one particular LF with one particular Speaker position was way way the best. No doubt.

DD

[Rod Gervais]

Originally Posted by DanDan

A client searched for optimum speaker and listening positions recently.
It was like a matrix. He had incremental Listening Positions for each loudspeaker position. Each Speaker position included distance from front and side walls, and height!

Obviously impossible combinations were ruled out, but there was a lot of data to view. The big surprise was that one particular LF with one particular Speaker position was way way the best. No doubt.

DD

+1

I always recommend multiple tests for each mic position - as well as testing in a grid patterns for each speaker position that would coincide with where your head would be in front of the board while working - this because we do not live with our heads in a vise when sitting at a desk........

Small changes in listening positions can relate to huge changes in room response with any particular speaker set up.

The reason for multiple tests of the same mic position is so one can have the chance to rule out anomalies that might otherwise skew the test results.

I have seen where some outside noise of a particular frequency adds to what would appear to be the room response - when in fact multiple tests reveal that it is not a room issue at all.

Multiple tests in a larger area surrounding what appears to be the sweet spot can be averaged to get a bigger picture of what you will really hear when working in that location.

Rod

[Radial185]

What a educational thread, thanks.

One question regarding one theory discussed of speakers as close to the front wall as possible. What about those that say the front wall badly messes with the stereo image? Even if the wall is treated for SBIR, I have seen this suggested. A caveat, it was on a hi-fi forum. Hold any truth or not?

[Jens Eklund]

Originally Posted by Radial185

What a educational thread, thanks.

One question regarding one theory discussed of speakers as close to the front wall as possible. What about those that say the front wall badly messes with the stereo image? Even if the wall is treated for SBIR, I have seen this suggested. A caveat, it was on a hi-fi forum. Hold any truth or not?

High gain early reflection will mess things up, but depending on the type of speaker used and the polar distribution of energy, the front wall (the wall behind the speakers) might not cause big issues, especially if the speaker is close to the wall thus keeping low frequency reflections from this boundary (that usually radiate omnidirectionally) more or less in constructive phase.

http://www.gearslutz.com/board/studi...ear-fields.htm

Monitor & Mixing Desk Placement - Reflection Issues

[OpusOfTrolls]

SBIR: Sucks because interfering reflection

[OpusOfTrolls]

Originally Posted by Jens Eklund

High gain early reflection will mess things up, but depending on the type of speaker used and the polar distribution of energy, the front wall (the wall behind the speakers) might not cause big issues, especially if the speaker is close to the wall thus keeping low frequency reflections from this boundary (that usually radiate omnidirectionally) more or less in constructive phase.

http://www.gearslutz.com/board/studi...ear-fields.htm

Monitor & Mixing Desk Placement - Reflection Issues

I am in a quandry regarding the use of freestanding monitor cabinets in my design. Since I plan on using a crossover between subs and mids, would it make sense to apply absorption around the cabinet as a pseudo soffit? The directivity is nearly free field. The cabinet baffle is around a foot away from the front wall.

[Jens Eklund]

Originally Posted by OpusOfTrolls

I am in a quandry regarding the use of freestanding monitor cabinets in my design. Since I plan on using a crossover between subs and mids, would it make sense to apply absorption around the cabinet as a pseudo soffit? The directivity is nearly free field. The cabinet baffle is around a foot away from the front wall.

Hard to say without knowing the details but if in general, pseudo soffit can reduce edge diffraction from the baffle and help minimize SBIR related issues.

[OpusOfTrolls]

Would there still be hemispherical reinforcement as if the monitors were soffited?

[Jens Eklund]

At very low frequencies (long wavelengths), the reflected energy from the boundary behind the speaker is still in constructive phase relation with the direct sound and will then provide reinforcement to this frequency range (and this is relatively easy and strait forward to compensate for with EQ if necessary). If the source is close to the boundary, the first theoretical cancellation frequency will end up at a higher frequency than if the source is further away from the boundary. This makes it easier to deal with since it´s easier to absorb a higher frequency and also, most speakers becomes more directional for higher frequencies so there might not be that much energy sent to the rear to begin with.

[Rod Gervais]

Originally Posted by Jens Eklund

At very low frequencies (long wavelengths), the reflected energy from the boundary behind the speaker is still in constructive phase relation with the direct sound and will then provide reinforcement to this frequency range (and this is relatively easy and strait forward to compensate for with EQ if necessary). If the source is close to the boundary, the first theoretical cancellation frequency will end up at a higher frequency than if the source is further away from the boundary. This makes it easier to deal with since it´s easier to absorb a higher frequency and also, most speakers becomes more directional for higher frequencies so there might not be that much energy sent to the rear to begin with.

+1 -

[DanDan]

I am in a quandry regarding the use of freestanding monitor cabinets in my design. Since I plan on using a crossover between subs and mids, would it make sense to apply absorption around the cabinet as a pseudo soffit? The directivity is nearly free field. The cabinet baffle is around a foot away from the front wall.

May I assume you meant to say the directivity is full field, i.e. Omni?
There are very few speakers like that. Most become quite directional at HF.

In any case. A client recently installed a 'wall' of fibre traps. An idea from another acoustician local to him. Some distance from the front wall, in line with the speaker fronts. A sort of soffit perhaps.
The measurements I have seen are quite stunning.

One reason perhaps, is that we have a layer of fibrous absorption at a region of high particle velocity because of wall bounce, which impedes the travelling wave at least twice, then multiples for modes.
EDIT Plus of course, cabinet diffraction diminished.


DD

[PaulRain]

Originally Posted by Jens Eklund

At very low frequencies (long wavelengths), the reflected energy from the boundary behind the speaker is still in constructive phase relation with the direct sound and will then provide reinforcement to this frequency range (and this is relatively easy and strait forward to compensate for with EQ if necessary).........

Interesting but I'm not sure I understand...
you say the phase relation between the direct sound and the reflected signal from the wall behind the monitors is always constructive?
If it is so than probably the EQ cut could help but, in my opinion, it can be destructive too, and I don't think you can solve a null caused by an out of phase signal by boosting with an EQ, the null will be there anyway regardless how much you boost..... am I wrong?

[Jens Eklund]

Originally Posted by PaulRain

Interesting but I'm not sure I understand...
you say the phase relation between the direct sound and the reflected signal from the wall behind the monitors is always constructive?
If it is so than probably the EQ cut could help but, in my opinion, it can be destructive too, and I don't think you can solve a null caused by an out of phase signal by boosting with an EQ, the null will be there anyway regardless how much you boost..... am I wrong?

No, I said:

Originally Posted by Jens Eklund

At very low frequencies (long wavelengths), the reflected energy from the boundary behind the speaker is still in constructive phase relation with the direct sound ...

Depending on the position of the source relative to the boundary naturally, but assuming fairly close.

[PaulRain]

define "very low"

[Jens Eklund]

Originally Posted by PaulRain

define "very low"

Well … I guess I could start to explain from the beginning about SBIR and how it works, but I suggest that you first read this thread, since I think everything one could possibly want to know about SBIR is already fully covered by previous posts.

If you after reading this thread, still have questions; I´ll happily answer them all.

[PaulRain]

I think I have been on this thread since day one
and I've read it few times already...
I was just trying to be a stupid smartass
Your answer is very clear, if the wavelenght is much bigger than the distance of the source to the boundary it will be more likely in phase because there is no room enough to reverse.

[Jens Eklund]

Yes, when the time difference between direct and reflected energy equals less than a 1/4 wavelength, you get constructive interference.

[localhost127]

Originally Posted by PaulRain

I think I have been on this thread since day one
and I've read it few times already...
I was just trying to be a stupid smartass
Your answer is very clear, if the wavelenght is much bigger than the distance of the source to the boundary it will be more likely in phase because there is no room enough to reverse.

Ripple Tank Simulation

explore with a (simplified) 2d wavetank. 1 source (omni) and draw one boundary (representing the front wall, for example). explore how the superposition (summation) of the two signals yields constructive and destructive interference via that of the developed polar lobes and nulls - of which is dependent upon frequency (wavelength), distance/spacing between the signal/reflection, and of course (the key take-away from this thread): receiver position. being situated in a polar null for a given wavelength will result in destructive interference within the frequency response (ambient pressure level), and being situated in a polar lobe will result in constructive interference (a peak within the frequency response).

[Bilou]

Originally Posted by DanDan

In any case. A client recently installed a 'wall' of fibre traps. An idea from another acoustician local to him. Some distance from the front wall, in line with the speaker fronts. A sort of soffit perhaps.
The measurements I have seen are quite stunning.

DD

It would be interesting to see some photos, and maybe measurements results.