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Author Topic: Boundary Effect  (Read 3111 times)

Ian Visible

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Boundary Effect
« on: May 09, 2008, 08:45:54 AM »

Could someone explain, in terms that can be understood by a simpleton, the boundary effect please?

I've read a quite a few posts that mention it both in relation to bass trapping and to mic placement, and the more I read, the more confused I become.

What's it all about, Alfie?

gullfo

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Re: Boundary Effect
« Reply #1 on: May 09, 2008, 12:16:47 PM »

from a mic position perspective. if you simply stand just off center in a room and speak, then shift sideways to a wall, you'll hear a distinct shift in what you hear. this is because the sound is bouncing off the wall and into your ear and mixing different frequencies at various phases as you do it. probably the simplest example of comb filtering. no equipment other than your ears needed. however, your ears will try to compensate for it, so the effect is short lived. a microphone has no such compensation and it will faithfully record the effect - not good unless you want it.

from a speaker perspective, the bouncing not only introduces cancellations and reinforcements, but changes the loading on the speaker which can cause the power needed to drive it to change and result in additional distortion. depending on where the speaker in placed front-back, side-side, has different effects on its response which is one reason for having adjustments on the speaker to cut or boost bass. a baffle mounted speaker designed for open mounting will experience a bass boost, and depending on how the baffle is constructed, you could also shift the apparent crossover points in a bad way.

Seigfried Linkwitz and Phillip Newell has some excellent treatises on this subject...
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franman

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Re: Boundary Effect
« Reply #2 on: May 10, 2008, 10:24:52 PM »

This is an very important subject and the concept can be difficult. Genelec has written some easy to understand papers on the subject that I would consider "recommended reading"

http://www.genelec.com/documents/publications/PlacementandRo omAcousticsInteraction.pdf

http://www.tangible-technology.com/sirhound/drums/genelec/GE NELEC_monitor_PLACEMENT.htm

Check these out. I think they will help.

FM
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Ian Visible

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Re: Boundary Effect
« Reply #3 on: May 13, 2008, 07:37:36 AM »

Excellent, a little light reading.

Thanks both for the info.

I shall no doubt return somewhat less baffled but with more questions!  Laughing

Ian Visible

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Re: Boundary Effect
« Reply #4 on: May 14, 2008, 08:20:13 AM »

I think I get it with respect to speaker placement.

What I'm still struggling with is how boundary effect affects bass trapping.

Unless I've misunderstood, standing a trap away from a wall lowers the effective frequency range absorbed.

But there's still the same amount of material.

Have I misunderstood? Embarassed

johnR

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Re: Boundary Effect
« Reply #5 on: May 14, 2008, 09:36:40 AM »

Ian Visible wrote on Wed, 14 May 2008 13:20

I think I get it with respect to speaker placement.

What I'm still struggling with is how boundary effect affects bass trapping.

Unless I've misunderstood, standing a trap away from a wall lowers the effective frequency range absorbed.

But there's still the same amount of material.

Have I misunderstood? Embarassed

At the surface of the wall the velocity of the air particles is zero. If it wasn't, the wall would have to be flapping back and forth.

Velocity increases as you move away from the wall (for distances less than a quarter wave length of the frequency you're concerned about anyway).

An absorbent trap works better if velocity is high, so moving it away from the wall makes it absorb more effectively.
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Ethan Winer

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Re: Boundary Effect
« Reply #6 on: May 14, 2008, 01:41:26 PM »

Ian Visible wrote on Wed, 14 May 2008 08:20

What I'm still struggling with is how boundary effect affects bass trapping.


johnR gave you the right answer, and this may help clarify further:

Optimizing the Air Gap

--Ethan

Ian Visible

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Re: Boundary Effect
« Reply #7 on: May 16, 2008, 09:41:36 AM »

That's a splendid page!

Thank you kindly!
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