# Speaker Sbir



## phazewolf (Feb 5, 2012)

Say my speakers were out 1 1/2 feet from the wall and I were to place a trap to help with sbir off the front wall at what point in thickness of the trap do you not gain anything?

Like say a 7.5 inch trap vs one that took all the room up behind the speaker in this case 1.5 feet.

Just trying to learn more about this stuff and thought this would be a good question to ask.


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## sdurani (Oct 28, 2010)

To address SBIR at that distance you need a panel thick enough to absorb 188Hz.


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## phazewolf (Feb 5, 2012)

Is there a point where you hit diminishing returns or do you need the speakers out far enough to where it is possible to absorb the frequency in question. 

What did you use to determine what freq would be needing to be absorbed?


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## sdurani (Oct 28, 2010)

Some of the sound from your speaker will make a round trip to the wall and reflect back, combining with the direct sound from the speaker before reaching your ear. Speaker/boundry interference occurs when that reflection combines with the direct sound out of phase, causing a cancellation (a notch or dip in the frequency response). 

In order for the reflection to be out of phase with the direct sound, its timing/delay has to be half a wavelength away (worst case). If the 3-foot round trip from your speaker to the wall to the speaker is half a wavelength, then the full wavelength is 6 feet. 

Speed of sound divided by wavelength gives you frequency: 1130 ÷ 6 = 188 Hz. That's the frequency where the notch will likely occur (you'd have to measure to confirm). Easy formula: speed of sound divided by 4x the distance to wall = SBIR frequency. 1130 ÷ (4 x 1.5) = 188 Hz.

If you move your speaker 6 inches from the wall: 1130 ÷ (4 x .5) = 565 Hz. Now you can get away with a thinner absorber.


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## phazewolf (Feb 5, 2012)

Ok that makes sense. I am going to measure my room again soon when time allows (working 7 days a week) and move speakers and seating around as much as I can to get the room sounding it's best. Then I am going to order some more traps and one of the things I want to do is place traps behind my new speakers to help out with sound.

All I am doing right now is learning so to make what I do work the best I can that way my money is well spent.

The Gik 244 traps will most likely be placed behind the speakers but I was thinking about using monster traps back there instead.

Have you played with traps behind your mains and how did it work out for you?


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## sdurani (Oct 28, 2010)

I covered as much of my front wall as possible with absorption and roughly the middle half of my back wall. Using absorption up front helped a lot, since it kept reflections from my surround speakers from bouncing off my front wall (I want those sounds coming from the surround field, not from the same direction as my front soundstage). Really improved imaging between my L/C/R speakers.


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## bpape (Sep 14, 2006)

First determine if you have SBIR. Second, for 180-190Hz, you can get by with standard 2" front wall covering.


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