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Author Topic: SBIR/Monitor Position Question  (Read 4269 times)

xAm

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SBIR/Monitor Position Question
« on: December 28, 2009, 08:36:52 am »

I'm familiar with what SBIR is, but I'm curious as to where the limit of SBIR actually is... if there is a limit.

In building my console, it's turned out a bit taller than I thought, and thus, I'm in a bit of a quandary concerning speaker placement.

The top of the meter bridge is at 42". While sitting in the sweet spot, my ears are at 48" (give or take an inch).

If I put the center of my monitors at ear height, the top of the speaker stands/bottom of the monitors would be at 42".

Would I be better off putting the monitors (Focal Twin 6BE's) on the bridge or putting up stands?

If I use stands, I'll put either 2" or 4" OC703 up for SBIR.

However, if the monitors would be better off on the meter bridge, should I still be concerned with SBIR?

Thanx,
Max
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Thomas Jouanjean

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Re: SBIR/Monitor Position Question
« Reply #1 on: December 28, 2009, 04:07:22 pm »

(Note: SBIR means Speaker Boundary Interference for those who don't like acronyms)

Well, any hard surface will interfere. You need a table to work on... Period. You'll hear the 'flange' of it for sure, but there is no way around it.

You should of course make sure that nothing is in the way of the speakers, that they stand clear of any obstacle in a reasonable distance.

Now, if you want to estimate the interference of a particular reflection point, here's how to do it (in the metric system...)

((1/2)c)/(D'-D)= Freq of cancellation.

With (speed of sound) c= sqrt(C/ρ)

And C= elasticity modulus and ρ=density. So as standard you will have c=~340 if temperature is around ~19C
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
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franman

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Re: SBIR/Monitor Position Question
« Reply #2 on: January 27, 2010, 10:20:32 pm »

There's a bit of confusion here between SBIR and reflection control. Putting 2-4" of 703 up on a wall will control first reflections (go with 4-6" please) but will have minimal effect on the SBIR. SBIR is the low frequency cancellation typically caused by the 1/4 wavelength path difference between a low frequency radiator (woofer) and the "Boundaries" in the room that are significant to low frequency.. typically the walls.

Genelec used to give out this really cute little tape measure that was divided into the frequency based on 1/4 wavelength. You could measure the distance from your woofer to the walls in 'frequency that will interfere based on SBIR. It was cool.. I still have one somewhere Cool

Thomas gives you good information for calculating the comb filtering from a first reflection surface. The LOW FREQUENCY Cancellation is more about what frequencies will be out of phase based on the round trip path length difference from boundaries that are adjacent to your speakers. As the difference gets larger these frequencies get lower and become less signficant after a couple of meters. Proper mounting of monitors flush in the wall is an effective way to minimize SBIR, but raises it's own set of challenges as has been discussed before...

Hope this helps.

FM
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Podgorny

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Re: SBIR/Monitor Position Question
« Reply #3 on: February 03, 2010, 01:51:23 pm »

franman wrote on Wed, 27 January 2010 21:20


Genelec used to give out this really cute little tape measure that was divided into the frequency based on 1/4 wavelength. You could measure the distance from your woofer to the walls in 'frequency that will interfere based on SBIR. It was cool.. I still have one somewhere Cool





index.php/fa/14231/0/
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Thomas Jouanjean

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Re: SBIR/Monitor Position Question
« Reply #4 on: February 07, 2010, 01:00:18 pm »

Podgorny wrote on Wed, 03 February 2010 12:51

franman wrote on Wed, 27 January 2010 21:20


Genelec used to give out this really cute little tape measure that was divided into the frequency based on 1/4 wavelength. You could measure the distance from your woofer to the walls in 'frequency that will interfere based on SBIR. It was cool.. I still have one somewhere Cool





index.php/fa/14231/0/


Sweeeeeeet Smile
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
http://www.northwardacoustics.com
http://www.facebook.com/pages/Northward-Acoustics/1062876633 71

Pro Audio Partners:
ATC Loudspeakers
FOCAL Professional Speakers

franman

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Re: SBIR/Monitor Position Question
« Reply #5 on: February 07, 2010, 10:26:10 pm »

Yes it's pretty cool and it really helps clients understand how time=distance=frequency (comb filters)... I always ending up telling them "my business is really all about time".. (just shorter increments that most people think about often)... The tape measure illustrates this nicely.

FM
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