room gain question

[christophorus]

Hi all,
I'm going to build a subwoofer and the SPL curve from WinISD pro is like the green line, it peaks at 20Hz. Is it correct when I add room gain, the curve line of SPL become flatten (magenta line)?
My room size is 4.5mWx6mLx3mH, my siting position is 4.2m(14feet) from sub position, so my room fgain at: 1/2x342/4.2=40.7Hz.
How much gain boost (in dB) will I get from my room?

thankyou.
Chris.

[rabbitz]

I thought it would increase the 20Hz more. See this chart on room gain.

[lndm]

Originally posted by rabbitz
I thought it would increase the 20Hz more. See this chart on room gain.

I've just been reading these
http://www.diyaudio.com/forums/showt...264#post603264
http://www.diyaudio.com/forums/showt...87#post1005887
FWIW

[Gunawan W]

thank's guys, I learn much from your links

[christophorus]

Hi rabbitz and Indm,
so in my case, room gain for my room start from 0.5x342/6=28.5Hz and raise up as per rabbitz chart.

Chris.

[lndm]

I found it hard to get a generic answer. The density of the walls seems to be a concern.

My room is 12' x 25' x 9' with a doorway that tunes it to 4Hz. I had to equalise a fair bit to reduce the bass. However, I use corner loaded horns and I have never measured them out of their corner so I'm not certain.

I have read several accounts, looked at charts, and made estimates. I am currently guessing on 50Hz/3-4dB, 40Hz/4-5dB, 30Hz/5-6dB, 20Hz/7-9dB.

[rabbitz]

That graph I posted previously gives idea of room gain. To get a better idea, you have to work out gain using boundary dimensions.... distance of woofer to walls or floor, how many boundaries.

There is software around to work all this out but have never used any.

There is a formula which I can't find but it works on the notion that the woofer power is twice as efficient in 2PI space as it is in 4PI space and the same goes for PI space vs 2PI space etc. The most gain you can get from 1 boundary is 3dB, even though the formula (I can't find) shows 6dB. This is due to an increase of 3dB due to confining the radiating power to half space but this is of no importance as it involves no change in power. The second 3dB is the result of actual doubling of the power radiated by the woofer and is the gain we hear. Add second boundary and up it goes again to 6dB gain in power and so on.

You can get an idea of the frequency where the gain takes place by using these formulae which relates to fractions of wavelength of sound (the 0.05 and 0.01) .

Frequency = 331 x 0.05 / Boundary dimension in metres (this is roughly the start of 3dB gain)
Frequency = 331 x 0.01 / Boundary dimension in metres (this is roughly about the lowest frequency of 3dB gain in a room that you need to calculate)

So for a dimension of 300mm (0.3m), you get a 3dB boost between 11Hz and 55Hz from 1 boundary. This info originally came from an article in the 70's or 80's by Roy F. Allison that was published by a local Hi Fi rag and is only a simple view into a complex subject.

There's a graph below that shows area of maximum gain between the green and red lines and if you look at the magenta and aqua line, they're a negative gain but that's another story. Any dimension above about 1200mm is not effected by boundaries.