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Re: 3032 capsules on PIP

Subject: Re: 3032 capsules on PIP
From: madl74
Date: Thu Jun 12, 2014 5:00 pm ((PDT))
> Ive been meaning to build a version of the brinibox for quite a long time. Id 
> like to "tune" the design if its possible to give at least 10 dB mechanical 
> gain (the gain advantage due to the box boundary layer) 


The measured on-axis gain on each horn was about 12dB over free air. The 
rectilinear conical horn theory is complex, but there are other ways to 
describe it. 

Note that a rectilinear horn does not have resonances as every incoming 
"ray" departs at 180 degrees like a radar reflector. You can't play a 
rectilinear horn as you can a tube, narrower cone, or exponential horn like 
a trombone. I used to play one. Note the musicians definition of a gentleman 
as someone who can play the trombone but doesn't. :-) 

Looking as a rectilinear horn as three reflectors, each with a "boundary 
layer" (and I have an explanation for that) the throat point is in effect a 
joint boundary point from three one-third-horn surfaces. In practice the 
12dB figure seems about right. 

The other way of looking at it, is as two conical horns which each have a 
higher acoustic impedance at the throat. This is why I specified sealing the 
mic into the thorn throat. 

>  By placing the mics in the brinibox and taking advantage of the boundary 
> effect 
>  do we reach the ability to record down to the mythical 0 dB SPL? 

0dB is impossible. Thermal air noise impinges on both sides of the diaphragm 
but the signal only acts on one side, which would give a noise figure of 
+3dB over incoming thermal noise. B&K claim the record for an instrumental 
mic of 5dB excess noise. The limit for a practical mic is about 10dB, if 
only because some of the incoming sound is reflected away. Even in a vacuum, 
the atoms in the diaphragm are still moving, but you can do wonders with 
creative specs and weighting. 

Going back to free air and throat impedances, in free air there is a large 
mismatch between the free air impedance and the diaphragm impedance with 
some incoming sound energy being bounced back as above. The same applies to 
the thermal noise of course, but that remains at 3dB above free air. It is 
impractical to evacuate the capsule interior. 

Now comes the tricky bit. If you up the signal and thermal sound impedances, 
they should stay in step, but the signal stays coherent and the thermal 
noise stays random, at half the dBs rise. Hence there is a noise benefit by 
using a horn. 

This is theory in English, but tests seem to back up the general principle 
of higher gain and lower relative noise. 

This is a narrative explanation and I have forgotten how to handle the 
required Bessel functions involved. The theory above is only a guide to 
trying out variations in practice. I chose the "box" shape as I thought it 
would give a better stereo image and I am happy with the shaking peanuts 
test. The rear sensitivity is quite a bit lower which can't be said about 
many stereo rigs. 

David Brinicombe 

"While a picture is worth a thousand words, a 
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.

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