[NewMusic] Core Sound ambisonic mics

[barry threw]

More info from some dude on the Max list:

The linear combination works but only for frequencies below 5000 Hz  
or such. Over, the microphone array can't be considered as coincident  
since the wavelenghts are much smaller than the distance between the  
capsules. We have "Spatial Audio" (a great book) at the research  
centre and I will check this but it looks like a coarse approximation.

In his original tetrahedron microphone patent (that you can find on  
the Internet), Gerzon explains that some complex filtering is  
required at least after the matrixing (linear combinations) and he  
gives the phase and frequency responses of the filters for W and X  
(or Y or Z). Perhaps this filter can be implemented comletely in the  
time domain, with a series of biquadratic filters for example, but I  
would have to ask a specialist about this. It's also possible to  
implement this filter by means of FFT-based convolution, which is  
more straightforward but requires using (unwanted) FFTs.

Farina has another approach, that consist in measuring the impulse  
response of a room in B-Format with the ambisonic microphone, and  
using it for implementing the required filtering by means of a  
convolution. This method is interesting because it also compensates  
the capsules misalignement in actual 4-capsule microphones.

b

On Nov 7, 2007, at 11:57 AM, Matt J. Ingalls wrote:

>
>
>> Is sound-field to b-format a simple add subtract?
>
> i seem to remember it's the equivalent to MS <-> stereo:
> L = .5*sqrt(2)*(M + S)
> R = .5*sqrt(2)*(M - S)
>
> but of course with soundfield mic it's all done in
> in the little preamp box so the output is b-format
>
> -m
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Barry Threw
Media Art and Technology


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