Re: Directional microphone

[James Beauchamp <jwb@xxxxxxxxxxxxxxxxxxxxx>]

Argiris Kranidiotis writes:

>        Is  it  possible  to  simulate  the  effect of a highly directional
>microphone  using an array of two or more omni-directional microphones ?  I
>suspect  that  this could be done by using a simple time-delay filter,but I
>am not sure how...

Off the top of my head, here is my answer. Please correct me if anyone knows
better.

If you have two omini-directional (pressure) mikes spaced a distance small
compared to a wavelength, and one of them is subjected to a time delay equal to
the maximum delay that can exist between the two mikes (this depends on the
angle of arrival of the sound wave), you can difference the two resulting
signals and achieve a cardioid pattern, which is the basic unidirectional
pattern. By elaborating on this scheme using several omni-directional mikes
together with suitable delays and attenuation factors one can achieve a highly
directional pattern. The closer the mikes are together, the wider the frequency
range of operation. Therefore, for most applications the mikes might as well be
built into a single enclosure. Design for a particular spatial pattern is very
much like digital filter design for a particular frequency response.

Jim Beauchamp
j-beauch@uiuc.edu