Monday, July 26, 2010

Listening Comparison Of Comb Filtering in Boundary Mounted & Free Air Mics

One of the criticisms of placing microphones next to boundaries is that it produces objectionable comb filtering. This comparison test demonstrates that any comb filtering produced is very subtle and occurs at very high frequencies. The use of moving pink noise  for detecting the phenomenon was suggested on the Nature Recordist list. I performed many "3 O'clock" to "12 O'clock" pans with a boom box speaker around the mics in an arc at a distance of 5 ft. and 10 ft. I selected one sweep from both distances where all three mics exhibited smooth amplitudes.  I used a 744T recorder with 45 dB of gain to record all three mics at the same time.  I had to use a mix of AT3032 and AT 4022 mics which are known to be very, very similar in terms of performance. 

A blind version of the QuickTime movie comparison is available for downloading (3mb .zip)  to see if you can match the samples with the right mic configurations. I'll soon post the ID's below. 

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

Here's the comparison test the samples identified [ download .zip 3mb]
Here's a larger image of the sonogram with more detail.

Follow-up discussion can be found on the NatureRecordist list as well as in Paul Jacobson's assessment below. Curiously, the Perpendicular to Boundary capsule positioning produced the least amount of comb filtering in this test. 

The guesses I received  suggest that any artifacts pink noise sweeping might reveal are difficult to accurately link to the arrays:

Sample A Guesses:  Free Air, Free Air, PBB, Perp2Air, Perp2Boundary
Sample B Guesses:  Perp2Boundary, PBB, Perp2 Boundary, PBB. PBB
Sample C Guesses PBB, Perp2Boundary, Free Air, Free Air, Free Air