Let's learn a bit about measurement microphones!

Measurement microphones are in principal the same as regular microphones, except they suck a whole lot less.  In fact, measurement microphones are pretty much the only type of acoustic transducers in your lab you can count on.  Here's a (somewhat long) video comparing the two most common types of measurement microphones:  Free Field (FF) and Pressure Field (PF) microphones, as well as a cheapo ($50) Behringer microphone.  A 1/4" FF microphone is pretty near acoustical perfection.  As is a 1/2" microphone, if designed right.  The Behringer microphone is, well, not so great.

All measurement microphones are 'omni-directional.'  Except that they aren't.  They do have some preferred direction, and also have preferred acoustical environment.   A microphone must be much smaller than 1 wavelength to be truly omni-directional, and at 20kHz, the wavelength is 17.5mm, so to be truly omni at 20kHz, we'd need a microphone with physical extents less than, say 1.7mm.  This gets tricky, so companies like B&K an G.R.A.S. need to build microphones for specific purposes.  If they could build a great 1mm microphone, they would, and that would be the end-all and be-all of acoustical microphones, but like in most of life, there are trade-offs.  This video covers a few of those trade-offs.

Enjoy the video

-Caleb