[Harp-L] RE: what makes a mic prone to feedback?

[Greg Heumann <greg@xxxxxxxxxxx>]

In my experience, I believe there are only a few factors. One is  
sensitivity of the mic - the mic that puts out more signal than  
another at a low level will feedback more. Another is resonance -  
some mics live in housings where the back side creates a resonant  
chamber. I found this out when the screw fell out of a little hole on  
the back side of my 520D - and I always thought it was sealed well  
enough that the back side didn't have much effect - but with that  
hole opened it was definitely worse than with it sealed, although  
there is another tiny hole that doesn't seem to have much effect.  
Finally, phase makes a difference. By phase I mean whether the amp's  
speaker moves out when the mic's diaphragm moves in, or vice versa. I  
installed a phase-switch switch in one of my amps (it reverse the  
polarity at the speaker) - and I was surprised at the difference in  
feedback. Either position will feed back, but at different  
frequencies and at different levels. If you ever play behind your amp  
and find it feeds back less, consider switching the phase.

But the most important factor is simply the output level - some mics  
are much hotter than others. Of course if you do NOT adjust volume at  
the amp, the hotter mic will generate a louder sound, perhaps enough  
to create feedback. Many folks blame this on the mic - but if you  
adjust the volume so that you got the same loudness for two different  
mics, you'll probably find they aren't very different.