Re: [Harp-L] Which harp mic is most feedback resistant?

[Greg Heumann <greg@xxxxxxxxxxx>]

Man there's a lot of B.S. floating around on this subject. Some truths  
too, but a lot of misinformation. Here's my two cents.

Feedback comes from, in order of priority:

1) Gain of the mic: Although engineers would like the mic's  
relationship of input signal to output signal to be linear, it isn't -  
some mics go from low output to full output with far less input signal  
than others. The Audix Fireball has the highest "headroom" of any mic  
I've ever tried - meaning it can tolerate extremely high input levels  
before it reaches its output peak, which isn't very high in any case  
(see #2). This is why it is so feedback resistant. (That doesn't mean  
it is the right mic for you - I personally care MUCH more about a  
mic's tonal characteristics than its feedback characteristics.) Every  
once in a while I get a Shure CM that seems to be very gainy compared  
to the average. These are feedback monsters, but they are the  
exception among these otherwise great elements.

2) The output level of the mic: For a given sound level, some mics  
produce more output than others. This will in turn make the amp louder  
at the same volume setting. All other things being equal, two  
identical mics with different output levels, adjusting the amp for the  
same volume between the two of them will result in the same feedback  
level - assuming the amp's volume control provides linear increase in  
gain.

3) The frequency response of the mic: Most mics, especially  
performance (as opposed to studio) mics have non-linear frequency  
response. If there is a bump, say, at 2500Hz, and the amp happens to  
also have a bump there, feedback is more likely.

4) The directionality of the mic: If the mic picks up sound from all  
directions ("omni-directional") it is more likely to "see" the  
amplifier's output - which is of course how feedback happens. SM57's  
are very directional - effectively blocking sounds coming at them from  
the sides and in front of you. An SM58 with its ball has a wider  
pickup pattern and will feedback before the SM57 does - especially in  
regards to floor monitors at the front of the stage. Anytime you hand  
hold a mic you totally change its directionality anyway, so if you  
hand hold a mic all bets are off.

Note that impedance itself does NOT have an effect on feedback OR  
tone. Sometimes converting a low-Z mic to high-Z or vice versa,  
whether with a dual-impedance internal transformer like the 545, or an  
external impedance matching transformer, will produce MORE output and  
sometimes it will produce LESS. Depends on the mic and the  
transformer. But equalize the volume at your amp between the two  
configurations and the feedback level will be THE SAME because the  
frequency response and directionality of the mic does not change.

When it is all said and done, the GAIN OF THE AMP is the biggest  
variable by far. Gainy amps will be feedback monsters, which is why  
harp-specific amps and recommended mods to stock guitar amps always  
involve reducing the gain from where guitar players like it. Pick a  
mic for tone. Dial in your amp to manage feedback. Need more volume  
before feedback? Employ anti-feedback electronics, EQ the offending  
frequency range down, or... Get a bigger amp with more speakers.  
(Multiple speakers tend to disperse the sound better so the energy  
level where you're standing compared to the total energy in the room  
is better than with a single speaker amp of the same power.)


/Greg

http://www.blowsmeaway.com
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