Re: Basic question about phantom and sensitivity


pandorapayne@xxxxxxxxxxx schreef:

What I'm trying to find out is, for the same volume level
from the talent, will using P48 instead of P12 require
less preamp gain?
I have the proper gain figured out for using P12 but
I will be switching to P48. Will I need to dial down the
gain?

A microphone sensitivity specification tells you how much electrical
output (millivolts) a microphone produces for a certain sound pressure
input (the loudness of the source in dB SPL). If two microphones are
subject to the same sound pressure level and one puts out a stronger
signal (higher voltage), that microphone is said to have higher
sensitivity. So when the maximum o/p level doubles when going from 12
to 48V phantompower the microphone will get more sensitive. This means
that for a similar SPL it produces a higher o/p voltage so indeed you
will have to reduce the i/p gain of the preamp in order to have similar
post i/p gain level in your micpreamp as you had when using 12V phantom
power.
There is a max dB SPL a microphone will be able to handle that is
related to the amount of (usually 1%) total harmonic distortion the
cirquit causes but this is pretty inaudible. In many cases it is almost
always the diaphram clipping (reaching its absolute displacement limit)
or the preamplifier that cannot handle the current that really is the
problem. Things are tricky though: In some microphones phantom power is
only used to power the mic's cirquit and in others it also supplies the
voltage used for polarizing the microphone's transducer element.
Depending on this constructions it varies what happens when a
microphone is given less phantom power than it is designed for. A
microphone will NOT be able to handle higher SPL levels before
diaphragmclipping occurs when the capsule is differently polarized (for
this is a mechanical thing). But the internal preamp will. Because you
say you ave a lavelier mic it is safe to assume your mic is an electret
which means the backplate is prepolarized and so its output level is
not affected by the amount of phantom power that is applied. The
internal cirquit of the microphone however is capable of better
performance when using 48V in stead of 12V (especially the internal
preamp) so maybe 12V phantom power will cause distortion at high SPL
levels because of the preamp incapability of properly handeling its i/p
level (since this remains constant). In that case applying 48V of
phantom power will just provide the preamp with more gainpotential and
thus causing a higher o/p level and thus more apparent sensitivity.
Normally though, when you compare two DIFFERENT microphones, higher
sensitivity compromises the clipping level.

Henk

.

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