Re: Ribbon Microphones

Scott Fraser wrote:
Only the part about slew rate limiting. ...snip...>There is a high frequency where diaphragm motion can no longer keep up with instantaneous differential pressure. It's not usually thought of as being level-sensitive, so maybe slewing isn't the best analogy. >>

Isn't what's being referred to here really mechanical inertia?

Yes. Modeled as an inductance in an equivalent circuit representation.

Here's the skinny. The EMF (signal) out of the mic is

EMF ~= v.

That is, the EMF is proportional to the velocity of the ribbon.

And

v ~= F/f.

That is, the velocity of the ribbon is proportional to the force on
the ribbon divided by the frequency _IF_ the ribbon is mass controlled
over its audible range which it is in a ribbon. This is a consequence
of Newton's observation that F=MA.

The ribbon is a pressure differential device so that the force on the
ribbon is proportional to the difference between the pressure at the
front and the pressure at the back. In a constant pressure sound
field, that difference is a function of frequency

F ~= dP/dx ~= P*f

because at low frequencies the wavelength is long and the difference
in pressure at two closely spaced points is small. As the frequency
rises and the wavelength gets shorter, the pressure difference gets
bigger.

Thus we get that

v ~= P*f/f ~= P

and

EMV ~= P

Just as we would like for a flat frequency response to pressure. Thus
the ribbon response to pressure is inherently flat over the band in
which it is mass controled and that certainly includes the higher
frequencies. The effect of higher ribbon mass is just to reduce the
sensitivity of the mic.

So where does the HF rolloff that we see in ribbons come from? In
large part it is because the front to back distance gets close to the
sound's wavelength within the audible band. At the frequency
corresponding to that wavelength the front and back pressures are
exactly in phase and the net pressure/force on the ribbon is zero. So
as we approach the frequency where the front to back distance becomes
appreciable compared to the wavelength of the sound, the output begins
to roll off, becoming zero when the wavelength equals the front to
back distance.

The above is also true of the dynamic mic of which the ribbon is just
a particular physical configuration.


Bob
--

"Things should be described as simply as possible, but no simpler."

A. Einstein
.

Relevant Pages

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