[Chris Hornbeck]

On Tue, 22 Jul 2008 01:55:21 -0400, "Soundhaspriority"
wrote:


Chris, I have a general physics background, but not a mike background, so I
am naturally curious. I'll just mention a couple of things that might fit
together for a solution. With straightforward dynamics and ribbons, the
dynamic is closed-back unless someone ingeniously opens it up, and the
ribbon is open back, and symmetrical, unless someone is equally ingenious.

Ribbons are "dynamic" in the purest sense of the term (from
"dynamo", a conductor moving in a magnetic field - very, very
old school stuff, so a newer, conflicting meaning has arisen).

So I'll just shut my trap and accept your terms gracefully.
(Grrrracefully...) Arf.


I think you are looking at mass and resonance as the primary cause of phase
shift in these elements, but the primary cause is elsewhere. I think that
for the purpose of this question, the dynamic can be approximated as a
massless, zero phase shift device. It's not like a speaker, where the mass
of the driver significantly figures into it, except way up in the treble.

But, but, but, why? All real-world diaphragms are significant
compared to their surrounding air, aren't they? A ribbon has its
massXcompliance resonance *below* its working range. Can any mic be
considered to be non-interacting? At this point, I guess that
your definition of "dynamic" will need to be clearer to me. It's
clearly my stumbling point.


The ribbon has a 90 degree phase shift not because of a mass effect, but
because it indirectly samples the particle velocity as the pressure
gradient. The particle velocity is not the same as the wave velocity "c".
It is the actual movement of bulk air in an oscillating,
net-zero-displacement fashion, that creates a wave with a propagation
velocity of "c". In a simple compressible fluid like air, the particle
velocity is proportional to the pressure gradient. The gradient is a
derivative, which means that ideally, it is measured in a vanishly small
space, but it is well approximated as the difference in pressure between the
front and back of the microphone, divided by the distance between the two.

Note: The "particle" is actually fictitious. Consider it a tiny hunk of air.
The actual air molecules are moving independently of this according to
Boltzman statistics, but through some kind of averaging miracle, the
fictitious "particle" has served well.

This is amazingly clear and very helpful.

My residual confusions about why this doesn't have a complimentary
parallel in the pressure case will need to await some snooze time
and thought. I'm just a very literal, rock-on-the-end-of-a-spring guy.


Much thanks, as always,
Chris Hornbeck