Re: Wiring for the stage

Jim Carr <newsgroups@xxxxxxxxxxxxxx> wrote:
"Benj" <bjacoby@xxxxxxxxxxx> wrote in message
news:1186181288.864742.212650@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Think of it as a sealed tin can with a saran wrap diaphragm over the
end with rubber band. As the outside air pressure is different from
that inside the can, the diaphragm is moved in and out.

I'm familiar with that concept.

A "velocity" mic is one where a very light element (say an aluminum
ribbon) is actually BLOWN back and forth like blowing a leaf away. The
element is just hanging in open air so any change in air pressure goes
right around the ribbon and does nothing. The air pressure is always
the same on both sides of the small ribbon. Because these mics are
sensitive to air velocity, blowing into one is a huge no-no! They will
respond to that high air velocity with damage!

Having never heard the term before, I did some research. I think this site
has a good explanation, but it doesn't seem to agree with yours, but that's
not a big deal to me.

http://www.record-producer.com/learn2.cfm?a=3012

The ribbon mic is still dealing with pressure changes, which is what sound
waves are. The term velocity came into being because of how the ribbon
itself reacts to the sound waves. Draw a line perpendicular to the source
and use that to measure distance. Sound waves move at a constant velocity if
measured perpendicular from the *source*. If it travels 1,100 feet per
second, it will be 1,100 feet away in one second.

But suppose you shoot the wave off at an angle from the source and your
perpendicular measuring stick. In one second it has moved away 1,100 feet as
measured along that angle. In order to measure the distance on your
perpendicular axis, you need to draw another perpendicular line from that
point back to your measuring line. This distance will *always* be shorter
than what you would get if the sound wave went straight along your
perpendicular measuring line.

Thus, it can be said that the *velocity* is therefore slower. If the sound
wave went out at 90 degrees away from the measuring line, the velocity would
be zero relative to the perpendicular measuring stick. So from the
perspective of the source to the perpendicular line you have a velocity that
goes from 0 to X and back to 0 as you sweep it across 180 degrees from the
source.

Now switch things around from the perspective of the sound waves hitting the
ribbon from various angles. From the perspective of the ribbon, waves
hitting it straight on are going "faster" than those coming at an angle, and
thus move it more. Those coming in from the sides don't do anything because
the wave has equal pressure on both sides of the ribbon. It won't move at
all.

For lack of better terms stuff from the left is at zero, stuff from the
front is at 100%, and stuff from the right is at zero. Stuff in between
comes at values between 0 and 100% to form a curve. Apply this to both sides
to get your figure-8, and Bob's your uncle.

This is from where the term velocity mic originates. The sound wave really
is moving at a constant speed as measured perpendicular to the source. It's
just this angle perspective of the ribbon that inspired the name.

It all makes perfect sense to me now.

We could argue until the cows come home whether it's really a pressure
gradient or air molecules hitting the ribbon. I read some whacked out stuff
about that which is *way* beyond anything I really care about. I can "see"
the ribbon flapping, and I know it doesn't work like your "sealed and
vented" diaphragm mics work. That's good enough for me.

I was simply thrown by the term "velocity" since it seemed to imply that
sound waves traveled at different speeds when everybody else says sound
waves travel at a constant speed in a given medium and conditions. Now I get
why some people used the term.

Sound particle velocity is different than the speed of sound. Sound
particle velocity is RELATED to pressure, but is not pressure. Sound
waves travel at a constant speed in a given medium and conditions, the
air particles moving to create that wave do not.


--
Aaron
.

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