# Re: Wiring for the stage

Jim Carr 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.

I should note that "velocity mic" has less than a thousand hits on Google, most of which were ads describing mics. Same for "velocity microphone". I don't feel so bad having not heard the term before.

This is all well and good, but the sound wave, moving at 1100 ft/sec, isn't moving the elements of the mic. Air molecules are moving the mic elements, and the air molecules aren't moving at 1100 ft/sec. They are moving fairly slowly and fairly small distances, in a series of compressions and rarefactions.

The next time you're fishing, watch your bobber. The waves pass by, but the bobber moves up and down. Actually, the bobber's motion is in a circle, but it isn't moving as fast as the waves from a passing boat. The air molecules near your mic are doing much the same.

-Raf

--
Misifus-
Rafael Seibert
mailto:rafseibert@xxxxxxxxxxxxxx
blog: http://rafsrincon.blogspot.com/
Photos: http://www.flickr.com/photos/rafiii
home: http://www.rafandsioux.com
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