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The DIY Ribbon Velocity Microphone, a home project

Physics Behind It

(some)

All Right.  I will try to do this the best that I can, although one of my links does a much, much better job than I could ever do (if you are technically minded).
If you want the simple answer, then read on.
 
The ribbon mic consists of a this metal ribbon suspended from two ends in the flux of a magnetic field (between the North and South of two magnets).  The Ribbon is very thin, ideally between 1 and 4 or 5 microns thick.  This thin ribbon will vibrate with the VELOCITY  and the pressure of a wave traveling through the air. 
If you've had electronics or physics experience, you should remember that a metal moving through a magnetic field produces a voltage.  This voltage shows up at both end of the ribbon, and is then put to the transformer.
 
All of this should make sense to you, up until now.   Here comes the hard part.
 
The frequency response of the ribbon microphone is not dependent upon the thickness of the ribbon.  It is natural to think that a thicker ribbon will vibrate less at high frequencies than a thinner ribbon.  The truth is, only two things are affected by the thickness:  The sensitivity and the output.  It takes more power to move a thick ribbon, and a thick ribbon must move further in a magnetic field to produce the same voltage that a thinner ribbon would.
 
Frequncy response is determined by time and distance.  The distance a wave has to travel around a ribbon mic's base, magnets, etc. determines the cancelling and doubling force of a wave, and the time is critical in the cancelling and doubling as well.
 
Ribbon mics are of the Pressure-Gradient variety.  They work by having pressure differences on both sides of the ribbon causing the ribbon to move.  The velocity of a wave comes into play because the ribbon moves at the velocity of a wave, because velocity and pressure are directly related.
 
Confused yet?  Ribbon mics work on a pricipal of waves cancelling themselves out on both sides of the ribbon (gradient pricipal).  Lets say that a wave of 500 Hz hits the ribbon from the on-axis position.  A wave of 500 Hz has a fairly long wavelength, and would easily wrap around a microphone.  The same wave will travel around to the back of the mic and strike the back of the ribbon very, very quickly and cancel a good deal of the on-axis power.  The fact that the on-axis hit with a little more force allows for more energy in that moment, so you still get a sound at the output. 
 
But if the frequency is higher, oh, say 10 kHz, then the wavelength is a whole lot shorter (logarithmically in fact).  About one inch long.  That means that if the ribbon's base and magnet structure is forces the wave to go more than one inch around to strike the ribbon's backside, then it's not subject to the same force, now, is it?  The higher wave has more of a change in the wave phase than the lower wave. 
 
O.K., I'm going to quote another website here.  It's the easiest way to explain this.  I've tried to contact Steve Spicer, but I get nothing in response.  This isn't plagerism, it's a quote.
(update:  Steve Spicer did contact me, and he gave me his blessing for using his information on the website.  He is a gracious man, and a scholar who truly enjoys enriching the lives of others).
 
"The path length from front to back is a fixed dimension, in inches if you like, determined by the size of various objects in the microphone. But in an acoustic sense it halves every octave. For example, consider a design with a physical path length of 1- inch from the front to back of the ribbon. At 1kHz this is about one-tenth of a wavelength. At 2kHz this becomes two-tenth's of a wavelength, even though it is still 1- inch. This means that at 1kHz, the pressure wave at the back is one-tenth of a wavelength behind that at the front. But at 2kHz, the pressure wave at the back is two- tenth's of a wavelength behind. Shown in Figure 2, it can be seen that the doubling of frequency, results in a doubling of the acoustic path, which in turn doubles the net pressure on the ribbon. So the force on the ribbon doubles each octave, giving the microphone a ruler-flat response from about 20Hz to around 10kHz." 
 
Update:  I understand that the link is broken, but I will leave this up hope that the prodigy will some day return...
 
 
That's easier.  I understand it, but I can't get it out.  If you read his page, and still have a difficult time gettin' it, let me know and I'll try to explain it better.
 
 

Wheewww!
 
It's pretty difficult for me to put it into words.  Others do it better.  What can I say?
 
So all of this should be a good indication that if you want to build a mic with a flatter frequency response, the you should make the structure itself as thin as possible.  Thinner magnets (or magnet poles), and a thin base.  This allows for optimum sound envelopment and lets the ribbon mic do what it naturally wants to do:  it wants to give you a flat frequency response.
 
After initial testing, Spike has shown that he has a fairly flat frequency response from 200 Hz all the way up to past 20 kHz (with different boosts below 200 Hz, mostly around 30 Hz).  I should have the graphs up pretty soon to show you.

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