How It's Made episode on AEA ribbon mics

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Cool video. Didn't realize the magnet component in this style of ribbon was so large/heavy.

Here's a version on YouTube as I couldn't get that version to stream smoothly:
http://www.youtube.com/watch?v=SHkOkqnJjy0
(albeit with a different narrator)
 
Really cool. Both that video and the video on the Royer talk about an electrical test to check the tension of the ribbon. In the Royer video, check the frequency of resonance. What should the frequency of resonance be? Does the ribbon vibrate noticeably more at the frequency of resonance? 
 
dmp said:
..  talk about an electrical test to check the tension of the ribbon. In the Royer video, check the frequency of resonance. What should the frequency of resonance be? Does the ribbon vibrate noticeably more at the frequency of resonance?
The resonance decides the LF cutoff of the mike.  It's done by measuring the impedance and looking at the ribbon with a microscope.  Resonance is determined when current & voltage are in phase which is the straight line on the scope.  Yes, ribbon vibrates more at resonance.  You can't put too much voltage into the ribbon cos it will stretch.  The microscope is probably checking for that rather than the resonance which is much more accurate from the scope display.
 
So in the royer video, it looks like the signal generator is set from 30 to 40Hz, and the scope is plotting voltage vs. current as an XY display. The display shows a line when the V & I are in phase.  I see what you are saying - thanks.
I'd like to try this - but how to measure the voltage and current?
1. connect the signal generator to the two sides of the ribbon, and measure the voltage across the ribbon with the scope channel 1. 
2. measure the current on scope channel 2 by including a small resistance in series with the ribbon and measure the voltage across the small resistor? Wrong?

Wouldn't the resonant frequency be problematic as it would be loud? Why tune the ribbon to have a resonant bump at 30-40Hz, where it is audible?
 
dmp said:
I'd like to try this - but how to measure the voltage and current?
The ribbon has resistance less than 0R2 so any generator will have MUCH greater resistance and be effectively a current source.

If you put 100R between generator & ribbon, the generator output will tell you the current.  Measure the voltage directly across the ribbon.  This is VERY small and you have to do it in a quiet place cos the ribbon is a .. microphone!

[/quote]Wouldn't the resonant frequency be problematic as it would be loud? Why tune the ribbon to have a resonant bump at 30-40Hz, where it is audible?[/quote]The ribbon resonance is exactly like the resonance of a moving coil speaker.  It forms a 2nd order HP filter.  Like a speaker, if properly damped, there is no peak in the response.  Lower resonant frequencies need the ribbon so slack that it would fall out of the gap.

But in the test, you feed the ribbon from a high R so the voltage & excursion will be maximum at resonance.  The microscope is probably to see that you are not putting too much voltage at resonance which will stretch the ribbon.  The scope XY trace is a much more accurate indicator of resonance.

From the video, the Royer's have a resonance between 30-40Hz.  From the measurements I've seen, this means they are quite highly damped as they have less LF than the BBC 4038 which is tuned to 40Hz.
 
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