How to test performace of piezo contact mics?

I recently published this short article, "How To Test Performance Of Piezo Contact Microphones":
http://www.zeppelindesignlabs.com/wp-content/uploads/docs/How%20To%20Test%20Performance%20of%20Contact%20Mics.pdf
In essence, I complain that we know how to test performance of dynamic and condenser mics, because they all function in a uniform medium: the atmosphere. But since contact mics function through a solid medium, they necessarily respond to the resonances of the medium. In the article I describe this test fixture we built to measure consistency among a batch of contact mics, but I am still flummoxed to come up with any absolute performance stats, espeically frequency response. What do you think? How would you measure the frequency response of a contact mic?
Contact-Mic-Test-Fixture-08-Working-Setup.jpg
 

1audio

Member
Paid Member
2004-03-24 5:16 am
SF Bay Area
I have not been able to open the links for some reason. A contact mike is essentially an accelerometer. Accelerometers are usually tested/calibrated in a back to back setup with a shaker of some sourt. With samll mikes I would take a small very still metal fixture that is suspended on springs/rubber. Attach a small shaker (salvaged from an NXT speaker ?) sweep the driver and compare the reference accelerometer to the mike output. The drive will not be "flat" but as long as its enough for the transducers you should be able to get what you want. Some software allows you to use one transducer as a reference for the measurement of the other.
 
That sounds like what I am after. I could produce a perfectly linear driver over a low frequency range by making my own shaker, based on an eccentric wheel on a spindle. I could even manipulate the waveform with differently-shaped wheels, but this is only viable up to 300-500 Hz. Above that, I can't spin a wheel fast enough.
 

1audio

Member
Paid Member
2004-03-24 5:16 am
SF Bay Area
You can take a small computer speaker or any 2" type driver and glue a weight to the cone then glue the magnet to a panel to make a shaker. It won't be flat but that's why you have the reference to compare to. if you need more broadband then you can get a used accelerometer from eBay and drive it from an amp. The accelerometers are piezo and can be used as a shaker.