Operating Characteristics for Ultrasonic Piezoelectric Transducer?

Question

I need to know a reliable technique for measuring frequency response (indepentent of amplifier or speaker), and operating impedence for piezoelectric ultrasonic transducer elements.

The piezoelectric transducer in question is from ALL ELECTRONICS, Corp.

The only information on this device available is: 50 MM Piezo Element
Murata 7BB-50-1A10.

Not much to go on. The frequency response that I have been able to determine experimentally, is from DC out to 300 KHz with a number of different points of resonance. An operating impedance of 5.6 K ohms was determined experimentally. I am wondering if anyone knows of a standard technique for determining frequency responce (operating bandwidth) and operating impedance measurement technique for piezoelectric devices?

The intent is to use the transducer for detecting ultrasonic signals from DC out to perhaps 80 KHz.

Answer 1

Ok, application is basically as a microphone. Frequency response is going to be very uneven and at high frequencies a larger diaphragm is not good because it increases the number of resonant modes. You might have better luck with an electret condenser microphone element - look for one with the smallest possible diaphragm. The main nice thing about a piezo is that you can use it as a transmitter and receiver...as a distance ranging element in a robot or other application...but you don't need to know frequency response for that (single frequency) application.

You can measure up to 48kHz if you have a 96kHz sampling soundcard by using one of the free acoustic measurement tools available on the internet. The impedance measurement is easy, look at the freeware program "speaker workshop" and other similar tools. A lower tech way of measuring impedance, but one that can go to very high frequencies is to construct an impedance measuring circuit - put a resistor of known value in series with the transducer, connect this to a sine wave signal source of variable frequency and measure the voltage across both devices (+ to - in diagram, call it Vs), the voltage across just the resistor (+ to T (as in Test), call it V1) and the voltage across just the piezo ( T to -, call it V2).

+ ------/\/\/\/\---T---||----- -

If the resistor has value R, the magnitude of the impedance across the piezo is simply Z= R/(Vs/V2-1). Simply vary the sine wave generator to frequencies of interest and do this simple measurement. The test resistance should be relatively comparable to the impedance of the device under test for best results. There is a way to find out impedance phase using V1, but I don't want to get into that right now.

Frequency response is a bit harder. The simplest way is with a reference microphone. But there is another way. The (slightly) tougher way to calibrate microphones is to use reciprocity. Do a search on reciprocity microphone calibration. Do you have more than one of those piezos? If so you can assume they are the same and calibrate them off each other, IIRC.

Good luck.

Answer 2

An X-ray will not cause things exposed to it to become radioactive, so there's no danger from that. As long as the X-ray is not causing the piezoelectric crystal to become too hot, you should be fine. I don't think it'll induce any voltage in the crystal either, but I'm not so sure about that.