Ultrasonic transducers.

[CJay]

Not so much a beginner's question but one from way back in the mists...

I seem to remember reading that although ultrasonic transducer pairs were marked T and R that they are exactly the same thing?

Am I right or is my mind playing tricks on me?

[Benta]

Not sure about the T and R, but I do remember that they were matched and sold in pairs.

[McBryce]

I think there are/were very minor differences between the physical and electrical properties of the T and R devices that were sold in pairs, but that they were essentially the same device. Probably sold in pairs because they were tuned to the same centre frequency. I know that modern automobile parkaid systems only use one device to transmit and recieve the signal, so the T and R devices must have been extremely similar.

McBryce.

[Gyro]

As the T/R indication is usually either a stick-on label or ink stamp, I suspect it's more of a 'selection' process. Maybe they keep the lower impedance ones for TX. The internal construction looks identical, same little metal cone. They both seem to work ok in bat detectors.

[CJay]

As the T/R indication is usually either a stick-on label or ink stamp, I suspect it's more of a 'selection' process. Maybe they keep the lower impedance ones for TX. The internal construction looks identical, same little metal cone. They both seem to work ok in bat detectors.

Exactly the application I have in mind, thanks. I've ordered a couple of pairs of 40KHz ones.

[Gyro]

Aha, in that case you might want to take a look at bertrik's bat detector page...  http://bertrik.sikken.nl/bat/

One of the articles is on de-tuning transducers to increase their frequency range.

[CJay]

Oh nice web page, lots of reading.

A bat detector is one of those things I've been intending to build for years, I was reminded by a post on another mailing list and now summer is here, they are flying through and around my garden and the holiday home in Wales regularly so I thought it'd be a nice project.

 I was intending to make something similar to the Franzis one but MEMS microphones have been mentioned and it seems that you can buy them with bandwidths up into the hundreds of kilohertz which opens up the possibility of a waterfall display of bat sounds so I'm investigating those right now...

[Gyro]

Don't put it off any longer, it's well worth doing - It's the sort of project that makes your wife suddenly realize that you're good for something! 

[Berni]

Ultrasonic air transducers are sold both in separate TX and RX versions or as a combined transceiver version.

The reason for the the TX transducer to be separate is that driving the piezo element inside at high amplitude causes its resonance frequency to shift a little bit. So to compensate for it they offset its frequency just right so that once driven at rated power it drifts back to exactly match the receivers resonance frequency.

Additionally the TX and RX can be made from a different grade of piezo ceramic material. There are harder blends of ceramic that have worse piezo properties but have less loss to the acoustic waves, this allows them to be driven at higher powers. On the other end there are softer blends that have more pronounced piezo electric effects for the same applied force, but suffer from high loss (But that's not a issue as the received power is tiny)

The combined transciever version is simply a compromise between them, letting you do the job with only one transducer but getting potentially less range out of it.

[CJay]

Oh that's interesting and, in my head, it makes sense the drive power would cause the frequency to shift, I think, more drive, larger excursions, lower frequency because of greater movement.

 Am I being simplistic or just plain wrong?

I don't think that for my application the lower sensitivity will matter too much but I'm planning to build at least two devices so I might swap the 'microphone' a few times  and see if I can measure a difference.

I've just ordered some MEMS microphones and am playing with Teensy FFT code, I may end up with a BATSDR...



 

[Berni]

Yes its something along those lines. I think it also has to do with some of the material properties not being completely linear.

But for your application it sounds like a MEMS microphone is the way to go mainly because they have a resonably flat freqency response while these ultrasonic distance measurement transducers are designed to only be used at the resonan't frequency so will generally have a frequency response with a huge peak in the middle.