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Piezo elements and their Resonant Frequency

Ok, I was looking forward to stock up on piezos. Not particular plan in mind at the moment, but I just want to have them around when I need them. (shipping times are bothersome) So, I was looking for me most versatile ones.

Then, I've realized that I know next to nothing about piezos, but when looking around on ebay, what it seemed to vary was the size of them alonside with the resonant frequency.

So looking for resonant frequency of the piezos I've found:

http://www.physikinstrumente.com/en/products/prdetail.php?sortnr=400600.60

which is a bit overkill, and has loads of info beyond the scope of pragmaticability (if that's a word).

found a more tractable answer at:

http://www.endevco.com/news/archivednews/2011/06/ask-the-expert-june.html

"For piezoelectric accelerometers whose mechanical structure is almost completely undamped, the amplitude of the resonant peak can be quite high, resulting in a sensitivity many times higher than that of the specified reference sensitivity (i.e. at 100 Hz). As such, any vibration at or near the frequency of the resonant peak will be highly amplified, resulting in distorted measurements and corrupted data. A design goal of manufacturers, then, is to push the resonance frequency point as high as possible in an accelerometer's structure, with intent that the point shall be well beyond any vibration frequencies of the user’s measurement application. The user also must ensure that no vibration frequency components are at or near the resonance frequency point."

 

While this is specific to piezoelectric accelerometers it leads me to believe, that higher resonant frequencies would be desirable.

so I should prefer

http://www.ebay.com/itm/10PCS-12mm-Piezo-Disc-8-5kHz-buzzer-pressure-sensor-speaker-DIY-/120920750716?pt=Vintage_Electronics_R2&hash=item1c27703a7c

over these

http://www.ebay.com/itm/10PCS-15mm-Piezo-Disc-6-0kHz-for-buzzer-pressure-sensor-speaker-DIY-/120928177476?pt=Vintage_Electronics_R2&hash=item1c27e18d44

if I do not have size restrains, would that be so?

also these:

http://www.ebay.com/itm/110806636510?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649


looked nicely priced and although not fully specced seemed similar to the latter (size-wise at least). They seem just like piezo discs in a box, if so they are more for less and they are located in europe which is a plus for me.

Anyhow, I would welcome a more pragmatical notion on the subject from any who might have experience and knowledge on the subject

many thanks :)

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Take a look at these in comparison to the ones you linked. 

Note the frequency response is from 500 Hz to 9+ KHz, instead of just a center frequency with a small +/- range.

I suppose it all depends on what you intend to use them for. I've really only used them for audio playback. For that you want something with a wide frequency response over the audible range. For use as a speaker, avoid piezos that are marked "buzzers" as these resonate strongly at one frequency. 

Buzzers are nice for alarms and such. Some are very loud, like what you find in smoke detectors.

I don't have any experience using a piezo as a pressure sensor or accelerometer. The ones you linked from eBay may be fine for this. I'm not so sure about how well they will work as speakers.

I really like these little guys for speakers. I have no specs for them, but they seem to work well. They must be ressonably efficient, since they sound pretty loud without extra amplification, and they produce good tones at different frequencies. Apparently they were used in cell phones, so that would make them perfect for voice or any audio playback.