Piezo Element

Some piezos have pretty sharp resonances. You might want to wire it up as an oscillator, or sweep slowly between 1 and 10 kHz and listen again.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

It's really more acoustical impedance matching than amplification. Making them louder... might not be a good thing.

-- Les Cargill

You could drive it between two outputs of opposite polarity (opposite phase if a single sine wave) to double the available voltage swing, 'BTL' fashion. In many commercial sounders, the piezo disc is surrounded by a Helmholtz resonator to provide improved matching to the surrounding air at a chosen frequency (or a narrow band). You might find this helpful:

The greeting card sounders I've seen (and re-used) were each mounted in a (thin) Helmholtz resonator.

Chris

Can't figure out what your talking about. Element needs a baffle, and some add a horn. Bare element ?? Is it housed ?

Most speaker drivers have a limit under a 100 volts. I have driven elements with hundreds of volts to produce movement of 5 mm or more.

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Nice!

Also, get some ideas from disassembling a few (cheap) digital alarm watches; they manage to pack such resonators into a very small volume yielding quite a lot of sound.

Mark L. Fergerson

Jon Slaughter writes

In general you try to keep the drive volts below 30V ac or you run into regulatory issues to do with safety...

But that usually only applies to the supply. Some drive circuits use the piezo element (which is quite capacitive - did you take this into account in your drive circuit?) as part of a resonant circuit. It can end up with well over 30V ac across it that way. This resonant technique is only good at one frequency. Most of the piezo sounders I've come across have been used as alarm sounders so were optimised for one tone. People used every trick they could to maximise (apparent) volume: Helmholtz resonators, differential drive, careful choice of drive frequency, voltage doublers, clever attention-grabbing cadences... sine waves generally give loudest apparent volume, followed by square waves.

Basically you won't get much volume at 5V unless you use a magnetic sounder, but that will take tens of mA.

I don't know how greeting cards combine decent volume with a broad frequency response and small size.

The most common resonant frequencies for piezos seem to be 2048 and

4092Hz. This seems to be an informal standard. It varies a few percent depending on ambient air density (humidity, pressure etc).

--
Nemo

Some greeting card sound modules use 'magnetic sounders' in the form of flattened mylar-cone loudspeakers (e.g.

and manage to work a (potentially annoying) number of times on a couple of button cells.

Chris