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In another question I posed, there were couple of "piezo sounder" parts suggested for the purpose of a compact but reasonably loud, alternative to electro-mechanical hooters.

Was trying to understand the various device characteristics that these "pieze sounders" have, and how I could use them to generate a sound-pattern (involving at-least 2 different frequencies), without sacrificing too much on the "loudness" aspect.

This is where I ran into "resonant frequency" of piezo elements, which is mentioned in all the data-sheets. So my question is, is this the frequency at which the device is loudest but it is still possible to generate tones at other frequencies, possibly with reduced loudness ? None of these data-sheets provide a graph of the loudness-vs-frequency characteristics for the device. So I am wondering if the piezo sounders with "resonant frequency" mentioned (but missing the graph), can produce sound at only one frequency ?

The two SE QnA's referred to:

  1. Piezo buzzer characteristics
  2. Multiple tones using piezo speaker
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You can use the piezo to produce sound at a wide frequency range, but indeed it will be the loudest at the resonance frequency. If you look at a few more datasheets you'll find that some manufacturers do publish SPL versus frequency data, like Murata, for instance.

enter image description here enter image description here

As you can see the spectrum can be very different between two piezo buzzers (the graphs are for types from the same PKM series), so it's worthwhile to compare many different types.

Further reading
Piezoelectric Sound Components, Murata catalog

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    \$\begingroup\$ Note that one of these graphs is a square wave and the other is a sine wave. I expect that the graphs would look much more similar if they were both the same type of input signal. \$\endgroup\$ – mjcopple Aug 31 '12 at 13:31
  • \$\begingroup\$ @mjcopple - You're right that they're not completely comparable, but there will still remain a huge difference. The PKM13 may produce 20, maybe 30 dB SPL at 100 Hz and its first harmonics, whereas for the PKM22 this is 60 dB. The higher harmonics don't have enough energy to cover that difference. \$\endgroup\$ – stevenvh Aug 31 '12 at 13:37
  • \$\begingroup\$ Thanks Steven answering and @mjcopple for pointing out the aspect of frequency response to signal type -- had entirely missed that aspect. \$\endgroup\$ – icarus74 Sep 1 '12 at 7:42
  • \$\begingroup\$ Just wondering, if I wish to create an audio pattern using 2-3 different tones, each produced at same "loudness" (sound pressure) level, I would have to adjust the Vrms applied to the piezo terminals, right ? \$\endgroup\$ – icarus74 Sep 1 '12 at 7:44
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If the goal is to produce an attention-getting sound using a small piezo, your best bet may be to modulate between two tones which have strong resonant peaks, using a varying modulation frequency. Especially when driven hard, piezo beepers produce a lot of distortion, so even though they have a limited ability to produce lower frequency sounds directly, they have a much greater ability to produce a wide range of frequencies as a consequence of distortion. Predicting exactly what frequencies will be produced at the greatest volume is apt to be difficult, but isn't really necessary if the only purpose of the noise is to get attention.

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