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I'd like to measure with a cheap piezoelectric disk the vibration signals sent by whiteflies when resting on leaves. These disks are usually pretty big (I have not found any less 2cm diameter) and heavy compared to a leaf. I've not found any past white-fly studies using piezzo (see this other SE question)

How would you attach them to the leaf to allow the best connection in term of material (glue/wax) and of gluing surface (better to glue the whole piezzo disk even if it takes most of the leaf surface or just the center of the disk)?

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In general, I see three aspects you have to consider: sensitivity, bandwidth and impedance (one of your tags).

The sensitivity increases with the size of the sensor so the larger disk seems to be better than a smaller one.

Concerning bandwidth, my understanding is that most piezo disks have a resonant frequency in the 3-4 kHz range, so if the vibrations of whiteflies are below this resonant frequency, bandwidth is sufficient.

Addressing the impedance issue, you wanted to transfer all vibrations intensity of the fly to the piezo. Ideally, you wanted the fly sitting on the piezo disk, but I guess, that is not the real environment for the fly to communicate and therefore not an option. I would therefore get the piezo disk in good contact i.e., glue most of the piezo disk, to the leaf.

Caveat: gluing most of the disk to the leaf assumes that the fly is not adapting the vibrations to the local resonance of the leaf to maximize intensity.

Note: This is a real opportunity for lab experiments, testing different contact methods.

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Piezo-discs come in a wide size range c.a. 7 mm up to around 55 mm. Size/thickness affects the resonant frequency- check out Digikey for an idea of the breadth of these. It is also possible to buy them in other morphologies, e.g. rods, flexible films..etc

The problem you have is that the mass of the sensor on the leaf will affect the vibrations you record. For this reason, most insect studies utilize non-contact methods (an LDV) for plant vibrations.

For attachment, wax is always a good method, which is common for attaching vibrational sensors. My guess is as much of the disc in contact with the leaf as possible. But, I don't know if a piezo would be sensitive enough in this case to detect your signal.

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