>For a school project, I want to investigate the use of ultrasonic waves to
>determine the state of various items of food (eg. fruit).

I want to use ultrasound to explode pumpkins 8^)

>The catch is, a 1Mhz pulse would not survive very far into such a
high-water
>content, porous material as an apple, let alone hitting the other end and
>reflecting back. Apparently, 50kHz is more reasonable. Why not build a
50kHz
>transducer then? The second catch is, a piece of PZT having a natural
>resonate frequency of 50kHz would have to be 50mm thick(that's two whole
>inches!), with a diamter greater than that still. OK then, so what?

Why not use a cheap piezo disk vibrating in the shear mode? If you look up
acoustics on the web, their are pages that show the drum-head or disk mode.
The best I found is Maple's animations of the Bessel functions. If you buzz
a piezo disk, you will find multiple resonances of these various modes.
Scratch or electrochemicaly etch off the disk's plating to drive these mode
more effectively, and you should be able to get up to a watt out of the
disk. You can find a powerfull mode up around 30 KHz on a 2 KHz piezo disk.

>3.    Save the freebie for later, try a different alternative. You've all
>seen piezo buzzers and their u-sonic siblings. They work on a totally
>different principle to the piston transducer discussed above. The piezo
>material is usually very thin (~0.3mm). It is mounted on a brass diaphragm.
>As a voltage is applied across the material, the disc thickens as expected,
>but this isn't important. The key is, it also expands/contracts sideways
>(the radius changes). This causes the diaphragm to flex in and out. The
>actual change in the thickness of the piezo sheet is too small to be
>measured, but the diaphragm flex enough to blast your ears out.

If you use a laser you can well see it. Or you can sprinkle a dust and drive
it with a few watts to image the mode.

> My guess is that pressing such a TX head against any
>material (even via a coupling liquid) would quench the delicate flexing
>osillations, because solid matter compresses so little compared with air.

Solid is much better than air- you will have a much better acoustic match.

Are you going to try to measure the complex acoustic impedance of food, or
just attenuation? You might need to examine over a broad bandwidth. Kynar
film would be great for that, I doubt much power would be required.

I strongly recommend a kilo-joule shock to disintegrate the vegitable under
test, then spatter-pattern analysis 8^)

****************************************************************************
Freedom is pursuing your carrot, not running from somebody's stick
Does society make you enthusiastic, or fearful?

The mob rules only what its members achieve.
****************************************************************************

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu