Date: Sun, 05 May 2002 21:54:20 -0700 (PDT)

Author: William Beaty

Subject: Re: ultrasound demos?

Post:

On Mon, 6 May 2002, Gordon Shepherd wrote:
> There must be some safety precautions for untrasonic devices. From
> intuition I've always thought that a device that would "mistify" water
> might do the same to my blood if I put my hand in the water near it. NO
> WHERE have I seen any safety practices. I figured you'd probably have
> some or know where they could be found. Thanks.

Good point. The instructions for humidifiers just say to keep your
fingers out. I say this on my own site:

WARNING: The ultrasound beam in the humidifier's internal water pool is
many watts of sound concentrated into a tiny spot. If you put your
fingers in the water, you can be burned by the intense sound.

If you place the bowl of a plastic spoon within a few centimeters of the
transducer for 20 seconds, a small soft spot appears in the plastic. I've
accidentally passed my fingers through the beam on several occasions with
no obvious results, but I've never dared hold them there. I imagine that
the region near the transducer is about as dangerous as a small, cool
candle flame.

On the other hand, I have an old piece of electronic junk in my
collection: and ultrasonic distruptor. Sounds like a Romulan weapon? :)
I just have the oscillator, not the transducer head. These are used in
biochem labs to chop things up at the scale of single cells. The question
remains: at what power level does cell disruption begin? But maybe it
requires much shorter wavelengths (the foggers run at somewhere slightly
above 1MHz.)


About "nebulization", I suspect that the mist is created because the
surface of the water breaks up into a pattern of soliton waves, then the
peaks of the tiny waves eject droplets. The underwater wavelength is
several mm, so whatever is producing the fog is far shorter than the
acoustic wavelength. I've never seen a good explanation for the process,
but it might resemble the spray that occurs in a fluid-filled Kundt's
tube, or when you rub your wet finger on the rim of a half-full wine
glass. If so, then it's a low-energy process and would affect the surface
of water far more than it would affect your fingers (i.e. you can stick
your fingers into the wine even when the vibrating glass is causing
spray.)

But don't forget that softened spot in the plastic spoon!



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