Re: What is the best way to stop ultrasonics or soundwaves from traveling?

I have good news and bad news.  Good first:  I can tell you how to measure
the frequency and amplitude of ultrasonics easily.  Bad:  I don't know what
is a good thing to stop ultrasonic sound waves.  

To measure ultrasonic sound waves you need only a piezo electric transducer
and an oscilloscope.  Remember that sound is typically measured in dB which
is a logarythmic scale.  An oscilloscope measures voltages linearly.  The
transducer efficiently converts the sound waves into electrical signals. 
The oscilloscope can be used to measure the amplitude and frequency of the
incoming signals.  Perhaps easier would be a spectrum analyzer.  A typical
unit automatically measures dB(m) and frequency of the incoming signal with
minimal fussing about. 

As for the stopping of ultrasonic waves, you can either absorb or reflect
sound waves.  Absorbing sound waves is typically best done with acoustic
tiling, anechoic chamber foam, carpet, etc.  Really, anything that appears
highly dispersive to the incoming wave.  Reflecting is something different,
though.  Waves of all kinds have characteristic velocities in different
media.  Sound travels at 330 m/s in air, and roughly 1480 m/s in water. 
The reflection coefficient R = (1 - Z_rel) / (1 + Z_rel).  Z_rel is the
ratio of relative impedances of the media, which is proportional to the
density and propagation velocity of the media, i.e. Z_rel ~ p1c1/p2c2.  For
example, water is about 800 times more dense than air and sound travels
about 5 times faster in water than in air.  So the relative impedance is
about 1/4000.  So, the reflection coefficient is 99.95%.  This means that
water is a really good reflector of sound.  This is why you can't hear
people talking to you under water.  Sound doesn't travel from air to water
well.  On the other extreme, there is a rubber referred to as "rho-c
rubber" whose impedance is the same as that of sea water.  This means that
a sound transducer can transmit sound impulses directly to the water
with little loss through the rubber, and still be insulated from the water. 

(After D. Towne, "Wave Phenomena," 1967)

Hope this helps
-Fred