>I'm cofude also. In dive training, and in all the related pubs on the
>subject I could find, it's commonly stated that sound travels much more
>efficiently and for longer distances in H2O...Why do you say that the
>effective distance would be reduced in a sonar ap? Also, the sonar buoys
>we used to make had small amounts of drive for the transmitters and not
>much amp gain for the receivers. They recorded sounds from many miles
>away..

Sonar buoys and sonobuoys are different afaik. I do not know what you
worked with. Recording (presumably wideband) sounds from miles away and
pinging miles away is different. Noises coming from a ship or marine
animal are not 'quiet'. Probably several watts continous at least, even
for small ones. A sonar will send out a pulse and receive it, and
determine distance, bearing, depth etc from the echo return time. A
barrier type device or a communications device (or listening device) uses
only the direct path. In a direct path the inverse square law applies for
distance, in a sonar application the radar equation applies instead.

The range of a sonar/radar device relates to the straight path
communication distance using the same technology as in Dsonar = R *
sqrt(Dstraight) where R is usually less than 1 and is the reflectance of
the target (it is >1 when the target functions as a concave reflector or
catadiopter - catadicustor ?).  This equation reflects the ratio between
the inverse square law and the radar equation. My figures (20 meters
straight, 3-4 meters sonar) were for still air using Murata etc 40kHz
transducers with 12Vpkpk square wave drive and 'average' amplifiers etc
(i.e. no special filters, no special processing, no noise in band from
external sources). In water you could probably reach farther than that
with the same technology (meaning the same transducers!), maybe 2-3 times
farther. 2-3 times farther means 2^2-3^2 less path attenuation for a sonar
application. Much better range could be achieved using an acoustical lens
or reflector optics, both in air and in water.

The transducers we are talking about here were designed for us remote
control in house environment and for alarms in closed spaces like cars,
and closed rooms. Their power specs reflect this. They were not designed
for use as sonars. Notice the popular Polaroid acoustical sonar modules
use a much larger transducer (more gain) and more power. I also think that
the grid structure in its aperture works as a lens and the transducer
diameter relates in some clever way to the operating wavelength in air.

There is a man in Portugal or Spain I think who built a ultrasound imaging
device using a single sensor that is scanned using motors. There is a
webpage with images. I do not have the url.

Peter

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