A Simple "SONAR" Demo

   For those with the interest in experimenting with sound and who have the equipment, but not the time, a very simple and relatively inexpensive demonstration of SONAR can be made in an evening. This experiment does require an oscilloscope and a pulse generator. In addition, an audio amplifier, and two inexpensive tweeters are required.  The tweeters can be purchased for under $10 at stores such as Radio Shack, and other discount electronics sources. A typical home stereo amplifier will work fine.

   Use one tweeter as an output and connect it to one channel of the audio amplifier.  Connect the other tweeter to the input of the oscilloscope and place it next to the first one. It may help if they are touching. Aim both tweeters at a blank wall a few feet away, or at the ceiling. Using the pulse generator, feed a series of pulses of about 10 millisecond duration into the audio amplifier at a rate of about 10 Hz. Adjust the volume slowly until ticks are heard. Set the trigger on the scope carefully to begin sweeping as a tick occurs, and select a sweep rate of 1 to 10 millisec/cm. The "tick" seen on the scope will probably not be the nicely shaped pulse input to the tweeter, but a complicated waveform containing resonance frequencies of the tweeter.

   Carefully adjust the sensitivity of the scope until the reflected pulse is seen in the trace. The trigger pulse may be off the top of the screen at this point. Slowly increase the volume if necessary. The reflected pulse can be picked out from other spurious signals by moving a large card back and forth in front of the tweeters a few feet away. The reflected pulse will move on the oscilloscope trace correspondingly. Using the scope to measure the transit time of the "tick" and knowing the distance one can determine the speed of sound. Alternatively, if one knows the speed of sound, the setup can be used to determine the distance to the card, which is the principle on which sonar is based.

   RefCD contains a special track designed for just this demonstration. It consists of a 10Hz pulse train of narrow, rapidly decaying pulses, and eliminates the need for a pulse generator. Any home CD player will thus work as the signal source, levaing the oscilloscope as the only piece of specialty equipment required for this experiment.

For more information, check out the RefCD page.