From: Marty Grogan (ges) To: sowacs@aqua.ccwr.ac.za Date: Sunday, May 12, 1996 12:01 am Subject: re: TDR and Acoustics I believe that the mathmetical description of wave mechanics wasdeveloped by a fellow named Schroedinger several years back. Basically, allof the natural properties required for wave-based appliances to operatecome from tradeoffs between two interlocked energy storage mechanisms. Apendulum trades off potential and kenetic energy, acoustic waves trade offtransverse and longitudinal compression and expansion and electromagneticwaves trade off storage between magnetic and electric fields. TDR depends on discontinuities in the energy storage mechanismswhich are available. Combined with knowledge of the propagation velocitiesof the waves in the medium being used, these discontinuities can belocated by observing the change in energy levels at fixed points in themedia. Energy which does not become dissapated returns to its source. The probetips of a TDR appliance present a discontinuity in the wave propagation pathof the energy initiated at the signal source. They can be thought of assimilar to railroad tracks. At the end of the tracks, the train does notcrash but merely reverses course. An added convenience in TDR is that thetrain can only run at a single speed--regardless of its direction. Mothernature has some incredible springs. The presence of water affects the speedof the train. The accuracy of TDR measurements depends on precise measurement oftime and precise calibration with the relative volumetric content of wateraround the probe. Freedom from interfering signals and capacitivedisturbances may also be a factor, i.e., if you are measuring in a shallow metaltray, TDR may not work well. The TDR probes tend to react in ways similarto "rabbit ear" antennae used for television reception. In addition, manysignalling issues must be resolved such as energy levels, pulse widths andshapes, repetition rates, oscillator stability, etc. I hope that this has been helpful. The acoustic distance measurement devices which you requestedinformation about can be purchased in the building department of most hardwarestores for less than 20.00US. Poloroid at one time sold an acousticdistance measuring kit for do-it-yourself projects. There are many deviceswhich use acoustic signalling methods in the medical field as well. M.G.