<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; = charset=windows-1252"> <META content="MSHTML 5.50.4134.600" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=#ffffff> <DIV>  Recently the following statement appeared in one of = the posts on soundboards: “An accelerometer transduces variations in pressure = to a voltage.”  This statement is somewhat misleading and has = bothered me for a couple of days now. There are a variety of sensors used in industry. = Those sensors commonly known as pressure sensors are transducers designed to = convert gas or liquid pressure into an electrical signal and this is some = removed from the function of an accelerometer. Force sensors are transducers designed to convert = force—and the word pressure could also be used here—into an electrical signal. = So are load cells but these are typically designed to detect static loads. You might = find a load cell in your bathroom scale or in the truck scale along side the = road. Force sensors are designed to measure and monitor changing levels of = compression and tension and impact forces—i.e., dynamic loads. The electric piano = I referred to some time ago used a number of force sensors mounted between a = more-or-less conventional bridge and a slightly compliant base. The vibrating strings = caused variations in the force applied to the force sensors which then created = an electrical signal that was processed and sent to amplifiers and = speakers. To approximate the tone of the acoustic piano the compliant base was = necessary to allow the bridge assembly to move in response to the motion of the = vibrating strings in approximately the same way it moves in the acoustical = piano.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p> And now we get to accelerometers. Accelerometers = are transducers or sensors that convert the acceleration aspect of motion = into an electrical signal for measurement, monitoring and control purposes. The = signal produced by the accelerometer is proportional (within the design limits = of the sensor) to the rate of acceleration. When used to detect acceleration in = a vibrating body or system the accelerometers signal can also be = electronically integrated to indicate both the velocity and displacement of the = vibrating object. In principle accelerometers are quite simple. They = consist of a base, a slice of piezoelectric material—usually quartz or an = artificially polarized ferroelectric ceramic—and a seismic mass. The crystal is = placed between the base and the seismic mass. When the assembly is set in = motion the seismic mass (wishing to remain stationary) induces a mechanical stress = in the crystal which causes it to generate an electrical charge across its pole = faces. This electrical charge is proportional to the applied force. Obviously = this charge is generated only when there is acceleration. When the = accelerometer has achieved some steady state—whether that be stationary or at some = steady velocity—there is no electrical charge generated. Accelerometers do not measure or indicate force or = pressure. Force or pressure applied to any part of an accelerometer will have no = effect on its output unless there is some acceleration involved. Or, I suppose, if = the force is great enough to cause physical damage to the accelerometer = housing in which case there will be some signal output but it may be quite = brief. When appropriately mounted to the top of a = vibrating body—such as the bridge of a piano as it is being played—an = accelerometer will produce an electrical signal proportional to the physical vibratory = motion of the bridge at the mounting point of the accelerometer. Since = accelerometers are unidirectional they indicate acceleration (or velocity or displacement) = in only one direction. Most tests I have done on the mobility of the = bridge/soundboard assembly have involved studying the vertical motion (in a grand) of the = bridge. Usually this involved mounting the accelerometer just beside the note in = question. To measure the for-and-aft motion of the bridge the = accelerometer is simply turned on its side so its axis is aligned in the direction you = want to investigate. (Or you can spend a small fortune and purchase a triaxial accelerometer. There are several ways to mount an accelerometer on = a test object. The least destructive is to simply glue it in place. The glue = can be as benign as beeswax or as intrusive as some epoxy or CA adhesive. In = trying to figure out the motion of the agraffe, a CA adhesive was used to mount an = accelerometer to the top of a prepared agraffe (the top had been milled = flat to provide a good mounting surface for the accelerometer). I’ve used = beeswax to temporarily bond accelerometers to the tops of bridges—removing the = strings from one adjacent unison usually provides enough room—and in various places = around the rim and structure. I’ve also used studs screwed into holes drilled = into the bridge between unisons such that the accelerometer ended up locked down = tight with its base just above the strings. This lowered the resonant = frequency of the accelerometer some but (according to the manufacturer) not enough to = worry about for the tests I had in mind. Magnetic bases are available from most accelerometer manufacturers but I’ve not used them. When looking at = vibrations in plates I’ve simply glued the accelerometer to the plate surface = using beeswax. Del</DIV> <DIV>Delwin D Fandrich Piano Designer & Builder Hoquiam, Washington  USA E.mail:  <A href="mailto:pianobuilders@olynet.com">pianobuilders@olynet.com</A> = Web Site:  <A href="http://www.pianobuilders.com">www.pianobuilders.com</A></D= IV></BODY></HTML>