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The Piano Plate
The iron plate is the backbone of the piano, bearing every ounce of the massive tension produced by 180 steel strings stretched to their limits, with each having an average tension of 160 lbs.
Indeed the plate must be strong, but it must perform acoustical functions (or non-functions) as well. This is an area of conflicting opinions, as one school of thought maintains that a piano plate performs optimally by remaining acoustically neutral, that is by neither absorbing vibrations from, nor transmitting vibrations to the soundboard.
Others disagree, saying that the resonance of a well made iron plate adds to the tone color of the instrument, and that care must be taken when mixing the alloys in order to achieve strength and proper resonance. Like testing a piece of soundboard wood by tapping it and listening for a certain well defined pitch, finished plates can be tried in much the same way. A clear and sustained ring would indicate a well-made plate. This however is not to be confused with a resonant frequency, which stands out in abnormal proportions. The plate's characteristic ring, or mode of vibration, should be composed of a smooth mixture of the frequency spectrum, which complements the sound already produced by the strings and belly.
It is the author's assumption that the latter explanation is the most valid. There are massive amounts of vibrations in and around the piano, so much, that the whole instrument is in a state of agitation during a performance - from the strings to the soundboard, to the rim, etc., etc. The piano sounds like the sum of it's parts, all moving as an interrelated whole. The ramifications of all of these modes of vibrations of all these different types of materials sounding together is virtually incalculable. The sound can not be so easily defined, or duplicated, as there are just too many variables involved. Subtle as they may be, the plate acoustics are just one more factor added to the multitude of variables comprising piano tone.
In any case, the acoustical function of the plate is to completely terminate the vibrating strings, while not adding any "unwanted" sound of its own.
The material best suited for this purpose is a mixture predominately
composed of cast iron, steel, and certain chemicals. As metals go, cast iron is
relatively soft, and dense, which reduces its ability to transmit sound. It also
contains no resonant frequencies which could be set-off by any of the keyboard
frequencies.
The plate terminates the vibrating strings at the agraffes in the midrange and lower treble, and at the V-bar in the upper treble. Once again, there should be minimal energy loss in these places. This is accomplished by the plate reflecting back as much of the vibratory motion of the strings as possible, i.e. no acoustical leakage. Like a ripple in a pool, once these small waves reach the wall, they are reflected back, without losing much strength. If the wall had holes in it, energy would be lost into these holes, and the reflected wave would be very weak, at best. The trick is to keep as much energy in the oscillator (vibrating strings) as possible. Translated, this means more volume and sustain. Excessive energy leakage in either of these places not only reduces volume and sustain, but adds other noises as well. This leakage is usually heard as a sizzling, and/or fuzzy type of sound. One way to check if the agraffe or V-bar is the culprit, is to place a finger on the string, on the keyboard side of the agraffe. If the sizzle stops, it is invariably due to inadequate draft angle, or the angle at which the string enters the agraffe. A good rule of thumb is it say this angle should be no less than 24 degrees. This can be easily measured by taking another string of the same angle and holding it next to the offending agraffe.
For more on piano plates, and the full book version of Vital Sound, click here.
©2008 Don Kulak All rights reserved
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