The Piano Plate

 

 

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by Don Kulak

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 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 its 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 are 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.

In the very upper notes of the piano, there are relatively few partials per note and consequently the sound is not as full and rich as the lower notes. In these cases, a little added sizzle is often desired in order to enhance this section of the keyboard; to make it sparkle and standout. One must bear in mind though that this agraffe sizzle is added at the expense of volume and sustain, as there is less energy being reflected back into the soundboard. If the tone still sounds slightly fuzzy, without the clarity and volume it should have, there still may be inadequate termination.  As the plate is made of relatively soft cast iron, it is not uncommon for the steel strings, especially the thinner strings in the upper treble/V-bar area, to cut groves into the iron plate, which makes for a poor contact/termination point.  This can be often be remedied by pushing the strings back and forth over the trouble area of the V-bar, in effect filing down the high spots until a smoother surface remains.  If thesis not effective, one must resort to having the plate filed down, or attaching a metal strip to the V-bar in the worn area.  If it comes to this, the plate must usually be lifted out of the piano, which is to say a rebuilding job is in order.

"Tone Holes"

The round holes in the plate, just behind the back-end of the strings, are referred to as tone holes by the manufacturers. This name may be a bit misleading, as the primary purpose of these holes is to even out the cooling process in the foundry.  The thicker parts of the plate tend to cool off slower than the thin sections, producing strains within the plate, and thus creating an inherent weakness.  The holes are placed to add more surface area to the thick sections, enabling the entire plate to cool off evenly.

Defects

Nobody is perfect, and plate manufacturers are entitled to their mistakes as well.  Imperfections probably occur less frequently here than in any other part of the piano, but nevertheless they still do occur.

An improper mix of cast iron, steel, and chemicals at the foundry sometimes produces a plate with a resonant frequency of its own.  This can be set-off when certain notes are struck, creating an unwanted "pedal point," of the metallic variety.  Like acoustical feedback in electric amplification, the sound is reinforced until it stands out in abnormal proportions.

Sometimes the V-bar is not set perfectly straight.  As this is a termination point for the strings, any deviation up or down from the proper angle, would effectively change the speaking length of the string, and would necessitate change in string tension in order to produce the proper note, as frequency is inversely proportional to speaking length, and tension is directly proportional to frequency.  This however, would then create another problem of unequal tension in this area of this piano.  The only true remedy would be to put the V-bar in its proper place.

©2011 Don Kulak All rights reserved


 


 
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