Key-Weighting and Tuning

My customer and piano teacher Jennifer C. from London Kensington asks:
How can I improve the tuning with key-weighting on my Yamaha C3?
Dear Jennifer,
This blog post is intended to suggest a possibly lesser known “aid” for tuning the entire piano. Not included in this discussion are such basics as temperament, tuning checks to use, methods of tuning and electronic instruments if one is used. This article will focus on “fine tuning” a piano that is already at the A440 pitch level and in fairly good, even tensions in all sections. While tuning certain pianos, particularly the older uprights, all of us have encountered certain strings, usually the longer tenor ones, that suddenly start to ring or sound while tuning due to sympathetic vibration. It was exactly this situation that prompted further study and the exploration of these “open” strings as an aid in aural tuning. It was further noted that the more in phase the octave being tuned, the louder and clearer the sing-through sound created by sympathetic vibration. This article assumes that the reader also understands at least some of the physical characteristics of vibrating piano strings, their modes, partials and inharmonicities. The principal of weighting keys is to deliberately raise the damper, freeing the string to sound sympathetically as related intervals are being tuned. The louder and clearer the sound of the weighted-open note, the better in tune the interval being tuned. This is the main principle of the system. Primary intervals used in key-weight tuning are the octave, fifth, octave fifth and the double octave fifth. We hope to stimulate enough reader interest for many to actually experiment with the use of key-weighted open sounding strings in tuning. One very discriminate college piano teacher once remarked, “I am more interested in the fifth being in tune than the simple octave.” Of course, what she was favouring was really the 6:3 octave tuning instead of using only the fundamentals in tuning the octave 2:1. A good example of “fifth” tuning is in the matching of the fundamental and the fifth in bass tuning which is fairly standard and well known by most tuners. In regard to the type of key weight used, anything of size and shape, heavy enough to hold any key fully depressed to open the dampers, is fine. It must also conform in size and shape to hold only one key down at a time, including the sharps, without falling off easily. It is also suggested that the weight be felt covered to eliminate the possibility of scratching any keytops.
After completing a fine-tuned temperament, extend accurately the first few notes up to the first B natural above (B4)*. The reason for tuning up these few notes is to have in tune the first fifths to weight keys open before starting the bass tuning. For example, the first note of the bass section below temperament is E3. Now place the weight on B4 and proceed to tune E3 from the temperament E4 in octave form as per usual. When B4 actually sounds the loudest, E3 is harmonically in tune both in single octave and octave-fifth relationship. Merely continue on down through F2, changing the weight each half step as you tune the octaves. Before E2 is tuned from E3, change the weight back again to B4 and tune this octave. This weights open the double octave fifth which is a bass tuning procedure. The rest of the bass is then tuned by weighting open the double octave fifth above and balancing these harmonics of the single octave with the double octave fifth.
In general there are two uses of key weights. One is to actually strike the key with the weight in hand, causing the string to vibrate just as the pianist plays the piano. Obviously this activates all of the string partials. The second technique is to depress the key silent/y with the weight without the hammer activating the string. In bass tuning, experimentation will soon determine the method you personally prefer. However, in treble tuning the more valuable technique is the silent weighting which will be further described below.
After tuning the bass, it generally is a good idea to recheck F#4 through B4, weighting below notes B2 through E3. In treble tuning the procedure seems to work better if the octave being tuned is struck with a staccato effect, listening for the actual harmonic sounding from the open strings. Also the second technique described above – placing the weight on keys silently – is preferred. By striking in a staccato manner either a fifth or a fourth below the key-weighted open string, only the related harmonics will sound, no fundamental or other partials. At least the most prominent partials will then ring, enabling a better balance between them and the octave being tuned. For example, weight B2 silently. Then recheck the F#3 – F#4 octave. If the weighted open B2 sympathetically reinforces volumewise the pitch levels, you have compromised the octave the best possible. The louder the better in tune. True, the octave can be somewhat out of phase and the open string will sound but not at the loudest level! To prove this, listen volumewise to the single octave both with and without the addition of the weighted key. Now merely continue tuning treble octaves, moving the weight one-half step each time as you change octaves, until you reach C6. In tuning C6, again drop the weight down an octave and weight the temperament F3. Thus the rest of the treble is tuned with the key weight in a “double octave” position. Test each octave both ways – first by trying to make the weighted note sound its loudest, and second by sounding the fifth below the open string. This will produce a sound for certain and then compare that harmonic with the octave you are tuning.
One can experiment with key-weighting even without a weight! The next time you tune a grand piano with a sustenuto system, merely depress the desired fifth below or above the octave you are tuning and listen to the results! Then you can make yourself a desirable key weight for pianos without a sustenuto. Key-weighting has one additional advantage in tuning. If as octaves are being tuned you cannot actually hear the desired harmonics, then tensions have changed as you were tuning. These corrections must be made as the errors are discovered before proceeding further! The piano tuner definitely is working with a change of tensions and is confronted in each tuning with Hooke’s law which states “the amount of compression is proportional to the amount of compressional force used”. I personally prefer not to tune pianos, either ascending or descending, by complete sections. That is, a “balance” of compressional forces is favoured in smaller amounts at a time. A portion of the bass is tuned first and then a similar portion of the treble, repeating this alternating of tensions during the entire tuning. Also, both use the unison to indicate needed corrections indicated by key-weighted open strings. In order to make corrections as tensions change (Hooke’s law), best results seem to happen by tuning octaves from “open” strings. That is, by tuning octaves from all strings in the unison rather than by just one with the others muted off. “Rolling” unisons indicate compression changes that need correction for accurate extremes in either treble or bass tuning. 
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