By way of explanation, it would first be worth mentioning that when a piano string vibrates, it doesn't just create a sound at one pitch or frequency. In fact, the string goes through a complex set of vibrations that create sound at approximately 2, 3, 4, 5 and so on times the frequency of the base (fundamental) note. This is one of the things that gives an acoustic piano a rich tone (difficult to imitate on an electronic instrument). Bass notes have more of these "overtones" than notes in the treble.
However, the prominence of these partials or overtones in the sound can vary from piano to piano; those where they are not very prominent are normally said to be "mellow" whilst those where the overtones are very prominent are "bright"; this may become "harsh" if the extra tones are too strong.
To overcome the problem of harsh (or excessively muddy) tone, it's possible to carry out different treatments on the hammers of the piano - however before doing this the piano needs to be in a good state of regulation and very well-tuned, paying particular attention to the unisons (that is, for most notes on a piano, there are two or three strings to increase the volume; these strings should be perfectly in tune with each other). If unisons are poorly tuned, this can give an impression of poor tone even if the hammers are fine.
For this reason, new hammers are sometimes "pre-needled", that is, treated with a larger needle to release some of the cupped layers of felt closer to the centre.
Piano hammers can then be sanded, using a strip of sandpaper pulled around the nose with pressure from the forefinger, to give an even surface. This method can also be used on pianos with hammers that have deep grooves in the nose from years of use (as long as there is enough felt left, particularly on the hammers in the high treble). Heavily grooved hammers can result in a dull or metallic sound with poor tonal quality.
Sanding hammers to ensure clean contact with the strings (the rear hammer of the two is the one being sanded)
It's then possible to carry out careful sanding on the nose of the hammers with a finer abrasive paper to ensure that the contact between the hammer and the strings of the trichord is even and perfectly simultaneous when each key is pressed.
Once all this has been done, the person carrying out the voicing will listen and assess the tone of each note on the piano, checking in particular for the following:
- Is the tone harsh (too bright) or too muddy across the piano or on certain notes?
- Is the tone even across the piano, or are there any changes (particularly sudden changes) of tone or timbre?
- Does the piano have a good dynamic range or is the sound too aggressive, and is the tone harsh when played loudly or softly?
Carrying out needling of the hammers
The answers to these questions will inform the piano voicer as to the way in which the hammers need to be treated. This is quite detailed, but in general terms over-bright or harsh tone is dealt with by needling the hammers with voicing needles, which soften the surface of the felt. On the other hand, if the tone is muddy or dull (too mellow) then the surface of the hammer needs to be hardened a little. This can be done by sanding of the surface of the hammer, ironing with a specially shaped hammer iron, or treating the hammer with a special hardening solution.
Many thanks to Richard Schönhardt of Bechstein's, Chris Vesty who organized the day and to Millers of Cambridge for hosting such an excellent training session.
This informative video on YouTube shows a piano technician from Nevada sanding and voicing badly worn hammers on a Baldwin grand piano, and the difference that can be made when the instrument starts out with a poor or harsh tone.
