Sustain and how to achieve it
This originally appeared as Workshop News for July 2006
Too many historically informed clavichords have a blunt, unmusical sound without sustain. This is what you risk getting if you make mechanical copies of old instruments, relying perhaps too enthusiastically on evidence of old string practices that suggest the old clavichord makers used rather thick gauges of wire. But a note should have a beginning, middle and end; that is musical, whereas a mere plink is not. Descriptions of the sound of old clavichords – I am thinking in particular of two published accounts of C. P. E. Bachs famous Silbermann – often refer to the sustaining power of the instrument.
What makes for sustain? A combination of factors, I should say. First, it is necessary to avoid wasting any of the energy in the string through internal damping and frictional losses. This means choosing the best spruce for the soundboard, and making the instrument well, with close joints, and probably using traditional animal glue for all the joints that matter. It probably helps to use leather rather than cloth washers under the keylevers. In a clavichord, there is little enough energy in the string to start with – you don't want any more than is absolutely necessary to be wasted as heat.
Having conserved energy, you can either let it all go out at once with a blunt but short-sustained sound; or you can sacrifice volume slightly in favour of sustain. You must control the conditions at either end of the sounding length: a heavier bridge will increase the sustain, and so will a more massive tangent, or masses in the form of lead weights added to the keylever near to the tangent. Then there is the balance between the terminations – tangent and bridge – and the wires themselves: thinner wires will sustain longer, but produce less sound. A heavier bridge may demand a heavier stringing. Then there is the voicing: the tangent must be adjusted so that there is a very small interval between the impacts on the two strings. The ideal size of this interval varies from instrument to instrument: in some large unfretted instruments it needs to be virtually nil, whereas in some small fretted ones there may be a perceptible gap between the two impacts.
That is not all. Sympathetic vibrations play an important part – even though the clavichord is not normally equipped with sympathetic strings like a viola damore. The random resonance of the undamped wires that lie between the bridge and the tuning pins surrounds the sound with reverb, producing an illusion of sustain, though in this case it is not under the control of the player. But the sounding strings themselves – those that arent being played – also help the sound; even though they are damped by the listing cloth, they do seem to respond sympathetically during the first few microseconds of each note, borrowing energy, so to speak, and paying it back after a very short delay.
And then there is the listing, with or without an over-rail, which influences the way the players finger adjusts the phase relationship between the two notes, enabling him or her to control, in the best set-up instruments, the way in which energy is fed into the soundboard, so that the sound can be prolonged by means of extra pressure – a trick that C. P. E. Bach called Tragung der Töne; its even possible sometimes to make the sound swell.
Oh, and of course, there's the tuning of the unisons; if they are too precise, the two wires can settle in opposite phases, producing a very dull and short-sustained sound.
ALL these things must be correct if a clavichord is to speak as it should.
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