Why Does That Sitar Keep Humming? There Are Strings Nobody Is Playing
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Some instruments carry extra strings that are never touched and vibrate on their own whenever a played note matches them, producing a shimmer that follows the melody around.
How they work
A string vibrates readily at its own natural frequency and at multiples of it. Sound reaching a string at one of those frequencies sets it moving without anything touching it, transferring energy through the air and through the instrument body. An instrument fitted with a set of such strings, tuned to the notes of the scale in use and running beneath or behind the played strings, therefore responds continuously as a player works through a melody, with different ones answering different notes and continuing to sound after the played note has stopped.
Where they are found
The idea was arrived at repeatedly across a wide area:
- •The sitar, with a dozen or so beneath the frets
- •The sarangi, which may carry more than thirty
- •The Norwegian hardanger fiddle, with four or five under the fingerboard
- •The viola d'amore, a baroque instrument with a set beneath
- •Various Turkish and Persian instruments
- •Some pianos, which use undamped upper strings for the same effect
What they do to the sound
The audible result is distinctive and hard to imitate any other way. Notes acquire a halo that starts fractionally after the note itself, since energy takes time to transfer, which gives the sound a sense of depth and of reverberation coming from the instrument rather than from the room. Sustain is lengthened because the extra strings keep sounding after the played one is stopped. And because they are tuned to a particular scale, the halo reinforces that scale continuously, which is musically significant in traditions where the identity of a mode is central.
Getting the effect without the strings
Instrument makers who wanted some of the same character without the tuning burden found several partial substitutes. A double top or a second resonating chamber lets more of the body vibrate in response to the played note. Undamped portions of a piano string beyond the bridge contribute a faint halo, and some makers tune those lengths deliberately. Releasing the dampers on a piano while playing produces exactly this effect using the instrument's own strings, which is what the right pedal does and why it changes the sound far more than simply sustaining notes would.
The costs of having them
Players pay for the effect in ways an audience never sees. Tuning takes far longer, since thirty extra strings must each be set and they interact with the tension of the rest of the instrument. Changing key or mode may mean retuning the whole set, which is why such instruments are strongly associated with traditions that stay within one mode for a long performance. The strings need damping for certain effects, and unwanted ringing in recording studios is a known nuisance. And the added tension requires a heavier instrument body, which changes its tone in its own right.
The takeaway
Extra strings tuned to the notes of a scale vibrate on their own whenever a played note matches their frequency, adding a halo that starts fractionally late and outlasts the played note. The sitar, sarangi, hardanger fiddle and viola d'amore all use them. The cost is lengthy tuning and difficulty changing mode, which ties them to traditions that stay in one scale.