What does sound post mean in music?

The instrument

Sound post

French, âme, "soul"

The sound post is a small wooden dowel wedged inside a string instrument between the top and back, transmitting vibration and holding the body under string tension.

It stands under the treble foot of the bridge, held in place by friction alone. Moving it by a fraction of a millimetre audibly changes the instrument's response and balance, which is why the French call it the soul of the instrument and why adjusting it is a luthier's job rather than an owner's.

If a sound post falls, which can happen if the strings are all removed at once, the instrument should be taken to a luthier and not played, since the top is no longer supported against the bridge's downward pressure. This is the practical reason string players change strings one at a time.

Rosin: Rosin is the hardened tree resin applied to bow hair to create the friction that makes a string speak; without it the bow slides silently across the string.
Open string: An open string is a string played with no left-hand finger stopping it, sounding the pitch the string is tuned to.
Concert pitch: Concert pitch is the agreed tuning reference for an ensemble, standardly the A above middle C at 440 Hz, though many orchestras tune to 442 Hz.
Harmonics: Harmonics are pure, flute-like tones produced by touching a string lightly at a node instead of pressing it to the fingerboard, notated with a small circle above the note.

Every term in this glossary is taught in context somewhere in Arco's curricula, on the staff, on the keyboard, and on your own fingerboard at the same time.

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