Hammer, strings, bridge, soundboard: how a piano makes sound.

The sound chain

Four parts, in order.

A piano produces sound through a mechanical chain. Each step depends on the one before it:

1. The hammer strikes the string. When you press a key, the action mechanism throws a felt-covered hammer at one or more strings tuned to that note. The hammer bounces off immediately. The movement of the action affects hammer speed and the resulting sound; compare the actual response rather than using pressure held on a key as a volume control.
2. The string vibrates. The struck string vibrates at its tuned frequency. Bass strings are wound with copper wire to add mass; treble strings are plain steel. Most notes use two or three strings tuned to the same pitch, which is why you hear a wavering when they drift apart.
3. The bridge carries the vibration. The vibrating string presses against the bridge, a strip of dense wood (usually hard maple) glued to the soundboard. The bridge transfers the string’s vibration into the soundboard.
4. The soundboard radiates the vibration. This thin wooden panel moves air over a larger surface than the strings alone. Yamaha’s tonewood explanation describes the role of the soundboard and bridge, and the woods it uses in particular models. A maker’s material description does not establish the condition or response of a used instrument.

The frame

Cast iron holds everything together.

A piano’s cast iron frame (also called the plate) bears the tension of all the strings, which collectively exert tens of thousands of pounds of force. The frame does not produce the sound; it provides the rigid structure against which the strings are stretched. The soundboard sits behind or below the frame, depending on whether the instrument is an upright or a grand.

The ribs on the back of the soundboard are strips of wood glued across the grain that distribute vibration and add stiffness. They also hold the slight crown (curve) of the soundboard, which is part of how it projects sound. If a rib appears separated, record it and ask a qualified person to assess its significance.

What a buyer can observe

Eyes and ears, before the technician.

You do not need to be a technician to notice the following. The viewing checklist covers the full routine; here is what relates specifically to the soundboard and the sound chain:

• Cracks along the grain: on an upright, look at the back of the soundboard. On a grand, look down through the strings and underneath with a torch. Record any visible mark or gap and ask what has been assessed. A crack alone does not tell you whether the piano is usable; its length, location and whether it buzzes all matter.
• Ribs pulling away: the ribs should sit flat against the back of the soundboard. Record an apparent gap and ask what assessment has been made; do not attempt to adjust or reattach it.
• Bridge splits: look at the bridge where the strings cross it. Splits around the bridge pins affect how the string’s vibration reaches the soundboard and should be declared.
• Buzzes on certain notes: play every note slowly and listen for buzzing. A buzz on a single note may be a loose soundboard crack vibrating, a rib issue, or something resting on the strings. Record the note and what you hear, then ask a qualified person to assess the cause.

These observations tell you what to ask about. They do not tell you what the problem costs to fix, whether it is worth fixing, or what the piano’s long-term prognosis is. Those are questions for a qualified assessment of the specific instrument.

What needs a qualified assessment

Do not diagnose a piano from a photograph.

A crack in a photograph does not establish whether a piano is ruined or fine. Soundboard condition depends on factors that cannot be assessed remotely:

• Whether the crack is stable or still moving
• Whether the crown (the slight curve) of the soundboard is intact
• Whether the ribs are properly seated along their entire length
• Whether the crack affects the sound at playing volume or only at extremes
• What the repair would involve and whether it is economical for the instrument

If you see a crack, a loose rib, or a bridge split, ask the seller to explain what has been assessed and what a technician concluded. If you want an independent opinion, ask whether a technician can inspect the instrument before you commit. The repair vs replace guide covers how to think about the decision once you have the facts.