Symptoms
- Some notes sound harsh, tinny or much louder than their neighbors
- Deep grooves are visible in the hammer felt where it hits the strings
- A note sounds twice (double strike) or stutters when played softly
- A hammer wobbles, clicks, or rests at a different height from the others
- Holes or bare patches in the felt
Likely causes
Worn and grooved felt
Years of playing compress the felt and cut grooves where it meets the strings. The tone becomes bright and uneven, and the hammer may hit adjacent strings poorly.
Hardened felt
Felt hardens with age and use, which makes the tone harsh and loud even at soft dynamics.
Loose hammer heads or flanges
Glue joints dry out and flange screws loosen, especially with dry air. A loose hammer clicks, wobbles or strikes at an angle.
Action out of regulation
If let-off, drop and checking are not set correctly, hammers can block against the strings, bounce and strike twice, or fail to repeat quickly.
Moth or mouse damage
Moth larvae eat wool felt, leaving holes in the hammers and other felts, which changes tone and noise.
What you can safely check yourself
- 1Play every key slowly at a soft and a medium volume and mark notes that sound different, double-strike or click.
- 2Open the lid (and the front panel on an upright) and look at the hammers for grooves, holes, bare wood or one hammer at an odd angle.
- 3Look for moth larvae, small white casings or droppings around the felts.
- 4Gently wiggle a suspect hammer shank sideways with a finger; noticeable play compared with its neighbors means a loose flange screw or joint.
- 5Note how old the piano is and when the hammers were last serviced, if you know.
What not to do
- Don't file, sand or needle the hammers yourself; it is easy to ruin their shape and voicing permanently.
- Don't spray WD-40, oil or silicone anywhere in the action — it soaks into felts and wooden centers and causes sluggish keys.
- Don't tighten flange screws with random screwdrivers; over-tightening can split the wood or bind the center pin.
Quick fixes vs. call a pro
You can usually handle it
- Mark the problem notes
- Visually inspect for grooves and holes
- Check for signs of moths
- Keep the room humidity stable
Call a technician
- Reshape (file) grooved hammers
- Voice hammers for even tone
- Tighten flanges and reglue loose heads
- Regulate the action or replace hammers
What a technician will do
A technician can reshape worn hammers by filing off the grooved layer, then voice them with needles or hardener so the tone is even across the keyboard. Loose heads are reglued and flange screws tightened, and double-striking is fixed by regulating let-off and checking. Reshaping and voicing typically take half a day to a day; badly worn or moth-eaten hammers may need a full replacement set ordered for your piano.
A piano holds about 18–20 tonnes of string tension. If a fix means opening the action, turning tuning pins or working near the strings, leave it to a technician.