Piano Hammer Strikes Twice


Sometimes you play a piano and it sounds and feels odd. It’s not the tuning, (though that may be a separate issue) but it just doesn’t feel right. It may in fact be that the piano hammer is striking the string twice in quick succession. Piano technicians usually refer to this effect as bubbling. Often times the cause of the note bubbling is insufficient key travel. When the key is depressed by your finger, it doesn’t travel far enough before it hits the keybed. Many times this happens in the middle of the piano. As pianos settle with age, the middle of the keyboard can sag making the appearance of a smiley face. The keys in the middle are lower than the keys in the bass and treble extremes. A piano technician will shore up these middle keys, making a straight line from the top to the bottom of the keyboard. In the instance of this picture, a Zimmerman piano from Europe needed the middle keys to be raised, allowing for greater key travel. I brought these keys up with paper punchings, which are placed under the balance rail felt. Now that the key travel is corrected, the hammers no longer bubble or double hit and the piano has been restored to the manufactures’ specifications.

4 replies on “Piano Hammer Strikes Twice”

Hi Joshua

I’m a technician with lots still to learn. Thanks for the info on bubbling hammers. I have 3 Yamaha uprights in a senior care facility, all have serial numbers that are Prefaced with the letter P.
As you suggested, the main bubbling in each of these pianos is in the mid range.
I’m curious to know if key dip, lost motion, let off and hammer-blow distance are also factors the can cause bubbling.
If so, is there a typical order to follow when making these adjustment?



Absolutely! All those factors can contribute to bubbling. Usually start with hammer blow distance, (Though usually only if hammers are quite worn), loss motion, levelling key height (including squaring them if necessary), key dip, let off, loss motion brush up. For further info, check out Arthur Reblitz’s book Piano Servicing, Tuning, and Rebuilding


Thank you for generously sharing this professional knowledge.
Following your instruction, I have successfully solved the annoying bubbling problem.
I hope more pianists will know it.
Carl 3.18.13


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