A mute strip is a felt strip that piano technicians use to mute off strings while tuning a piano.

By inserting the strip between each trichord (three-stringed note), the felt mutes the outside strings of each note.

This leaves the center string of each note free to vibrate.

The technician then tunes these center strings until they sound the best possible and then removes the mute strip, tuning the outside strings to the center string as they go.

But is this the most accurate way to tune? Is there a difference between the initial frequency of the center string and the final frequency of the complete note?

The answer depends on your required accuracy and precision.

If you require accuracy less than 1 cent (the limit for 100% in the Registered Piano Technicians Tuning Exam), then the answer is no.

This is because the final frequency of the complete note can be unchanged, move up, or move down by as much as 1.5 cents, and there is no way to predict it.

The following two articles and videos are studies that were done to prove this phenomenon exists.

This study was published in 1977 by Professor Gabriel Weinreich in The Journal of the Acoustical Society of America 62, 1474 (1977)

Coupled Piano Strings – by Prof. Gabriel Weinreich

The following article was written by myself and published in the March 2016 edition of the Piano Technicians Journal, Volume 59, Number 3.

Unison Drift – by Mark Cerisano, RPT, B.Sc.(Mech. Eng.), Dip.Ed.

This video shows examples of actual piano strings of clean unisons drifting as a mute is added and removed:

In the video above, I finish with a frustrated tone. That is because people still don’t believe the facts when presented. I was actually banned from a piano technician forum because they thought it was a joke.

I have since learned that this is a common response when people are faced with facts that disprove core beliefs. It is called the Semmelweis Effect after a 19th-century Hungarian doctor who discovered with data that doctors not washing their hands was killing thousands of women in childbirth.

Read the article here.

I have also since learned that the problem of people not accepting my research is not with them, but with me. One thing I can do to make this research more accepted is to collaborate with others in the field. If you are interested, Contact Me.

The following video describes what may be going on with the strings when they change pitch: