Wednesday, May 23, 2012

Adjusting a Tuning Fork

I wanted something to help demonstrate the accuracy of my HotPaw music tuner apps for iPhone, iPad, and even Macs.  So I ordered a Concert A 440 tuning fork from Amazon.  It arrived in the mail a couple days later.  I quickly tried it out.  Big Problem!  The fork measured 9 cents sharp or 442 Hz.  Thinking something might be wrong with my tuners, I checked the tuning fork against 5 competing tuner apps.  But they all reported the pitch to be around the same amount sharp.  Was my iPhone broken?  To rule that out, I again checked the tuning fork with strobe tuner apps on a different iPhone and also an iPad.  Same result.

This tuning fork was out-of-tune.

But I found a quick-and-dirty fix.

As an online education forum suggested, a interesting school science experiment might be to see if adding weights and changing their position on a tuning fork will change the frequency of the fork. So I played science student, and tried the experiment.

Two big rubber bands on the tines clearly lowered the frequency... by way too much. Next I tried a couple strips of masking tape wrapped around the very end of the each tuning fork tine. That also lowered the frequency, by only a bit too much, to 20 cents flat. So I removed those pieces of tape and carefully measured them. Then I cut some tape strips about 9/29ths as long (actually a few mm longer). Wrapped the new lengths of tape on the fork tines, and the fork rang 3 cents flat. A little more fine trimming of the length of the tape with some sharp scissors, and I got the fork to measure within +-1 cent of 440 Hz, as measured against several calibrated iPhone tuners (both dial and strobe). The fork also now sounds good when played simultaneously with an mp3 of a 440 Hz tone to check for beats.

The tone probably isn't as pure, and tape will eventually age and wear, but this quick fix should work well enough until my next trip to a good music store for a better quality tuning fork.

1 comment:

  1. I'm wondering if the methods you used to measure the frequency of that tuning fork depended on the speed of sound in air. The latter depends on temperature, barometric pressure and humidity. I don't really know by how much.

    But many years ago I bumped into the first chair trombonist with the Chicago symphony, who happened to be practicing in a studio near my trumpet lesson. Somehow the subject came up, and he was delighted to have found an explanation for the slightly differing pitches he experienced while practicing under different weather conditions.