Calibration Using NIST

The National Institute of Standards and Technology provides precise audio tones that can be used to calibrate electronic tuning devices.  These tones are available over the phone by calling NIST at (303)499-7111 in Colorado.  They are also available on shortwave radio.  Since most piano techs do not have access to shortwave radios, we focus on the use of the telephone service.

NIST says this is a very popular number.  They get over 2,000,000 calls per year.  When you call this number, you will hear the same thing that is broadcast on shortwave radio station WWV.  If you can get your computer or other tuning device to pick up the sound from the telephone receiver, then you can check your calibration.  Try holding the telephone right up to the microphone.  Verify the pickup is working by getting the dial tone to register.  You will want to figure out how to place the phone on the microphone of your tuning device so that it will stay put without your hanging on to it and causing noises that will interfere with the pickup of the tones.

The tones from NIST that are useful for calibration are the 500 Hz and 600 Hz tones.  Here are the musical notes that are equal to these tones:

500 Hz = C5 offset -78.69 cents
600 Hz = D5 offset +36.95 cents

These offsets are already provided for TuneLab users in a tuning file called NIST-303-499-7111-C5-D5 which comes packaged with TuneLab.  If your tuning device has the ability to store offsets for each note, then store these two offsets in a page in memory or in a tuning file.  Make sure that you start with a no-stretch tuning.  If you are checking an Accu-Tuner, then you may want to use the notes C3 and D3 instead because the Accu-Tuner uses the 4th partial for these notes, which works out just right.

In order to be sure of hearing a 500 Hz or 600 Hz tone when you call, check the following WWV broadcast schedule, which is repeated every hour.  When a tone is present, it is present for the first 45 seconds of the minute and is silent for the last 15 seconds.  As you can see, the best time to call is between 19 and 24 minutes after the hour, or between 30 and 39 minutes after the hour.

Once you have the tuning device all set up to look for 500 Hz and 600 Hz on C and D, then call the Colorado number at a time when you can be sure of hearing the desired tones.  You will only have three minutes maximum because they automatically disconnect you after three minutes.  With the tone sounding  in the microphone of the tuning device, select either the C or D depending on which tone is playing.  Once you determine which tone is playing, note the rotation of the lights in the Accu-Tuner or the spinner in the CyberTuner or the movement of the squares in TuneLab.  Any movement in these displays indicates the error in calibration.  If you can adjust the offset to make the display stand still, then that offset tells you exactly how far off your tuning device is.

If you do have a shortwave radio, try picking up WWV on 2.5, 5, 10, 15, or 20 MHz and save yourself the phone call.  If you have trouble receiving during the day, try listening at night when shortwave reception is usually better.

For more information on WWV/WWVH broadcasts and NIST standard time and frequency services, see:

WWV and WWVH broadcast schedule
NIST Time and Frequency Services

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