Electrical hum

If you hear electrical equipment humming, it’s probably at a pitch of about 60 Hz since that’s the frequency of AC power, at least in North America. In Europe and most of Asia it’s a little lower at 50 Hz. Here’s an audio clip in a couple formats: wav, mp3.

The screen shot above comes from a tuner app taken when I was around some electrical equipment. The pitch sometimes registered at A# and sometimes as B, and for good reason. In a previous post I derived the formula for converting frequencies to musical pitches:

h = 12 log(P / C) / log 2.

Here C is the pitch of middle C, 261.626 Hz, P is the frequency of your tone, and h is the number of half steps your tone is above middle C. When we stick P = 60 Hz into this formula, we get h = -25.49, so our electrical hum is half way between 25 and 26 half-steps below middle C. So that’s between a A# and a B two octaves below middle C.

For 50 Hz hum, h = -28.65. That would be between a G and a G#, a little closer to G.

Update: So why would the frequency of the sound match the frequency of the electricity? The magnetic fields generated by the current would push and pull parts, driving mechanical vibrations at the same frequency.

15 thoughts on “Electrical hum

  1. Sounds a bit high – but i’m in aussieland so its 50Hz & you indicated such…

  2. I cannot stand the 60Hz hum. I wonder if it would be any better if they ran at 65.4 Hz (24 steps below and sitting very close to C)?

  3. If a musician had been part of the committee to determine the standard frequency, maybe that would have happened. :)

  4. About the choice of frequency, it could be somewhat arbitrary, but once chosen it needs to be kept accurate. Electrically powered clocks (and much other electronic equipment) uses 60Hz from the power line as a time reference. Every 60 beats (in the US), a mechanical electric clock counts one second. Appliances and tools with electric motors usually depend on it too, but with more tolerance. (This is true for “AC only” motors, not true for “AC/DC” electric motors.)

    Power companies take pains to ensure 60Hz is stable.

    Data centers use 400Hz, but that is off-grid, local to the building.

    More juicy info can be found on Wikipedia …

    http://en.wikipedia.org/wiki/Utility_frequency

  5. Last time I looked in any detail, “60Hz Hum” from mechanical effects, failing filter caps in the power supply, or insufficient thermal mass of the cathode in your tubes had a hefty dose of 120Hz (similarly for 50->100).

    Of course, musically 120/100 would just be the same note an octave higher, but still…

  6. MikeA: Yes, there are strong overtones in the signal. My tuner usually described the pitch as 60Hz, but sometimes 120 Hz or higher.

  7. That was my point. Even if you account for the intentional rectifiers (whose 120Hz _should_ be attentuated by adequate filtering in your power supply), the world is full of unintentional rectifiers and other non-linear effects. Look for stories of people “hearing radio through their teeth”.

  8. John,
    Is this the same thing as “ground hum”? My sub for my home theater is not grounded for this very reason, as you get a slight hum from it due to the ground plug.

  9. It’s called gStrings and I have no complaints with it. It’s amazing that one device now can replace a metronome, a tuner, a video camera, …

  10. So, a classically trained musician electrician (maybe not so rare an occupation among classically trained musicians) should be able to expertly diagnose the power quality issues: “Every time I hear that clarinet timbre, I know the building is full of idle computers putting third harmonics on the line”.

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