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Old 12-16-2020, 12:59 PM   #1
hazel
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Something odd I noticed today


My computer has a resonant hum, probably because it's a largely empty case (basically it consists of laptop electronics inside a tower). Occasionally the hum gets quite loud but mostly it's quiet. Probably the vibration comes originally from the disk drive.

Today, when I switched the computer on, I happened to have the washing machine running in the kitchen and I noticed to my surprise that the frequency of the sound it made was almost identical, perhaps very slightly sharper. Yet the computer's case is much smaller than the washing machine. I wonder what determines these things.
 
Old 12-16-2020, 01:31 PM   #2
GPGAgent
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resonance somewhat like the https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge
 
Old 12-16-2020, 01:32 PM   #3
Ser Olmy
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The resonance frequency is determined by the mass of the object vibrating and the forces that keep it still.

The sound from washing machine (I first thought you meant a dishwasher, but then I remembered you live in the UK) is the motor driving the drum. Apart from during the spin cycle, it doesn't really go all that fast. The hum stems from the windings in the motor (specifically, the stator) being turned on and off by the driver circuit at a controlled rate to obtain a certain rotational speed.

The computer case contains some parts that readily vibrate, such as the side panels. Not only are they light and flat, they are also almost perfectly parallel to one another, opening up the possibility for standing waves.

I really hate that humming sound, so I went to an auto shop and got some anti-vibration plates that are meant to be put inside door panels and on the floor underneath the carpet. They are soft enough to cut with a knife, and they have an adhesive side that sticks easily to metal.

So now I have a computer case that's almost totally silent, and also impractically heavy.
 
Old 12-16-2020, 02:09 PM   #4
rtmistler
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Yes, it's cool to discover or annoying if that's your perspective, in simple situations, like in your home and the fact that it seems benign. Along the lines of what GPGAgent noted, I've seen things like critical resonant speeds for ships or critical wind velocities related to tall buildings. Far more serious. The fact that they once calibrated a tall building by having some men run across the top and then leap/stop to cause deflection of the building is both amazing and scary, and hoping that they got paid a lot for that.
 
Old 12-16-2020, 02:39 PM   #5
ondoho
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I have two guitars of very different size (adult & children's model) which can be tuned to the same tune nevertheless.
 
Old 12-16-2020, 02:51 PM   #6
enorbet
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As a fun and possibly productive experiment, hazel, why not disconnect your hard drive from it's power connection and determine if that is actually the source of the hum? I suspect it may not be. Line noise including induced hum and it's harmonics based on the frequency of AC house current, 50Hz in the UK iirc, is common and exists at a wide variety of levels largely due in nasty cases to less than ideal Ground, or Earth. It is possible to improve Ground and the impedance between Hot and Ground, in many cases quite cheaply. The benefits can include far more than reducing or eliminating annoying sounds. Such remedial action could potentially save your appliances and your life.
 
Old 12-17-2020, 06:45 AM   #7
hazel
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@ondoho. I have no experience with guitars, but in cellos the half-size and three-quarter-size instruments are tuned with looser strings to give the same pitch at shorter length. As a result, they go out of tune rather quickly.

@enorbet. I don't look inside my machines. You ought to know that by now.
 
Old 12-17-2020, 12:56 PM   #8
ondoho
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You wondered about size, and what determines "these things".
I was just pointing out that size is not the only aspect that can determine the frequency of the sound a thing makes.
 
  


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