The 20Hz theory

[henene4]

Not sure if the US has a simmilar phenomenon, but here in the EU, I always recognized something about washers at higher spin speeds:

At first, they seem silent. They speed up, past 800rpm, past 1000rpm.
And then, suddenly, at about 1200rpm, the noise level changes dramaticly, usually for the louder.
And that has been true for any washer I washed with that spun at or above 1200rpm.

I always wondered why around that speed and a few days ago, it suddenly hit me:

1200rpm -> 20rps

At 1200rpm, the tub assembly completes 20 entire swing-movements per second, vibrating at 20Hz.

The lower boarder for human hearing is often given at 20Hz.

So at 1200rpm, the entire tub assembly produces sound waves that are audible to the human ear. Lower speeds and the frequency is to low.

 

[rolls_rapide]

I think there's more to it than that.

The resonance frequencies of the outer cabinet shell, and how effective the dampers are at absorbing mechanical shocks.

In my experience, a load being spun can be quite quiet at the lowest speeds. Then, the outer shell vibrates quite violently at around 800-1000rpm. As the speed increases further 1200, 1400, 1600rpm, the machine becomes perfectly stable again.

Much has to do with the type of outer shell, how it is designed, put together, the pressed pattern on the side walls, motor type, etc.

 

[kb0nes]

Audibility at 20hz while the generally accepted as the start of human hearing is pretty dicey. At that low a frequency it is really more of a tactile sensation and it will vary a lot person to person.

There are lots of other noises, vibrations, rubs, harmonics etc. that would be audible above and before you hit the magic 20Hz frequency.