[jerod]

The Earth's silent hum

Just how can a hum be silent. I thought the word hum described a noise (which can't be silent) or a smell.

By the way, has anybody tried HUMMER, the scent?

[Gondring]

I hum can be silent to humans, since it's far below the frequency we can hear. Just like you can have visible light and invisible light, you can have audible sound and inaudible sound.

[dr_lew]

Just how can a hum be silent. I thought the word hum described a noise (which can't be silent) or a smell.

It's a hum only by extrapolation. 2-7 millihertz is 7-25 cycles per HOUR. "Hum" is pretty misleading, but scientists like colorful terminology. If you were to take the amplitude data and run a speeded up graphic, you might say, "Oh look, the earth is humming!"