Probably a trivial thing but a simple google search didn't show anything relevant about it.

If I cover both of my ears with my hands, I hear a very deep rumble. If I slowly move my hands away the sound increases in pitch.

What happens? Looks like some kind of low-pass action. Does my hand simply shadowed the high frequencies or is there some more interesting going on?

  • $\begingroup$ Not-so-trivial. Probably some kind of amplification of noise effect; yet someone with expertise in acoustics should take a shot at this! +1 $\endgroup$ – safkan Jan 14 '15 at 23:00
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    $\begingroup$ This is equivalent to seashell resonance. $\endgroup$ – BMS Jan 14 '15 at 23:24
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    $\begingroup$ @Calmarius In short you create a resonant cavity, but I tried and didn't hear the increase that you mention. $\endgroup$ – Sofia Jan 15 '15 at 0:22
  • $\begingroup$ Put on a pair of well-sealed "earmuff" headphones (not connected to a sound source, of course). You will not hear the rumble. Although there is a sort of "low-pass" action that goes on with your hands, much of what you hear is actually the blood moving through your hands. $\endgroup$ – Hot Licks Jan 15 '15 at 0:57
  • $\begingroup$ @Sofia Push your palms firmly against your ears. As you relax them higher frequency appear in the sounds as well. $\endgroup$ – Calmarius Jan 15 '15 at 7:31

The short answer: "shadowing out" of high frequencies and passive resonance.

The detailed answer: The hands act as "low-pass" filters (they block out the higher frequencies). Almost everywhere has some form of background sound, but we tune it out. We notice changes in noise level and/or frequency. This is why you can "hear the ocean" through a seashell: you only notice "sound" when it changes as the shell is put near your ear.

Why are hands and shells a low pass filter? Sound waves have no trouble pushing/passing-through thin solid objects. But how thin is "thin" depends on the wavelength: there should be as much kg/m^2 of air in a 1-wavelength thick slab as there is object to pass through. Very long wavelength, low frequency sounds can penetrate through hands.

There is also a resonant chamber effect: the resonant frequency of an open bottle will drop as the neck gets narrower, which is roughly what happens when the hands move closer. The frequencies of external sounds that are at or near the resonant frequency will generate strong standing waves in the chamber.

Passive resonance is not amplification, it's energy transferral. A large volume of low-intensity background waves must give up it's energy in order to create a small volume of high-intensity chamber waves. A similar phenomena happens in an acoustic guitar string: the chamber "amplifies" sound by efficiently robbing energy from the string (via the vibrations in the wood) and giving it to the air.

  • $\begingroup$ You mean kg/m^3? 'As there is object to pass through?' Please explain. $\endgroup$ – Harshfi6 Jan 15 '15 at 2:29
  • $\begingroup$ No it's a barrier and it's density is mass per unit area, or kg/m^2. $\endgroup$ – Kevin Kostlan Jan 15 '15 at 15:01

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