There is a popular folk myth that if one holds a sea shell — specifically, most often, a conch shell — to one's ear, one can hear the sound of the ocean. What in fact one hears is the noise of the surrounding environment, resonating within the cavity of the shell. The same effect can be produced with any resonant cavity, such as an empty cup or even by simply cupping one's hand over one's ear. The similarity of the noise produced by the resonator to that of the oceans is purely accidental. The resonator is simply amplifying some frequencies of the ambient noise in the environment, including air flowing within the resonator and sound originating within the human body itself, and attenuating others. The human ear in fact picks up sounds made by the human body, including the sounds of blood flowing, muscles acting, and even air seeping through ear wax. These sounds are normally discarded by the brain. However, they become more obvious when louder external sounds are filtered out. This occlusion effect occurs with sea shells, cups, or hands held over one's ears, and also occurs with circum-aural headphones, whose cups form a seal around the ear, raising the acoustic impedance to external sounds.
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