The transmission media through which acoustic signals pass can be modelled as filters, with frequency-specific and temporal delay properties. The quality of sounds changes when heard through a wall or when one's own voice is heard through the tissues of the head rather than directly through the air or from a tape-recording.
Acoustic signals pass through different media (gases, liquids and solids) at dif
ferent speeds. For instance, they pass through gases which are lighter than air
(such as helium) faster than they pass through air; if helium is breathed (e.g.
in the air mixture used by divers), then speech sounds higher-pitched than in air). The distance in space between positive-going zero-crossings of a sinusoid si
gnal is its wavelength, abbreviated by 
.