The stimulus for the auditory system is sound waves, which are actually vibrations of molecules.  Sound waves must travel throughout some physical medium, such as air.
	Like light waves, sound waves are characterized by their amplitude (loudness), wavelength (pitch), and purity (timbre). Wavelength is described in terms of frequency and is measured in cycles per second, or hertz (Hz). In general, the higher the frequency (more cycles per second), the higher the pitch.
	Amplitude is a description of sound pressure and is measured in decibels (db). Perceived loudness is higher with increasing decibel level.