Hearing: Pitch, Loudness, and Localization

Sound travels as a wave of pressure, and two features of that wave carry all the information you hear. Frequency, how many wave cycles pass per second, determines pitch: more cycles per second means a higher pitch. Amplitude, how tall the wave is, determines loudness: a bigger wave pushes the eardrum harder and sounds louder. These are independent dials. Turning one does not move the other.

Inside the cochlea, the basilar membrane translates frequency into a pattern of neural firing in two complementary ways. Place theory says each frequency vibrates a specific spot on the membrane — high frequencies peak near the base, low frequencies near the tip — and the location of that peak tells the brain the pitch. This works well for high frequencies. Frequency theory says the whole membrane vibrates at the same rate as the sound wave, and the rate of neural firing itself encodes pitch. This works better for low frequencies, since neurons can fire fast enough to match them. Together the two theories cover the full range of human hearing.

Locating a sound uses both ears at once. A sound off to one side reaches the near ear a fraction of a millisecond before the far ear, and arrives slightly louder there too, since the head partly blocks it. The brain compares these tiny timing and intensity differences between the two ears to compute direction, which is why damage to one ear makes localization much harder. This sits right after the ear's sensory transduction process: once hair cells convert vibration into neural signal, pitch, loudness, and location are all built from how that signal varies across time, intensity, and ear.