Fourier in the ear

The cochlea is a coiled cavity within the inner ear that is primarily responsible for much of our auditory system. Within the cochlea is the basilar membrane, a filmy structure that divides the cochlea and vibrates with incoming sound energy. The width of the basilar increases from the base (entrance) to apex (end), while the width of the cochlear cavity decreases.

The basilar membrane has thousands of basilar fibers embedded within. These fibers affect the local stiffness in the membrane. The membrane starts thick and stiff, but becomes thinner and more flexible toward its apex. When sound waves travel along the basilar membrane, sound energy is dissipated at the place along the membrane which has the same natural resonant frequency. The stiff fibers will resonate with high frequencies, and the more flexible fibers resonate at lower frequencies.

The hair cells along the length of the basilar membrane detect this vibration and convert it into electrical potentials for transmission to the brain. There are also outer hair cells that contract in response to signals from the brain. This allows the brain to adjust or tune the stiffness of the membrane, thus providing a feedback mechanism to enhance the resolution of frequency content.

Together, the basilar membrane and hair cells give a continuum of natural resonance from high frequencies at the base to low frequencies at the apex. This distributes the energy over space as a function of frequency. (the distribution is logarithmic, which also accounts for why we hear sound on a log scale). Thus, the cochlea acts as a filterbank, performing a Fourier decomposition of the incoming sound.

Interestingly, this is a reversal of the normal frequency representation, since the highest frequencies appear at the entrance to the cochlea, and the lowest frequencies at the rear.

King Tubby – Playing the mixing desk

King Tubby (1941 –1989) was a Jamaican electronics and sound engineer, and his innovative studio work is often cited as one of the most significant steps in the evolution of a mixing engineer from a purely technical role to a very creative one.

In the 50s and 60s, he established himself as an engineer for the emerging sound system scene, and he built sound system amplifiers as well as his own radio transmitter. While producing versions of songs for local deejays, Tubby discovered that the various tracks could be radically reworked through the settings on the mixer and primitive early effects units. He turned his small recording studio into his own compositional tool.

Tubby would overdub the multitracks after passing them through his custom mixing desk, accentuating the drum and bass parts, while reducing other tracks to short snippets. He would splice sounds, shift the emphasis, and add delay-based effects until the original content could hardly be identified.

King Tubby would also rapidly manipulate a tuneable high pass filter, in order to create an impressive narrow sweep of the source until it became inaudible high frequency content. In effect, he was able to ‘play’ the mixing desk like a musical instrument, and in his creative overdubbing of vocals, became one of the founders of the ‘dub music’ genre.

See G. Milner, Perfecting Sound Forever: The Story of Recorded Music: Granta Books, 2010.