Cochlear traveling wave: von Bekesy and the basilar membrane

Sound reaches the cochlea after passing through the outer ear, setting the eardrum in motion, and transmitting vibration through the three bones of the middle ear. At the oval window, that mechanical vibration enters fluid. What happens next, a wave traveling along a membrane, is the mechanical foundation of the human ability to distinguish frequencies. Understanding it explains not only how the ear works but also why certain types of hearing loss follow predictable patterns.

The cochlea as a fluid-filled chamber

The cochlea is a coiled bony structure roughly the size of a pea. It contains three fluid-filled compartments running its length. The scala vestibuli and scala tympani are filled with perilymph, a fluid similar in composition to cerebrospinal fluid. Between them sits the scala media, filled with endolymph, a fluid with high potassium concentration that plays a critical role in hair cell transduction.

The basilar membrane forms the floor of the scala media along the cochlea’s entire length. It is not a uniform structure. At the basal end, near the oval window, it is narrow (about 0.1 millimeters wide) and mechanically stiff. It widens progressively toward the apical end, reaching approximately 0.5 millimeters, and its stiffness decreases by a factor of roughly 100 from base to apex.

This gradient in width and stiffness is the fundamental mechanical design that allows frequency separation.

Von Bekesy’s traveling wave: the experimental description

Georg von Bekesy worked with human cadaver cochleas in the 1940s, opening them and observing basilar membrane motion under stroboscopic illumination. His observations, compiled in his 1960 book Experiments in Hearing, described what became known as the traveling wave.

When sound enters the cochlea, it sets the fluid in motion. That fluid motion creates a wave of displacement along the basilar membrane. The wave does not stay stationary. It travels from the base toward the apex. As it moves, it grows in amplitude until it reaches a specific location on the membrane, peaks, and then rapidly collapses.

The location of that peak depends on frequency. High-frequency sounds peak near the base, where the membrane is stiff. Low-frequency sounds travel farther and peak near the apex, where the membrane is compliant. This spatial arrangement, with different frequencies activating different membrane positions, is called tonotopy. Von Bekesy received the Nobel Prize in Physiology or Medicine in 1961 for describing this mechanism.

Passive vs active mechanics

Von Bekesy’s observations were made on cadaver cochleas, which means the hair cells were not functional. His measurements revealed the passive mechanical response of the basilar membrane to sound.

For decades this seemed to explain frequency selectivity. It did not fully explain sensitivity. The passive basilar membrane has tuning curves that are broad and relatively undiscriminating. A living cochlea shows tuning that is approximately 100 times sharper and sensitivity that is roughly 40 dB better than the passive model predicts.

Researchers in the 1970s and 1980s identified the source of this discrepancy: outer hair cells. Outer hair cells (OHCs) actively change their length in response to electrical stimulation, a property called electromotility. This active process feeds mechanical energy back into the basilar membrane, amplifying its motion and sharpening its response to specific frequencies. The combined passive-plus-active system is what the living cochlea actually uses.

This active amplification is also the source of otoacoustic emissions. When a healthy cochlea’s outer hair cells are active, they generate sound that can be measured in the ear canal. These OAE signals are used clinically to assess outer hair cell function, often before any change appears on a standard audiogram.

The basilar membrane and tonotopic organization

The basilar membrane’s frequency map is consistent across individuals. Frequencies above 8,000 Hz are processed near the basal end. Frequencies around 1,000 to 2,000 Hz are processed in the middle region. Frequencies below 500 Hz are processed near the apex.

This mechanical tonotopy is preserved through the entire auditory pathway. The spiral ganglion neurons that innervate the hair cells maintain frequency organization. The cochlear nucleus, inferior colliculus, medial geniculate body, and primary auditory cortex all preserve a tonotopic map derived ultimately from the mechanical gradient of the basilar membrane.

This means that a clinician looking at an audiogram is reading, indirectly, a spatial map of basilar membrane function. A high-frequency hearing loss at 4,000 to 8,000 Hz reflects damage at the basal end of the cochlea, typically to the outer hair cells in that region.

Relevance to noise-induced hearing loss

Noise-induced hearing loss preferentially damages the basal region of the cochlea because of how the traveling wave distributes acoustic energy. Intense broadband sounds produce the greatest displacement at a region roughly 8 to 14 millimeters from the base, corresponding to the 3,500 to 6,000 Hz region. Hair cell damage at this location produces the characteristic 4,000 Hz notch seen on audiograms following significant noise exposure.

NIDCD notes that this pattern of damage is one of the distinguishing features of noise-induced versus age-related hearing loss. Age-related loss (presbycusis) also begins at the base but tends to produce a sloping high-frequency loss rather than a notch, because it reflects gradual stria vascularis degeneration and diffuse hair cell loss rather than a discrete acoustic trauma.

Tinnitus and the traveling wave

The relationship between basilar membrane mechanics and tinnitus is indirect but real. When outer hair cells are damaged in a specific cochlear region, the normal input from that region is reduced. The central auditory system may compensate by increasing its own gain in the deafferented frequency region. This central upregulation of gain is one proposed mechanism of tinnitus generation and explains why tinnitus pitch often correlates with the audiometric notch.

Understanding this helps clarify why treatments targeting peripheral hearing loss, including hearing aids and cochlear implants, sometimes reduce tinnitus. Restoring peripheral input to the deprived frequency region may reduce the central drive that generates the phantom sound.

If symptoms persist or change, see an audiologist or physician.