Psych 129 - Sensory Processes

Sound and the ear

The nature of sound

• The perceptual phenomenon we call “sound” is created by variations in air pressure. When an object in the environment moves or deforms its shape, it will displace the air molecules around it. This temporary displacement is propagated through space by air molecules bumping into each other, creating a sound wave.

• These variations in air pressure provide information about the environment, so many organisms have evolved specialized organs - hearing organs - that allow these variations to be detected.

• Both vision and sound acquire information about the environment by detecting forms of energy that propagate in the form of waves. An important distinction between vision and hearing is that in the former case we rely mainly on reflections of light waves off of objects, whereas in the latter case we rely on mainly on waves emanating directly from the objects themselves.

• The intensity of sound, or sound level, is measured in decibels (dB). This scale is so named for Alexander Graham Bell, who first observed that the discriminability between sound levels depends on the ratio of their amplitudes, not on the absolute difference. Thus, the decibel scale measures the difference between two sound levels by the logarithm of the ratio between the two. Mathematically, this can be expressed as

sound level in dB =

where is the amplitude of one sound and is the amplitude of the other. Thus, decibels are a relative measure. If you want to compute how many dB higher one sound is than another, you simply compute their ratio, take the , and multiply by 20. If you want to compute the absolute sound level, then you take to be the reference level of 20 mbars, which is the threshold amplitude for human hearing (one bar is the average air pressure at sea level).

• Because many air pressure variations reaching the ear are the result of vibrating membranes, sound is often characterized by the frequency of vibration, or how many cycles per second (measured in Hz) the air pressure is changing. A sound that consists of a single sinusoidal oscillation of air pressure, , is referred to as a pure tone (bear in mind though that there is nothing intrinsically “pure” about a pure tone - it is just a man-made term for a particular type of waveform that can be described in a simple mathematical form). Most physical objects have many resonant modes of vibration, and so give off sound waves that can be characterized as a mixture of pure tones at different frequencies, or harmonics.

• Loudness, pitch, and timbre are the subjective percepts we usually attach to the physical aspects of intensity, frequency, and harmonic content, respectively.
The ear

• The ear is subdivided into outer, middle, and inner portions, each playing a different role in transducing sound waves into the eventual electrical impulses that send information to the brain.

• The outer ear consists of the pinna and auditory canal, and is responsible for collecting sound and funneling it into the tympanic membrane. The shape of the pinna filters sound in such a way that can provide cues to the direction of the sound.

• The middle ear is responsible for impedance matching between the tympanic membrane and oval window of the cochlea, and consists of a number of small bones and muscles that transform mechanical displacements of the tympanic membrane into displacements of the oval window. The large ratio between the area of these two interfaces provides a huge source of leverage and amplification for overcoming the loss that would normally accompany an air/water interface.

• The inner ear consists of the liquid-filled cochlea, a snail-like structure containing the organ of Corti. Mechanical displacements at the oval window cause the fluid within the cochlea to move accordingly.

• As the fluid in the cochlea moves back and forth, it causes motions in the basilar membrane, which is the principal structure of the organ of Corti. The basilar membrane is narrow and stiff at the base of the cochlea and wide and floppy at the apex of the cochlea. These differences in width and stiffness cause each portion of the basilar membrane to have a different resonant frequency.

• Hair cells along the basilar membrane move back and forth as the basilar membrane moves. It is these back and forth motions of the hair cells that finally transduce mechanical movement into electrochemical signals.

• The different resonant frequencies at each portion of the basilar membrane give rise to a frequency tuning for each hair cell along the membrane. i.e., any given hair cell will become electrically excited only to a limited range of frequencies.

• Thus, the cochlea performs a frequency analysis of sound, and forms a tonotopic representation of sound by the fact that neighboring hair cells will have similar frequency tuning.