The Ear Is Sensor for Hearing and Equilibrium

The human ear has a degree of complexity probably as great as that of the eye. Understanding our sense of hearing requires familiarity with the physics of sound and its interactions with the biological structures involved in hearing.

The Nature of Sound. Sound waves are mechanical disturbances that travel through an elastic medium (usually air or water). A sound wave is produced by a mechanically vibrating structure that alternately compresses and rarefies the air (or water) in contact with it. For example, as a loudspeaker cone moves forward, air molecules in its path are forced closer together, this is called compression or condensation. As the cone moves back, the space between the disturbed molecules is increased,- this is known as rarefaction. The compression (or rarefaction) of air molecules in one region causes a similar compression in adjacent regions. Continuation of this process causes the disturbance (the sound wave) to spread away from the source.

The speed at which the sound wave travels is determined by the elasticity of the air (the tendency of the molecules to spring back to their original positions). Assuming the sound source is moving back and forth at a constant rate of alternation (i.e., at a constant frequency), a propagated compression wave will pass a given point once for every cycle of the source. Because the propagation speed is constant in a given medium, the compression waves are closer together at higher frequencies, that is, more of them pass the given point every second.

The distance between the compression peaks is called the wavelength of the sound, and it is inversely related to the frequency. A tone of 1,000 cycles per second, traveling through the air, has a wavelength of approximately 34 cm, while a tone of 2,000 cycles per second has a wavelength of 17 cm. Both waves, however, travel at the same speed through the air. Because the elastic forces in water are greater than those in air, the speed of sound in water is about 4 times as great, and the wavelength is correspondingly increased. Since the wavelength depends on the elasticity of the medium (which varies according to temperature and pressure), it is more convenient to identify sound waves by their frequency. Sound frequency is usually expressed in units of Hertz (Hz or cycles per second).

Another fundamental characteristic of a sound wave is its intensity or amplitude. This may be thought of as the relative amount of compression or rarefaction present as the wave is produced and propagated, it is related to the amount of energy contained in the wave. Usually the intensity is expressed in terms of sound pressure, the pressure the compressions and rarefactions exert on a surface of known area (expressed in dynes per square centimeter). Because the human ear is sensitive to sounds over a million-fold range of sound pressure levels, it is convenient to express the intensity of sound as the logarithm of a ratio referenced to the absolute threshold of hearing for a tone of 1,000 Hz. This reference level has a value of 0.0002

dyne/cm2, and the scale for the measurements is the decibel (dB) scale. In the expression dB = 20 log (P/Pref),

the sound pressure (P) is referred to the absolute reference pressure (Pref). For a sound that is 10 times greater than the reference, the expression becomes dB = 20 log (0.002 / 0.0002) = 20.

Thus, any two sounds having a tenfold difference in intensity have a decibel difference of 20,- a 100-fold difference would mean a 40 dB difference and a 1,000-fold difference would be 60 dB. Usually the reference value is assumed to be constant and standard, and it is not expressed when measurements are reported.

Table 4.1 lists the sound pressure levels and the decibel levels for some common sounds. The total range of 140 dB shown in the table expresses a relative range of 10 million-fold. Adaptation and compression processes in the human auditory system allow encoding of most of this wide range into biologically useful information.

Sinusoidal sound waves contain all of their energy at one frequency and are perceived as pure tones. Complex sound waves, such as those in speech or music, consist of the addition of several simpler waveforms of different frequencies and amplitudes. The human ear is capable of hearing sounds over the range of 20 to 16,000 Hz, although the upper limit decreases with age. Auditory sensitivity varies with the frequency of the sound, we hear sounds most readily in the range of 1,000 to 4,000 Hz and at a sound pressure level of around 60 dB. Not surprisingly, this is the frequency and intensity range of human vocalization. The ear's sensitivity is also affected by masking: In the presence of background sounds or noise, the auditory threshold for a given tone rises. This may be due to refractoriness induced by the masking sound, which would reduce the number of available receptor cells.

The Outer Ear. An overall view of the human ear is shown in Figure 4.17. The pinna, the visible portion of the outer ear, is not critical to hearing in humans, although it does

The Relative Pressures of Some Common Sounds

Pressure (dynes/cm2)

Sound Pressure Level (dB)

Sound Source

Relative Pressure

Sound Source

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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