Fig. 8.15 ). Most naturally occurring sound, however, is a mixture of pure tones. Noise is unwanted sound and may have any composition of pure tones. Sound propagates at about 335 m/sec in air. The waves are associated with certain pressure changes, called sound pressure. The unit of sound pressure is Newtons per meter squared (N/m2), but sound pressure is more commonly expressed as the sound pressure level (SPL). The unit of SPL is the decibel (dB):
Equation 8.1 where P is sound pressure and PR is a reference pressure (0.0002 dyne/cm2, the absolute threshold for human hearing at 1000 Hz). A sound with intensity 10 times greater would be 20 dB; one 100 times greater would be 40 dB.
The normal young human ear is sensitive to pure tones with frequencies that range between about 20 and 20,000 Hz. The threshold for detection of a pure tone varies with its frequency (
Fig. 8.16 ). The lowest thresholds for human hearing are, for pure tones, approximately 3000 Hz. The threshold at these frequencies is approximately −3 to −5 dB, in comparison with the reference 0 dB at 1000 Hz. In reference to this scale, normal speech has an intensity of about 65 dB, and its main frequencies fall in the range of 300 to 3500 Hz. Sounds that exceed 100 dB can damage the peripheral auditory apparatus, and those higher than 120 dB can cause pain and permanent damage. As people age, their thresholds at high frequencies rise, thereby reducing their ability to hear such tones, a condition called presbycusis.
• Fig. 8.16 Sound Threshold Intensities at Different Frequencies. The bottom curve indicates the absolute intensity needed to detect a sound. The dashed curve represents the threshold for functional hearing.Thetop curveindicateslevelsatwhichsoundispainfuland damaging.
The Ear
The peripheral auditory apparatus is the ear, which can be subdivided into the external ear, the middle ear, and the inner ear (Fig. 8.17
The external ear includes the pinna and the external auditory meatus (auditory canal). The auditory canal contains glands that secrete cerumen, a waxy protective substance. The pinna helps direct sounds into the auditory canal and plays a role in sound localization. The auditory canal transmits the sound pressure waves to the tympanic membrane. In humans, the auditory canal has a resonant frequency of about 3500Hz, and this resonance contributes to the low perceptual threshold for sounds in that range.
The external ear is separated from the middle ear by the tympanic membrane (see
Fig. 8.17A ). The middle ear contains air. Three ossicles are present and serve to link the tympanic membrane to the oval window of the inner ear.
and organ of Corti Scala tympani Scala vestibuli • Fig. 8.17 Ear and Cochlear Structure. A, Location of the right human cochlea in relation to the vestibular apparatus, the middle ear, and the external ear. B,Relationshipsbetweentheouter, middle, and inner ear spaces; the cochlea is depicted unrolled for clarity.
CHAPTER 8 The Special Senses 145 • Fig. 8.17, cont’d C, Drawing of a cross-section through the cochlea. The organ of Corti (see
Fig.
8.18A and B ) is outlined.
Adjacent to the oval window is the round window, another membrane-covered opening between the middle ear and inner ear (see
Fig. 8.17A and B