Most sounds that occur naturally are a combination of pure tones, which can be unwanted and have various compositions. Sound travels at a speed of approximately 335 m/sec in air and is associated with pressure changes known as sound pressure. The unit for measuring sound pressure is Newtons per meter squared (N/m2), but it is more commonly expressed as the sound pressure level (SPL) in decibels (dB). The equation for SPL is given by P = 20 log10 (P/PR), where P is the sound pressure and PR is the reference pressure of 0.0002 dyne/cm2, which is the absolute threshold for human hearing at 1000 Hz. For example, a sound with intensity 10 times greater than the reference pressure would have an SPL of 20 dB, and one that is 100 times greater would have an SPL of 40 dB.

The human ear is sensitive to pure tones within a frequency range of approximately 20 to 20,000 Hz. The threshold for detecting a pure tone varies depending on its frequency. The lowest thresholds for human hearing are around 3000 Hz, with a range of approximately -3 to -5 dB compared to the reference 0 dB at 1000 Hz. Normal speech has an intensity of about 65 dB and its main frequencies fall between 300 and 3500 Hz. Sounds exceeding 100 dB can cause damage to the peripheral auditory apparatus, while those higher than 120 dB can lead to pain and permanent damage. As individuals age, their thresholds at high frequencies increase, resulting in a reduced ability to hear such tones, which is known as presbycusis.

The ear, which consists of the external ear, middle ear, and inner ear, makes up the peripheral auditory apparatus. The external ear includes the pinna (earlobe) and the auditory canal (ear canal). The auditory canal contains glands that secrete cerumen, a protective waxy substance. The pinna helps direct sounds into the auditory canal and aids in sound localization. Sound pressure waves are transmitted through the auditory canal to the tympanic membrane (eardrum). In humans, the auditory canal has a resonant frequency of approximately 3500 Hz, contributing to the low perceptual threshold for sounds in that range.

The middle ear is separated from the external ear by the tympanic membrane and contains air. Three ossicles (bones) connect the tympanic membrane to the oval window of the inner ear. The inner ear consists of the cochlea, the organ of Corti, and the scala tympani and scala vestibuli. The oval window is adjacent to the round window, which is another membrane-covered opening connecting the middle ear and inner ear.
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