19 HEARING: ANATOMY, PHYSIOLOGY, Copyright © 2006 by Academic Press, Inc. AND DISORDERS OF THE AUDITORY SYSTEM Second Edition All rights of reproduction in any form reserved. 1. ABSTRACT 1. Sound normally reaches the cochlea via the ear canal and the middle ear, but it may also reach the cochlea through bone conduction. Sound that enters the middle-ear cavities can also set the tympanic membrane in motion and thereby reach the cochlea. 2. The sound pressure at the tympanic membrane depends on the acoustic properties of the pinna, ear canal, and the head. 3. The ear canal acts as a resonator, which causes the sound pressure at the tympanic membrane to be higher than it is at the entrance of the ear canal. The gain is largest near 3 kHz (the resonance frequency) where it is approximately 10 dB. 4. In a free sound field, the head causes the sound pressure at the entrance of the ear canal to be different (mostly higher) than it is when measured at the place of the head without the person being present. 5. The effect of the head on the sound pressure at the entrance of the ear canal depends on the frequency of the sound and on the angle of incidence of the sound (direction to the sound source). 6. The difference in time of arrival of a sound at the two ears is the physical basis for directional hearing in the horizontal plane, together with the difference in intensity of the sound at the two ears. 7. The middle ear acts as an impedance transformer that matches the high impedance of the cochlea to the low impedance of air. 8. The gain of the middle ear is frequency dependent and the increase in sound transmission to the cochlear fluid due to improvement in impedance matching is approximately 30 dB in the mid-frequency range. 9. It is the difference between the force that acts on the two windows of the cochlea that sets the cochlear fluid into motion. Normally the force on the oval window is much larger than that acting on the round window because of the gain of the middle ear. 10. The ear’s acoustic impedance is a measure of the tympanic membrane’s resistance against being set into motion by a sound. 11. Measurements of the ear’s acoustic impedance have been used in studies of the function of the middle ear and for recordings of contraction of the middle ear muscles. 2. INTRODUCTION In the normal ear, sound can be conducted to the cochlea mainly through two different routes, namely: (1) through the middle ear (tympanic membrane and the ossicular chain); and (2) through bone conduction. Bone conduction of airborne sound has little impor- tance for normal hearing but it is important in audiom- etry where sound applied to one ear by an earphone may reach the other ear by bone conduction (cross transmission). CHAPTER 2 Sound Conduction to the Cochlea 3. HEAD, OUTER EAR AND EAR CANAL The ear canal, the pinna and the head influence the sound that reaches the tympanic membrane. The influ- ence of these structures is different for different fre- quencies and the effect of the head depends on the direction of the head to the sound source. 3.1. Ear Canal The ear canal acts as a resonator and the transfer function 1 from sound pressure at the entrance of the ear canal to sound pressure at the tympanic membrane has a peak at approximately 3 kHz (average 2.8 kHz [113]) at which frequency the sound pressure at the tympanic membrane is approximately 10 dB higher than it is at the entrance of the ear canal (Fig. 2.1). This regards sounds coming from a source that is located at a distance from the observer (free sound field). The effect of the ear canal is different when sound is applied through headphones or through insert earphones (Fig 2.1). 3.2. Head In a free sound field the head acts as an obstacle to the propagation of sound waves. Together the outer ear and the head transform a sound field so that the sound pressure becomes different at the entrance of the ear canal compared with the sound pressure that is measured in the place of the head. The effect of the head on the sound at the entrance of the ear canal is related to the size of the head and, the wavelength 2 of sound. This means that the “amplification” is fre- quency (or spectrum) dependent and, therefore, the spectrum of the sound that acts on the tympanic mem- brane becomes different from that which can be meas- ured in the sound field in which the individual is located. The sound that reaches the entrance of the ear canal also depends on the head’s orientation relative to the direction to the sound source. Depending on its orientation relative to the sound source, the head can function as a baffle for the ear that points towards the sound source or it can act as a shadow for sounds reaching the ear that is located away from the sound source. 20 Section I The Ear 1 The transfer function (or frequency transfer function) of a transmission system is a plot of the ratio between the output and the input, plotted as a function of the frequency of a sinusoidal input signal, known as a Bode plot. Such a plot is not a complete description of the transmission properties of a system unless the phase angle between the output signal and the input signal as a function of the frequency is included. Nevertheless, often only the amplitude function is shown, often expressed in logarithmic measures (such as decibels). 2 The wavelength of sound is the propagation velocity divided by the frequency. The propagation velocity of sound in air is approximately 340 m/s slightly depending on the temperature and the air pressure. Assuming a propagation velocity of 340 m/s the wavelength of a 1,000 Hz tone is 340/1,000 = 0.34 m = 34 cm. FIGURE 2.1 Effect of the ear canal on the sound pressure at the tympanic membrane: (A) average differ- ence between the sound pressure at the tympanic membrane and that measured at the entrance of the ear canal; (B) difference between the sound pressure at the tympanic membrane and a location in the ear canal that is 1.25 cm from the tympanic membrane (similar to that of an insert earphone); and (C) theoretical esti- mate of the difference between the sound pressure at the tympanic membrane and that at a point that is the geometric center of the concha (reprinted from Shaw, 1974, with permission from Springer). The results from studies of the effect of the head on the sound pressure at the entrance of the ear canal always refer to a situation where the head is in a free sound field with no obstacles other than the individ- ual on which the measurements are performed. Such a situation occurs in nature with the sound source placed at a long distance and where there is no reflec- tion from obstacles. This is a different situation from an ordinary room where sound reflections from the walls modify the sound field by their reflection of sound. A free sound field can be artificially created in a room with walls that absorb all sound (or at least most of it) and thus avoid reflection. Such a room is known as an anechoic chamber. Anechoic chambers are used for research such as that of the transformation of sound by the head and the ear canal. 3.3. Physical Basis for Directional Hearing The physical basis for directional hearing in the horizontal plane is differences in the arrival time of sounds that reach the two ears and differences in the intensity at the entrance of the ear canal. The intensity difference is not only a factor of the direction to a sound source in the horizontal plane (azimuth) but it also depends on the frequency (spectrum) of the sound while the difference in arrival time is independent of the frequency of the sound. The differences in the sound that reaches the two ears are processed and dis- criminated in the central nervous system (see p. 143). The basis for discriminating direction in the vertical plane (elevation) is poorly understood but may have to do with the outer ear’s acoustic properties with regard to high frequency sounds. Sound arrives at the two ears with a time difference except when sounds come from a location directly in front of or directly behind the observer. The reason is that the sound trav- els a different distance to reach the two ears. The dif- ference in arrival time is related to the travel time from a sound source and it has a simple linear relation to the azimuth. The maximal difference in arrival time of the two “ears” in the standard model of the head shown in Fig. 2.2 is approximately 0.6 ms (Fig. 2.3). Values calcu- lated from measurements taken from a hard spherical model of the head (solid line) agree closely with actual measurements made on a live subject. Information about the difference in arrival time and the difference in sound pressure at the two ears is used by the central auditory nervous system to determine the direction to a sound source in the horizontal plane (azimuth). It is believed that the intra-aural time dif- ference is most important for transient sounds and sounds with most of their energy in the frequency range below 1.5 kHz while it is the difference in the intensity that is most important for high frequency sounds (see p. 142). A solid sphere the size of a head (Fig. 2.2) has been used as a model of the head in studies of the transfor- mation of sound from a free sound field to that found at the tympanic membrane and how that transforma- tion changes when the head is turned at different angles relative to the direction to the sound source [128]. Such studies have shown that the sound pres- sure at the tympanic membrane is approximately 15 dB higher than it is in a free sound field in the fre- quency range 2–4 kHz when a sound source is located directly in front of an observer (Fig. 2.4). A dip occurs Chapter 2 Sound Conduction to the Cochlea 21 FIGURE 2.2 Schematic drawing showing how a spherical model of the head can be used to study the effect of azimuth of an incident plane sound wave (reprinted from Shaw, 1974, with permission from the American Institute of Physics). FIGURE 2.3 Calculated intra-aural time difference as a function of azimuths for a spherical model of the head (Fig. 2.2) with a radius of 8.75 cm (solid line), and measured values in a human subject (open circles) (reprinted from Shaw, 1974, with permission from the American Institute of Physics; after Feddersen et al., 1957). 22 Section I The Ear in the transfer function of sound to the tympanic mem- brane at approximately 10 kHz. The difference in the intensity of sounds that reach the two ears is a result of the head being an obstacle that interferes with the sound field. The head acts as a shield to the ear that is turned away from the sound source, which decreases the sound that reaches that ear and it acts as a baffle for the ear turned toward the sound source and that increases the sound intensity at that ear. This means that the effect of the head on the transfer of sound to the entrance of the ear canal depend on both the angle (azimuth) to the sound source and the frequency of the sounds (Fig. 2.5). The difference between the sound pressure in a free field and that which is present at the entrance to the ear canal is small at low frequencies because the effect of the head is small for sound of wavelengths that are long in comparison to the size of the head (Fig. 2.2). In the frequency range between 2.5 and 4 kHz the ampli- fication of sounds by the head and the pinna varies from 8 to 21 dB depending on the angle to the sound source in the horizontal plane (azimuth). The shadow and baffle effects of the head and the outer ear con- tribute to the difference in the sound intensity experi- enced at the two ears for sounds that do not come from a source located directly in front (0° azimuth) or directly behind (180°). In a broad frequency range above 1 kHz the intensity of sounds that come from a direction (azimuth) of 45–90° relative to straight ahead is approximately 5 dB higher at the entrance of the ear canal than at the free sound field occupied by the individual (Fig. 2.5). The transformation of sound from a free sound field to the sound that reaches the tympanic membrane varies between individuals because of differences in the size and shape of the head making the results such as those shown in Fig. 2.5 represent the average person only. 4. MIDDLE EAR Two problems are associated with transfer of sound to the cochlear fluid. One is related to sounds being ineffective in setting a fluid into motion because of the large difference in the acoustic properties (impedance) of the two media, air and fluid. The other problem is related to the fact that it is the difference between the force that acts at the two windows that causes the cochlear fluid to vibrate. The difference in the imped- ance of the two media would cause 99.9% of the sound energy to be reflected at the interface between air and fluid and only 0.1% of the energy will be converted into vibrations of the cochlear fluid if sound was led directly to one of the cochlear windows. Both these problems are elegantly solved by the middle ear. The middle ear acts as an impedance transformer that matches the high impedance of the cochlear fluid to the low impedance of air, thereby improving sound transfer to the cochlear fluid. By increasing the sound transmission selectively to the oval window of the FIGURE 2.4 The combined effect of the head and the resonance in the ear canal and the outer ear, obtained in a model of the human head. The difference in sound pressure measured close to the tym- panic membrane and a sound pressure in a free sound field with the sound coming from a source located directly in front of the head (based on Shaw, 1974). FIGURE 2.5 Calculated differences between the sound pressure (in decibels) in a free field to a point corresponding to the entrance of the ear canal on a model of the head consisting of a hard sphere (Fig. 2.2). The difference is shown as a function of frequency at different azimuths (reprinted from Shaw, 1974, with permission from the American Institute of Physics). BOX 2.1 STUDIES OF PHYSICAL FACTORS THAT ARE IMPORTANT FOR DIRECTIONAL HEARING The difference between the sound pressure at the tym- panic membranes of the two ears has also been studied using a manikin equipped with microphones in place of the tympanic membrane [106] (Fig. 2.6). The results of such studies are in good agreement with those using a spherical model of the head. This model includes the pinna and the results show that the pinna mostly affects transmission of high frequency sounds. While the studies using a manikin more accurately mimic the normal situa- tion, the results do not include the effect of the absorption of sound on the surface of the normal head. A change in the direction to a sound source in the ver- tical plane (elevation) does not cause any change in the inter-aural time difference and determination of the elevation must therefore rely on other factors such as the differences in the spectrum of broad band sounds that reaches the two ears for different elevations [8]. This occurs because the transformation of a sound from the free field to the tympanic membrane depends on the ele- vation to the sound source. The pinna plays an important role in this dependence of the sound transformation on the elevation of the sound source. The effect of elevation (angle to the sound source in the vertical plane) on the sound that reaches the two ears is greatest above 4 kHz (Fig. 2.7) [128]. The sound pres- sure at the tympanic membrane for 0° azimuth and an elevation of 0° falls off above 4 kHz (solid line in Fig. 2.7). With increasing elevation this upper cut off frequency shifts toward higher frequencies (dashed lines in Fig. 2.7). At an elevation of 60° the cut off is above 7 kHz and at that frequency, the sound pressure is more than 10 dB above the value it has at an elevation of 0° [128]. FIGURE 2.6 Sound intensity at the "tympanic membrane" as function of the azimuth measured in a more detailed model of the head (manikin) than the one shown in Fig. 2.2. The difference between the sound inten- sity at the two ears is the area between the two curves (based on Nordlund, 1962, with permission from Taylor & Francis). cochlea, the middle ear creates a difference in the force that acts on the two windows of the cochlea and it thus provides an effective transfer of sound to vibration of the cochlear fluid. 4.1. Middle Ear as an Impedance Transformer Theoretical considerations show that the transm- ission of sound to the oval window would be improved by 36 dB if the middle ear acted as an ideal impedance transformer with the correct transformer ratio. However, the transformer ratio of the human middle ear is slightly different from being optimal and that causes some of the sound to be reflected at the tympanic membrane and thus lost from transmission to the cochlea. The impedance transformer action of the middle ear is mainly accomplished by the ratio between the effective area of the tympanic membrane and the area of the stapes footplate, but the lever ratio of the middle ear bones also contributes. The ratio of areas of the 24 Section I The Ear FIGURE 2.7 Effect of elevation on the sound pressure at the tym- panic membrane (reprinted from Shaw, 1974, with permission from Springer). BOX 2.2 SOUND DELIVERED BY EARPHONES The sound delivered to the ear by earphones is not affected by the acoustic properties of the head. This means that spectral filter action of the head, pinna and ear canal is not effective when earphones are used. This is one of the reasons that music and speech sounds differ- ently when listening through ordinary earphones com- pared to listening in a free sound field. This was recognized as a problem for music delivery when ear- phones came into frequent use. The problem was solved by modifying the sound spectrum that drives the ear- phones in a way that imitates the effect of the head [8]. This principle was first applied to the Sony ® Walkman type of tape players but later used in modern digital devices that deliver music. The modification of the sound spectrum made music and speech played through ear- phones sounds similar to what it does in a (natural) free field. Such a correction of the spectrum of the input to earphones is the reason sound produced by earphones can sound natural, giving an impression of “sound space.” The effect of turning the head when listening in a free field, however, is absent when listening through earphones. The earphones that are commonly used for audiomet- ric purposes are either supra-aural headphones and now, more commonly, insert earphones. There are two concerns regarding the use of earphones for hearing testing; one is calibration and the other is that an earphone applied to one ear also conducts sound to the other ear, by bone conduction. This “cross-talk” is different for different earphone types, being much greater for supra-aural head- phones than for insert earphone (Fig. 2.8A). This cross transmission is the reason that it may be necessary to mask the better hearing ear when testing the hearing in individuals with large differences between hearing thresholds in the two ears. For frequencies below 1 kHz the attenuation of the cross-transmitted sound is greater than 80 dB for insert earphones. Insert earphones have roughly the same frequency characteristics as supra-aural earphones but concerns about the accuracy of the calibra- tion remain. Normally, hearing tests are performed in sound insu- lated rooms but occasionally it is necessary to test the hearing in environments with high ambient noise. In such situations, it is important that the earphone that is used attenuates sounds from the environment. Insert earphones also provide much higher attenuation of external noise than supra-aural headphones (Fig. 2.8B). BOX 2.3 MIDDLE EAR , S EFFECTIVENESS IN TRANSFERING SOUND TO THE COCHLEA The specific impedance of air is 42 cgs units and that of water 1.54 × 105 cgs units (41.5 dynes/cm 3 and 144,000 dynes/cm 3 ), thus a ratio of approximately 1:4,000. Transmission of sound to the oval window will therefore be optimal if the middle ear has a transformer ratio that is equal to the square root of 4,000 (equals 63). This assumes that the input impedance to the cochlea is equal to that of water; in fact it is less. Studies in the cat show that the input impedance of the cochlea is lower at low frequencies than at high frequencies. In the middle frequency range the impedance of the cochlea is approxi- mately the same as that of seawater. Rosowski [122] calculated the overall effectiveness of transferring sound from a free field to the cochlear fluid for the cat (Fig. 2.9). Merchant et al. [85] arrived at gain values of approxi- mately 20 dB between 250 Hz and 500 Hz with a maxi- mum of 25 dB at 1 kHz above which the gain decreases at a rate of 6 dB/octave. The results obtained by different investigators differ and show a gain of the middle ear in the range 25–30 dB. Chapter 2 Sound Conduction to the Cochlea 25 FIGURE 2.8 (A) Average and range of intramural attenuation obtained in six subjects with two types of earphones (TDH 39 and an insert earphone, ER-3) (reprinted from Killion et al., 1985. (B) External noise attenuation of four different earphones often used in audiometry (reprinted from Berger and Killion, 1989, with permission from the American Institute of Physics). FIGURE 2.9 The efficiency of the cat's middle ear, showing the fraction of sound power entering the middle ear that is delivered to the cochlea (after Rosowski, 1991, with permission from the American Institute of Physics). BOX 2.4 THE GAIN OF THE MIDDLE EAR One of the first animal studies that qualitatively meas- ured the gain of the cat’s middle ear in transferring sound to the cochlea, was published by Wever, Lawrence and Smith (Fig. 2.10A) [153]. Early studies of the transfer func- tion of the middle ear used pure tones of different fre- quencies measuring the sound pressure at the tympanic membrane that is required to produce cochlear micro- phonic (CM 2 ) potentials of a certain amplitude [153]. Usually the sound pressure that evokes a 10 µV CM response is determined in the frequency range of interest (for instance, from 100 to 10 kHz). Measurements are first done while the middle ear is intact and then repeated after the middle ear is removed surgically and the sound led directly to the oval window (dashes in Fig. 2.10A), or to the round window (dots in Fig. 2.10A) using a specu- lum that was attached to the bone of the cochlea. This arrangement ensured that sound only reached one of the two cochlear windows at a time. When the sound is con- ducted directly to either the round or the oval window a much higher sound level is needed to obtain a 10 µV CM potential than when conducted via the normal route with the middle ear being intact. The difference between the solid curve in Fig. 2.10A and the dotted or the dashed curves (Fig. 2.10B) is a measure of the gain in sound con- duction to the cochlea provided by the cat’s middle ear. It is seen that the gain of the cat’s middle ear is frequency dependent and it is largest in the frequency range between 0.5 and 10 kHz where it is between 35 and 38 dB. FIGURE 2.10 (A) Illustration of the gain of the middle ear of a cat. Sound pressure needed to produce a CM of an amplitude of 10 mV is shown with the middle ear intact and the sound conducted to the tympanic membrane (solid lines), and after removal of the middle ear and the sound conducted to the oval window (dashes) and round window (dots) using a closed sound delivery system (based on Wever, E.G., Lawrence, M., Smith, K.R. 1948. The middle ear in sound conduction. Arch of Otolaryng. 48, 12-35, with permission from Archives of Otolaryngology Head and Neck Surgery. Copyright © (1948) American Medical Association. All rights reserved). (B) Difference between the dotted-dashed curves and the solid curve in (A) (from Møller, 1983; based on Wever, E.G., Lawrence, M., Smith, K.R. 1948. The middle ear in sound conduction. Arch of Otolaryng. 48, 12-35, with permission from Archives of Otolaryngology Head and Neck Surgery. Copyright © (1948) American Medical Association. All rights reserved). Chapter 2 Sound Conduction to the Cochlea 27 tympanic membrane and that of the stapes is frequency dependent because it is the effective area of the tym- panic membrane 3 and not its geometrical (anatomical) area that makes up the transformer ratio. The middle ear has mass and stiffness that make its transmission properties become frequency dependent. Its efficiency as an impedance transformer thus becomes a function of frequency. Stiffness impedes the motion at low frequencies and mass impedes motion at high frequencies. The friction in the middle ear causes loss of energy that is independent of frequency. The lever ratio may be frequency dependent because the mode of vibration of the ossicular chain is different at differ- ent frequencies. The effective area of the tympanic membrane depends on the sound frequency and that contributes to the frequency dependence of middle-ear transmission. Because sound transmission through the middle ear is frequency dependent, it is an oversimpli- fication to express the transformer action as a single number and the transformer ratio of the middle ear must be described by a function of frequency, namely, its transfer function. Estimates of the gain of the middle ear by different investigators vary and there are systematic differences between results obtained in humans and in animals. The total efficiency of the human middle ear is approx- imately 10 dB less than ideal for frequencies up to approximately 0.2 kHz and its highest efficiency is attained around the frequency 1 kHz where it is approximately 3 dB below that of an ideal impedance transformer. This means that the middle ear transmits approximately one-third of the sound energy to the cochlea in this frequency range and less above and below this range [122]. Above 1.5 kHz the efficiency (in percentage of energy transferred to the cochlea) varies between 20% at 4 kHz and 20% (Fig. 2.9), correspon- ding to losses between 5 and 25 times (7 and 14 dB), respectively. In the experiments described above sound was led to only one of the two windows of the cochlea at a time. If sound is led to the middle-ear cavity, a different situation arises because sound then will reach both the oval window and the round window with about the same intensity. (Hearing loss without the middle ear is discussed in Chapter 9.) Direct measurements of the sound transmission through the middle ear as the function of the frequency have also been performed both in anesthetized ani- mals and in human cadaver ears. The transfer function of the middle ear has been studied in anesthetized cats by measuring the vibration amplitude of the stapes using microscopic techniques with stroboscopic illu- mination [44] or by using a capacitive probe to meas- ure the vibration of the round window (Fig. 2.11) [104]. 4.2. Transfer Function of the Human Middle Ear The middle ear in humans is different from those of animals, which are usually used in auditory experi- ments, and that makes it important to distinguish between results obtained in humans and animals. How to “translate” the results of experiments in animals 2 The CM is generated in the cochlea and its amplitude is closely related to the volume velocity of the cochlear fluid. The CM in response to pure tones is a sinusoidal waveform the amplitude of which increases with the increase in the sound pressure of the sound that elicits the CM. Recording of the CM is often used to determine changes in sound transmission of the middle ear. The generation of the cochlear microphonic potential (CM) is discussed in detail in Chapter 4. 3 The effective area of a membrane like the tympanic membrane is the area of a rigid, weightless piston that transfers sound in the same way as the membrane. FIGURE 2.11 Vibration amplitude of the round window (circles and solid lines) and the incus (triangles and dashed lines) of the ear of a cat, for constant sound pressure at the tympanic membrane. The vibration amplitude was measured using a capacitive probe (from Møller, 1983; based on Møller, 1963, with permission from the American Institute of Physics). into estimates of sound transmission in humans will be discussed below. Some of the earliest studies of the frequency trans- fer function of the middle ear were done in human cadaver ears by von Békésy in 1941 [6]. 4 Measurements of the transfer function of the human middle ear are limited to studies in cadavers. The ratio between the vibration amplitude of the ossicles (the umbo and the stapes) in human cadaver ears and the sound pressure close to the tympanic membrane (Fig. 2.12) reveals transfer functions that are similar to those obtained in animals [46, 71]. The vibration amplitude of the ossi- cles is nearly constant for low frequencies up to the resonance frequency of the middle ear (approximately 900 Hz). These results are similar to those obtained by von Békésy [6] almost 50 years earlier. The similar- ity between these results and those obtained using modern techniques is remarkable in the light of the 28 Section I The Ear FIGURE 2.12 (A) Average displacements of the umbo, the head of the stapes and the lenticular process of the incus. (B) The lever ratio at 124 dB SPL at the tympanic membrane in 14 temporal bones. Vertical bars indicate one standard deviation (reprinted from Gyo, et al., 1987, with permission from Taylor & Francis). [...]... properties of a triangular shaped portion of the tympanic membrane known as the pars flaccida membrana tympani are assumed to contribute to the irregular pattern of the acoustic impedance of the human ear (Figs 2. 15 and 2. 16) This part of the tympanic membrane is relatively loose and its vibrations are not transferred to the manubrium of the malleus as effectively as vibrations of other parts of the membrane... measurement of the vibration velocity of the malleus (for constant sound pressure) is a measure of the ability of the tympanic membrane to convert sound into vibration of the malleus, thus a measure of the function of the tympanic membrane (The velocity of the vibration is the first derivative of the amplitude and the velocity for sinusoidal vibrations at constant sound pressure level can be computed from the. .. non-invasive way to determine the pressure in the middle-ear cavity The use of tympanometry for that purpose is based on the finding that the ear’s impedance changes as a function of the difference between the air pressure in the ear canal and the tympanic cavity and that the impedance has its lowest value when the pressure is the same in the ear canal as it is in the middle-ear cavity (see Fig 2. 21B)... in the high frequency range The attenuation caused by contraction of the stapedius muscle is approximately 8 dB in the cat for frequencies below 1 kHz (Fig 2. 25) [89] Comparisons of the change in the acoustic impedance and the change in the transmission properties of the middle ear (Fig 2. 25) support the hypothesis that contraction of the stapedius muscle causes some kind of “decoupling” between the. .. selectivity is a result of a combination of at least three different mechanisms: (1) The traveling wave motion on the basilar membrane; (2) the active function of the outer hair cells that inject energy into the motion of the basilar membrane and causes the basilar membrane motion to become non-linear; and (3) the resonance of the tectorial membrane and its attachment (mainly the stereocilia of the outer hair... absolute value (length of a vector) and the phase angle (of the vector) (Fig 2. 14B) The impedance of a capacitor and an inductance has pure imaginary values of opposite signs; impedance of a capacitor decreases as a function of the frequency and that of an inductor increases as a function of the frequency The impedance of a circuit that contains a capacitor and an inductor will therefore be zero at a... of the ear’s acoustic impedance more than BOX 2. 9 , EFFECT OF THE BONY SEPTUM IN THE CAT S MIDDLE-EAR CAVITY The cat has a bony septum separating the middle-ear cavity in two compartments that communicate by a small hole in the septum The reactive component of the acoustic impedance of the cat’s ear changes rapidly as the frequency is changed around 4 kHz because of the resonator Comparison of the. .. the beginning of the 1970s that it became evident that the motion of the basilar membrane is non-linear and that it was Sounds set the cochlear fluid into motion and the motion of the cochlear fluid in turn sets the basilar membrane into motion The mechanical properties of the basilar membrane and how they vary along the membrane determine which kind of wave motion a sound gives rise to The traveling... this hypothesis was plausible When the reed was set into up and down vibrations, the mass would exhibit lateral movements when the frequency of the vibrations was equal to the resonance frequency of the reed-mass combination Later, the results of these theoretical and model studies were confirmed in animal experiments [169] showing that the mechanical properties of the tectorial membrane and the stereocilia... This non-linearity of the basilar membrane vibration also provides compression of the amplitude of the basilar membrane vibration relative to the sound stimulus 46 Section I FIGURE 3.5 The amplitude of the displacement of the basilar membrane in a monkey obtained in a similar way as the results shown in Fig 3.4 The top curve shows the results when the monkey was alive (anesthetized), and the two other . effect of the head on the sound pressure at the entrance of the ear canal depends on the frequency of the sound and on the angle of incidence of the sound (direction to the sound source). 6. The. the size of the head (Fig. 2. 2). In the frequency range between 2. 5 and 4 kHz the ampli- fication of sounds by the head and the pinna varies from 8 to 21 dB depending on the angle to the sound source. between the effective area of the tympanic membrane and the area of the stapes footplate, but the lever ratio of the middle ear bones also contributes. The ratio of areas of the 24 Section I The