Rupture pressures of membranes in the ear.

The rupture pressures of the tympanic membrane, Reissner's membrane, the round window membrane, and the annular ligament have all been measured in cadaver ears from Norwegian cattle. For the tympanic membrane, a static overpressure was applied to the ear canal; for Reissner's membrane, to the endolymph; and for the round window membrane, to the perilymph. The rupture pressure of the annular ligament equals the rupture force to the footplate divided by the area of the oval window. The mean rupture pressures are 0.39 atm for the tympanic membrane, 0.047 atm for Reissner's membrane, greater than 2 atm for the round window membrane, and 29.4 atm for the annular ligament. This last pressure corresponds to 0.68 kilogram force applied to the footplate. The ruptures of the tympanic membrane appeared without exception as small tears in the pars flaccida. The rupture pressure of the tympanic membrane was also measured in a few ears from foxes.

Acoustic impedances at the oval window, and sound pressure transformation of the middle ear in Norwegian cattle.

In 15 cadaver ears from Norwegian cattle, sound pressure transfer functions have been measured (1) for sound input to the tympanic membrane, (2) for sound input to the oval window with the footplate in place, but with the ossicular chain removed, and (3) for sound input to the oval window with also the footplate removed. The output pressure was measured in an enclosure cemented to the round window. The data allow calculation of equivalent sound pressures at the input positions, as well as the acoustic input impedances at the oval window with intact footplate, Z(sc), and with the footplate removed, Z(c). The difference Z(s)=Z(sc)-Z(c) is the acoustic impedance contribution of the footplate and annular ligament. Z(sc) is mainly determined by the stiffness of the annular ligament at low frequencies, and by the cochlear input impedance Z(c) at higher frequencies. Z(c) is predominately resistive, a minor reactive part at low frequencies is attributed to the stiffness of the round window membrane. Z(s) and Z(c) are equal in magnitude at about 0.4 kHz. Rather close RLC fits have been obtained for all the three impedances, Z(sc), Z(s), and Z(c). The fitted values for the resistive parts of Z(sc) and Z(c) are 62.9 and 58.2 acoustic Gomega, respectively. The relatively small difference, 4.7 Gomega, is attributed to the resistance of the annular ligament. The fitted resistance of Z(s) is somewhat larger, 8.6 Gomega, but is anyway of minor importance relative to the dynamic stiffness of the annular ligament. This stiffness depends on the static pressure difference across the footplate. Each of the averaged Z(sc) corresponds to minimum stiffness. The fitted acoustic compliance is 6.89 x 10(-15) m3/Pa. The acoustic inertance plays a minor role. It is attributed to the mass of the footplate and the co-vibrating liquid in the inner ear, and has a fitted value of 4.7 x 10(5) Pa s2/m3. A sound pressure at the eardrum is equivalent to a larger pressure at the footplate, about 16 dB larger at frequencies below 100 Hz, increasing to about 30 dB at 10 kHz. In the vestibulum at the inner side of the footplate, the sound pressure at 20 Hz is about 20 dB below the equivalent pressure at the outer side. The two pressures approach toward higher frequencies, and above 1 kHz they are nearly equal.

Frequency characteristics of sound transmission in middle ears from Norwegian cattle, and the effect of static pressure differences across the tympanic membrane and the footplate.

For 23 cadaver ears from Norwegian cattle, frequency characteristics for the round-window volume displacement relative to the sound pressure at the eardrum have been measured, and are compared to earlier results for human ears [M. Kringlebotn and T. Gundersen, J. Acoust. Soc. Am. 77(1), 159-164 (1985)]. For human as well as for cattle ears, mean amplitude curves have peaks at about 0.7 kHz. At lower frequencies, the mean amplitude for cattle ears is about 5 dB smaller than for human ears. The amplitude curves cross at about 2 kHz, and toward higher frequencies the amplitude for cattle ears becomes increasingly larger. If amplitude curves are roughly approximated by straight lines above 1 kHz, the slope for cattle ears is about -5 dB/octave as compared to about -15 dB/octave for human ears. The phase of the round-window volume displacement lags behind the phase of the sound pressure at the tympanic membrane. The phase lag is close to zero below 0.2 kHz, but increases to about 3.5 pi at 20 kHz for cattle ears, as compared to less than 2 pi for human ears. Further investigations are needed in order to explain the observed differences. Sound transmission in the ear decreases with an increasing static pressure difference across the tympanic membrane, especially at frequencies below 1 kHz, where pressure differences of 10 and 60 cm water cause mean transmission losses of about 10 and 26 dB, respectively, the losses being somewhat larger for overpressures than for underpressures in the ear canal. At higher frequencies, the transmission losses are smaller. For small overpressures, and in a limited frequency range near 3 kHz, even some transmission enhancement may occur. Static pressure variations in the inner ear have only a minor influence on sound transmission. Static pressures relative to the middle ear in the range 0-60 cm water cause mean sound transmission losses less than 5 dB below 1 kHz, and negligible losses at higher frequencies.

A graphical method for calculating the speech intelligibility index and measuring hearing disability from audiograms.

The speech intelligibility index (SII) is interpreted as the proportion of total speech information available to the listener's ear for a given speech material. Consequently, SII varies in the range 0-1. A simple graphical method for determining SII for monaural listening at 1 m distance from a talker producing "average speech" (or PB-words) at normal speech effort is described. The speech area is visualized by 10 x 10 = 100 points in the audiogram form, each point contributing 0.01 to the SII. The SII thus equals 0.01 times the number of points in the audible range. A sensorineural hearing loss gives rise to an additional loss in speech recognition due to reduced frequency discrimination and time resolving ability. This suprathreshold deficit is corrected for, if the SII contribution in each frequency band is multiplied with a hearing threshold level dependent "desensitization factor". The SII is related to the intelligibility of speech, and may be used to evaluate a hearing disability.

The equality of volume displacements in the inner ear windows.

The equality of volume displacements in the inner ear windows is commonly assumed. In the present work this assumption is experimentally verified. The stapes is given a known displacement. The volume displacement of the round window is determined by measuring the sound pressure set up in a tube cemented to the round window. Inner ears of pigs have been used in the investigation. Supplementary measurements on one human temporal bone have been performed. The equality of the volume flows in the inner ear windows is also supported through an analysis of earlier measurements of the round window displacement for a given sound-pressure level at the eardrum.

Acoustic impedance in the human ear canal.

For 13 normal-hearing test persons, the acoustic impedance about 6 mm inside the ear canal entrance has been measured in the frequency range 90-20,000 Hz by using standing wave measuring tubes connected to the ear canal via small ear adaptor tubes. The results show good reproducibility but large individual differences.

Network model for the human middle ear.

The purpose of this work is to offer a contribution to network modelling of the human middle ear. The model proposed has been successfully adapted to the following empirical frequency characteristics: 1) stapes displacement per unit sound pressure at the eardrum, 2) sound pressure increase from ear canal entrance to the tympanic membrane, 3) acoustic impedance at the eardrum for a normal ear, an otosclerotic ear, and an ear with interrupted incudo-stapedial joint. The acoustical energy reflectance at the eardrum, as calculated from a model of the ear canal when terminated by the middle ear model, agrees reasonably well with experimental data up to about 12 kHz. Satisfactory agreement between model results and experimental data has also been achieved for the sound pressure transformation in the middle ear. Stapedius muscle contraction is simulated by changing a single parameter. It is concluded that further progress in middle ear model development requires a strengthening of the empirical basis.

Frequency characteristics of the middle ear.

For 68 temporal bones, frequency curves for the round window volume displacement have been measured for a constant sound pressure at the eardrum. Phase curves were measured for 33 of the specimens. The levels averaged amplitude curve is approximately flat below 1 kHz, where the round window volume displacement per unit sound pressure at the eardrum is 6.8 X 10(-5) mm3/Pa, and falls off by about 15 dB/oct at higher frequencies. For the 20 ears having the largest sound transmission magnitude at low frequencies, the corresponding amplitude curve is displaced about 5 dB towards higher levels. The phase of the round window volume displacement lags the eardrum sound pressure phase. In average for 33 temporal bones, the phase lag increases from zero at the lowest frequencies to pi near 2 kHz and to about 1.5 pi at 10 kHz.

Guidelines for the dimensioning of induction loop systems.

For various loop configurations the long-term rms current required to produce a specified magnetic field is given by a simple formula, containing the average loop width as the only loop geometry parameter. A simplified and easy to use formula for loop inductance facilitates the determination of the impedance presented to the loop amplifier. The amplifier requirements are heavily dependent upon field strength specification and impedance of the loop. Taking these factors into account, an economical and correct dimensioning of the amplifier and loop system may be obtained.

Noise-induced hearing losses. Can they be explained by basilar membrane movement?

The development of noise-induced hearing losses must in some way be related to the basilar membrane movement. Our analysis of this relationship is based on Mössbauer effect measurements of the basilar membrane movement in human temporal bone preparations. At a single basilar membrane position in each of seven preparations, the displacement frequency response was measured for a given sound pressure level at the ear drum. The measurements covered the place range 2.2--6.2 kHz. Despite the inadequacy of the experimental data, there seems to be no doubt that low frequency components contribute substantially to the displacement and mechanical strain of the hair cells near the 4 kHz location, where the hair cells are known to be most vulnerable to noise damage. In fact, the analysis performed indicates that these cells will suffer the greatest mechanical strain, almost irrespective of the spectrum shape of the stimulus noise.

The size of the middle ear and the mastoid air cell.

The total volume for the middle ear and mastoid air cells was measured in 55 temporal bones with normal ear drums from a Norwegian post-mortem material. The measurements were made by an acoustic method and checked by means of fluid filling induced by vacuum pumping. As a measuring fluid X-ray contrast medium was employed so that the degree of filling could be examined by X-ray. The volume varied between approximately 2--22 cm3 with an average round 6,5 cm3.