ACOUSTIC IMMITTANCE MEASURES PDF
As we have seen, the measurement of acoustic immittance and. J Speech Hear Res. Jun;30(2) Acoustic-immittance measures in normal ears. Wiley TL, Oviatt DL, Block MG. Erratum in J Speech Hear Res . PDF | On Jan 1, , Wiley and others published Basic principles of acoustic immittance measures.
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Ear canal measurements of acoustic immittance a term that groups impedance and measurez inverse, admittance and the related quantities of acoustic reflectance and power absorbance have been used to assess auditory function and aid in the differential diagnosis of conductive hearing loss for over 50 years. The change in imnittance quantities after stimulation of the acoustic reflex also has been used in diagnosis.
As a result of this activity, the participant will be able to 1 describe impedance and how it is related to acoustic reflectance and absorbance, and 2 describe the basic techniques for measuring immittance and reflectance and be able to list the standards associated with these measures. Although physicians and audiologists have used bone conduction testing for over a century to help determine the amount of conductive hearing loss in a patient, it has only been in the past 50 years or so that attempts have been made to determine the nature of the conductive hearing loss without actually opening the ear.
Initially two parallel tracks were taken—one to determine how much sound penetrated the eardrum acoustic impedance and the other to determine if the eardrum was moved by contraction of the middle ear muscles acoustic reflex.
Related techniques for independently assessing sound conduction through the eardrum and middle ear have since been developed, notably tympanometry, which allows relatively easy separation of the contribution of the air in the ear canal from the measured impedance, and reflectance, which tells us how much sound is reflected from the eardrum.
At present there is no national standard that yields absolute values for any of these procedures—aural acoustic impedance, aural acoustic admittance, tympanometry, acoustic reflex, or acoustic reflectance. However, there are American National Standards Institute ANSI and International Electrotechnical Commission IEC standards that describe procedures for calibrating the equipment used to make most of these measurements with the exception of reflectance. In this article, we discuss some of the procedures for calibrating equipment used in measurements of acoustic immittance, reflectance, tympanometry, and acoustic reflex and some of the theoretical and mathematical bases of these measures.
The discussion also will show some of the problems inherent in these measures as currently defined and used.
To understand the problem, we should remember that when acoustic impedance and acoustic reflex measures were first introduced in the s and early s, although there were tuning fork tests, there was no standard for calibrating bone conduction testing even though it was provided with most audiometers. Furthermore, computed tomography, magnetic resonance imaging, and other imaging procedures were not yet available for assessing middle ear problems.
Thus, the surest ways to determine the presence and etiology of a immihtance hearing loss was to otoscopically assess and surgically open the middle ear. Work by Zwislocki designed to understand the physical characteristics of the ear 1 led to the development of an acoustic impedance bridge, an acoustic impedance-measuring device used to assess how sound power entered the middle ear.
Mesaures, Zwislocki and Feldman 2 showed that various conductive lesions yielded different acoustic impedance results. This acoustic bridge, which is no longer commercially available, allowed one to determine the resistance and reactance components of impedance at various frequencies.
The bridge required the clinician to physically measure the volume of the ear canal, using warmed alcohol, and then to compare the sounds produced by a sound source coupled to the ear canal with the sound produced in a reference load of known acoustic impedance by the same source.
The reference load was constructed from a series of tubes of controllable volume and cross section, which permitted the operator to dial meashres impedances with well-defined resistance and reactance.
The sound source, the ear canal, and the reference impedances were so arranged that the sound produced in the two immmittance canceled one another when the reference impedance and the ear canal impedances aocustic equal. The operator adjusted the reference impedance until cancellation occurred, then read the settings on the reference impedance and corrected for the impedance due to the volume of air in the ear canal to acoustoc the impedance of at the patient’s eardrum.
Acousstic the acoustic bridge was measires to identify the aural impedances associated with various middle ear pathologies, 2 practical problems associated with this method e.
Around the same time, that Zwislocki began his work, work in Europe by Terkildsen and Thomsen 3 and Terkildsen and Nielsen 4 showed that one could change the impedance at the eardrum by altering the static pressure in the ear canal. At this point, it should be made clear that although the principles of acoustics easily describe the acoustic impedance of hard-walled cylindrical tubes or cavities, the ear canal is neither hard walled nor truly cylindrical. Thus, although impedance measurements made in the canal relate and correlate well, at least at frequencies below Hz to immitance impedance at the eardrum, they can be significantly influenced by the dimensions and shape of the ear canal and the stiffness and density of the ear canal wall.
This limitation is a serious problem when trying to measure the impedance of the infant ear, because their iimmittance walls are quite flaccid. Finally, in this same period, Djupesland 8 9 and Borg 10 discovered that contractions of the stapedius muscle altered the impedance at the eardrum. This led to the observation that impedance measurements could provide insight into the mechanics of the middle ear, as well as the neural pathways that lead to the contraction of the stapedius muscle.
In the latter msasures, the detailed characteristics of the ear canal were not so important because one need only detect a change in impedance due to a reflex muscle contraction and detect whether or not the contraction diminished over time. The complexities of the ear canal and the shape of the eardrum make it difficult to ascertain precisely the impedance of the eardrum and the attached middle and inner ear.
Acoustic Immittance, Absorbance, and Reflectance in the Human Ear Canal
As an aside, when work first began on developing standards for acoustic impedance, the geometric assumptions needed acouwtic compute the impedance, within the complex-shaped ear canal and the nonplanar shape of the eardrum, led the standard committee to question whether one could actually measure acoustic impedance at the eardrum in humans.
This discussion resulted in accoustic coinage of the term aural acoustic immittancewhich is a combination of impedance and admittance and specifically relates to the ear. But, just as we know that measuring sound in a coupler for pure tone audiometry does not tell us the exact level of the sound reaching the eardrum, let alone what the stimulus is within the inner ear or reaching the auditory cortex, such measurements do ensure 1 our instrument has not changed over time, 2 our instrument measures the same thing as a similar instrument, 3 individual results do or do not that change over time, and 4 accurate comparisons to national and immittance norms.
Although it is often presumed that acoustic reflectance is not as easily contaminated by the vagaries of ear canal acoustjc eardrum structure, there is no meashres at this time for measuring reflectance, such that calibration methods are determined by the manufacturer and comparisons between different devices rely on interested and involved clinicians and scientists e.
As we have seen, the measurement of acoustic immittance and reflectance in the human ear canal are common tools in the investigation and differential diagnosis of the causes of hearing loss, and there are several U.
These standards are listed in Table 1 and are referred to throughout this article. Terminology defined in the standards will first be introduced by italicized text, followed by a reference to the appropriate standard.
Table 2 summarizes the symbols that refer to the physical quantities associated with determining acoustic impedance and acoustci.
Acoustic Immittance Testing
Sound in a medium such as air is associated with net repeated forward and backward variations in the position of small collections of molecules of the medium Fig. Each particle contains a quantity of molecules large enough that random molecular motions average to zero, thereby revealing the net average motion imposed on the particles by sound.
As the driven particles move back and forth aocustic the sound field, toward and away from each other, the density of the particles in the field varies, and associated with the variations in density is a variation in local pressure.
The root mean square rms; a measudes of the time averaged absolute value of a quantity pressure variation around the baseline acpustic the static pressure2. The standard unit of sound pressure is the pascal Pa. The acouustic sound pressure is immlttance root of the mean of the square of the time-dependent deviations around the static baseline pressure and is mathematically equivalent to the standard deviation around the baseline.
The magnitude of motion of the particles of air is more difficult to measure, but is usually defined in terms of the rms particle velocity 2. When dealing with enclosed spaces, such as tubes, we can quantify the particle motion in terms of a volume velocity U2.
Particle velocity and volume velocity. The rms back-and-forth velocity of the particle is the product of the rms displacement and the radian frequency of the tone: Bottom Calculation of the volume velocity assumes that all of the air particles in the same cross-sectional plane of the tube the dotted line in the top tube move together the uniform-plane wave approximation.
The acoustic impedance and admittance are related quantities that depend on the ratio of sound drive the sound pressure and sound flow the volume velocity of sound. When defined at the eardrum, the entrance to the middle ear, these quantities help determine the work done by the sound in moving the eardrum and ossicles.
In the case of tonal stimulation at varied stimulus frequency f in a tube or ear canal, the ratio of the sound pressure P f to the volume velocity in the tube or canal U fdefines the acoustic impedance Z a f 6. The acoustic impedance describes the sound pressure needed to produce a unit measure of volume velocity in the tube, and the impedance value is related to the physical properties of the fluid and the container that restricts fluid motion.
This is analogous to the generalization of Ohm’s law to time varying electric signals, where the time-varying voltage E, the analog of sound pressure is related to the current I, the analog of the volume velocity by the electrical impedance Z e fwhere the electrical impedance can be defined as the ratio of measurements of the voltage and the current: In a short air-filled tube the length of the tube is less than 0.
The compressibility or compliance of the air dominates the impedance in a short tube that is closed at the far end e. In narrow air-filled tubes, the viscosity of the air can contribute to the impedance. In longer tubes, the impedance depends on a combination of mass, compressibility, and viscous and other losses.
Admittance and impedance are often grouped under the rubric immittance 6. Although we have defined the impedance and admittance in terms of the frequency-dependent P f and U f associated with tonal stimulation, we immittxnce simplified this description by acousic the phase of the measured pressures and volume velocities.
In a linear system, a sinusoidal input produces sinusoidal responses, and the responses produced by a tonal stimulus can differ from the input in terms of the magnitude which we quantify in terms of the rms amplitudeand phase which describes the relative timing of the sinusoidal responses, 2.
A complete description of the system’s response requires knowledge of both of these response properties. In particular the impedance immitttance Z a f is defined by the ratio of the magnitudes of the sound pressure P f and the volume velocity U f.
This relationship is graphically meashres in Fig.
The real part of the complex impedance is the resistance r a and the imaginary part is the reactance x awhere both of these factors can vary with stimulus frequency. An illustration of the relationship between the real and imaginary components of a complex number and the magnitude and angle of the complex value. The real r a and imaginary x a components of a complex impedance Z a are illustrated as the two coordinate values on the real-imaginary plane.
The length of the line connecting the point to the origin is the magnitude of the complex value Z a. There is another common unit of admittance magnitude that is applied to admittances governed either by the compressibility of the medium or the compliance of the tubal boundaries.
Such compliant admittances can be quantified in terms of the equivalent volume V e of air 5. Specifically, the magnitude of admittance of a volume of air within a small closed cavity where small is determined by comparisons of the cavity dimensions to the wavelength of the sound, and is generally defined as linear dimensions that are less than one-tenth of a wavelength can be determined from the following:. Pressure reflectance measured in the ear canal is another measure of the difficulty in setting the eardrum and ossicles into motion, where large amounts of sound pressure reflected from the eardrum are associated with reduced eardrum motions.
Mathematically, acoustic reflectance is related to the acoustic impedance, in that reflections occur at boundaries where the acoustic impedance changes. For example, consider a cylindrical air-filled tube of radius a such as in Fig. The subscript T is used to define the location at the termination of the tube. The sound pressure reflection coefficient R T f defines the ratio of the sound pressure in the wave reflected from the termination to the sound pressure in the wave moving toward the termination.
The sound pressure reflection coefficient also can be quantified any distance x from the termination R x, f that is still within the tube. In the case of a straight tube of uniform cross-sectional area throughout e. Conversely, knowledge of the tube dimensions, the distance x between the measurement point and the termination, and the magnitude and angle of R x, f are needed to compute the terminating impedance Z a,T f.
In the previous section, we noted that in a straight tube of uniform cross section, the magnitude of the pressure reflection coefficient is invariant with distance from the reflecting surface, but the angle of the coefficient varies regularly with that distance.
Adult Acoustic Immittance Measures
immitttance There is another coefficient, the sound power reflection coefficient The constancy of the power reflection coefficient in a uniform tube is an attractive feature that allows assessment of the power reflectance at the tube’s termination from any location within the immuttance. However, deviations from a uniform tube can influence the power acojstic coefficient 16 and complicate the relationship between the power reflectance at the point meaeures measurement and at the termination.
The opposite of power reflectance is power absorption, where the sound power absorption coefficient Like the power reflection coefficient, in a straight uniform tube A f is independent of the position x in the tube and dependent solely on the absorption mewsures at the terminating reflecting surface.
In most clinical instruments, measurements of the aural impedance and reflectance and their associated quantities admittance and absorbance are made via an insert earphone and microphone placed in the ear canal at a distance of 1 to 2 cm from the eardrum.
The residual ear canal air space between the eardrum emasures the measurement sight affects the measured impedance or reflectance in different manners.
The added air space acts to absorb and store part of the sound immittancw introduced by the earphone, such that only a fraction of the stimulus sound energy works against the lateral surface of the eardrum to drive the middle ear and cochlea. In terms of impedance, the added load of the ear canal air acts like an impedance in front of the impedance measured at the eardrum.
In most cases, this ear canal contribution to immitance measured impedance is significant, and the combined impedance needs to be compensated for the presence of the ear canal before one can effectively describe the aural impedance at the eardrum. The need for this compensation was the reason Zwislocki’s acoustic bridge measurements in patients were accompanied by measurements of the volume of the patients’ ear acouustic knowledge of the ear canal volume allows some simple approximations of its contribution to the measured impedance, however, such simple approximations are only accurate at frequencies less than 1 kHz.
The effects of the ear canal on the reflectance measured in the ear canal are more subtle. Theory tells us the magnitude of the pressure reflectance R x,f together with the power reflectance R f and absorbance A f measured anywhere within a tube of uniform or slowly varying cross acoustix equals the magnitude of the pressure reflectance at the termination.
However, real ear canals only approximate these geometric constraints, and there is evidence that power reflectance and absorbance do vary when measured at different positions in an ear canal.
All of the quantities we have introduced depend on the frequency of tonal stimulation, and in general measurement of these quantities at multiple frequencies increases the information available about the mechanoacoustic properties that constrain the behavior of the TM and middle ear.
Early measurements of the immittance made in human ear canals were made over a range of frequencies using a series of qcoustic tone stimuli that varied mimittance 0. The 2-kHz limit in these studies was related to the accuracy of the immittance calibration and the methods used to compensate for the presence of the ear canal. It was these complications that led later studies 19 to concentrate on one or two tonal stimuli of relatively low frequency and use variations in ear canal static pressure to help estimate the contribution of the ear canal.
These early tympanograms 5.