Keywords
wideband acoustic immittance - acoustic impedance - wideband power absorbance - middle
ear transfer function - resonance frequency
Thinking about the Middle Ear in Wideband
Thinking about the Middle Ear in Wideband
One of the important functions of the middle ear is its role in amplifying incoming
sound from the air-filled ear canal with an upward of 30 dB of gain to sufficiently
conduct sound through the oval window into the fluid-filled inner ear. This function
of the middle ear is known as impedance matching, as the amplification it provides
compensates for the difference between the characteristic impedance of the air and
fluid media. The healthy middle ear, however, is not a perfect conductor of all frequencies
of sound, with some frequencies receiving more gain than others, dependent on the
impedance of the middle ear itself.
Because the middle ear processes and conducts all sound frequencies that are audible
to the human ear, a realistic representation of the middle ear function is to describe
middle ear gain over a wide range of frequencies, where middle ear gain is defined
as the difference between the intensity of (input) sound and that of the middle ear
(output) response. Unfortunately, immittance testing in routine audiological evaluations
is often limited to a single probe-tone test frequency. Despite the clinical benefits
of single-frequency tympanometry, the assessment of the middle ear it conveys is quite
limited. Over the last two decades, clinical systems that are equipped with wideband
acoustic immittance (WAI) testing capabilities have become available commercially.
Using WAI, clinicians will be able to evaluate sound conduction through the outer/middle
ear over a wide range of frequencies. The objective of this article is to provide
clinicians with an overview of the principles underlying assessment and interpretations
of WAI measurements.
In the first sections, we review the manner in which the normal middle ear transforms
sound over a wide range of frequencies, and describe the physical attributes that
give rise to preferential gain for some frequencies over others. Primarily, we highlight
the relationship between the physical attributes of the middle ear and the concepts
of acoustic impedance and resonance. The following sections will introduce typical
WAI measurements in normal-hearing adults, and outline approaches to assessment and
interpretation of abnormal measurements in pathological ears.
Acoustic Impedance
Acoustic impedance can be defined as the inherent opposition to flow of acoustic energy
in a vibrating object. Recall that for any object to vibrate, giving off sound (for
a sound source) or conducting it (for a sound medium), there has to exist a balanced
interplay between the elastic forces connecting its elementary subparts, and the mass
that is constituted within these subparts. While the mass of the vibrating elements
maintains their momentum (“inertia”) during displacement from a neutral position,
the interconnective elastic forces “store” energy, that is, when in states of compression
or rarefaction, and “restore” the object back to its neutral position.[1]
The natural frequency of vibration for an object, also called the resonance frequency,
varies from one object to another depending on its unique physical attributes including
the amount of mass and elasticity (or stiffness). These physical attributes also determine
how vibrations at different frequencies are impeded or reinforced.
Impedance, Reactance, Resistance, and Frequency Dependency
Impedance is represented mathematically using the complex number Z. It has magnitude and phase that vary as a function of frequency.
[Equation (1)] shows that impedance (Z) is composed of a real number, resistance (R), and an imaginary component, reactance (X).[2]
[Fig. 1] shows a phasor plot representation of Z as a vector (blue arrow), with magnitude
|Z| represented by the length of the vector and a phase ϕ. The subcomponents R and X are represented on the x- and y-axis, respectively. Note that Z vector magnitude and phase vary as a function of
frequency, and that the example in [Fig. 1] is a representation of Z for one frequency. For a detailed review of the mathematical and graphical representation
of acoustic impedance, the reader is referred to Van Camp et al.[2] The scope of this presentation is to describe the subcomponents of impedance (resistance
and reactance) and highlight its dependency on frequency in simplified terms.
Figure 1 Phasor plot representation of the complex number impedance (Z). Represented on the
plot for one frequency, Z is shown as a vector with both a magnitude that is indicated
by the length of the blue solid arrow and a phase angle (ϕ). The real component of
Z, resistance (R), is represented on the x-axis; the imaginary component, reactance (X), is represented on the y-axis, with M and S subscripts indicative of mass- and stiffness-reactance, respectively.
On one hand, resistance represents aspects of a vibrating object that loses or dissipates
vibratory energy into forms other than sound, e.g., friction within the vibrating
elements which dissipate energy in the form of heat. The dissipation effect of resistance
on sound vibration amplitudes is independent of their frequencies. In other words,
resistance “impedes” energy equally at all sound frequencies.
Reactance, on the other hand, relates the intertwined physical attributes of vibratory
objects that maintain either vibratory motion as momentum, called mass reactance (Xm), or the elastic restorative forces that counter that motion, called stiffness reactance
(Xs). Although mass and stiffness reactance both “impede” sound vibrations requiring energy either to accelerate the mass elements or to overcome
the elastic restorative elements, the two types of reactance impede sound vibrations in opposition to each other (note the negative sign between Xm and Xs in [Equation 1]). The interplay between Xm and Xs, which is dependent on physical composition of the object (e.g., mass/density and
elasticity/stiffness) determines whether some sound frequencies vibrate at greater
amplitudes than other frequencies.
A vibratory object or system that has great levels of stiffness is said to be a stiffness-dominated
system. In such systems, stiffness overcomes the inertia carried by its mass and quickly
restores the vibration motion. The quick restoration of vibratory motion causes a
stiffness-dominated object to vibrate with greater amplitudes at a faster rate. Said
differently, stiffness-dominated systems impede low-frequency vibrations, and vibrate with greater amplitudes at higher frequencies.
Alternatively, a system with greater amount of mass that overcomes the elastic restorative
elements is called a mass-dominated system. Without a large restorative force to counter
the inertia of a dense object, vibrations occur at slower rates as more time is needed
to slow down (decelerate) mass-dominated objects in one motion direction and reverse
(accelerate) it in the opposite direction. Said differently, mass-dominated objects
vibrate with greater amplitude at low frequencies, and impede vibrations at high frequencies.
Impedance at the Frequency of Resonance
A vibrating object is said to be in a state of acoustic resonance when it vibrates
in phase with the incoming sound. As well, an object in a state of resonance vibrates
at greater amplitude compared to a nonresonant state. The frequency at which an object
resonates varies depending on the object's unique mass and elasticity composition.
At the resonance frequency, mass- and stiffness-reactance perfectly counter each other
resulting in overall low impedance with no contribution from reactance. Stated in
mathematical terms, using [Equation (1)], Z = R + 0, because Xm – Xs = 0, and impedance phase (ϕ) is equal to 0 degree. In stiffness-dominated systems,
resonance occurs at higher frequencies, whereas in mass-dominated systems resonance
occurs at lower frequencies. [Equation 2] describes this relationship, where f
0 represents the frequency of resonance, the k and m constants represent stiffness and mass of the vibrating system, respectively.[3] If stiffness and mass are modified, the k to m ratio changes and the frequency of resonance changes accordingly. If k increases, f
0 is shifted to a higher frequency. Alternatively, if m increases, f
0 is shifted to a lower frequency.
To illustrate the relationships between mass, stiffness, and frequency of resonance,
consider the following example about tuning of guitar strings. Given the unique combination
of tension (stiffness) and density of a guitar string, each string is “tuned” to vibrate
at a specific frequency (i.e., resonance frequency). A guitarist may manipulate the
tension of the guitar strings to make sure they are accurately tuned. Tensing up (stiffening)
a string tunes it to vibrate at a higher frequency, whereas reducing tension in the
string tunes it to a lower frequency. Similarly, consider the effect of density of
the guitar strings; thicker strings with higher (mass) density vibrate at lower frequencies,
where thinner strings vibrate at higher frequencies.
Middle Ear Acoustic Mechanics
Middle Ear Acoustic Mechanics
Natural Mass, Stiffness, and Resistance of the Middle Ear
Impedance of the healthy middle ear is dependent on the natural combination of mass,
stiffness, and resistance elements in its physical structure. For example, stiffness
elements are found in the tension of the tympanic membrane and ossicular joints, ligaments,
tendons, and the volume of air in the tympanic cavity.[2]
[4] The mass elements are found in the pars flaccida of the tympanic membrane, ossicular
bones, density of the perilymph in the cochlea that is coupled to the footplate of
the stapes at the oval window, and mesenchyme commonly found in newborns and young
infants. Resistance elements also exist at the tendons and ligaments that hold vibrating
parts in place: for example, the tympanic annulus ligament at the peripheral rim of
the pars tensa of the tympanic membrane, the narrow passages between the middle ear
cavity and mastoid, and the viscosity of the perilymph and the mucous lining of the
middle ear cavity.
[Fig. 2] illustrates an acoustic-mechanical model of the middle ear by Marquet et al,[5] in which mass (denoted by m0), stiffness (indicated by the sketched spring symbols), and resistance (indicated
by the sketched fork symbol) elements that are inherent to the healthy middle ear
are distributed through its interconnected anatomical parts. Interactions among these
physical elements determine how sound of different frequencies is either impeded or
reinforced, as well as the frequency of resonance, where the middle ear gain is expected
to be greatest.
Figure 2 Model of the middle ear as an acoustic-mechanical system, with natural stiffness
(shown by spring symbols), mass (denoted by m0), and friction (fork symbol) elements. (Reprinted with permission from Marquet J,
Van Camp K, Creten W, Decraemer W, Wolff H, Schepens P. Topics in physics and middle
ear surgery. Acta Oto-Rhino-Laryngologica Belgica 1973;27(2):139–319.)
The Middle Ear Transfer Function: Gain versus Frequency
As discussed in the opening section of this article, one of the functions of the middle
ear is to amplify sound. However, due the middle ear's natural impedance, sound is
not amplified equally at all frequencies. Middle ear gain as a function of frequency,
called the transfer function, has been measured in human cadavers and guinea pigs.[6]
[7] The middle ear gain at various frequencies is measured by recording the amplitude
of vibrations at an input point at the beginning of the middle ear (e.g., tympanic
membrane displacement/velocity) and at an output point at the end of the middle ear
(e.g., stapes footplate displacement/velocity). Gain is computed as the difference
in the amplitudes between the output and input points of measurement in decibels.
Aibara et al[6] successfully measured the middle-ear pressure gain (GME), defined as the ear canal
sound pressure to cochlear vestibule pressure gain for the 0.05- to 10-kHz frequency
range in 11 fresh human temporal bones. [Fig. 3] is a re-illustration of the GME gain function across frequency from their work,
with a mean maximum gain of 22 dB at 1,100 Hz.
Figure 3 Upper panel illustrates mean middle ear gain as a function of frequency measured
in 11 postmortem human temporal bones. Gain was determined as vestibule vibrations
relative to input sound pressure measured at the surface of the tympanic membrane.
The frequency at which gain is maximum is indicated by the vertical solid blue arrow.
The corresponding mean phase angle as a function of frequency is shown in the lower
panel. The frequency at which phase angle = 0 degrees is indicated by the dashed blue
arrow, which extends vertically from the peak gain in the upper panel to highlight
their correspondence. The frequency where gain is maximum and phase = 0 degrees is
the frequency of resonance at which impedance is lowest. (Illustration based on report
by Aibara R, Welsh JT, Puria S, Goode RL. Human middle-ear sound transfer function
and cochlear input impedance. Hearing research 2001;152(1):100–109.)
Impedance and Middle Ear Resonance
The configuration of the transfer functions shows that the middle ear gain varies
across frequency. This configuration is dependent on the impedance of the middle ear
at various frequencies. The frequency at which the middle ear gain is greatest, 1,100 Hz,
is the frequency of resonance. Recall that at this frequency, impedance is lowest
(reactance = 0 Ω) and the output and input vibrations occur in phase with each other
(ϕ = 0 degrees). The highest gain is marked by the dashed blue line in [Fig. 3] (top panel) which extends vertically along the 1,100 Hz frequency point (resonance
frequency) on the x-axis. In the lower panel, the blue arrow symbol indicates the 0-degree phase occurs
at the same frequency. At frequencies lower than the resonance frequency, stiffness-dominated
reactance results in greater impedance and a reduction in gain, indicated by the vertical
orange arrows in the figure. At frequencies higher than 1,100 Hz, mass-dominated reactance
results in greater impedance and reduction in gain, indicated by the vertical purple
arrows.
Clinical Immittance Testing
Clinical Immittance Testing
Immittance, which is derived from the terms impedance and admittance, is an encompassing term which refers to a family of clinical measures that assess
the acoustic-mechanical properties of the middle ear.[8] Immittance measures are derived from acoustic measurements in the ear canal where
a speaker presents a “probe” stimulus and a microphone records a “response.” For example,
in 226-Hz tympanometry, the probe stimulus is the 226-Hz tone, and the response is
the complex change in acoustic pressure in the ear canal, which is related to the
change in velocity of the air volume (called volume velocity) that is trapped between
the probe tip and the tympanic membrane while the latter vibrates in response to the
stimulus. The recorded acoustic signal is then computed in units of admittance. Similarly,
in multifrequency tympanometry (MFT), multiple probe tone frequencies from 200 to
2,000 Hz are presented to provide more information about the function of the middle
ear, including frequency of resonance of the middle ear. As shown in [Fig. 3], the magnitude and phase of the response of the middle ear to a series of tones
varies with frequency.
In tympanometry, admittance is measured while the static (air) pressure in the ear
canal is varied from 200 to −400 daPa. At the extreme static pressure points, the
tympanic membrane and middle ear are stiffened, allowing for characterization of the
ear canal response to the 226-Hz tone as a simple enclosed volume cavity with minimal
contribution from the middle ear. Therefore, at 226 Hz, volume units are directly
related to admittance units and can be computed from each other. The clinical utility
of this relationship is known for compensation of the ear canal admittance and estimation
of ear canal volume. This relationship also allows for simple calibration of the probe
in acoustic cavities of known volume, where 1 mmho = 1 mL at 226 Hz.[9] This simple calibration has its drawbacks, including errors for probe frequencies
greater than 2,000 Hz, where standing waves in the ear canal create pressure nulls.[10] As well, the underlying assumption of the ear canal as a rigid-walled cavity that
does not change with pressurization holds true for older childrens' and adults' ears
but not for newborns and infants.[11]
Wideband Acoustic Immittance
In the case of WAI, the probe stimulus is a transient stimulus (a click or a chirp)
with acoustic components over a wide range of frequencies. Recordings in the ear canal
are dependent on knowledge of probe acoustics (e.g., impedance and pressure), which
are determined in the calibration step,[12]
[13] and sound pressure that is reflected back from the surface of the tympanic membrane
and recorded by the probe microphone. Subsequently, the ratio between reflected pressure
and the incident pressure of the stimulus is computed, called pressure reflectance.
Rather than using a pressure measure, power reflectance has been utilized for clinical
measurements. This is because power measures have uniform magnitude between the probe
tip and the tympanic membrane and have no phase. By comparison, despite having uniform
magnitudes, the phase of pressure measures varies depending on the location of the
probe. The computation of power reflectance from ear canal recordings is reviewed
in details in Rosowski et al.[14] The uniformness of power reflectance along the dimension of the enclosed ear canal
volume is advantageous because reflectance theoretically has the same value at the
tympanic membrane and position of measurement with certain limitations as discussed
in Voss et al.[15]
Thanks to advancements in probe calibration techniques,[12]
[13] acoustic measurements can be recorded with accuracy even at frequencies greater
than 2,000 Hz, which is not otherwise possible with conventional tympanometry methods.
For a detailed review of these calibration techniques, the reader is referred to Rosowski
and Wilber.[10] Because of the difference in calibration methods, WAI testing does not require pressurization
of air in the ear canal and compensation for ear canal admittance. This presents an
advantage for testing in newborns whose immature ear canal walls contract and expand
as static pressure is varied. However, because of this methodological difference,
it is important to keep in mind that WAI tests do not only measure the acoustic mechanics
of the middle ear but are also affected by resonance and gain of the enclosed ear-canal
volume between the probe-tip and the tympanic membrane and the mechanics of the ear
canal itself.
The advantage of testing using wideband stimuli is that the immittance measures can
approximate the transfer function of the outer-/middle ear over a wide range of frequencies
using quick and noninvasive procedures. This allows for the clinical evaluation of
middle ear acoustic mechanics in healthy and pathological ears.
Wideband Power Absorbance
Power absorbance (also known as energy absorbance or simply absorbance) is defined
as the ratio of power absorbed by the middle ear to the incident power.[16] Power absorbance is computed from power reflectance (power absorbance = 1–power
reflectance). The value of absorbance ranges from 0 (meaning no acoustic energy was
absorbed by the middle ear) to 1 (meaning all acoustic energy was absorbed by the
middle ear). Note absorbance values may also be represented as percentages from 0
to 100%.
In normal adult ears, the absorbance pattern is characterized by a broad absorbance
maximum between 1,000 and 4,000 Hz and low absorbance outside this frequency range.[17]
[Fig. 4] illustrates examples of power absorbance measurements as a function of frequency
from 5 normal-hearing adults (aged 21–35 years[18]). The five absorbance measurements exhibit prominent absorbance maxima within the
1,000- to 2,000-Hz range with individual narrow-band variations. Often individual
absorbance measurements will exhibit a primary peak between 2,000 and 4,000 Hz and
a secondary peak or an inflection point between 1,000 and 2,000 Hz which vary from
one individual to another. Normal variations in body size and anatomy may explain
these variations (e.g., differences in middle ear volume, age, and gender).[17]
[19]
[20]
Figure 4 Examples of normal absorbance measurements plotted across frequency from five normal-hearing
adults. The normative absorbance range is shown by the grey-shaded area that is bound
between the 5th and 95th percentile values across frequency.
Absorbance measurements in the ear canal are dependent on the compound impedance of
the middle ear system, and the impedance of the ear canal wall. At frequencies where
impedance is great, absorbance is low, whereas when impedance is low, absorbance is
great. This relationship is illustrated in the example measurement from a normal-hearing
adult ([Fig. 5]), where Panel A shows absorbance versus frequency and Panel B shows the corresponding
impedance magnitude versus frequency function. Impedance is high in the low- and mid-frequency
range caused by stiffness of the outer and middle ear system, thereby suppressing
absorbance in this region. Impedance attains high values in the high-frequency region
(> 3,000 Hz) due to mass elements of the middle ear, thereby suppressing absorbance
in this region, too. Impedance attains a minimum value near 3,000 Hz in this case
which corresponds to the frequency at which maximum absorbance occurs. Panel C of
[Fig. 5] shows the corresponding phase versus frequency function. Phase attains negative
values in the low to mid frequencies, crosses the zero line at 3,000 Hz where minimum
impedance and resonance of the system occur, and attains positive phase values beyond
3,000 Hz.
Figure 5 Example of WAI recording from a single healthy young adult showing three derived
measures: (A) absorbance across frequency, (B) impedance (Z) magnitude across frequency, and (C) impedance phase across frequency. The vertical dashed blue line, which transverses
the three panels, highlights with arrow heads the correspondence among peak absorbance
in (A), Z magnitude minimum in (B), and 0 degrees Z phase in (C). The frequency at which these corresponding values occur represents the compound
resonance of middle ear system.
Assessment of Abnormal Sound Conduction in Pathologies of Middle Ear
Assessment of Abnormal Sound Conduction in Pathologies of Middle Ear
Assessment of the middle ear function requires accurate measures of its impedance,
absorbance, middle ear gain, and/or resonance. Normative values of these measures
have been established for healthy neonates, children, and adults.[21]
[22]
[23] However, in ears with a conductive condition, their natural mass, stiffness, and
resistance elements are modified resulting in different impedance, absorbance, gain,
and resonance. Currently, the diagnosis of middle ear dysfunction may be aided by
an assessment of absorbance values across a wide range of frequencies. While due emphasis
is placed on the value of absorbance at a particular frequency or frequency range,
not enough attention has been paid to examining the configuration of the absorbance
spectrum which changes depending on the conductive condition or disorder.[24]
Modeling of the middle ear as physical spring–mass–resistance mechanical system provides
a theoretical basis for qualitative assessment and interpretation of absorbance pattern
across frequencies.[3]
[25] According to such models, pathology-related changes in mass and/or stiffness are
expected to result in differential changes in absorbance across frequencies and affect
the frequency at which the main absorbance peak occurs. Consider a middle ear condition
where mass is abnormally increased. As explained earlier, the addition of mass increases
impedance at high frequencies due to increased mass reactance. Hence, the absorbance
configuration is altered with decreased absorbance in the high frequencies and the
main absorbance peak shifted to a lower frequency. In contrast, when the stiffness
of the middle ear system is abnormally increased, the absorbance configuration is
modified with decreased absorbance in the low-mid frequencies and the main absorbance
peak shifted to a higher frequency.
The following case examples briefly demonstrate methods for assessment and interpretation
of absorbance measurements across frequencies. The data represented in the cases were
obtained from clinical research measurements with appropriate IRB approvals. Using
the interpretative paradigms discussed earlier, it is possible to also make inferences
about the acoustic mechanics of the outer/middle ear (e.g., stiffness, mass, resonance).
A more expansive discussion of the application of WAI tests in various clinical populations,
together with case examples, is discussed in the subsequent titles of this issue of
Seminars in Hearing.
Case A: Hypermobile Eardrum
In the case of a hypermobile eardrum, the stiffness of the middle ear decreases, resulting
in a decrease in impedance and a corresponding increase in absorbance in the low and
mid frequencies. The decrease in stiffness also affects the resonance of the outer
and middle ear system, resulting in a shift of the main absorbance peak to a lower
frequency. [Fig. 6] shows the absorbance results obtained from the right ear of a normally hearing 32-year-old
adult with a monomeric tympanic membrane that was confirmed by otoscopic examination.
The corresponding 226-Hz tympanogram (not shown) revealed tympanometric peak pressure
(TPP) was within normal limits and a static admittance of 1.75 mmho, just outside
the normal limits.
Figure 6 Case example of wideband power absorbance measured in a normal-hearing adult with
a hypermobile tympanic membrane. The normative range is shown by the grey-shaded area
(defined in caption of [Fig. 4]). Absorbance peak is abnormally shifted to a low frequency, resulting in abnormal
increase in absorbance in the low frequencies, above the 95th percentile of normal.
As well, absorbance in the mid-high frequency is reduced below the 5th percentile
of normal, which is indicated by the lower bound of the grey-shaded area.
Case B: Ossicular fixation
For an ear with ossicular fixation, the ossicular chain is abnormally stiffened, resulting
in increased stiffness reactance and, hence, reduced absorbance in the low-mid frequencies.
[Fig. 7] shows the absorbance results obtained from a 34-year-old male who was diagnosed
at the age of 4 years to have ossicular fixation with a mild conductive hearing loss
in the left ear. Hearing thresholds have remained unchanged since childhood. Due to
increased stiffness of the middle ear, absorbance was reduced in the low- to mid-frequencies
with a notch at 1.2 kHz. The associated change in resonance also results in a shift
of the main absorbance peak to a higher frequency. Tympanometry findings indicated
a TPP of −35 daPa with a static admittance of 0.25 mmho, slightly below the normal
range. Ipsilateral acoustic reflexes were absent at 500 to 4,000 Hz in the left ear,
but present at stimulus levels of 80 to 85 dB HL in the right ear.
Figure 7 Case example of wideband power absorbance measured in an adult ear with surgically
diagnosed otosclerosis, shown along with grey-shaded area of normal (defined in caption
of [Fig. 4]). Absorbance in the low frequencies is generally reduced below the lower bound of
the normal range. There is also a subtle high-frequency shift of the absorbance peak.
Wideband Tympanometry
Wideband tympanometry (WBT) is a method by which wideband absorbance is measured repeatedly
as ear canal air pressure is swept from +200 to −300 daPa. It generates a three-dimensional
plot of absorbance as a function of frequency and ear canal pressure. Absorbance between
250 and 8,000 Hz can be extracted under a chosen applied ear canal pressure, for example,
wideband absorbance at 0 daPa (WBA0).
Furthermore, absorbance measurements extracted at TPP (WBATPP) can be used to evaluate middle ear function while compensating for the difference
between atmospheric air pressure and middle ear pressure. By comparing WBA0 with WBATPP results, a clinician can uncover underlying middle ear conditions that may be present
in addition to negative middle ear pressure.[23] Currently, there are commercially available instruments with WBT capabilities (Titan
device by Interacoustics A/S, and TympStar Pro by Grason-Stadler). The following case
example demonstrates measurements, assessment, and interpretations of WBT.
Case C: Negative Middle Ear Pressure and Otosclerosis
A 64-year-old male, who is referred to by the alias name Joshua, was recently diagnosed
with a mild sensorineural hearing loss in his right ear and a mild to moderate mixed
hearing loss in his left ear. He reported a chronic muffled/blocked sensation in his
left ear that he has experienced since childhood, but has never had it investigated.
Routine immittance testing revealed a normal, Type A tympanogram in the right ear,
and an abnormal, Type C tympanogram in the left ear. Ipsilateral acoustic reflexes
were present at 500, 1,000, and 2,000 Hz in the right ear, but absent in the left
ear. Following an otologic assessment, Joshua was diagnosed with chronic Eustachian
tube dysfunction and otosclerosis (early stage) in the left ear.
WBT was conducted on Joshua's left ear by an experienced audiologist. [Fig. 8(A)] displays the WBT results in the form of a three-dimensional plot of absorbance as
a function of frequency and static ear canal pressure. The color gradient indicates
the range of absorbance values between 0% (0) at the red end and 100% (1) at the violet
end of the spectrum. In general, this plot shows reduced absorbance in the low to
mid frequencies (0.25–1.8 kHz) with notches between 1 and 2 kHz, and grossly normal
absorbance in the higher frequencies. The black bold line shows absorbance at a pressure
corresponding to the TPP (−136 daPa). These features may be made more clear on a two-dimensional
plot as discussed next.
Figure 8 Case example of a wideband absorbance tympanogram (WBT) in an adult ear with negative
middle ear pressure, and otosclerosis. (A) The WBT is shown as a three-dimensional plot with absorbance on the vertical (z-axis), and frequency and static pressure on the horizontal axes (x- and y-axes, respectively). The violet end of the color spectrum indicates high levels of
absorbance, and the red end indicates low levels. The black solid lines indicate absorbance
across frequency at −136 daPa, corresponding to the tympanometric peak pressure (TPP).
(B) Wideband power absorbance (WBA) measurements extracted from the WBT in panel A at two static pressure points: ambient pressure at 0 daPa (WBA0) and at TPP (WBATPP). Both WBA measurements are reduced in the low frequencies, but the WBA0 shows greater reduction below the lower bound of the shaded area of normal (defined
in caption of [Fig. 5]) in comparison to WBATPP. In addition, WBA0 shows a greater high-frequency shift of absorbance maxima.
[Fig. 8(B)] is a two-dimensional absorbance versus frequency plot showing two absorbance measurements
that were recorded in the WBT measurement (from Panel A) either at TPP or at 0 daPa.
The black curve represents the absorbance at TPP (WBATPP) where the influence of negative middle ear pressure is counterbalanced. The WBATPP shows absorbance at or below the 5th percentile from 0.25 to 1 kHz, and normal absorbance
beyond 1 kHz. These results reveal atypically high stiffness of the middle ear even
after compensation for the negative middle ear pressure, consistent with the diagnosis
of otosclerosis.
By comparison, the absorbance at 0 daPa (WBA0), shown by the blue curve, shows severely reduced absorbance in the low-mid frequencies,
and the main peak of absorbance shifted to a higher frequency. This is consistent
with the effect of additional increase in stiffness/tension on the tympanic membrane
and middle ear, caused by the negative pressure in the middle ear cavity. This illustrates
the benefit of testing absorbance at TPP in addition to ambient pressure ([Fig. 8B]). In this case, testing at TPP did not resolve the abnormal wideband absorbance
pattern, thus revealing the underlying middle ear dysfunction free from the effect
of middle ear pressure.
Conclusions
This article has provided an overview of the principles underlying the measurement
of acoustic impedance in a middle ear system with contributions from interactions
of the mass, stiffness, and resistance components. The association between impedance
and gain is demonstrated by the middle ear transfer function with implications for
distinctive sound conduction characteristics using wideband absorbance measures. The
wideband absorbance measure is based on the principles of measurement of acoustic
impedance, and is therefore not an entirely novel measure, at least conceptually.
Instead, it is an advancement to existing aural acoustic-immittance measurements,
which enables evaluation of middle ear function across the audible frequency spectrum
(250–8,000 Hz). The assessment and interpretation of wideband absorbance measures
demonstrated in the above case studies are guided by these principles.
