**2. Wideband tympanometry**

The ME ossicular structures drive the incoming sound stimuli from the eardrum (maleus) to the inner ear (footplate of the stapes). In terms of acoustic energy transmission, there is a physical problem interfacing the middle and the inner ear. The middle ear propagation medium is gaseous while the inner ear medium is liquid. To optimize the propagating stimulus energy, it is necessary to adjust the ME impedance so that the stimulus at the stapes undergoes an "*optimal power transference into the inner ear*" also called "*minimum impedance reflection*." This operation is termed as "*the middle ear impedance matching transformer*" [2, 3] and describes the efficiency by which the acoustic sound energy at the stapes is transformed into an acoustic pressure wave inside the helix structures of the inner ear, without significant energy losses.

There is a specific terminology in the ME measurements: There is an excellent review of these terms by Block and Wiley [4] and by Hall and Chandler [5]. Most of the ME measurements refer to *acoustic immittance* values, which describe the *easiness* by which a sound stimulus can propagate across a medium (air or liquid). Most media impose a resistance to any type of propagation energy. According to this concept, the structures of the ME impose a *resistance* to the propagation of the sound energy, and this opposition/resistance is termed as *acoustic impedance Z*(*ω*). By definition, the reciprocal value of acoustic impedance is *acoustic admittance Y*(*ω*). In this context an acoustic immittance measurement can refer to either *Z*(*ω*) or *Y*(*ω*) and the measurement is conducted with the same manner. The *Z*(*ω*) and *Y*(*ω*) variables are complex and they are characterized by a real and an imaginary part. In clinical terms, this characteristic means that the values of these depend on the frequency (*ω*) of the propagating stimulus. There is another measurement called "static immittance" which refers to measurement under a normal atmospheric pressure (i.e., not varying) and according to Hall and

Traditional tympanometry assesses the impedance of the middle ear at the frequency of 226 Hz. The measurement modality is described in **Figure 1**, and it is conducted with a sensitive probe, which seals completely the ear of the patient. Once the 226 Hz tone is emitted, the pressure variation in the external acoustic meatus displaces the eardrum. This causes the tone absorption of the ME to vary, and a sensitive microphone incorporated in the probe evaluates the total

Tympanometry provides quantitative information about the presence of fluid in the ME, about the mobility of the tympanic-ossicular system, and about the volume of the external acoustic meatus. While it is an effective procedure to identify ME changes in children, adults, and seniors, it has its limitations. For example, there are reported cases in the literature of myringotomy surgeries where the 226 Hz tympanometric data were reported as normal [6]. Assessment outliers like these myringotomy cases, are probably caused by a lack of specific norms for the different types of populations under assessment. It is well known that the eardrum and the external acoustic meatus of neonates and children are anatomically different than those from adult subjects. In this context, the ME impedance norms of one population do

Data in the literature suggest that in infants of approximately 6 months of age, the high-frequency ME transmission is more efficient. Tympanometry measurements with a high‐frequency tone (1000 Hz) can be more sensitive to identify ME changes than those conducted

Chandler [5] clinically this can be measured at 226 Hz.

admittance of the system [2].

30 Advances in Clinical Audiology

not describe well the norms of the other.

#### **2.1. Description and instrumentation**

As described in the previous section, the traditional tympanometry probe-tone at 226 Hz evokes different results depending on the anatomical characteristics of the ME cavity, which can influence the test results. The use of a wideband stimulus (i.e., acoustic click, chirp) has been shown to be more efficient and precise for a ME assessment. Because of the presence of multiple frequencies in the transient stimuli, wideband tympanometry (WBT) is less susceptible to myogenic noise, which originates from the patient movements [3, 16].

The WBT evaluates the ME function with a transient stimulus (click or chirp) testing frequencies from 226 to 8000 Hz, in small incrementing steps. Assessment of ME function over such a broad bandwidth provides detailed information on the ME status and can assist considerably any needed diagnosis.

Currently, there are two families of devices in the market, which offer WBT measurements: (i) the Otostat, and the HearID systems from Mimosa Acoustics, USA; and (ii) the Titan system from Interacoustics, Denmark. As in the traditional tympanometry, WBT is performed by placing a sealing probe into the external auditory canal. The probe contains a microphone, a pressure system, and a speaker transducer. The Mimosa devices are PC‐independent, while the Titan requires a PC connection to perform the WBT measurements. **Figure 2** shows the WBT data from the Otostat system, displayed on a PC running the Otostation data management software. All the other figures in the text are generated using WBT data from the Titan device.

**Figure 2.** WBT data from the Otostat system (Mimosa Acoustics). The panels indicate WBT reflectance, absorbance, and pressure response × tested frequency (the Otostat uses a chirp stimulus). The lower panels show the distortion product OAEs in terms of spectrum and S/N ratios at the four tested frequencies. The WBT + OAE combination favors a good assessment of the ME function in neonates, and it can be used to avoid many REFER or FAIL results.

Interacoustics follows the philosophy of presenting the WBT data not in the traditional 2D manner but in a 3D format, depicting pressure (*y*-axis), frequency (*x*-axis), and absorbance (*z*-axis). An example of this 3D representation is shown in **Figure 3(A)** and **(B)** where neonatal WBT responses are depicted. The 3D graph can rotate, so the user can identify patterns in the 3D contour, which might be of interest. So far there are no data in the literature connecting the 3D pattern variations with some clinical observations. The main reason for this is the enormous amount of data (and the large number of variables represented) in the 3D graph. Higher absorbance values suggest a more efficient ME (**Figure 3A**). Lower values suggest some sort of energy impediment in the ME structure, with a very good probability of a hearing impairment (**Figure 3B**). Interacoustics offers in the 3D graph, an absorbance scale which is color-coded with maxima in the blue and minima in the red color region. The scale is subject-dependent and it is not normalized (thus serves only as a visual aid).

**Figure 3.** (A): Neonate normal WBT data. The subject passed a TEOAE screening test and it is considered as normal. The 3D curve is color-coded, showing good values in blue (high absorbance) and lower or possibly problematic absorbance values in red. The scale is relative to this subject and it is used for a visual aid. In the Appendix there are links from where the readers can download a video (avi file) showing how this 3D structure can be rotated or collapsed, in order to obtain specific frequency information. (B) Neonate WBT data from a infant who failed the TEOAE screening test. The 3D curve is color-coded, showing lower or possibly problematic absorbance values in orange red. The scale is relative to this subject and it is used for a visual aid. In the Appendix there are links from where the readers can download a video (avi file) showing how this 3D structure can be rotated or collapsed, in order to obtain specific frequency information.

#### **2.2. Absorbance and related measurements**

can influence the test results. The use of a wideband stimulus (i.e., acoustic click, chirp) has been shown to be more efficient and precise for a ME assessment. Because of the presence of multiple frequencies in the transient stimuli, wideband tympanometry (WBT) is less suscep-

The WBT evaluates the ME function with a transient stimulus (click or chirp) testing frequencies from 226 to 8000 Hz, in small incrementing steps. Assessment of ME function over such a broad bandwidth provides detailed information on the ME status and can assist considerably

Currently, there are two families of devices in the market, which offer WBT measurements: (i) the Otostat, and the HearID systems from Mimosa Acoustics, USA; and (ii) the Titan system from Interacoustics, Denmark. As in the traditional tympanometry, WBT is performed by placing a sealing probe into the external auditory canal. The probe contains a microphone, a pressure system, and a speaker transducer. The Mimosa devices are PC‐independent, while the Titan requires a PC connection to perform the WBT measurements. **Figure 2** shows the WBT data from the Otostat system, displayed on a PC running the Otostation data management software. All the other figures in the text are generated using WBT data from the Titan device.

Interacoustics follows the philosophy of presenting the WBT data not in the traditional 2D manner but in a 3D format, depicting pressure (*y*-axis), frequency (*x*-axis), and absorbance (*z*-axis). An example of this 3D representation is shown in **Figure 3(A)** and **(B)** where neonatal WBT responses are depicted. The 3D graph can rotate, so the user can identify patterns in the 3D contour, which might be of interest. So far there are no data in the literature connecting the 3D pattern variations with some clinical observations. The main reason for this is the enormous amount of data (and the large number of variables represented) in the 3D graph.

assessment of the ME function in neonates, and it can be used to avoid many REFER or FAIL results.

**Figure 2.** WBT data from the Otostat system (Mimosa Acoustics). The panels indicate WBT reflectance, absorbance, and pressure response × tested frequency (the Otostat uses a chirp stimulus). The lower panels show the distortion product OAEs in terms of spectrum and S/N ratios at the four tested frequencies. The WBT + OAE combination favors a good

tible to myogenic noise, which originates from the patient movements [3, 16].

any needed diagnosis.

32 Advances in Clinical Audiology

It is possible to collapse a number of frequencies and obtain absorbance data over an averaged frequency range (wideband averaged tympanogram), which might offer better clinical estimates for well babies and NICU residents. The WBT average range used in infants is from 800 to 2000 Hz, because it is optimized for ME transmission anomalies such as ME negative pressure and ME with effusion. In this frequency range these pathologies generate 3D graphs presenting major and significant differences between normal and abnormal ears.

According to Interacoustics, the WBT average range in adults is defined from 375 to 2000 Hz. Using average WBT data in this range it is possible to discriminate well WBT responses between children and adults. Interacoustics suggests to average the WBT data starting from 375 Hz and not from 226 Hz, since the latter frequency does not offer a high discriminative value.

It is also possible to obtained absorbance information at the resonance ME frequency, which corresponds to the frequency where mass and stiffness contribute equally to the absorbance (response with a zero phase). The resonance frequency can be useful in the diagnosis of ME abnormalities such as the disjunction of the ossicular chain or otosclerosis. For cases of ossicular chain discontinuity or of other pathologies presenting a dominant ME mass, the resonance frequency of the middle ear tends to be reduced. In the case of otosclerosis, the resonance frequency shifts to higher frequencies [17, 18]. Monitoring the resonant frequency seems to be promising as a method to follow the clinical progression of otosclerosis. It is also possible to obtain the "resonance frequency tympanogram," which is useful in the differentiation between cases of ossicular disruption and a flaccid eardrum [17–19].

From the 3D-WBT graph, it is possible to obtain information about the absorbance at a particular frequency measured in ambient pressure or at the pressure of the middle ear (see Appendix section for a video showing how this is accomplished). The acoustic absorbance (A) is defined as the ratio of (absorbed sound power)/to (incident sound power). Pathologies that can be further monitored or identified with this data modality are: otosclerosis, flaccid eardrums, ossicular chain discontinuity, and semicircular canal dehiscence and babies with negative middle ear pressure and middle ear effusion [20, 21]. The WBT devices from Mimosa Acoustics utilize the concept of acoustic reflectance. Reflectance is the amount of energy reflected by the system in relation to total energy propagating through the system, and it is measured in percentage. The reader might find useful terminology reviews by Hall and Chandler [5] and by Stinson [22].

Several publications indicate that the graph of absorbance allows a better differentiation between middle ear diseases than the traditional tympanometry. There are groups of patients where the pressurization of the ear can be difficult or unwise. Thus, an absorbance test held in nonpressurized conditions will be useful for monitoring middle ear state immediately after surgery, with perforated eardrum during neonatal hearing screening. In several studies performed in ambient pressure proved to be able to detect changes in middle ear function significantly for infants and neonatal measurements [23, 24]. In the case of patients with ventilation tubes in the eardrum, data from Groon et al. [25], suggest that: (i) for any leak larger than 0.25 mm there are absorbance alteration effects up to 10 kHz; (ii) above 1 kHz these effects are unpredictable; and (iii) absorbance values were mostly increased in the lower frequency bands (0.1–0.2 and 0.2–0.5 kHz).

Data from Keefe and Simmons [26] suggested that the absorbance measurements, if they are conducted at peak pressure level, are more sensitive to ME pathologies and complications. Analytically they have reported "comparing tests at a fixed specificity of 0.90, the sensitivities were 0.28 for peak‐compensated static acoustic admittance at 226 Hz, 0.72 for ambient‐pressure WBT, and 0.94 for the pressurized WBT. Pressurized WBT was accurate at predicting conductive hearing loss with an area under the receiver operating characteristic curve of 0.95."
