**2.2.2 Tympanometry**

The Impedance Audiometry or most commonly used Tympanometry, is a magnitude that examines the condition of the middle ear, the status and mobility of the eardrum (tympanic membrane). The equipment produces variations of air pressure in the ear canal by means of a probe measuring the middle ear's acoustic resistance. The secondary purpose of this examination is to evaluate the acoustic reflex pathway which includes the 7th and 8th cranial nerves and the brain stem. Furthermore, tympanometry permits a distinction between sensorineural and conductive hearing loss when results are not apparent via Weber and Rinne testing. It is also helpful in the diagnosis of otitis media by demonstrating the presence of middle-ear effusion. When sound impacts on the eardrum, part of the sound is absorbed and sent via the middle to the inner ear, while the remaining part of the sound is reflected. When the eardrum is inflamed over a long period of time, it can become stiff and heavy and the majority of the sound is reflected; therefore less sound reach the inner ear. Among other things, tympanometery assesses cochlear integrity and evaluates acoustic impedance, i.e., the degree of difficulty that the middle ear and ossicles encounter for the passage of sounds, as a result of the mass, stiffness and ossicular disruption of the auditory system. If there is fluid behind the ear-drum, it will not move back and forth from its resting position when pressure is applied. Tympanometry is thus affected by the mass, mobility, and resistance systems of the external and middle-ear cavities.

The instrument, or tympanometer, applies air pressure to the eardrum and measures the reflected sound. Inside the instrument's probe, a small loudspeaker is installed that emits a tone, typically at 226 Hz, through a tube into the auditory canal in front of the eardrum. The canal's air pressure is altered between +200 and -400 decapascals (dapa), at which the sound strikes the tympanic membrane, causing middle-ear vibration, which in turn results in the perception of hearing. Some of this sound is reflected back and picked up by another tube, which is connected to the microphone inside the probe that receives the sound. Together with a third tube, all three are inserted nearly into the eardrum and are made airtight against outside pressure by the ear tip. A manometer and a pump, which can produce both positive and negative pressure, are connected to tube **C**. Less sound is reflected into the microphone when the eardrum is stiff and the eardrum transmits the majority of the sound via the middle to the inner ear. Highest compliance is normally reached with air pressure that corresponds to the outside pressure. During tympanometric measurement, a continuous change in positive and negative pressure is produced by the instrument's pump in the outer middle auditory canal. This test should not be performed in infants below 7 months of age because the suppleness of the external canal's cartilage may produce misleading results.

Technology for Hearing Evaluation 13

have marketed the invention registered by Heller et al, (Heller et al., 1987). This instrument has the capacity has the capacity of performing the test and of displaying and storing data.

Otoacoustic Emissions (OAE) may occur spontaneously or may be evoked by acoustic stimulation, appearing to originate from within the cochlea and propagating through middle ear structures to the external auditory meatus. OAE are acoustic energy leakages from the biochemical reactions (echoes) of a healthy cochlea that possess a latency of 2-20 msec. If an emission by the cochlea is present, it is likely that hearing is normal at that frequency; those with a hearing loss greater than 25–30 dB usually do not produce these soft sounds. Auditory neuropathy may have OAE even though the hearing loss may be

To measure OAE, the equipment produces a quiet, clicking sound that is emitted by the loudspeaker and OEA is measured with a small probe inserted into the ear canal; recording of the response signal is captured with the aid of a sensitive microphone (Choi, 2011). A number of response epochs must be averaged to improve the signal-to noise ratio (SNR) and produce a clear waveform. Mathematical methods are used for evaluation of the signal, such as Fast Fourier (FF) and Wigner-Ville transforms (WVT), Digital filtering, and Correlation analysis (CA) (Buller, 1997), or Hilbert–Huang transform (HT) for detection of OAE and Time–Frequency Mapping (Janušauskas et al., 2006). The measuring system employed to detect otoacoustic emissions consists of the following several parts, as illustrated in Figure 10: a measuring probe that includes a microphone and loudspeaker; a low-noise

Fig. 9. General block diagram of a hand-held tympanometer.

**2.2.3 Otoacoustic emissions** 

profound.

Normally, air pressure in the ear canal is the same as ambient pressure. Also, under normal conditions, air pressure in the middle ear is approximately the same as ambient pressure because the Eustachian tube opens periodically to ventilate the middle ear and to equalize pressure. In a healthy individual, maximum sound is transmitted through the middle ear when ambient-air pressure in the ear canal is equal to the pressure in the middle ear.

The general term employed to describe how energy is transmitted through the middle ear is admittance. The impedanciometer measures reflected sound and expresses it in ml as admittance to or compliance with the pressure in daPa, plotting the results on a chart known as a tympanogram, which is illustrated in Figure 8. The maximum compliance occurs when the pressure of the external auditory canal and the middle ear becomes equal. It is only at this pressure that maximal acoustic transmission takes place through the middle ear. Thus, the compliance peak indicates the pressure of the middle ear, implying the efficacy of the Eustachian tube function. The height of the compliance peak indicates the mobility/ stiffness of the tympanic membrane or of the middle-ear cavity.

Fig. 8. Tympanogram of a normal subject, their canal's air pressure is altered between +200 and -400 dapa at 226Hz.

This test is not a hearing evaluation, but is rather a measure of energy transmission through the middle ear. It should not be used to assess hearing sensitivity and the results of this test should always be viewed in conjunction with pure-tone audiometry. Figure 9 presents the block diagram of a portable, hand-held tympanometer. Manufacturers, such as Welch Allyn,

Normally, air pressure in the ear canal is the same as ambient pressure. Also, under normal conditions, air pressure in the middle ear is approximately the same as ambient pressure because the Eustachian tube opens periodically to ventilate the middle ear and to equalize pressure. In a healthy individual, maximum sound is transmitted through the middle ear

The general term employed to describe how energy is transmitted through the middle ear is admittance. The impedanciometer measures reflected sound and expresses it in ml as admittance to or compliance with the pressure in daPa, plotting the results on a chart known as a tympanogram, which is illustrated in Figure 8. The maximum compliance occurs when the pressure of the external auditory canal and the middle ear becomes equal. It is only at this pressure that maximal acoustic transmission takes place through the middle ear. Thus, the compliance peak indicates the pressure of the middle ear, implying the efficacy of the Eustachian tube function. The height of the compliance peak indicates the mobility/

Fig. 8. Tympanogram of a normal subject, their canal's air pressure is altered between +200

This test is not a hearing evaluation, but is rather a measure of energy transmission through the middle ear. It should not be used to assess hearing sensitivity and the results of this test should always be viewed in conjunction with pure-tone audiometry. Figure 9 presents the block diagram of a portable, hand-held tympanometer. Manufacturers, such as Welch Allyn,

and -400 dapa at 226Hz.

when ambient-air pressure in the ear canal is equal to the pressure in the middle ear.

stiffness of the tympanic membrane or of the middle-ear cavity.

have marketed the invention registered by Heller et al, (Heller et al., 1987). This instrument has the capacity has the capacity of performing the test and of displaying and storing data.

Fig. 9. General block diagram of a hand-held tympanometer.
