Evaluation of reproducibility and accuracy of measurements of the transfer function of the outer ear by the F-MIRE method

Introduction. The transfer function of the outer ear is an important characteristic of the hearing organ, determined by the acoustic properties of the auricle and the external auditory canal, their shape and size. The acoustic impedance of the eardrum is also likely to matter.

The frequency characteristics of the transmission function of the outer ear must be taken into account when choosing individual noise protection devices and when prosthetics of the hearing organs. Measurements of the sound pressure distribution along the auditory canal carried out by various researchers with a probe microphone have shown that for frequencies above 3 kHz, it can vary significantly. At the same time, it was noted that the relative measurements of the actual acoustic characteristics of hearing aids and individual noise protection devices, when the end of the microphone probe does not change position, are quite accurate (Sullivan, 1988; Traynor, Ackley, Wiersbowsky, 1989; Johnson, Nelson 1991; Hellstrom, Axelsson, 1993; Searchfield, Purdy, 1997).

The study aims to evaluate the reproducibility of the F-MIRE method and the accuracy of the measurement results of the transfer function of the outer ear when fixing the probe microphone with a wire bracket from a set of microphone devices; to determine possible changes in the acoustic characteristics of the outer ear under the influence of noise of various levels, their relationship with the size of the auricle and the external auditory canal, the state of auditory function.

Materials and methods. Using the F-MIRE method (microphone in a real ear) in the third-octave frequency bands and in frequency corrections A and C, specialists repeatedly measured the transfer function of the outer ear (TFOE) when exposed to broadband noise of various levels in 17 subjects (16 men and 1 woman) aged 25 to 80 years in a sound chamber. The  researchers determined the geometric dimensions of the auricle and the auditory canal. The otolaryngologist examined all the subjects with an audiological examination. Each subject gave informed consent to participate in the study.

Results. Experts have established according to the data of the conducted research:

1. The most informative indicators of the transfer function of the outer ear — TFOE is its value at the frequency of the first resonance, in the third-octave frequency bands 500–4000 Hz and in the frequency correction A, while the most accurate are the measurement results in the third-octave frequency bands close to the resonant one.
2. The range of TFOE spectral measurement results increases in the third-octave bands of 4.0 kHz and above. In this frequency range, both interference of the direct and reflected sound waves from the eardrum and inaccuracy of positioning of the probe microphone can affect.
3. The transfer function of the outer ear is individual, the TFOE of the left and right ears are different in the subjects.
4. The main resonance in the majority of subjects of the left ear of 14–22 dB and the right ear of 11–18 dB is in the third octave band of 2.5 kHz.
5. In most subjects, the TFOE of the main resonance of the left ear exceeds the value of the right ear by 0.5–6 dB.
6. There was no significant dependence of TFOE changes on the level of impacting noise (changes in the acoustic impedance of the eardrum).
7. A reliable relationship between the TFOE value at the main resonance and the size of the auricle and the external auditory canal was determined only in a part of the subjects for the right ear.
8. The reproducibility of the F-MIRE method for measuring spectral values of TFOE can be estimated as high if the subjects were motionless during the measurements.

Conclusion. The present study is an attempt to evaluate the capabilities of the MIRE (F-MIRE) method, its reproducibility and accuracy. The researchers have shown the potential of the method for analyzing the effects of noise on hearing and understanding the influence of various factors on the accuracy of sound pressure measurements in the external auditory canal.

Funding. The study had no funding.

Conflict of interests. The authors declare no conflict of interests.

Received: 27.06.2024 / Accepted: 03.07.2024 / Published: 31.07.2024

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