Basic investigation of sound field inside and outside ear canal

under ultrasound irradiation Yuya Ogawa 1 Department of Electrical Engineering, College of Science and Technology, Nihon University, Japan 11-2-3, Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan Ayumu Osumi 2 Department of Electrical Engineering, College of Science and Technology, Nihon University, Japan 11-2-3, Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan Youichi Ito 3

Department of Electrical Engineering, College of Science and Technology, Nihon University, Japan 11-2-3, Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan

ABSTRACT

In recent years, high-intensity airborne ultrasound applied technology has been developed actively. Accordingly, there are concerns about the effects of sound wave exposure by high-intensity air- borne ultrasound. The sound wave intensity is different near the entrance of the ear canal and near the eardrum, because the wavelength is short in the ultrasound region. Therefore, it is nec- essary to know accurately the sound pressure at the eardrum position in considering the effects of sound waves exposure by ultrasound. However, it is difficult to measure sound pressure near the eardrum with a probe microphone because it may damage the ear canal and eardrum. Against this background, we have studied the sound pressure characteristics inside and outside the ear canal when airborne ultrasound is exposed to the mannequin head (KEMAR) with a pseudo-ear canal created to imitate the dimensions of the human ear canal. And, we have attempted to estimate the sound pressure value near the eardrum using the sound pressure in the ear canal. In this report, we verified the sound field inside and outside the ear canal and the sound pressure near the ear- drum, when ultrasound (frequency:20 kHz) were irradiated to the mannequin head.

1. INTRODUCTION

Many technologies using a high-intensity airborne ultrasound have been studied. In addition, the devices that can radiate the high-intensity airborne ultrasound to realize applied technology are also developed. [1-6]. As examples, there are parametric speakers [7,8]that are super-directional speakers, airborne ultrasound phased arrays [9] that can perform high-speed electronic scanning, and devices [10,11] that can emit extremely the high-intensity airborne ultrasound. On the other hand, the ultra-

1 csyu21008@g.nihon-u.ac.jp

2 osumi.ayumu@nihon-u.ac.jp

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sound exposure by the high-intensity airborne ultrasound used in these techniques has become a prob- lem. Therefore, various safety standards for ultrasound exposure have been studied [12-17]. One of evaluation of ultrasound exposure is an evaluation of sound pressure at the eardrum. The sound pres- sure near the eardrum often does not match the sound pressure near the entrance to the ear canal. The reason for this is that the wavelength of the sound wave is short in the ultrasound region. Therefore, in order to verify the effect of ultrasound exposure, it is necessary to accurately measure the sound pressure at the eardrum.

In order to verify the sound pressure near the eardrum, we proposed to evaluate the sound pressure inside and outside the ear canal when the mannequin head is irradiated with airborne ultrasounds.

In this report, the sound field formed in the ear canal was verified by the measurement and the analysis. 2. Simulation of ultrasound exposure using pulsed ultrasound

2.1. Analysis procedure

(a): Schematic view. (b): Cross section view.

Figure 1: Simulation model.

Figure 1(a) and (b) show the analytical model. Fig. 1(a) shows a schematic view and Fig. 1 (b) shows a cross-sectional view.

(a) Pseudo-auricular. (b) Pseudo-ear canal.

Figure 2: Pseudo-auricular and pseudo-ear canal.

Figure 2(a) and (b) show the pseudo-auricular and pseudo-ear canal used in the analysis. Here, the head model used in the analysis is simulated of a mannequin head (GRAS, KEMAR Type 45BA), as

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shown in Fig. 1, with pseudo-auricular and pseudo-ear canal structures as shown in Fig. 2(a) and Fig. 2(b).

In the analysis, as shown in Fig. 1 (b), the plane shaped ultrasound is set to be irradiated from the front direction of the mannequin head. In addition, the analysis area of the sound field distribution is around the head and the ear canal. The material of the mannequin head was water (sound velocity: 1500 m/s, density: 997 kg/m3), simulating a human head, and an air medium (sound velocity: 340 m/s, density: 1.293 kg/m3) in the other areas. Other analysis conditions are shown in Table.1.

_ ae a— | = S—— a — —_—.. — i= _. = — = = = ——-

Table.1: Analysis conditions

Mesh type Free tetrahedral

Mesh size 3.4 mm

Number of element Approx.320000

Time resolution 1 us

Input cycle 7 waves

The COMSOL Multiphysics (ver. 5.6) finite element method software was used for the acoustic analysis. 3. Result and discussion

Aut): Apt aan

3.1. The sound field around the head

(a) (b) (c) Figure 3: Observation of pulse ultrasound propagation around head.

Figure 3 shows an image of ultrasound propagating around the head. Fig. 3(a)-(c) are images at each time periods.

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First, sound waves arriving at the head are divided into two types. One is sound waves that goes directly to the auricular (Primary wave). The other is a sound wave that propagates to the auricular while diffracting along the side surface of the head (Succeeding wave), (Fig. 3(a)). Subsequently, a part of the primary wave that reaches the auricle enters the ear canal. In addition, a part of the primary wave is reflected by the auricle and interferes with the succeeding wave. Sound waves entering the ear canal is reflected by the eardrum and interfere with the succeeding wave to form standing waves.

3.2. The sound field formed inside the ear canal

Based on the simulation results verified in Section 3.1, we performed a detailed verification of the sound field in the ear canal. This verification was performed both by analysis and experiment.

The experiment was carried out according to the following procedure. First, as in the analysis, the plane shape ultrasound is irradiated from the front of the mannequin head equipped with the pseudo- auricle and the pseudo-ear canal. Next, the sound field inside and outside the ear canal of the manne- quin head is measured with a 1/4-inch microphone (ACO, type7017) with a probe tube with a diam- eter of 2 mm.

(a) Measurement results (b)Analysis results

Figure 4: Sound pressure distribution in ear canal.

Figure 5: Sound pressure distribution along the center line.

Figure 4 (a), (b) show the measurement results. The results show the instantaneous sound pressure distribution at the time when the standing wave was formed. The results are normalized by the max- imum value among the results and shown in a color map. Fig. 4(a) shows the experimental result, and Fig. 4(b) shows the analysis result.

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From the results, it can be confirmed that the sound pressure distributions of the experimental results and the analysis results are almost the same. Next, the sound pressure distributions of the lines shown by the broken lines in Fig. 4 were extracted and the sound pressure distributions were qualitatively compared. Fig. 5 shows the result. From the result, it was confirmed that the amplitude distribution of the sound pressure in the ear canal was approximately the same as the analysis result and the experimental result. In addition, it was also confirmed that the sound pressure was different near the eardrum and near the entrance of the ear canal. 4. CONCLUSIONS

In this report, the sound field formed in the ear canal was verified by actual measurement and anal- ysis using a mannequin head under airborne ultrasound irradiation. As a result, the following contents were clarified. (1) The simulation results clarified the complicated sound field around the head and the auricle and the process of sound wave intrusion into the ear canal. (2) The sound pressure distribution in the ear canal was clarified by actual measurement and analysis results. 5. REFERENCES

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