PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Evaluation of frequency resolution characteristic ofcartilage-conduction hearing using physiological and psychologicalmeasurement
Gaik Sean YAP1; Sho OTSUKA1,2,3,4; Masato YUMOTO4; Seiji NAKAGAWA1,2,3,4;
1

Department of Medical Engineering, Graduate School of Science and Engineering, Chiba University
2

Center for Frontier Medical Engineering, Chiba University
34Med-Tech Link Center, Chiba University Hospital

Graduate School of Medicine, The University of Tokyo

ABSTRACTIn bone-conduction, a transducer needs to be clamped strongly against the mastoid process of the temporalbone and this has caused severe pain and discomfort to the users. To solve such defects of bone-conduction,"cartilage-conduction" has been proposed and applied to several devices such as hearing aids andsmartphones. In this study, psychophysical measurements and brain magnetic field measurements wereconducted to objectively evaluate the frequency resolution characteristics of the cartilage-conduction hearing.
The results indicate that in a certain range of frequencies, the cartilage-conduction hearing has nearly equal
frequency resolution to that of conventional bone-conduction and air-conduction hearing.
Keywords: Cartilage-conduction, Perception, Frequency resolution

1. INTRODUCTION
The sounds that we usually perceive initially enter the auditory canal and finally the cochlea in theform of air particle vibrations, and thus this is often referred to as the air-conduction (AC). On theother hand, sound that is transmitted using body structure such as skin, muscle, and especially bone iscalled bone-conduction (BC). Generally, BC is presented by attaching a sound transducer to themastoid process of the skull, and in this way, the sound is said to be transmitted as 4 components 1 : (1)the osseotympanic component, which involves sound radiated into the ear canal; (2) the inertial
osteogenic component, which is based on the relative motion between the middle ear ossicles and thetemporal bone; (3) the compressed osteogenic component, which results from compression andexpansion of the cochlear shell; and (4) the air-conducted component. BC has been applied to devices
such as headphones and hearing aids, but problems such as the causes of pain and discomfort and thedifficulty of securing the transducer, have hindered its widespread application. Therefore, as a solution,"cartilage conduction (CC)" has been proposed 2 . In CC, the ear cartilage tissue is used as the maintransmission medium, together with skin and bones by bringing a transducer into contact with thecartilage region of outer ear. Similar to BC, it transmits via 4 pathways and has been applied to hearing
aids3 and smartphones 4. However, a limited number of studies have been conducted on CC and thedetails of the perception mechanisms remain unclear. In this study, the frequency resolution of CC
hearing was evaluated. Using both psychological measurement (frequency difference limens: DLF)
and brain magnetic field (mismatch field: MMF) measurement, the CC hearings were compared with
AC and conventional BC hearings. In addition, in order to investigate the influence of each soundcomponent on CC perception, measurements were also carried out in conditions where theair-conducted component was blocked by an earplug.

1
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yap@chiba-u.jp
s-nakagawa@chiba-u.jp

2. PSYCHOLOGICAL MEASUREMENT (DLF MEASUREMENT)
Seven normal-hearing subjects participated. Two tone-bursts with slightly different frequency
across a certain center frequency were presented to the subjects, and the subjects were required tochoose the stimulus of higher frequency. The stimuli were presented in 4 conditions: (a) AC, (b) BC,(c)CC and (d) CC with earplugs (CCP) (Fig.1). In each condition, the DLF were estimated using 2
forced-alternative-choice of 2 down-1 up method. Across a frequency range from 0.5 to 8.0 kHz, nosignificant differences among presentation conditions were observed (Fig.2).

ACCCearplug(a) AC(b) BC(c) CC(d) CCP

DLF/fo [%]1,5BCCCP

1,00,50,00,51,02,04,08,0Center Frequency, fo [Hz]Figure 1 – Presentation of stimuli

Figure2 – Estimated DLF for each condition

3. PSHYIOLOGICAL MEASUREMENT (MMF MEASUREMENT)
MMF is a kind of auditory evoked brain magnetic field induced by low occurrence stimuli when
randomly presented with other stimuli of a higher rate of occurrence. MMF has been used as anobjective index in sound discrimination 5 . In this measurement, 5 normal-hearing subjects participated.
With the same presentation method as in DLF measurement (Fig.1), Stimulus I & II (100 mstone-burst), were presented in 2 separate sessions in a random order.
[Stimulus I] high-occurrence stim.(85%): (std) 1.00 kHz; low-occurrence stim.(4% each):(dev A) 1.02, (dev B) 1.04, (dev C) 1.08, (dev D) 1.16, (dev E) 1.32 kHz[Stimulus II] high-occurrence stim.(85%): (std) 4.00 kHz; low-occurrence stim.(4% each):(dev A) 4.08, (dev B) 4.16, (dev C) 4.32, (dev D) 4.64, (dev E) 5.28 kHz
The MMF responses evoked by low-occurrence stimuli were measured using a 306-ch whole-headbrain magnetic field measurement system (Vectorview TM, Neuromag Ltd.). The activation source ofeach MMF was estimated using the equivalent current dipole (ECD) method, and the ECD moments
were then compared. It was found that, the greater the difference between dev and std, the greater theintensity of ECD moment observed. A slight decrease of ECD moment was observed in Stimulus II;
however, no statistical differences among conditions were observed.

4. CONCLUSIONSIn the both DLF and MMF measurements, no significant differences among CC, AC, and BC wereobserved. At least in the range from 0.5 to 8.0 kHz, it is suggested that the frequency resolution of CC
is nearly equal to that of the conventional BC and the AC hearing. In addition, no significant change
was observed with the presence of an earplug in CC, which may have been due to the occlusion effectthat compensated for the effect of the air-conducted component.

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