Frequency characteristics and speech recognition in cartilage conduction.

OBJECTIVE: Cartilage conduction (CC) is a new transduction form, and hearing devices that utilize CC present a new option for patients with aural atresia. However, in occluded ears, low-tone sounds are transmitted very effectively, resulting in excessive low-tone sound emphasis and speech recognition reduction. This study aimed to clarify low-tone speech recognition for CC in occluded ears, and determine if excessive low-tone sound emphasis decreases maximum speech recognition scores. METHODS: Eight volunteers with normal hearing participated. The performance-intensity function and maximum speech recognition scores for CC with an earplug (i.e., occluded) and air conduction (AC) were measured under high- and low-pass filter (HPF and LPF) conditions, respectively. RESULTS: An HPF improved the maximum speech recognition scores for CC in occluded ears. The scores for CC (occluded) under the conditions of the first- and second-order HPF did not differ from that for AC under the no-filter condition. Conversely, an LPF reduced the scores for AC. The scores and confusion matrices under the second-order LPF condition were similar to those for CC (occluded). CONCLUSION: The conditions under the second-order LPF resembled those of CC in occluded ears. An HPF can compensate excessive low-tone emphasis and improve speech recognition.

Perception of speech in cartilage conduction.

OBJECTIVE: By attaching a transducer to the aural cartilage a relatively loud sound is audible even with a negligibly small fixation pressure applied to the transducer. This form of conduction is referred to as cartilage conduction (CC). Utilizing CC, novel audio devices can be developed, and one possible application is a CC hearing aid. However, there are no studies on speech perception in CC. In this study, CC speech recognition performance was measured and compared with that for air and bone conduction (AC and BC, respectively). METHODS: Nine volunteers with normal hearing participated in the study. The performance-intensity functions were measured for AC, BC and CC. These measurements were performed in the conditions with and without an earplug. RESULTS: Without the earplug, no differences in speech recognition scores were observed among AC, BC, and CC. With the earplug, the level at which the maximum speech recognition score was obtained did not increase in CC, which agreed with the result of BC but not AC. The maximum speech recognition CC score decreased with the earplug. The performance-intensity functions for AC and BC shifted in parallel with the earplug. These shifts approximated the average threshold shifts. In contrast, for CC, the performance-intensity function did not shift in parallel with the earplug. As for the CC threshold shifts with the earplug, although the threshold at 500Hz decreased by 15.4dB, those at 2000 and 4000Hz increased by 13.8 and 31.1dB, respectively. Compared with AC and BC, CC excessively emphasized low over high frequency sounds when the earplug was inserted. Confusion matrices analysis demonstrated that 4%, 22%, and 74% of the errors occurred at low, intermediate, and high frequency speech sounds, respectively. Thus, this excessive low frequency sound emphasis probably prevented the recognition of high frequency speech sounds. CONCLUSION: The decrease in the maximum speech recognition score for CC with the earplug was derived from the biased frequency composition. It can be improved by frequency composition adjustment.

Cartilage conduction is characterized by vibrations of the cartilaginous portion of the ear canal.

Cartilage conduction (CC) is a new form of sound transmission which is induced by a transducer being placed on the aural cartilage. Although the conventional forms of sound transmission to the cochlea are classified into air or bone conduction (AC or BC), previous study demonstrates that CC is not classified into AC or BC (Laryngoscope 124: 1214-1219). Next interesting issue is whether CC is a hybrid of AC and BC. Seven volunteers with normal hearing participated in this experiment. The threshold-shifts by water injection in the ear canal were measured. AC, BC, and CC thresholds at 0.5-4 kHz were measured in the 0%-, 40%-, and 80%-water injection conditions. In addition, CC thresholds were also measured for the 20%-, 60%-, 100%-, and overflowing-water injection conditions. The contributions of the vibrations of the cartilaginous portion were evaluated by the threshold-shifts. For AC and BC, the threshold-shifts by the water injection were 22.6-53.3 dB and within 14.9 dB at the frequency of 0.5-4 kHz, respectively. For CC, when the water was filled within the bony portion, the thresholds were elevated to the same degree as AC. When the water was additionally injected to reach the cartilaginous portion, the thresholds at 0.5 and 1 kHz dramatically decreased by 27.4 and 27.5 dB, respectively. In addition, despite blocking AC by the injected water, the CC thresholds in force level were remarkably lower than those for BC. The vibration of the cartilaginous portion contributes to the sound transmission, particularly in the low frequency range. Although the airborne sound is radiated into the ear canal in both BC and CC, the mechanism underlying its generation is different between them. CC generates airborne sound in the canal more efficiently than BC. The current findings suggest that CC is not a hybrid of AC and BC.

Cartilage conduction efficiently generates airborne sound in the ear canal.

OBJECTIVE: By attaching a transducer to the aural cartilage, a relatively loud sound is audible even with a negligibly small fixation force. Previous study has identified several pathways for sound transmission by means of cartilage conduction. This investigation focused on the relative contribution of direct vibration of the aural cartilage to sound transmission in an open and in an occluded ear. METHODS: Thresholds with and without an earplug were compared for three experimental conditions: the transducer being placed on the tragus, pretragus, and mastoid. Eight volunteers with normal hearing participated. RESULTS: The thresholds increased with distance of the transducer from the ear canal (tragus, pretragus, mastoid, in that order). The differences were statistically significant for all conditions except for the occluded ear at 4 kHz. With the earplug inserted, the thresholds for the tragus condition were most sensitive below 2 kHz, indicating a significant contribution of direct vibration of the aural cartilage. CONCLUSION: Direct vibration of the aural cartilage can enhance sound transmission. At low frequencies, cartilage conduction can deliver sound efficiently across a blockage in the ear canal. Stray airborne sound radiating from the transducer dominates cartilage conduction in the open ear at high frequencies.

Is cartilage conduction classified into air or bone conduction?

OBJECTIVES/HYPOTHESIS: The aim of this study was to establish the sound transmission characteristics of cartilage conduction proposed by Hosoi (2004), which is available by a vibration signal delivered to the aural cartilage from a transducer. STUDY DESIGN: Experimental study. METHOD: Eight volunteers with normal hearing participated. Thresholds at frequencies of 0.5, 1, 2, and 4 kHz for air conduction, bone, and cartilage conductions were measured with and without an earplug. The sound pressure levels on the eardrum at the threshold estimated with a Head and Torso Simulator were compared between air and cartilage conductions. The force levels calibrated with an artificial mastoid at the threshold were compared between bone and cartilage conductions. RESULTS: The difference in the estimated sound pressure levels on the eardrum at the thresholds between air and cartilage conductions were within 10 dB. In contrast, the force levels at the thresholds for cartilage conduction were remarkably lower than those for bone conduction. These findings suggested that sounds were probably transmitted via the eardrum for cartilage conduction. The threshold shifts by an earplug showed no significant difference between bone and cartilage conductions at 0.5 kHz. At 1 and 2 kHz, the threshold-shifts increased significantly in the order of bone, cartilage, and air conductions. These results suggested that airborne sound induced by the vibration of the cartilaginous portion of the ear canal played a significant role in sound transmission for cartilage conduction. CONCLUSIONS: Cartilage conduction has different characteristics from conventional air and bone conductions.

Benefit of a new hearing device utilizing cartilage conduction.

OBJECTIVE: Our previous study demonstrated that sound was effectively transmitted by attaching a transducer to the aural cartilage even without fixation pressure. This new method for sound transmission was found by Hosoi in 2004, and was termed cartilage conduction (CC). CC can be utilized even in hearing-impaired patients who cannot use air-conduction hearing aids owing to continuous otorrhea or aural atresia. A prototype hearing aid employing CC was investigated in this study. METHODS: Four patients with conditions such as continuous otorrhea and acquired aural atresia after surgery participated in this study. The CC hearing aid was fitted, and its benefits were assessed by audiometric tests and interview. RESULTS: Thresholds and speech recognition scores improved in all subjects. However, in subjects with continuous otorrhea, it was difficult to obtain the gains according to the target gains owing to their severe hearing loss and the limitation of the output level. On the other hand, unexpectedly large gains were obtained below 2kHz in the patient with acquired aural atresia. These large gains were probably caused by soft tissue filling the postoperative space. No subjects complained of pain associated with the attachment of the transducer, although such problems are usually observed for a bone-conduction (BC) hearing aid. This feature is considered one of the advantages of the CC hearing aid. CONCLUSION: The results of the audiometric tests and interview suggest that the CC hearing aid has potential as a useful amplification device for hearing disability. Unfortunately, if the soft tissue pathway is not involved, the current device is insufficient for the patients with severe hearing loss. The improvement of the output level will lead to develop a reliable CC hearing aid as an alternative to BC hearing aids or bone anchored hearing aids.