Bone conduction reception: head sensitivity mapping

Mechanical impedance of the skin-covered human head at different stimulation positions, static forces, and interface areas

Bone conduction (BC) transducers applied to skin-covered areas of the head differ in their positioning, contact area, and static force, all of which influence output characteristics. This study assesses the mechanical impedance of the skin-covered head under varying conditions. Measurements were conducted on 30 participants at 3 positions, earfront, forehead, and mastoid, using circular interfaces with diameters of 10, 15, and 25 mm, and 6 static forces ranging from 0.5 to 7 N. Results showed that mechanical impedance was stiffness-controlled below the resonance frequency and mass-controlled above it. Low-frequency impedance depended on position and static force, with the forehead producing the highest impedance magnitude and the earfront producing the lowest impedance magnitude. At high frequencies, impedance across positions was similar, except for the mastoid with 25 mm interface. Impedance magnitude increased with interface area below 350 Hz and above resonance frequency. These findings highlight an impedance mismatch between standard artificial mastoids and human mastoids, potentially leading to inaccurate force estimations in BC devices. Additionally, three-element and six-element lumped mechanical models for the earfront, forehead, and mastoid were developed, with parameter values as function of contact area and static force.

A Novel Method to Determine the Maximum Output of Individual Patients for an Active Transcutaneous Bone Conduction Implant Using Clinical Routine Data

OBJECTIVES

The maximum output provided by a bone conduction (BC) device is one of the main factors that determines the success when treating patients with conductive or mixed hearing loss. Different approaches such as sound pressure measurements using a probe microphone in the external auditory canal or a surface microphone on the forehead have been previously introduced to determine the maximum output of active transcutaneous BC devices that are not directly accessible after implantation. Here, we introduce a method to determine the maximum output hearing level (MOHL) of a transcutaneous active BC device using patients' audiometric data.

DESIGN

We determined the maximum output in terms of hearing level MOHL (dB HL) of the Bonebridge using the audiometric and direct BC threshold of the patient together with corresponding force levels at hearing threshold and the maximum force output of the device. Seventy-one patients implanted with the Bonebridge between 2011 and 2020 (average age 45 ± 19 years ranging from 5 to 84 years) were included in this study. The analyses of MOHLs were performed by (1) dividing patients into two groups with better or worse average audiometric BC threshold (0.5, 1, 2, 4 kHz), on the ipsilateral side or (2) by separating the MOHLs based on better or worse frequency-by-frequency specific audiometric BC thresholds on the ipsilateral (implanted) side.

RESULTS

When using a frequency-by-frequency analysis obtained average ipsilateral MOHLs were in the range between 51 and 73 dB HL for frequencies from 0.5 to 6 kHz in the group with better audiometric BC threshold on the ipsilateral ears. The average contralateral MOHLs in the group with better contralateral hearing were in the range from 43 to 67 dB HL. The variability of the data was approximately 6 to 11 dB (SDs) across measured frequencies (0.5 to 6 kHz). The average MOHLs were 4 to 8 dB higher across frequencies in the group with better audiometric BC threshold on the ipsilateral ears than in the group with better audiometric BC threshold on the contralateral ears. The differences between groups were significant across measured frequencies ( t test; p < 0.05).

CONCLUSIONS

Our proposed method demonstrates that the individual frequency-specific MOHL on the ipsilateral and contralateral side of individual patients with a transcutaneous BC device can be determined mainly using direct and audiometric BC threshold data of the patients from clinical routine. The average MOHL of the implant was found 4 to 8 dB higher on the ipsilateral (implanted) side than on the contralateral side.

Analysis of cross-talk cancellation of bilateral bone conduction stimulation

When presenting a stereo sound through bilateral stimulation by two bone conduction transducers (BTs), part of the sound at the left side leaks to the right side, and vice versa. The sound transmitted to the contralateral cochlea becomes cross-talk, which can affect space perception. The negative effects of the cross-talk can be mitigated by a cross-talk cancellation system (CCS). Here, a CCS is designed from individual bone conduction (BC) transfer functions using a fast deconvolution algorithm. The BC response functions (BCRFs) from the stimulation positions to the cochleae were obtained by measurements of BC evoked otoacoustic emissions (OAEs) of 10 participants. The BCRFs of the 10 participants showed that the interaural isolation was low. In 5 of the participants, a cross-talk cancellation experiment was carried out based on the individualized BCRFs. Simulations showed that the CCS gave a channel separation (CS) of more than 50 dB in the 1-3 kHz range with appropriately chosen parameter values. Moreover, a localization test showed that the BC localization accuracy improved using the CCS where a 2-4.5 kHz narrowband noise gave better localization performance than a broadband 0.4-10 kHz noise. The results indicate that using a CCS with bilateral BC stimulation can improve interaural separation and thereby improve spatial hearing by bilateral BC.

The Effect of Stimulation Position and Ear Canal Occlusion on Perception of Bone Conducted Sound

The position of a bone conduction (BC) transducer influences the perception of BC sound, but the relation between the stimulation position and BC sound perception is not entirely clear. In the current study, eleven participants with normal hearing were evaluated for their hearing thresholds and speech intelligibility for three stimulation positions (temple, mastoid, and condyle) and four types of ear canal occlusion produced by headphones. In addition, the sound quality for three types of music was rated with stimulation at the three positions. Stimulation at the condyle gave the best performance while the temple showed the worst performance for hearing thresholds, speech intelligibility, and sound quality. The in-ear headphones gave the highest occlusion effect while fully open headphones gave the least occlusion effect. BC stimulated speech intelligibility improved with greater occlusion, especially for the temple stimulation position. The results suggest that BC stimulation at the condyle is generally superior to the other positions tested in terms of sensitivity, clarity, and intelligibility, and that occlusion with ordinary headphones improves the BC signal.

The sensitivity of bone conduction for dental implants

Dental implants are connected to the alveolar bone by osseointegration. Dental implants could be used as a potential bone conduction (BC) hearing assistive device in the mouth. However, the BC threshold of dental implants has not been reported. The present study aimed to examine the pure tone auditory thresholds of normal human subjects to BC stimulation of the implants. Dental implants showed a significantly lower BC threshold than natural teeth and mastoids. Mandibular dental implants had BC sensitivity similar to that of maxillary dental implants. The BC threshold of anterior dental implants was significantly lower than that of posterior dental implants. Dental implants exhibited excellent BC properties.

Effects of Stimulation Position and Frequency Band on Auditory Spatial Perception with Bilateral Bone Conduction

Virtual sound localization tests were conducted to examine the effects of stimulation position (mastoid, condyle, supra-auricular, temple, and bone-anchored hearing aid implant position) and frequency band (low frequency, high frequency, and broadband) on bone-conduction (BC) horizontal localization. Non-individualized head-related transfer functions were used to reproduce virtual sound through bilateral BC transducers. Subjective experiments showed that stimulation at the mastoid gave the best performance while the temple gave the worst performance in localization. Stimulation at the mastoid and condyle did not differ significantly from that using air-conduction (AC) headphones in localization accuracy. However, binaural reproduction at all BC stimulation positions led to similar levels of front-back confusion (FBC), which were also comparable to that with AC headphones. Binaural BC reproduction with high-frequency stimulation led to significantly higher localization accuracy than with low-frequency stimulation. When transcranial attenuation (TA) was measured, the attenuation became larger at the condyle and mastoid, and increased at high frequencies. The experiments imply that larger TAs may improve localization accuracy but do not improve FBC. The present study indicates that the BC stimulation at the mastoid and condyle can effectively convey spatial information, especially with high-frequency stimulation.

Measurement and modeling of the mechanical impedance of human mastoid and condyle

Bone conduction devices are used in audiometric tests, hearing rehabilitation, and communication systems. The mechanical impedance of the stimulated skull location affects the performance of the bone conduction devices. In the present study, the mechanical impedances of the mastoid and condyle were measured in 100 Chinese subjects aged from 22 to 67 years. The results show that the mastoid and condyle impedances within the same subject differ significantly and the impedance differences between subjects at the same stimulation position are mainly below the resonance frequency. The mechanical impedance of the mastoid is significantly influenced by age, and not related to gender or body mass index (BMI). While the mechanical impedance of the condyle is significantly affected by BMI, followed by gender, and not related to age. There are some differences in mastoid impedance between the Chinese and Western subjects. An analogy model predicts that the difference in mechanical impedance between the mastoid and condyle leads to a significant difference in the output force of the bone conduction devices. The results can be used to develop improved condyle and mastoid stimulators for the Chinese.

Bone Conduction Auditory Navigation Device for Blind People

A navigation system using a binaural bone-conducted sound is proposed. This system has three features to accurately navigate the user to the destination point. First, the selection of the bone-conduction device and the optimal contact conditions between the device and the human head are discussed. Second, the basic performance of sound localization reproduced by the selected bone-conduction device with binaural sounds is confirmed considering the head-related transfer functions (HRTFs) obtained in the air-borne sound field. Here, a panned sound technique that may emphasize the localization of the sound is also validated. Third, to ensure the safety of the navigating person, which is the most important factor in the navigation of a visually impaired person by voice guidance, an appropriate warning sound reproduced by the bone-conduction device is investigated. Finally, based on the abovementioned conditions, we conduct an auditory navigation experiment using bone-conducted guide announcement. The time required to reach the destination of the navigation route is shorter in the case with voice information including the binaural sound reproduction, as compared to the case with only voice information. Therefore, a navigation system using binaural bone-conducted sound is confirmed to be effective.

To Beep or Not to Beep? Evaluating Modalities for Multimodal ICU Alarms

Technology plays a prominent role in intensive care units (ICU), with a variety of sensors monitoring both patients and devices. A serious problem exists, however, that can reduce the sensors’ effectiveness. When important values exceed or fall below a certain threshold or sensors lose their signal, up to 350 alarms per patient a day are issued. These frequent alarms are audible in several locations on the ICU, resulting in a massive cognitive load for ICU nurses, as they must evaluate and acknowledge each alarm. “Alarm fatigue” sets in, a desensitization and delayed response time for alarms that can have severe consequences for patients and nurses. To counteract the acoustic load on ICUs, we designed and evaluated personal multimodal alarms for a wearable alarm system (WAS). The result was a lower response time and higher ratings on suitability and feasibility, as well as a lower annoyance level, compared to acoustic alarms. We find that multimodal alarms are a promising new approach to alert ICU nurses, reduce cognitive load, and avoid alarm fatigue.

Morphological differences affect speech transmission over bone conduction

In bone conduction (BC), acoustic signals travel through an individual's bones and soft tissues rather than travelling through the air. While bone conduction hearing and communication are important in everyday life, nature, and technology, little is known about how individual differences affect the transmission of bone-conducted sound. Individuals differ in the sizes, shapes, and proportions of their craniofacial bones, leading to potentially different bone-conducted sound transmission effects in different individuals. Individual differences may influence the audibility and quality of bone-conducted sound, and this was studied using speech intelligibility as an assessment criterion for bone-conducted sound transmission. Thirty-two human participants were first subjected to a series of anthropometric craniofacial measurements. Eight morphologically diverse talkers were recorded with bone microphones placed at different skull locations, and 24 morphologically diverse listeners listened to these samples over bone conduction headphones. Modified Rhyme Test results suggest that skull morphology influences BC speech intelligibility and does so differently at different skull locations. Understanding morphological effects can improve bone conduction sound transmission models and may help to enhance BC technology for a diverse user population.

The effect of vocal and demographic traits on speech intelligibility over bone conduction

Bone conduction (BC) communication systems provide benefits over air conduction systems but are not in widespread use, partly due to problems with speech intelligibility. Contributing factors like device location and background noise have been explored, but little attention has been paid to the role of individual user differences. Because BC signals travel through an individual's skull and facial tissues, demographic factors such as user age, sex, race, or regional origin may influence sound transmission. Vocal traits such as pitch, spectral tilt, jitter, and shimmer may also play a role. Along with microphone placement and background noise, these factors can affect BC speech intelligibility. Eight diverse talkers were recorded with bone microphones on two different skull locations and in different background noise conditions. Twenty-four diverse listeners listened to these samples over BC and completed Modified Rhyme Tests for speech intelligibility. Forehead bone recordings were more intelligible than condyle recordings. In condyle recordings, female talkers, talkers with high fundamental frequency, and talkers in background noise were understood better, as were communications between talkers and listeners of the same regional origin. Listeners' individual traits had no significant effects. Thoughtful application of this knowledge can help improve BC communication for diverse users.

The effect of bone conduction microphone placement on intensity and spectrum of transmitted speech items

Speech signals can be converted into electrical audio signals using either conventional air conduction (AC) microphone or a contact bone conduction (BC) microphone. The goal of this study was to investigate the effects of the location of a BC microphone on the intensity and frequency spectrum of the recorded speech. Twelve locations, 11 on the talker's head and 1 on the collar bone, were investigated. The speech sounds were three vowels (/u/, /a/, /i/) and two consonants (/m/, /∫/). The sounds were produced by 12 talkers. Each sound was recorded simultaneously with two BC microphones and an AC microphone. Analyzed spectral data showed that the BC recordings made at the forehead of the talker were the most similar to the AC recordings, whereas the collar bone recordings were most different. Comparison of the spectral data with speech intelligibility data collected in another study revealed a strong negative relationship between BC speech intelligibility and the degree of deviation of the BC speech spectrum from the AC spectrum. In addition, the head locations that resulted in the highest speech intelligibility were associated with the lowest output signals among all tested locations. Implications of these findings for BC communication are discussed.

Impact of a bone conduction communication channel on multichannel communication system effectiveness

OBJECTIVE

In this study, the impact of including a bone conduction transducer in a three-channel spatialized communication system was investigated.

BACKGROUND

Several military and security forces situations require concurrent listening to three or more radio channels. In such radio systems, spatial separation between three concurrent radio channels can be achieved by delivering separate signals to the left and right earphone independently and both earphones simultaneously. This method appears to be effective; however, the use of bone conduction as one channel may provide both operational and performance benefits.

METHOD

Three three-channel communication systems were used to collect speech intelligibility data from 18 listeners (System I, three loudspeakers; System 2, stereo headphones; System 3, stereo headphones and a bone conduction vibrator). Each channel presented signals perceived to originate from separate locations. Volunteers listened to three sets of competing sentences and identified a number, color, and object spoken in the target sentence. Each listener participated in three trials (one per system). Each trial consisted of 48 competing sentence sets.

RESULTS

Systems 2 and 3 were more intelligible than System I. Systems 2 and 3 were overall equally intelligible; however, the intelligibility of all three channels was significantly more balanced in System 3.

CONCLUSION

Replacing an air conduction transducer with a bone conduction transducer in a multichannel audio device can provide a more effective and balanced simultaneous monitoring auditory environment.

APPLICATION

These results have important design and implementation implications for spatial auditory communication equipment.

A free-field method to calibrate bone conduction transducers

Bone conduction communication systems employ a variety of transducers with different physical and electroacoustic properties, and these transducers may be worn at various skull locations. Testing these systems thus requires a reliable means of transducer calibration that can be implemented across different devices, skull locations, and settings. Unfortunately, existing calibration standards do not meet these criteria. Audiometric bone conduction standards focus on only one device model and on limited skull locations. Furthermore, while mechanical couplers may be used for calibration, the general human validity of their results is suspect. To address the need for more flexible, human-centered calibration methods, the authors investigated a procedure for bone transducer calibration, analogous to free-field methods for calibrating air conduction headphones. Participants listened to1s third-octave noise bands (125-12,500 Hz) alternating between a bone transducer and a loudspeaker and adjusted the bone transducer to match the perceived loudness of the loudspeaker at each test frequency. Participants tested two transducer models and two skull locations. Intra- and inter-subject reliability was high, and the resulting data differed by transducer, by location, and from the mechanical coupler. The described procedure is flexible to transducer model and skull location, requires only basic equipment, and directly yields perceptual data.

The effect of bone conduction microphone locations on speech intelligibility and sound quality

This paper presents the results of three studies of intelligibility and quality of speech recorded through a bone conduction microphone (BCM). All speech signals were captured and recorded using a Temco HG-17 BCM. Twelve locations on or close to the skull were selected for the BCM placement. In the first study, listeners evaluated the intelligibility and quality of the bone conducted speech signals presented through traditional earphones. Listeners in the second study evaluated the intelligibility and quality of signals presented through a loudspeaker. In the third study the signals were reproduced through a bone conduction headset; however, signal evaluation was limited to speech intelligibility only. In all three studies, the Forehead and Temple BCM locations yielded the highest intelligibility and quality rating scores. The Collarbone location produced the least intelligible and lowest quality signals across all tested BCM locations.