Spatial Release From Masking With Bilateral Bone Conduction Stimulation at Mastoid for Normal Hearing Subjects.

This study investigates the effect of spatial release from masking (SRM) in bilateral bone conduction (BC) stimulation at the mastoid. Nine adults with normal hearing were tested to determine SRM based on speech recognition thresholds (SRTs) in simulated spatial configurations ranging from 0 to 180 degrees. These configurations were based on nonindividualized head-related transfer functions. The participants were subjected to sound stimulation through either air conduction (AC) via headphones or BC. The results indicated that both the angular separation between the target and the masker, and the modality of sound stimulation, significantly influenced speech recognition performance. As the angular separation between the target and the masker increased up to 150°, both BC and AC SRTs decreased, indicating improved performance. However, performance slightly deteriorated when the angular separation exceeded 150°. For spatial separations less than 75°, BC stimulation provided greater spatial benefits than AC, although this difference was not statistically significant. For separations greater than 75°, AC stimulation offered significantly more spatial benefits than BC. When speech and noise originated from the same side of the head, the "better ear effect" did not significantly contribute to SRM. However, when speech and noise were located on opposite sides of the head, this effect became dominant in SRM.

Broadband sound absorption based on impedance decoupling and modulation mechanisms.

In sound absorbers, acoustic resistance and reactance are usually coupled together and affect each other, which brings difficulties to impedance matching. An impedance decoupling method is proposed to make acoustic resistance and acoustic reactance vary independently. For the same thickness and perforation rate, acoustic reactance of the perforated panel with tube bundles (PPTBs) varies with the diameter of the tube, but acoustic resistance remains constant. Theoretical and simulated results show that a PPTB absorptive unit can exhibit resonance modes with varying damping states through impedance decoupling. It is found that through well-modulation, the PPTB unit in a slightly over-damped state cannot only maintain high sound absorption coefficients, but also expand the absorption peak bandwidth. Utilizing the mechanism of impedance decoupling, a broadband absorber is designed and evaluated by comprising the PPTB and microperforated panel (MPP). Measurement results indicate that it possesses an average absorption coefficient of 85% spanning more than a 3-octave bandwidth from 160 Hz to 1400 Hz with a deep sub-wavelength thickness. The impedance decoupling method helps to implement sound absorbers with highly efficient low-frequency broadband absorption.

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.

Modeling individual head-related transfer functions from sparse measurements using a convolutional neural network.

Individual head-related transfer functions (HRTFs) are usually measured with high spatial resolution or modeled with anthropometric parameters. This study proposed an HRTF individualization method using only spatially sparse measurements using a convolutional neural network (CNN). The HRTFs were represented by two-dimensional images, in which the horizontal and vertical ordinates indicated direction and frequency, respectively. The CNN was trained by using the HRTF images measured at specific sparse directions as input and using the corresponding images with a high spatial resolution as output in a prior HRTF database. The HRTFs of a new subject can be recovered by the trained CNN with the sparsely measured HRTFs. Objective experiments showed that, when using 23 directions to recover individual HRTFs at 1250 directions, the spectral distortion (SD) is around 4.4 dB; when using 105 directions, the SD reduced to around 3.8 dB. Subjective experiments showed that the individualized HRTFs recovered from 105 directions had smaller discrimination proportion than the baseline method and were perceptually undistinguishable in many directions. This method combines the spectral and spatial characteristics of HRTF for individualization, which has potential for improving virtual reality experience.

An objective bone conduction verification tool using a piezoelectric thin-film force transducer.

All hearing aid fittings should be validated with appropriate outcome measurements, whereas there is a lack of well-designed objective verification methods for bone conduction (BC) hearing aids, compared to the real-ear measurement for air conduction hearing aids. This study aims to develop a new objective verification method for BC hearing aids by placing a piezoelectric thin-film force transducer between the BC transducer and the stimulation position. The newly proposed method was compared with the ear canal method and the artificial mastoid method through audibility estimation. The audibility estimation adopted the responses from the transducers that correspond to the individual BC hearing thresholds and three different input levels of pink noise. Twenty hearing-impaired (HI) subjects without prior experience with hearing aids were recruited for this study. The measurement and analysis results showed that the force transducer and ear canal methods almost yielded consistent results, while the artificial mastoid method exhibited significant differences from these two methods. The proposed force transducer method showed a lower noise level and was less affected by the sound field signal when compared with other methods. This indicates that it is promising to utilize a piezoelectric thin-film force transducer as an in-situ objective measurement method of BC stimulation.

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.

EmotionBox: A music-element-driven emotional music generation system based on music psychology.

With the development of deep neural networks, automatic music composition has made great progress. Although emotional music can evoke listeners' different auditory perceptions, only few research studies have focused on generating emotional music. This paper presents EmotionBox -a music-element-driven emotional music generator based on music psychology that is capable of composing music given a specific emotion, while this model does not require a music dataset labeled with emotions as previous methods. In this work, pitch histogram and note density are extracted as features that represent mode and tempo, respectively, to control music emotions. The specific emotions are mapped from these features through Russell's psychology model. The subjective listening tests show that the Emotionbox has a competitive performance in generating different emotional music and significantly better performance in generating music with low arousal emotions, especially peaceful emotion, compared with the emotion-label-based method.

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.

Investigation of an MAA Test With Virtual Sound Synthesis.

The ability to localize a sound source is very important in our daily life, specifically to analyze auditory scenes in complex acoustic environments. The concept of minimum audible angle (MAA), which is defined as the smallest detectable difference between the incident directions of two sound sources, has been widely used in the research fields of auditory perception to measure localization ability. Measuring MAAs usually involves a reference sound source and either a large number of loudspeakers or a movable sound source in order to reproduce sound sources at a large number of predefined incident directions. However, existing MAA test systems are often cumbersome because they require a large number of loudspeakers or a mechanical rail slide and thus are expensive and inconvenient to use. This study investigates a novel MAA test method using virtual sound source synthesis and avoiding the problems with traditional methods. We compare the perceptual localization acuity of sound sources in two experimental designs: using the virtual presentation and real sound sources. The virtual sound source is reproduced through a pair of loudspeakers weighted by vector-based amplitude panning (VBAP). Results show that the average measured MAA at 0° azimuth is 1.1° and the average measured MAA at 90° azimuth is 3.1° in a virtual acoustic system, meanwhile the average measured MAA at 0° azimuth is about 1.2° and the average measured MAA at 90° azimuth is 3.3° when using the real sound sources. The measurements of the two methods have no significant difference. We conclude that the proposed MAA test system is a suitable alternative to more complicated and expensive setups.

Speech quality evaluation of a sparse coding shrinkage noise reduction algorithm with normal hearing and hearing impaired listeners.

Although there are numerous papers describing single-channel noise reduction strategies to improve speech perception in a noisy environment, few studies have comprehensively evaluated the effects of noise reduction algorithms on speech quality for hearing impaired (HI). A model-based sparse coding shrinkage (SCS) algorithm has been developed, and has shown previously (Sang et al., 2014) that it is as competitive as a state-of-the-art Wiener filter approach in speech intelligibility. Here, the analysis is extended to include subjective quality ratings and a method called Interpolated Paired Comparison Rating (IPCR) is adopted to quantitatively link the benefit of speech intelligibility and speech quality. The subjective quality tests are performed through IPCR to efficiently quantify noise reduction effects on speech quality. Objective measures including frequency-weighted segmental signal-to-noise ratio (fwsegSNR), perceptual evaluation of speech quality (PESQ) and hearing aid speech quality index (HASQI) are adopted to predict the noise reduction effects. Results show little difference in speech quality between the SCS and the Wiener filter algorithm but a difference in quality rating between the HI and NH listeners. HI listeners generally gave better quality ratings of noise reduction algorithms than NH listeners. However, SCS reduced the noise more efficiently at the cost of higher distortions that were detected by NH but not by the HI. SCS is a promising candidate for noise reduction algorithms for HI. In general, care needs to be taken when adopting algorithms that were originally developed for NH participants into hearing aid applications. An algorithm that is evaluated negatively with NH might still bring benefits for HI participants.

Evaluation of the sparse coding shrinkage noise reduction algorithm in normal hearing and hearing impaired listeners.

Although there are numerous single-channel noise reduction strategies to improve speech perception in noise, most of them improve speech quality but do not improve speech intelligibility, in circumstances where the noise and speech have similar frequency spectra. Current exceptions that may improve speech intelligibility are those that require a priori knowledge of the speech or noise statistics, which limits practical application. Hearing impaired (HI) listeners suffer more in speech intelligibility than normal hearing listeners (NH) in the same noisy environment, so developing better single-channel noise reduction algorithms for HI listeners is justified. Our model-based "sparse coding shrinkage" (SCS) algorithm extracts key speech information in noisy speech. We evaluate it by comparison with a state-of-the-art Wiener filtering approach using speech intelligibility tests with NH and HI listeners. The model-based SCS algorithm relies only on statistical signal information without prior information. Results show that the SCS algorithm improves speech intelligibility in stationary noise and is comparable to the Wiener filtering algorithm. Both algorithms improve intelligibility for HI listeners but not for NH listeners. Improvement is less in fluctuating (babble) noise than in stationary noise. Both noise reduction algorithms perform better at higher input signal-to-noise ratios (SNR) where HI listeners can benefit but where NH listeners have already reached ceiling performance. The difference between NH and HI subjects in intelligibility gain depends fundamentally on the input SNR rather than the hearing loss level. We conclude that HI listeners need different signal processing algorithms from NH subjects and that the SCS algorithm offers a promising alternative to Wiener filtering. Performance of all noise reduction algorithms is likely to vary according to extent of hearing loss and algorithms that show little benefit for listeners with moderate hearing loss may be more beneficial for listeners with more severe hearing loss.

On the level-dependent attenuation of a perforated device.

To investigate the physical principle governing the level-dependent attenuation of a perforated earplug, a mathematical model is first established with the transfer-matrix method to calculate the noise reduction through a simplified device, one perforated panel with back cavity, mounted in an impedance tube. The model prediction is compared with the measured noise reduction through two series of large-scale devices and one device with the dimensions of the ear canal under continuous noise and sinusoidal excitations. The model helps to improve significantly the level-dependent attenuation of the large-scale device. It also illustrates that the attenuation is not solely determined by the resistance of the orifice, which has been a well accepted design concept, but resulted from an incorporated effect of the acoustic filter comprised of the acoustic impedance of the orifice and other elements in the earplug-ear-canal system. This mechanism can interpret a resonance at low incident levels on improper design and reveal approaches to eliminate it. Finally, the model's potential contributions to the design of a perforated earplug are discussed, along with the threshold of level-dependent attenuation supported with experimental evidence.