Conventional Directional Microphones Research Articles

Speech Tracking in Complex Auditory Scenes with Differentiated In- and Out-Field-Of-View Processing in Hearing Aids.

In naturalistic auditory scenes, relevant information is rarely concentrated at a single location, but rather unpredictably scattered in- and out-field-of-view (in-/out-FOV). Although the parsing of a complex auditory scene is a fairly simple job for a healthy human auditory system, the uncertainty represents a major issue in the development of effective hearing aid (HA) processing strategies. Whereas traditional omnidirectional microphones (OM) amplify the complete auditory scene without enhancing signal-to-noise-ratio (SNR) between in- and out-FOV streams, directional microphones (DM) may greatly increase SNR at the cost of preventing HA users to perceive out-FOV information. The present study compares the conventional OM and DM HA settings to a split processing (SP) scheme differentiating between in- and out-FOV processing. We recorded electroencephalographic data of ten young, normal-hearing listeners who solved a cocktail-party-scenario-paradigm with continuous auditory streams and analyzed neural tracking of speech with a stimulus reconstruction (SR) approach. While results for all settings exhibited significantly higher SR accuracies for attended in-FOV than unattended out-FOV streams, there were distinct differences between settings. In-FOV SR performance was dominated by DM and SP and out-FOV SR accuracies were significantly higher for SP compared to OM and DM. Our results demonstrate the potential of a SP approach to combine the advantages of traditional OM and DM settings without introduction of significant compromises.

Benefits and Limitations of Common Directional Microphones in Real-World Sounds

We present extensive experimental data to objectively evaluate the benefits and limitations of common directional microphones in real-world sound fields. The microphones include a conventional directional microphone(DM), a balanced DM, etc., plus the Omni microphone (mic) as a benchmark. The evaluation focuses on noise outputs, signal-to-noise ratios(S/Ns) and distortions; the real-world sounds include male voices, female voices, babble noises, white noises and talking interferences. Each type of noises is at 4 or 5 levels, from 30 to 70 dB SPL, at 10 dB step, and each talking interference is at 3 levels: 50, 60 and 70 dB SPL. The research methods include analytically deriving sensitivity-gains, statistically calculating the three mics’ outputs, experimentally viewing waveforms and spectra, and using large-sample wave files for a high confidence level. According to the experimental results, this paper concludes that 1) for a conversation in a quiet field, in soft or low noise field, the common DMs achieve comfortable S/Ns: 7 to 33 dB, similar to what the Omni mic does; 2) for a conversation in low, competing or strong talking interference fields, the common DMs achieve about 16 dB better S/N than the Omni mic does; 3) for a conversation in competing or strong surrounding noise field, the common DMs do not achieve beneficial S/N to understand speech; the common DMs’ noises are close to the Omni mic noise; 4) in various experiments, the balanced DM preserve speech fidelity well as the Omni mic does, while the conventional DM does poorly. This paper further introduces the Simulink experimental manipulations, such as digital FIR filters’ design, stereo channels’ wave files creation, etc., in the Appendix.

POLAR PLOTS OF A DIRECTIONAL MICROPHONE WITH REAL-WORLD SOUNDS AND ITS SPECTRUM DISTORTION

Some references discussed and evaluated performances of conventional directional microphones in hearing aids, from their conclusions, it is undoubted for the microphones to improve S/N, yet conditions of the advantage somewhat confused to hearing aid clinicians and professionals. To supplement the incompletion, we investigated behaviors of a directional microphone in extensive situations. We built an SimuLink laboratory, selected realistic talking voices and noises from wave files and made many experiments to find out illustrative evidences. Electroacoustic models are used for components of modelling a directional microphone; we operated the microphone with these sounds from real-world to calculate, view, measure and record behaviors of the directional microphone. We acquired many waveforms, statistics and recordings of the experiments with woman’s and man’s talking voices, and environmental noises; we also listened to sounds at input and output of the directional microphone to perceive changes of the speech naturalness. 
 Comparing to an omni microphone, the directional microphone does not enhances the speech signals on average; sensitivity-gain of the directional microphone is higher than that of the omni microphone when the tone frequency > 1.78 kHz; the directional microphone cancels the undesired speech well, as well as the babble noise and the environmental white noise from a beamed source at rear side; the directional microphone cannot improve the S/N of a speech within babble or white noise fields; in addition, we also observed and heard the speech spectrum distortion caused by the directional microphone

An evaluation of the performance of two binaural beamformers in complex and dynamic multitalker environments

Objective: Binaural beamformers are super-directional hearing aids created by combining microphone outputs from each side of the head. While they offer substantial improvements in SNR over conventional directional hearing aids, the benefits (and possible limitations) of these devices in realistic, complex listening situations have not yet been fully explored. In this study we evaluated the performance of two experimental binaural beamformers. Design: Testing was carried out using a horizontal loudspeaker array. Background noise was created using recorded conversations. Performance measures included speech intelligibility, localization in noise, acceptable noise level, subjective ratings, and a novel dynamic speech intelligibility measure. Study sample: Participants were 27 listeners with bilateral hearing loss, fitted with BTE prototypes that could be switched between conventional directional or binaural beamformer microphone modes. Results: Relative to the conventional directional microphones, both binaural beamformer modes were generally superior for tasks involving fixed frontal targets, but not always for situations involving dynamic target locations. Conclusions: Binaural beamformers show promise for enhancing listening in complex situations when the location of the source of interest is predictable.

The Effect of Hearing Aid Technologies on Listening in an Automobile

Communication while traveling in an automobile often is very difficult for hearing aid users. This is because the automobile/road noise level is usually high, and listeners/drivers often do not have access to visual cues. Since the talker of interest usually is not located in front of the listener/driver, conventional directional processing that places the directivity beam toward the listener's front may not be helpful and, in fact, could have a negative impact on speech recognition (when compared to omnidirectional processing). Recently, technologies have become available in commercial hearing aids that are designed to improve speech recognition and/or listening effort in noisy conditions where talkers are located behind or beside the listener. These technologies include (1) a directional microphone system that uses a backward-facing directivity pattern (Back-DIR processing), (2) a technology that transmits audio signals from the ear with the better signal-to-noise ratio (SNR) to the ear with the poorer SNR (Side-Transmission processing), and (3) a signal processing scheme that suppresses the noise at the ear with the poorer SNR (Side-Suppression processing). The purpose of the current study was to determine the effect of (1) conventional directional microphones and (2) newer signal processing schemes (Back-DIR, Side-Transmission, and Side-Suppression) on listener's speech recognition performance and preference for communication in a traveling automobile. A single-blinded, repeated-measures design was used. Twenty-five adults with bilateral symmetrical sensorineural hearing loss aged 44 through 84 yr participated in the study. The automobile/road noise and sentences of the Connected Speech Test (CST) were recorded through hearing aids in a standard van moving at a speed of 70 mph on a paved highway. The hearing aids were programmed to omnidirectional microphone, conventional adaptive directional microphone, and the three newer schemes. CST sentences were presented from the side and back of the hearing aids, which were placed on the ears of a manikin. The recorded stimuli were presented to listeners via earphones in a sound-treated booth to assess speech recognition performance and preference with each programmed condition. Compared to omnidirectional microphones, conventional adaptive directional processing had a detrimental effect on speech recognition when speech was presented from the back or side of the listener. Back-DIR and Side-Transmission processing improved speech recognition performance (relative to both omnidirectional and adaptive directional processing) when speech was from the back and side, respectively. The performance with Side-Suppression processing was better than with adaptive directional processing when speech was from the side. The participants' preferences for a given processing scheme were generally consistent with speech recognition results. The finding that performance with adaptive directional processing was poorer than with omnidirectional microphones demonstrates the importance of selecting the correct microphone technology for different listening situations. The results also suggest the feasibility of using hearing aid technologies to provide a better listening experience for hearing aid users in automobiles.

2채널 마이크로폰을 이용한 청각 기기에서의 빔포밍에 대한 객관적 검증

Hearing devices like cochlear implant, vibrant soundbridge, etc. try to offer better sound for people. In hearing devices, several beamformers including conventional directional microphone are applicable to noise reduction. Each beamformer has different directional response and it could change sound intelligibility or quality for listeners. Therefore, we investigated the performance of three beamformers, which are first and second order directional microphone, and broadband beamformer(BBF) with a computer simulation assuming hearing device microphone configuration. We also calculated objective measurements which have been used to evaluate speech enhancement algorithms. In the simulation, a single speech and a single babble noisewere propagated from the front and azimuth degrees respectively. Microphones were configured in an end-fire array and the spacing was varied in comparison. With 3 cm spacing, BBF had about 3 dB higher enhanced SNR than that of directional microphones. However, enhancement of segmental SNR and frequency weighted segmental SNR were similar between the first order directional microphone and broadband beamformer. In addition when steady state noise was used, broadband beamformer showed the increased performance and had the highest enhanced SNR, and segmental SNR.

Optimized second-order gradient microphone for hands-free speech recordings in cars

Hands-free telephony and automatic speech recognition in adverse acoustic conditions require noise reduction and speech enhancement methods to achieve acceptable quality and speech recognition rates. This contribution is primarily focussed on the optimization of the recording microphone. The directivity of the microphone can be used to reduce sound components outside the microphone's main axis which suppresses noise and room reverberation effectively. Considering the outer dimensions, gradient microphones possess a high directivity and good noise suppression properties. In this study, the feasibility and the noise-cancelling characteristics of an optimized second-order gradient microphone are investigated. The development of a simple sensitivity-matching principle allows the use of low-priced microphone capsules. The directivity index, characterizing the diffuse noise suppression, is nearly frequency-independent in the telephone bandwidth and is about 3 dB higher than that of conventional directional microphones used in telecommunication. The microphone is evaluated in an automobile environment with the help of a speaker-dependent single-word recognizer and is compared with lower-order gradient microphones. The noise robustness of the speech recognizer is improved by at least 4.7 dB, provided the microphone is in an optimum position. Conclusions are then drawn regarding such an optimum microphone positioning.

Directivity characteristics of adaptive microphone-array for noise reduction(AMNOR).

AMNOR is a new type of noise-reduction microphone system which uses a microphone arrayand digital signal processing. In this paper, the theory of AMNOR is described in the frequency domain, and its directivity characteristics are studied. Then, it is shown that, under some simple noise conditions, AMNOR forms the same directivity patterns as those formed by conventional directional microphones. In contrast to the directivity characteristics of conventional directional microphones which are assumed to be optimal for some restricted noise environments, AMNOR can realize the optimal directivity characteristics adaptively for a wide variety of existing noise environments. This is accomplished in AMNOR by the following three functions;(1) AMNOR forms M-1 (where M is the number of microphone elements) dead angles with respect to the noise arrival directions, (2) AMNOR optimizes the directivity lobe figure (3) AMNOR performs directivity control of (1) and (2) for each frequency band independently.

Hearing‐aid microphone systems with increased directionality

Under some conditions of background noise and/or reverberation, increasing the directionality of hearing‐aid microphone systems can lead to gains in speech intelligibility. Conceptually, directional microphone systems are realized by combining the outputs of multiple, spatially distributed omnidirectional sensors. Two approaches to obtaining increased directivity D derive from the classical antenna theory: (1) With uniform or tapered sensor weightings, increasing the spatial extent L of the sensor array increases D. The maximum D is limited to 2Lf/c, where f is frequency and c is the speed of sound. (2) With arbitrarily small arrays, broadband “superdirectivity” can be achieved whereby the maximum D is independent of frequency and equals the square of the number of sensors in the array. A conventional hearing‐aid directional microphone exploits suboptimum superdirectivity; two closely spaced sensors attain D = 4.5 dB up to about 4 kHz. We shall review the basic principles and limitations of such microphone systems and report on our effort to develop a four‐sensor array (using two conventional directional microphones) that achieves D = 8.5 dB and a 3‐dB beamwidth of ± 37° for f < 4 kHz with an array length L = 4.1 cm that is practical for hearing‐aid applications. [Work supported by NIH.]