Active Hearing Protectors Research Articles

Improving speech intelligibility in active hearing protectors and communication headsets with subband processing

Parallel subband processing, in which the full bandwidth of environmental noise and a communication channel are processed separately in contiguous, restricted frequency bands, has been proposed as a means to improve speech intelligibility in noise for active hearing protectors and communication headsets (Bernstein et al., Int. J. Ind. Ergonom., in press). An active, adaptive feed-forward control structure has been employed to improve the audibility of sounds in the communication channel of a circumaural hearing protector / headset while active noise reduction (ANR) is used to complement the passive attenuation of the device from 50 to 800 Hz. The communication channel subbands have been implemented as octave bands from 125 Hz to 8 kHz. The performance of the device has been evaluated in a diffuse field when worn by human subjects. Word intelligibility in industrial noise was evaluated when the active system was not operating, when the device was operating as a fullband ANR system with fixed communication channel gain, and as a subband ANR system with adaptive gain of the communication channel signal to improve the speech signal-to-noise ratio. A significant improvement in speech intelligibility can be obtained with the subband system. [Work supported by NIOSH 5R01OH 008669.]

Performance of Different Types of Hearing Protectors Undergoing High-Level Impulse Noise

The paper describes the problems that may occur when hearing protectors, usually designed for industrial noise environments, are used for high-level impulse (weapon) noise. The military impulse noise environment is described, as are the different types of passive and active hearing protectors and the measurement procedures. The different mechanisms that may alter the effectiveness of the hearing protectors as well as their global efficiency when submitted to high-level impulse noise are presented. The paper also discusses how the performance values accessible to the user may be used in different damage risk criteria for continuous and impulse noise.

Active hearing protectors: Prospects for improving speech intelligibility while maintaining noise reduction and audibility of warning alarms

Users of hearing protection devices (HPDs), including the communication headsets and helmets commonly worn by aircraft air and ground crews, emergency responders, military personnel, and persons in industry working in hostile environments, have long complained that HPDs interfere with their ability to communicate and to hear warning alarms. There thus appears to be a need for additional specialized HPDs designed specifically for these situations. The technology of active noise control together with signal processing applied to the communication channel of the HPD may provide a potential solution. The combination could improve noise reduction at low frequencies and enhance the intelligibility of speech reproduced by the communication channel, while maintaining the overall noise reduction within occupational exposure requirements, and assisting the perception of external warning alarms. A strategy for achieving these goals using an active circumaural earmuff equipped with sub-band processing will be described, in which parallel controllers optimize the speech signal-to-noise ratio in bands that contribute most to intelligibility. The perception of a tonal alarm is assisted by binaural feed through to the earphones, within the constraints imposed by occupational exposure limits, and by sub-band processing to reduce the upward spread of masking.

Digital filters used in the feedback loop of an ANR earplug

Commercially available active hearing protectors are usually implemented as ear muffs with an analog driven feedback loop. The active attenuation bandwidth of these devices is limited to about 800 Hz. Moreover, it is not possible to adapt the analog system for specific noise events or to the user’s morphology. Active earplugs allow one to extend the bandwidth of the active protection to frequencies higher than 2 kHz. A digital feedback system has been implemented for active earplugs, in order to allow an adaptation to different noises and users. As the acoustic delays are too short to obtain a causal system when using FIR filters, the filters have been implemented as IIR type filters. The paper presents numerical simulations of different possible algorithms, used for the cascaded IIR filter of the ANR system. It shows how numerical errors are propagated in cascaded Bi-Quad implementations, and how an optimum signal-to-noise ratio may be obtained. The numerical predictions and experimental results will be presented and compared. As these filters will be used in a closed-loop feedback system, a numerical simulation of the behavior of the closed-loop system, including the electroacoustic transfer function of the ear plug, will be presented.

Comparison between subjective and objective measures of active hearing protector and communication headset attenuation.

A masked-threshold and a loudness-balance method have been developed to estimate the attenuation of communication headsets and hearing protectors with built-in active noise reduction (ANR) systems. Both methods are used to estimate the attenuation of the ANR systems and the masked-threshold method is also used to estimate the total attenuation (active plus passive) of the device. The procedures are designed to be used in the presence of environmental noise, and to minimize the noise exposure of subjects during the measurements. For comparison, physical measurements of insertion loss have also been performed using a miniature microphone in the concha. Experiments showed that the masked-threshold methods tends to give increased estimates of the attenuation if the noise reduction of the left and right earcup ANR systems differs, as commonly occurs in practice. In contrast, the loudness-balance method reduces the estimates of the active attenuation. Insertion loss measurements may be influenced by the position of the microphone, owing to the spatial variability of the sound field under an earmuff when the ANR system is operating. Differences between physical and subjective measurements of up to 20 dB have been obtained in this study at frequencies of 250 Hz and below for a device in which the sound pressure varied substantially near, and within, the ear canal.

On spatial variability in the attenuation performance of active hearing protectors

As noise cancellation (ANR) hearing protectors enter the market-place, the question is being asked, ‘How should we assess their attenuation performance?’. With the introduction of new European and international standards for hearing protectors, there is a need to evolve appropriate attenuation test methods. One particular issue which relates to this type of device is the variability in cancellation achieved throughout the ear-muff cavity, and consequently the level of attenuation perceived by the user. Experimental data are presented to illustrate the effects of alternative design approaches to the problem.

Noise control on military shooting ranges for rifles

Impulse noise from rifle shots on shooting ranges exposes trainers and trainees to a significant risk to their hearing. This study concerns the possibilities of controlling noise exposure during military shooting practice. Without any noise-control measures on open ground, the average noise exposure was 100 dB for a trainer and 110 dB for a trainee (LAeq5min), i.e. 80 dB for a trainer and 90 dB for a trainee over a day (LAeq8h). The noise peak values were 150–160 dB near the trainee and 120–150 dB near the trainer. The peak values of rifle shots were attenuated to 15–40 dB with hearing protectors, 18–28 dB with silencers, 10–15 dB with enlosures around the rifle and 1–10 dB with walls. The duration of the shots and the impulse energy attenuation were also studied. In a wooden shelter the noise exposure of a trainer was 0–12 dB higher than on an open range. When the shelter was covered with absorbent material, the noise energy was attenuated to 1–8 dB. Active hearing protectors would aid the teaching of trainees by keeping speech audible.

Active hearing protectors

A hearing protective device consisting of a headset having opposite earmuff and/or earplug assemblies resiliently connected by a headband assembly including a normally open power switch closed when the headband assembly is put on the user's head. The switch controls the energizing of the electrical circuitry of the device. Each earmuff and/or earplug assembly has an outwardly-facing microphone and an inwardly-facing sound reproducer, connected to circuitry defining respective stereo channels, each channel including preamplifier circuitry and a respective power amplifier drivingly connected to one of the sound reproducers. The circuitry also includes a hyper AGC circuit which receives and sums signals from the amplifier chain and derives attenuation signals therefrom which are fed back to the amplifier circuitry and which reduces the gain of this circuitry when the input sound exceeds a certain level. This causes the inputs to the power amplifiers to fall when the input to the preamplifiers increases beyond a certain level. A balanced attenuation circuit arrangement is employed which reduces the size of too-large sound waves without altering their shape, whereby the volumes of the two stereo channels are maintained in the proper relation to each other to preserve binaural hearing.

Identification and discrimination of azimuth with passive and active hearing protectors

Three studies are reported in which auditory localization ability is measured with and without hearing protectors. Assessment is made under two methods. Identification is indication on an analog of perceived azimuth of the stimulus. Identification data are presented as response frequency surfaces for 36 stimulus azimuths at 10° intervals. Discrimination measures were made under a two‐alternative forced‐choice matching paradigm for temporally and spatially disparate stimuli. Data are presented as psychometric functions of proportions of correct responding. Results of these studies indicate that passive circumaural devices degrade localization ability by eliminating pinna cues and by modifying spectral cues. Active devices tested severely disrupted localization and produced sometimes bizarre patterns of responding by eliminating interaural difference cues or by severely distorting signals. Stimuli were long duration broadband noise bursts.

Hearing protector effects on identification of sound source azimuth

Six normal hearing listeners who were presented with 750-ms bursts of wideband noise at 60 dB SPL from randomly selected positions on the horizontal plane indicated the apparent locus by adjusting a dial. Five conditions including active and passive protective modes with monaural and binaural devices and an unprotected control were run. Results provide a description of normal auditory localization ability and estimates of resolution of angle in the horizontal plane. Results also indicate severe distortion of localization functions when active hearing protectors were worn. Effects appear to be more strongly influenced by loss of information due to characteristics of amplifier circuits than by physical configuration of hearing protectors.