Risk factors for hearing loss at different frequencies in a population of 47,388 noise-exposed workers.

Weighted regression analysis was applied to determine the dependence of the hearing thresholds of 47,388 noise-exposed workers on age, sex, noise immission level, ear disease, head injury, tinnitus, hearing protector usage, and audiometric frequency in the range from 0.5 to 6 kHz. It could be shown that the hearing thresholds at any frequency are dominated by the age of the worker and that women, after equivalent exposure conditions, hear better than men. The relative effects of sex, noise immission level, ear diseases, tinnitus, and hearing protector usage are related to the audiometric frequency. Users of hearing protectors at the last audiometric investigation hear worse than nonusers. Hearing protector usage is strongly related with the hearing threshold in the low-frequency range. The noise immission level does not noticeably affect the hearing threshold below 3 kHz. The most important frequency of the noise immission level is as expected 4 kHz. For 4 kHz, it was shown that the variables age, noise immission level, tinnitus, head injuries, and ear diseases act in a good approximation additively on the pure-tone hearing threshold.     

Acoustic method for calibration of audiometric bone vibrators

The standard method for the calibration of audiometric bone vibrators requires the use of an artificial mastoid, a device that converts vibratory energy to an electrical analog. The mechanical input impedance of the device is designed to represent the average mechanical impedance of the human head. For calibration purposes, it is not necessary that the coupling device represent the impedance of the head. It is only necessary that it provides a repeatable measurement of the output of the vibrator that can be related to the normal threshold of hearing at each test frequency. In addition to the mechanical output that serves as the stimulus for the hearing test, bone vibrators produce an acoustic signal that is proportional to the mechanical force delivered to the head. By determining the transfer function relating the acoustic sound pressure to the mechanical force, the acoustic signal can serve as a proxy for the vibratory stimulus. This article describes the design and validation of an acoustic coupler for the calibration of audiometric bone vibrators.     

Is real-ear attenuation at threshold a function of hearing level?

In the course of measuring the real-ear attenuation at threshold (REAT) of experimenter-inserted E-A-R foam earplugs on 100 subjects, a statistically significant correlation was observed between attenuation and hearing level (for normal listeners, HTL less than or equal to 20 dB) at test frequencies from 2-8 kHz. Listeners with more sensitive hearing obtained better protection. The relationship was most robust at 6 and 8 kHz. For hearing levels greater than 20 dB, attenuation appeared independent of hearing level. A hypothesis was developed to explain the relationship for the normal listeners, based upon the fact that the high-frequency attenuation of the earplug was nearly bone-conduction limited. The hypothesis suggested that the attenuation of a hearing protector that provided substantially lower protection would not exhibit the same relationship. Data for such a device were collected for 70 subjects, and indeed demonstrated reduced correlation between attenuation and hearing level. Implications of the results of the experiments are discussed with regard to hearing level requirements for hearing protector attenuation test subjects, utilization of hearing-impaired listeners to measure REAT at suprathreshold (with respect to normal listeners) sound pressure levels, and linearity of hearing protector attenuation as a function of sound level.     

Earmuff comfort

In many industrial and military situations it is not practical or economical to reduce ambient noise to levels that present neither a hazard to hearing nor annoyance. In these situations, personal hearing protection devices are capable of reducing the noise by 20–30 dB. Although the use of a hearing protector is recommended as a temporary solution until action is taken to control the noise, in practice, it ends up as a permanent solution in most cases. Therefore, hearing protectors must be both efficient in terms of noise attenuation and comfortable to wear. Comfort in this case is related to the agreement of the user to wear the hearing protector consistently and correctly at all times. The purpose of this paper is, firstly, to provide some background on the publications related to earmuff comfort, most of which are based on measurement of the total headband force and subjective evaluation using questionnaires. Most of the published results show a weak correlation between total headband force and subjective evaluation. Secondly, this paper presents a new method to measure the contact pressure distribution between the earmuff cushions and the circumaural flesh of the human head and estimate a comfort index. The comfort parameters were investigated and equations developed to calculate comfort indices and overall quality indices. The most important calculated comfort index is measured from the contact pressure distribution and correlated with a subjective evaluation. Measurement results for the pressure distribution of 10 earmuffs show good correlation with the subjective evaluation.     

Effective compression and noise reduction configurations for hearing protectors

The author proposed to adopt wide dynamic range compression and adaptive multichannel modulation-based noise reduction algorithms to enhance hearing protector performance. Three experiments were conducted to investigate the effects of compression and noise reduction configurations on the amount of noise reduction, speech intelligibility, and overall preferences using existing digital hearing aids. In Experiment 1, sentence materials were recorded in speech spectrum noise and white noise after being processed by eight digital hearing aids. When the hearing aids were set to 3:1 compression, the amount of noise reduction achieved was enhanced or maintained for hearing aids with parallel configurations, but reduced for hearing aids with serial configurations. In Experiments 2 and 3, 16 normal-hearing listeners' speech intelligibility and perceived sound quality were tested when they listened to speech recorded through hearing aids with parallel and serial configurations. Regardless of the configuration, the noise reduction algorithms reduced the noise level and maintained speech intelligibility in white noise. Additionally, the listeners preferred the parallel rather than the serial configuration in 3:1 conditions and the serial configuration in 1:1 rather than 3:1 compression when the noise reduction algorithms were activated. Implications for hearing protector and hearing aid design are discussed.     

Effects of peak pressure and energy of impulses

Peak pressure has been one of the key parameters of impulse noise used to assess the hazard to hearing. It is used in most international noise exposure limits. France uses an A-weighted energy limit. There is a rough correspondence between peak pressure and the hazard to hearing for a given type of impulse noise. However, when the effects of different types of impulses are compared, this correspondence breaks down. One of the alternate measures of impulse intensity is weighted energy. Weighted energy is appealing for a number of reasons. It does not depend on details of the pressure-time history such as the peak pressure and the more common duration measures. It should be easier to integrate with continuous or intermittent noise standards. It would make it easier to use standard hearing protector attenuation to estimate the hazard when a specific hearing protector is worn. Results of previously published articles and reports will be discussed. These reports lead to the conclusion that weighted energy is a more potent determiner of hearing hazard than peak pressure if spectral effects are controlled.     

Hearing protector attenuation: A perspective view

The design, performance and evaluation of hearing protectors are matters of substantial current interest. The specific scientific questions which command attention include the accuracy of the shift in free field hearing threshold as a measure of protection, the relationship between physical measurements of sound attenuation with real heads and psycho-physical methods of measurement, the relationship between physical measurements with real and artificial heads, the acoustical behaviour of hearing protectors and the factors which limit their performance, and the relationship between hearing protector attenuation and speech intelligibility. These questions have received considerable attention during the past twenty years but are not yet fully answered.     

Design of a pulse generator and shock tube for measuring hearing protector attenuation of high-amplitude impulsive noise

The effectiveness of hearing protectors against high amplitude impulse noise levels remains the subject of research with objective testing techniques using acoustic test fixtures offering the only realistic method of providing rapid performance data for protector design and qualification. The work presented in this paper examines a prototype test method based on a shock tube and acoustic test fixture for the evaluation of protectors against high-level impulsive noise where established real ear attenuation at threshold methods would be impractical to apply. The results show that the system is capable of producing controlled repeatable high amplitude pressure pulses of variable duration for testing hearing protection devices in a grazing wave type test. A series of pilot tests illustrate how the system can have a sufficient self-insertion loss to reject flanking noise and allow the measurement of protector attenuations of up to 45 dB with little corruption from flanking noise.     

Earbud-type earphone modeling and measurement by head and torso simulator

This paper presents an analytical model using equivalent circuit method to design an earbud earphone. The electroacoustic parameters of a miniature loudspeaker are measured through a laser triangulation method. Design configurations are analyzed in accordance with the open and closed states of vent and sound holes of earphone. The equivalent circuit model is validated by the head and torso simulator measurements in an anechoic chamber. The effects of vent and sound holes on frequency response are examined and elucidated. The vent and sound holes found to affect the fundamental resonance frequency, low, and medium frequency response. The effect of sponge cover over the earphone’s front side is also investigated. Finally, it is concluded that the sponge can elevate the sound pressure level to 120 dBSPL and above, raising the possibility of permanent and incurable hearing loss. The major contribution of this work leads to successful development of earbud earphone.     

An acoustic head simulator for hearing protector evaluation. II: Measurements in steady-state and impulse noise environments

The attenuation characteristics of hearing protection devices (HPDs) were measured using a modular acoustic head simulator. The effect in changes in the head configuration was assessed in a steady-state diffuse sound field. The use of artificial circumaural skin had a relatively small influence on the insertion loss of earmuffs (max. 6-7 dB at low frequencies). This contrasts to the very large effects found for the artificial intraaural skin on the insertion loss of earplugs (in excess of 40 dB at low frequencies for some devices). Results were also compared with real-ear attenuation at threshold (REAT) data (ANSI S3.19-1974). In general, there is good agreement between the two methods, especially for earmuffs. Design improvements are proposed for earplugs. The result of an exploratory study aimed at measuring the complex (amplitude and phase) insertion loss of HPDs using an impulse noise source are also reported.     

Methods of measuring the attenuation of hearing protection devices

The published literature describing three real-ear-attenuation-at-threshold (REAT), nine above-threshold, and four objective methods of measuring hearing protector attenuation is reviewed and analyzed with regard to the accuracy, practicality, and applicability of the various techniques. The analysis indicates that the REAT method is one of the most accurate available techniques since it assesses all of the sound paths to the occluded ear and, depending upon the experimenter's intention, can reflect actual in-use attenuation as well. An artifact in the REAT paradigm is that masking in the occluded ear due to physiological noise can spuriously increase low-frequency (less than or equal to 500 Hz) attenuation, although the error never exceeds approximately 5 dB, regardless of the device, except below 125 Hz. Since the preponderance of available data indicates that attenuation is independent of sound level for intentionally linear protectors, the use of above-threshold procedures to evaluate attenuation is not a necessity. An exception exists in the case of impulsive noises, for which the existing data are not unequivocal with regard to hearing protector response characteristics. Two of the objective methods (acoustical test fixture and microphone in real ear) are considerable time savers. All objective procedures are lacking in their ability to accurately determine the importance of the flanking bone-conduction paths, although some authors have incorporated this feature as a post-measurement correction. The microphone in real-ear approach is suggested to be one of the most promising for future standardization efforts and research purposes, and the acoustical test fixture technique is recommended (with certain reservations) for quality control and buyer acceptance testing.     

Use of the sound levels of noise for assessing the adequacy of hearing protectors

The dB(A) sound level of a noise is accepted as a measure of the damage risk to unprotected ears but often it is not a reliable guide to the risk to ears fitted with hearing protectors. For any dB(A) level inside a protector, normally there will be substantially higher sound levels outside that protector. This paper shows how, from sequential frequency attenuation bands of the protector, and sound level weightings, external sound levels can be calculated, below which the noise inside the protector does not exceed a chosen dB(A) level. Further valuable information may be obtained by mapping external dB(A) and dB(C) levels to cover all possible noise spectra that give the chosen dB(A) level inside the protector. Thus, from a pair of measured sound levels, use of the method indicates whether the protector is sufficient or not, or whether more detailed measurment of the noise is required. This knowledge enhances the scope of the sound level meter and reduces the need for frequency analysis of industrial noise. Its application should be a helpful addition to the data provided by suppliers of hearing protectors.     

Applied topology optimization of vibro-acoustic hearing instrument models

Designing hearing instruments remains an acoustic challenge as users request small designs for comfortable wear and cosmetic appeal and at the same time require sufficient amplification from the device. First, to ensure proper amplification in the device, a critical design challenge in the hearing instrument is to minimize the feedback between the outputs (generated sound and vibrations) from the receiver looping back into the microphones. Secondly, the feedback signal is minimized using time consuming trial-and-error design procedures for physical prototypes and virtual models using finite element analysis. In the present work it is demonstrated that structural topology optimization of vibro-acoustic finite element models can be used to both sufficiently minimize the feedback signal and to reduce the time consuming trial-and-error design approach. The structural topology optimization of a vibro-acoustic finite element model is shown for an industrial full scale model hearing instrument.     

红外凝视传感器定量仿真及模型验证

针对新一代红外凝视成像传感器,依据红外辐射能量传递和转换的物理过程,完成了对传感器各组成单元的物理效应建模.不同于以往成像模型的定性仿真,该模型实现了对系统各模块成像特性的定量描述,综合考虑了传感器的信号传递特性、空间传递特性、空间采样特性和时空噪音特性,构建了较为完善的高仿真度红外成像仿真模型.为验证仿真模型的有效性,搭建了有效性实验验证平台,获取了真实热像仪的性能曲线和输出图像.通过计算和比较仿真模型和真实热像仪性能曲线和输出图像相似程度,定量化评价了模型的仿真度,验证了仿真模型的有效性.     

An object-oriented designed finite-difference time-domain simulator for electromagnetic analysis and design in MRI--applications to high field analyses

This paper presents a finite-difference time-domain (FDTD) simulator for electromagnetic analysis and design applications in MRI. It is intended to be a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The program has been constructed in an object-oriented framework. The design procedure is detailed and the numerical solver has been verified against analytical solutions for simple cases and also applied to various field calculation problems. In particular, the simulator is demonstrated for inverse RF coil design, optimized source profile generation, and parallel imaging in high-frequency situations. The examples show new developments enabled by the simulator and demonstrate that the proposed FDTD framework can be used to analyze large-scale computational electromagnetic problems in modern MRI engineering.     

The use of acoustical test fixtures for the measurement of hearing protector attenuation. Part II: Modeling the external ear, simulating bone conduction, and comparing test fixture and real-ear data

This paper investigates two main features of the human head which influence the measured attenuation of circumaural and intraaural hearing protection devices (HPDs): the external ear and the different pathways of bone conduction. A theoretical model for the external ear shows that its influence on the insertion loss of HPDs, on the sensitivity level of headphones or earphones, and on the insertion gain of hearing aids, all can be described by one equation. While it is not necessary to simulate the eardrum impedance in order to measure the insertion loss of earmuffs and the sensitivity level of headphones with acoustical test fixtures (ATFs), the required accuracy of an ear simulator is more stringent when the same measurements are performed on intraaural devices. For the evaluation of HPDs, bone conduction plays an important role. We have developed a model to estimate HPD-dependent bone conduction effects. The model includes two bone conduction sources: one in the external ear and one in the middle ear. The model explains, for example, the occlusion effect of HPDs and the masking error at low frequencies due to physiological noise that arises when real-ear attenuation at threshold (REAT) measurements are made. Consequently, objectively measured insertion loss can now be used to predict REAT with improved accuracy. ATF and REAT data are compared using nine earmuffs and nine earplugs. In the majority of cases, the two sets of data agree well. Discrepancies are discussed.     

The typical noise: first step in the development of a short procedure for estimating performance of hearing protectors

An exact measurement of the effectiveness of a hearing protector requires the determination of how well it works in the specific noise in which it is to be worn. As a practical matter, though, people do not generally remain in a single noise spectrum throughout a working career or even throughout a working day, so it does not matter so much that most measurers are not likely to be equipped to perform spectrum analyses. Other techniques for judging earplugs and earmuffs are obviously necessary. Previous research has tried to find the average attenuation given by a particular device in several (or many) noises, or it has tried to deal with the attenuation given in noises given with particular C-minus-A values. In this paper, a procedure is developed for compressing these multiple-spectra calculations into a single-spectrum computation that proves to be at least as accurate as the more complex methods.     

An improvement to computational efficiency of the drain current model for double-gate MOSFET

As a connection between the process and the circuit design, the device model is greatly desired for emerging devices, such as the double-gate MOSFET. Time efficiency is one of the most important requirements for device modeling. In this paper, an improvement to the computational efficiency of the drain current model for double-gate MOSFETs is extended, and different calculation methods are compared and discussed. The results show that the calculation speed of the improved model is substantially enhanced. A two-dimensional device simulation is performed to verify the improved model. Furthermore, the model is implemented into the HSPICE circuit simulator in Verilog-A for practical application.     

Measurement of the attenuation characteristics of nonlinear hearing protective devices using the auditory brain stem response

The purpose of this study was to determine the feasibility of using the auditory brain stem response (ABR) as a method of measuring the attenuation characteristics of nonlinear hearing protective devices. Sound field ABRs were recorded from seven normal hearing subjects with and without hearing protection. Three hearing protectors (two nonlinear and one linear) were evaluated. Test stimuli, consisting of 4000-Hz tone pips, were presented in a sound field. Linearity and the amount of attenuation for each hearing protector were derived by comparing the protected and unprotected latency-intensity functions for wave I of the ABR. Results indicate that the ABR may be used effectively to measure the attenuation characteristics of linear and nonlinear hearing protectors for high-frequency impulse-type stimuli.     

The use of acoustical test fixtures for the measurement of hearing protector attenuation. Part I: Review of previous work and the design of an improved test fixture

This paper gives a comprehensive progress report on the development of objective methods for measuring the attenuation of hearing protection devices (HPD's), and focuses on the use of acoustic test fixtures (ATF's), i.e., artificial heads. While there are many publications on ATF's for the evaluation of circumaural HPD's (earmuffs), only one serious attempt to construct an ATF for the evaluation of intra-aural HPD's (earplugs) could be found. Consequently, no ATF for testing earplugs has been standardized so far, while two standardized ATF's currently exist for testing earmuffs [see ANSI S3.19-1974 (1975) and ISO/DIS 6290 (1983)]. Both ATF's are suited, however, only for production testing and are not designed for HPD-type testing. It is believed that both ATF's do not provide sufficiently high accuracy for HPD-type testing. A new ATF with appropriate circumaural and intra-aural flesh simulations was constructed, including a suitable ear simulator and a cast of an average pinna. Objectives for design and construction of the new ATF are discussed. The effect of using artificial flesh on the insertion loss of earmuffs (max. 5 dB at 125 and/or 250 Hz) and the effect of using a pinna (max. 12 dB lower insertion loss at 2 kHz) were evaluated.