Hearing Protector Attenuation Research Articles

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.

Effects of work conditions simulated in a laboratory environment and wearer fit on attenuation of slow-recovery foam earplugs

Laboratory-derived ratings of hearing protector attenuation typically overestimate the protection levels provided in-field. A review of prior research on wearing time effects suggested that attenuation may decrease over time with aural inserts, but that this effect may be device-specific. A psychophysical real-ear measurement procedure was used in an experiment conducted to determine the change in protection levels afforded by particular slow-recovery foam earplugs under different levels of subject fit, subject movement activity and wearing time. Statistical analyses of the spectral attenuation data supported several important results. These compliant inserts were found to be stable in the face of vigorous temporomandibular and highly kinetic bodily activity, thus exhibiting negligible degradation in attenuation over time. However, improvements in attenuation provided by trained-subject fit vs. naive-subject fit of the earplugs were large at 1000 Hz and below, ranging from gains of 12–14 dB, and were smaller, but still statistically significant, at 2000–8000 Hz, ranging from gains of 3–5 dB. Practical implications of the findings are discussed.

Hearing protector attenuation performance under different simulated work and fitting conditions

Four hearing protectors—a foam‐cushion earmuff, foam and triple‐flanged polymer earplugs, and an earmuff over foam earplug combination—were evaluated to determine the effects of wearing time, movement activity, and fitting procedure on spectral attenuation. Using a psychophysical real‐ear‐attenuation‐at‐threshold measurement procedure (as per ANSI S12.6‐1984), attenuation data were collected from 40 naive subjects before, during, and after the activity movement tasks, which induced highly kinematic subject movements. Loss in attenuation over the 2‐h wearing period was up to 6 dB for all devices except the foam earplug, and all protectors except the earmuff significantly improved protection (from 4 to 14 dB) at 1000 Hz and below due to trained fitting. The foam plug benefited more than other devices from the training for proper fitting but was resilient to subject movement. [Work supported by NIOSH‐NIH.]

A multi-purpose facility for research on hearing protection

Assessment of the efficacy of personal hearing protectors is based on a wide range of factors, requiring a diversity of experimental methodologies and acoustic test environments. This paper describes the development of a multi-purpose facility which complies with current Canadian and American standards for the measurement of the real-ear attenuation of hearing protectors. The facility also allows for a variety of experiments in auditory perception, including speech intelligibility and sound localization, known to be particularly relevant in the workplace. Two main features are: (1) an acoustic test chamber with a modular sound absorption system, and (2) computer-controlled signal presentation and subject monitoring instrumentation. As an application of the facility, the attenuation results generated using two psychophysical methodologies, a variation of Bekesy tracking and two-interval forced-choice, were compared for two insert hearing protectors (E-A-R formable foam and Bilsom Soft) and two earmuffs (E-A-R 3000 and Hellberg No-Noise). The difference due to method was not statistically significant.

Measurement of hearing protector attenuation using the physical ear attenuation test (PEAT)

The attenuation provided by a sample of circumaural hearing protectors (ear muffs) was measured using the physical ear attenuation test (PEAT). The PEAT utilized a small electret microphone mounted in a silicon ear plug in the outer ear of a human subject. The subjects were exposed to a high level (about 100 dBA) of pink noise with and without the hearing protector fitted. The attenuation provided by the protector was calculated by subtracting the spectrum measured under the protector from the spectrum measured without the protector. The results of the PEAT evaluation were compared to the results obtained using the test method described in ANSI Standard S12.6‐1984, the Method for the Measurement of the Real‐Ear Attenuation of Hearing Protectors. Agreement was found between the methods for the test frequencies from 250 to 8000 Hz. The PEAT results at 125 Hz were lower than the results obtained with the ANSI S12.6‐1984 method.

Application of the probe microphone method to measure attenuation of hearing protectors against high impulse sound levels

The purpose of the present study was to develop a probe microphone method to measure the attenuation of earplugs, earmuffs and their combination against high impulse sound levels from military firearms. The field measurements were made on three earmuffs and one earplug. A probe microphone was supplied with a replaceable soft tube (outer diameter 1·5 mm) to transmit the sound pressure from the ear canal to the microphone. The tube was led between the skin and the cushion ring of the earmuff and through the earplug to the ear canal. A formula was developed to estimate the potential effect of tube length on the results. Subjective laboratory tests of earplugs showed good agreement with the probe microphone method below the 3·15 kHz octave band. The field measurements showed that earplugs may provide attenuation equal to that of large-volume earmuffs against low-frequency impulses from large-calibre weapons. It was also shown that a prominent increase of attenuation can be achieved by using earmuffs and earplugs simultaneously.

Hearing protector attenuation for impulse noise

The study of the effectiveness of hearing protection for impulse noise pose several problems. Due to the transient nature of the noise, real‐ear‐at‐threshold (REAT) data are not readily applicable since current impulse‐noise criteria are based on either (1) peak pressure, some measure of impulse duration, and the number of impulses, or (2) total A‐weighted energy. It is possible to measure the effect of an earmuff on these parameters by positioning a miniature transducer under the muff at the wearer's ear canal. An alternative arrangement which permits the study of both ear muffs and plugs involves the use of an acoustic test fixture. The test fixture must simulate those characteristics of the head and outer and middle ears that affect the attenuation of hearing protectors on real ears. These include the viscoelastic characteristics of the circumaural and intra‐aural skin, the inertial properties of the head, the frequency response of the auditory canal, and the mechanisms of bone conduction and physiological masking. DCIEM has sponsored by contract the development of such a test fixture [C. Giguere et al., J. Acoust. Soc. Am. Suppl. 1 78, S5 (1985)]. Attenuation results with the test fixture compare reasonably well with REAT‐derived data. Agreement between the two methods could be improved for earplugs if the thickness of the artificial intra‐aural skin were reduced. On‐going research is directed towards studying human intra‐aural skin, and measuring the peak attenuation, waveform‐duration increase, and linear and A‐weighted energy reduction of protectors for short‐ and long‐duration impulses.

Analytical concepts and models for calculating hearing protector attenuation

In principle, the ideal hearing protector is a rigid airtight cover or plug that encloses a volume of air in contact with the eardrum and is sealed to an immovable object (the head or the external ear) by a resilient airtight cushion or interface. According to this simple view, the sound attenuation curve is solely dependent on a few lumped elements: the mass and area of the cover or plug, the acoustic impedance presented by the enclosed space, and the stiffness and mechanical resistance of the cushion or interface. In practice, the attenuation is also dependent on air leakage, body‐conducted sound, the lack of rigid support, wave effects, and mechanical imperfections. These factors and others have been measured, estimated, and modeled with considerable success though perfect agreement between theory and experiment remains elusive. So, perhaps there is still room for further work of a fundamental nature. In the meantime, despite their limitations, the classical analytical models continue to provide an invaluable foundation for hearing protector design.

Suggested standard hearing protector attenuation function relating to the NRR.

An assumption was made that a hearing protector (HP) attenuation function derived from some combination of the A-weights and C-weights used in the calculation of the NRR would produce a minimum difference between the NIOSH long R-value and the NRR-dBC + dBA + 3 (RNRR), regardless of the spectrum of the noise. Four hypothetical HP attenuation functions were derived. A mean R-RNRR difference, standard deviation and range were determined for these functions and for five real HP functions using 262 noise spectra. The mean R-RNRR difference for one of the hypothetical functions ("ED") was zero, with a standard deviation of .2 dB and a range of 1.6 dB. It was concluded that an HP designed to match the "ED" attenuation function would minimize the R-RNRR difference, regardless of the noise spectrum.

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.

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.

Effects of user insertion/donning instructions on noise attenuation of aural insert hearing protectors.

An experiment was performed to examine the influence of different levels of insertion instructions on the noise attenuation achieved with five common universal-fit protective earplugs. Five insertion conditions were investigated; these required earplug placement by subjects with no instruction, with auditory feedback, with manufacturers' on-package instructions, with detailed written and pictorial instructions, and with experimenter-modeled earplug insertion procedures. Attenuation data were obtained using REAT (real-ear attenuation at threshold) test procedures on 50 subjects. The results indicated that insertion-instruction strategies posed a major influence on the level of attenuation to be realized with the earplugs, and that earplugs were differential in their sensitivity to instruction. In some instances, attenuation afforded by no instruction was tripled when more comprehensive instructions were presented to the subjects.

The effects of protector-noise interaction on the accuracy of the NRR estimate of hearing protector attenuation.

A total of 262 noise spectra were divided empirically into three groups and the accuracy of the NRR estimate of hearing protector attenuation was evaluated for each group using 15 hearing protectors. The mean and the standard deviation of the error in the NRR estimate was most stable across the 15 protectors, when noise spectra were characterized by a simple monotonic band-level function yielding a dBC-dBA difference of 3.5 dB or less. Both the octave band attenuation function of the hearing protector and the octave band spectrum of the noise appeared to contribute to the error in the NRR estimate.

Sound attenuation of TDH-39 earphones in a diffuse field of narrow-band noise.

The sound attenuation characteristics of the audiometric earphone Telephonics TDH-39 with cushion MX-41/AR have been tested, using the hearing threshold method according to the International Standard ISO 4869-1981 ("Acoustics--Measurement of sound attenuation of hearing protectors--Subjective method"). This method specifies 1/3-oct bands of noise as test signals and a diffuse sound field. This is in contrast to the use of pure tones in a frontally incident free sound field, by means of which most earlier attenuation data have been obtained. The mean sound attenuation values obtained from a group of 20 normal-hearing subjects were up to 3-5 dB lower than those previously reported for pure tones in a free field.

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.

Hearing protector attenuation as influenced by frequency and time variation of the noise

Over a decade ago, Jim Botsford proposed two innovative concepts that provide the basis of this study. First, he proposed correcting the attenuation of hearing protectors by a “C‐A” adjustment [J. H. Botsford, Sound Vib., 32–33 (Nov. 1973)]. Using three separate time history dosimeters, set to 5‐s periods, the C‐weighted and A‐weighted levels at the shoulder were measured; also, the level under the muff was recorded. The results show that most of the time the C‐weighting outside the muff is the best predictor of the A‐weighted level under the muff when used with a single value of noise reduction. Another concept of Botsford was the TTS meter, in which the intermittency pattern of a noise could be assessed [J. H. Botsford, Am. Ind. Hyg. Assoc. J. 32, 92–95 (1971)]. Harris and the first author further enhanced this concept by providing a TTS‐based intermittency correction factor to adjust a noise dose [D. L. Johnson and C. S. Harris, Proc. Noise Expo, Chicago, IL, 16–122 (1979)]. Using this adjusted dose, our current data indicate better performance by muff‐type hearing protectors than simply calculating inside and outside average noise levels might predict. We also noted that a person's voice can disproportionately influence the attenuation predicted by an under the muff measurement. We believe that Botsford's early concepts are still valid and technology now exists which can make his ideas useful.

A signal generator for testing of hearing protectors

The measurement of the acoustical attenuation of hearing protectors requires a signal generator whose characteristics are specified in the ANSI S3.19-1974 Standard. This paper describes such a generator which was constructed by modifying a regular screening audiometer, including a pink noise generator followed by nine one-third octave band filters and an additional 2·5 dB attenuator. Detailed information on the construction and performance of the generator is provided in the paper. The generator, which is relatively easy to build, has proved to be reliable for uses in the laboratory, as well as in the field.

Hearing conservation: Regulatory status and issues

The Occupational Safety and Health Administration published the final decisions on its hearing conservation amendment on 8 March 1983. Certain changes are likely to cause confusion when OSHA personnel enforce the standard. Employers now must use personal monitoring in cirumstances where area monitoring is “generally inappropriate” unless they can show that “area sampling produces equivalent results.” Also, OSHA has changed the term “significant threshold shift” to “standard threshold shift,” which some individuals interpret to mean an ordinary or expected shift in hearing level. Most changes from the January 1981 version are towards a performance rather than a specification approach, which may result in less rigorous hearing conservation practices, increasing the likelihood that employees will suffer noise‐induced hearing loss before intervention can occur. Consequently, three critical issues have emerged. First, employers and consultants need a standard, practical method of testing hearing protector attenuation as it occurs in the field. Second, OSHA needs to derate the Noise Reduction Rating to reflect real world use, and third, employers need an objective method by which to evaluate the effectiveness of their hearing conservation programs.

Comparison of hearing protector single‐number rating methods

During the last ten years, various organizations have developed single‐number rating methods for hearing protectors. These values have been used to approximate the amount of noise reduction obtained by using these devices. From the hearing protector attenuation data obtained from the American National Standards Institute test procedures, the organizations have mathematically arrived at single numbers such as the “R factor”, the “NRR”, and “D factor”, and so on. These are based on various items such as actual noise spectrums, pink noise spectrums, standard deviation of the attenuation data, etc. This paper compares the various single number ratings which have been calculated for six different hearing protectors, and the methods used. As can be seen, the single number rating varies considerably. Advantages and disadvantages of each method is also discussed. Finally, application of the values to on‐the‐job situations will be discussed.

A psychophysical evaluation of the dependence of hearing protector attenuation on noise level.

The attenuation characteristics of ten commercially available hearing protectors (five muffs and five plugs) were examined for 1/3-octave bands of noise centered at 1000, 2000, 3150, 4000, and 6300 Hz in ten normal-hearing young adults utilizing four psychophysical procedures. One of these procedures, the real-ear attenuation at threshold procedure described in the American standard for the evaluation of hearing protectors, is designed for use at 1/3-octave band noise levels below approximately 50 dB SPL. Three real-ear psychophysical procedures were also developed and evaluated. These procedures were: (1) a reaction-time paradigm; (2) a loudness magnitude-estimation procedure; and (3) a masked bone-conduction threshold technique. Each procedure was designed to evaluate protectors for 1/3-octave bands of noise having levels ranging from approximately 50 to 90 dB SPL. Mean data indicated that: (1) attenuation was linear over the range investigated, although some exceptions to this generalization were apparent; (2) attenuation estimates derived with the reaction-time and magnitude-estimation paradigms were typically less than those obtained with the other two methods; and (3) all attenuation estimates, regardless of procedure, were less than manufacturer's specifications for the majority of the protectors. Implications for existing standards are discussed.