Acoustic Test Fixture Research Articles

Development of an in-ear microphone for individualized measurement of hearing protection device output

Hearing protection devices (HPDs) are designed to reduce the amplitude of sound reaching the ear, but can also distort the acoustical cues necessary for sound localization. In order to quantify the acoustic impact of an HPD on the signal reaching the tympanic membrane, the signals deep in the ear canal with and without the HPD must be compared. While some commercial microphone solutions for measuring in-ear pressures with HPDs exist, they are device-specific and/or may not provide reliable isolation of the microphone transducer from the (potentially high-intensity) ambient sound field. Here, we describe a new assembly using a small MEMS microphone attached to the end of a thin flexible circuit, which can be inserted either alongside or through virtually any HPD. Following a description of the electromechanical design of the assembly, we present measurements of insertion loss for HPDs with and without the microphone using an acoustic test fixture. These measurements demonstrate that the presence of the microphone does not significantly disrupt the sound attenuation characteristics of a range of HPDs. Preliminary measurements of insertion loss and head-related transfer functions in human subjects are also presented.

A finite element model to predict the double hearing protector effect on an in-house acoustic test fixture.

The sound attenuation of double hearing protectors (DHPs), earplugs combined with earmuffs, generally falls short of the sum of each single protector's attenuation when used independently. This phenomenon, referred to as the DHP effect, is found to be related to structure-borne sound transmission involving the outer ear and can also be observed on acoustic test fixtures (ATFs). At present, it still remains not fully understood, and no available model can help demonstrate the associated sound transmission mechanisms. In this work, a finite element model is proposed to study the DHP effect on an ATF between 100 Hz and 5 kHz. Power balances are calculated with selected configurations of the ATF in order to (i) quantify the contribution of each sound path, and study the effects of (ii) the artificial skin and (iii) acoustic excitation on the ATF exterior boundaries. The DHP effect is shown to originate from the structure-borne sound power injected from the ATF boundaries and/or earmuff cushion. The important influence of earcanal wall vibration is highlighted when the skin is accounted for. The simulation results allow for gaining more insight into the sound transmission through a DHP/ATF system.

Effects of mandibular motion on earplug function

Earplug attenuation depends on developing and maintaining an acoustic seal between the earplug and the walls of the ear canal. During use, earplugs may lose some of their effectiveness by loosening over time and with activity, thereby compromising the acoustic seal in the ear canal and allowing more sound to enter the ear canal from the sound field. Mandibular motion (as during talking, singing, chewing, breathing) provides a nearly continuous source of ear canal movement resulting from forces applied by the condyle of the mandible. Individual differences in ear canal geometry also may impact the effects of mandibular motion on earplug function. The effects of mandibular motion on earplug function were investigated using an acoustic test fixture that combined (1) individualized pinna and ear canal structure beyond the second bend, inclusive of the mandibular bump, with (2) a standardized ear simulator (GRAS RA0045) with ear canal extension and microphone (GRAS 40AH ¼”). Mandibular motion was simulated using a computer-controlled drive-pin with a silicone tip that articulated with the mandibular bump of the individualized silicon ear canal. Results indicated that as few as 50 cycles of mandibular motion could significantly compromise earplug function and that this compromise was highly subject-dependent.

Detection thresholds with active hearing protection devices in quiet and noise

The Real-Ear Attenuation at Threshold (REAT) method is a standardized (ANSI/ASA S12.6) method of assessing the passive attenuation of hearing protection devices (HPD), including electronic HPDs with the electronics turned off. Characterizing HPDs with electronics turned on is performed with the Microphone in Real Ear (MIRE) method, or with an acoustic test fixture, as described in ANSI/ASA S12.42. It has been proposed that the REAT method may be useful for characterizing the effect of HPDs on detection of ambient sounds for HPDs with hear-through microphones, including the effects of electronics noise from the HPD. In this study, detection thresholds were measured with three electronic earmuff-type HPDs using a method similar to the REAT method. Detection thresholds were obtained for warble tones presented from a loudspeaker placed in front of the subject in a sound treated room. Thresholds were measured with unoccluded ears and with three HPDs with hear-through capability in quiet and in noise. In addition, thresholds were measured with one of the HPDs with foam earplugs inserted into both ears to simulate a conductive hearing loss. The results point to the interaction between device noise, ambient noise, and human absolute thresholds as limitations on the detectability of ambient sounds.

The effect of wearing an anti-flash hood on the noise attenuation of earmuffs.

The insertion losses of four pairs of earmuffs, including one noise-excluding headset, were measured in one-third octave bands in a diffuse broadband noise field using a head-like acoustic test fixture. The acoustic test fixture contained realistic ear simulators with microphones at the eardrum positions. The insertion losses were measured (i) with the earmuffs on their own, (ii) with the earmuffs worn over an anti-flash hood, and (iii) for one earmuff, with the earmuff worn under the hood. The other three earmuffs could not be fitted under the hood. The insertion loss of the anti-flash hood on its own was also measured. Wearing an anti-flash hood under the earmuffs greatly reduced the protection against noise, by 20–23 dB at high frequencies, by 17–20 dB at middle frequencies, by 12–16 dB at low frequencies, and by 16–20 dB overall. Only one earmuff was slim enough to fit under an anti-flash hood. Wearing an anti-flash hood over this earmuff had only a marginal effect on the earmuff insertion loss, of the order of 1 dB. If anti-flash hoods could be designed to fit over other types of earmuffs and headsets, the protection of these earmuffs and headsets would be virtually maintained.

Experimental study of earplug noise reduction of a double hearing protector on an acoustic test fixture

Double hearing protectors (DHPs), earplugs and earmuffs worn in combination, may be needed in high level noise environments. The DHP sound attenuation is known to be less than the sum of each single protector attenuation. This effect, referred to as the DHP effect, is still not fully understood. A recent study has shown that it can be observed on an acoustic test fixture (ATF) and characterized by the decrease of the earplug noise reduction (NR) when the earmuff is added. In this paper, a measurement methodology is proposed to (i) identify the main sound paths related to the DHP effect on an ATF and (ii) explain the latter by the relative contributions of the air-borne and structure-borne transmissions in the system. The focus is put on the NR values of the earplug alone and in the DHP. Measurement results suggest that the DHP effect is related to the energy transmitted from the earcup, through the earmuff cushion and finally into the earcanal via the sound radiation of the earplug and/or earcanal lateral walls. This flanking structure-borne path is found to dominate over the “direct” air-borne path through the hearing protectors at frequencies above 300 Hz.

An explanation of the decrease in the earplug noise reduction when combined with an earmuff on an acoustic test fixture

For people working in high noise level environments, typically above 105 dBA, double hearing protection (DHP) systems (earplugs worn in combination with earmuffs) are always recommended. However, it is very difficult to predict the DHP overall sound attenuation due to the complexity of the system. A recent experimental study has shown on human subjects and an acoustic test fixture (ATF) that the noise reduction (NR) of the earplug decreases considerably when an earmuff is worn over it but the reason is still not fully understood. In this work, specially designed experiments using an ATF are proposed in order to explain this observation. The focus is put on the NR of the single earplug and that of the earplug in the DHP system. The respective effects on the DHP attenuation of various sound transmission paths involved in the system (both air-borne and structure-borne) are analyzed by modifying the system coupling conditions or controlling the sound pressure level under the earmuff. This work will help better understand the interactions between the system components and will provide the grounds for developing a prospective numerical model to predict the sound attenuation of a DHP system.

Earcanal anthropometry analysis for the design of realistic artificial ears

How in-ear devices fit in the ear strongly influences the acoustical and mechanical (dis)comforts induced to the wearer. As important variations in the ear geometry exist based on gender, age and ethnicity, several studies collected ear anthropometric data as a basis for designing ear-mounted products. However, most of these studies focused on the ergonomic design of earbuds, and are thus limited to the geometry of the pinna, and concha (where earbuds fit). Few studies explored geometrical earcanal data for the design of intra-auricular hearing protectors that fit up to the earcanal second bend. The design of earplugs that fit to the widest range of earcanals requires realistic acoustical test fixtures representative of the population. This study uses statistical analysis and artificial intelligence based algorithms to cluster 32 Canadian workers pairs of earmolds scans as a function of earcanal curvilinear axis length, entrance, first and second bend cross sections area and circumference, but also earcanal tortuosity and cross sections aspect ratios. Dimensions relevant to cluster earcanals will be collected on a hundred of ears in a future study to design artificial ears which capture the inter-individual variability in mechanical and acoustical objective indicators related to the most important earplugs (dis)comfort attributes.

Observations on the correlation of subjective and objective attenuation measurements on earplugs.

Real-ear attenuation at threshold (REAT) has been recognized as the gold-standard method to evaluate the attenuation of hearing protection devices (HPDs). The question is often posed as to whether or not REAT measurements could be replaced with objective, insertion loss (IL) measurements conducted on acoustical test fixtures for assessing the performance and consistency of HPDs. To understand how IL data correspond to REAT data for earplugs, both methods were used to assess three earplugs. The resulting data were compared, and the HPD attenuation ranked at each frequency. In most cases, after correction for bone conduction limits, occlusion effect, and physiological noise, IL data show good agreement with REAT values. Discrepancies at some frequencies were found and varied by HPD. The IL data did not identify the high standard deviation of the REAT results on one earplug. The IL data did not correspond well to the REAT values, resulting in differences as large as 12 dB at some frequencies. In conclusion, IL and REAT procedures are used for different purposes when assessing attenuation characteristics of HPDs. While close agreement was found at most frequencies, IL data may not reflect the REAT performance of HPDs equivalently in all aspects.

A Transfer Matrix Model of the IEC 60318-4 Ear Simulator: Application to the Simulation of Earplug Insertion Loss

The IEC 60318-4 ear simulator is used to measure the insertion loss (IL) of earplugs in the ear canal of an acoustical test fixture (ATF) and is designed to represent an average acoustic impedance (in a reference plane) of the human ear. The ear simulator is usually modeled using a lumped parameter model (LPM) which has frequency limitations and inadequately accounts for the thermo-viscous effects in the simulator. The simulator numerical models that can better deal with the thermo-viscous phenomena often lack essential geometric details. Most related studies also suffer from the lack of experimental validation of the models. Therefore, a transfer matrix (TM) model of the IEC 60318-4 simulator is proposed based on a direct assessment of its geometric dimensions. Such a model is of particular interest for designing artificial ear simulators. The variability in the simulator impedance due to the geometric uncertainties is quantified using the Monte Carlo method. The TM model is validated using i) a finite element (FE) model of the simulator and ii) impedance measurements with a sound intensity probe. It is found to better describe the simulator impedance above 3 kHz compared to the LPM. The TM model is then coupled to a FE model of an occluded ATF ear canal to simulate the IL of an earplug in the frequency range [100 Hz, 10 kHz]. In the model, the simulator is considered as a cylindrical cavity terminated by an equivalent tympanic impedance which is determined from the TM model to simulate the sound pressure measured at the real microphone position (not at the reference plane) in the ATF ear canal. The simulated IL is validated against i) that obtained with a complete FE model of the corresponding system and ii) measurements using an ATF. The TM model is shown to better agree with the simulator FE model than the LPM above 6 kHz regarding the earplug IL simulated using this method.

Acoustic test fixtures: From KEMAR and beyond!

In 1972, KEMAR was introduced to the world. Based on the average of about 5000 males and females from the U.S. Air Force. It was the first head and torso simulator designed especially for acoustic research and enabled the hearing aid laboratories to perform simulated in-situ measurements of hearing aids. Today, more than 47 years after its introduction, KEMAR is able to test any device that contains both loudspeakers and microphones as well as performing binaural recordings of product sound and music. Fixturing, ear simulators, and special microphones, also known as couplers, are used by acousticians and engineers to design and test the many forms of modern acoustic electronics used from entertainment (Ear Buds, Headphones, IEMs, Smart Speakers, etc.), to hearing protection (Noise Cancelation), to medical devices (Hearing and Augmentation and Aids), to automotive cabins and sound systems, to military headsets alike. This presentation will discuss the evolution, ongoing design challenges, and future direction of Acoustic Test Fixtures required for the acoustic discovery and development going forward.In 1972, KEMAR was introduced to the world. Based on the average of about 5000 males and females from the U.S. Air Force. It was the first head and torso simulator designed especially for acoustic research and enabled the hearing aid laboratories to perform simulated in-situ measurements of hearing aids. Today, more than 47 years after its introduction, KEMAR is able to test any device that contains both loudspeakers and microphones as well as performing binaural recordings of product sound and music. Fixturing, ear simulators, and special microphones, also known as couplers, are used by acousticians and engineers to design and test the many forms of modern acoustic electronics used from entertainment (Ear Buds, Headphones, IEMs, Smart Speakers, etc.), to hearing protection (Noise Cancelation), to medical devices (Hearing and Augmentation and Aids), to automotive cabins and sound systems, to military headsets alike. This presentation will discuss the evolution, ongoing design challenges, and future direct...

Selection of Earmuffs and Other Personal Protective Equipment Used in Combination.

In a work environment, in addition to noise, people may be exposed to other harmful factors. Therefore, they wear both hearing protectors and other personal protective equipment (OPPE). Incorrect use of such a combination may increase the risk of hearing loss. The aim of this study was to determine whether the simultaneous use of earmuffs and other personal protective equipment could affect the effectiveness of hearing protection. The study was carried out under laboratory conditions using an acoustic test fixture. This fixture replicated the anatomical shapes of the head and the pinnae, and was also equipped with ear simulators. The study was carried out for five models of earmuffs and eight models of other personal protection equipment. We found that a change in the sound pressure level (SPL) under the earmuffs when using a full face respirator could reach up to 40 dB. On the other hand, the use of a half respirator had practically no adverse impact on the efficiency of hearing protection. In the selection process, it is recommended to consider safety spectacles equipped with thin temples, and half respirators equipped with band adjustment elements positioned on the facial part, rather than the back, of the user’s head.

In-ear and on-body measurements of impulse-noise exposure

Accurate quantification of noise exposure in military environments is challenging due to movement of listeners and noise sources, spectral and temporal noise characteristics, and varied use of hearing protection. This study evaluates a wearable recording device designed to measure on-body and in-ear noise exposure, specifically in an environment with significant impulse noise resulting from firearms. A commercial audio recorder was augmented to obtain simultaneous measurements inside the ear canal behind an integrated hearing protector, and near the outer ear. Validation measurements, conducted with an acoustic test fixture and shock tube, indicated high impulse peak insertion loss with a proper fit of the integrated hearing protector. The recording devices were worn by five subjects during a live-fire data collection at Marine Corps Base Quantico where Marines fired semi-automatic rifles. The field test demonstrated the successful measurement of high-level impulse waveforms with the on-body and in-ear recording system. Dual channels allowed for instantaneous fit estimates for the hearing protection component, and the device worked as intended in terms of hearing protection and noise dosimetry. Accurate measurements of noise exposure and hearing protector fit should improve the ability to model and assess the risks of noise-induced hearing loss.

Occlusion effects of hearing protection device on ear canal transfer function and cochlear injury from impulse noise exposure

The objective of this paper is to present a new finding on the effects of hearing protection devices (HPDs) on the ear canal (EC) transfer function (TFEC2ED). Specifically, we show that when an earmuff is worn, the TFEC2ED is not the same as that for the bare head, leading to a significant difference in cochlear injury prediction. We used shock tube blast with an acoustical test fixture (ATF) fitted with the recovered earmuffs from the historic blast overpressure project (BOP) to measure the TFEC2ED. A microphone was placed at the EC entrance under the earmuff (undermuff) to measure the pressure at the EC entrance, while the built-in ATF’s microphone was used to measure the eardrum (ED) pressure. The TFEC2ED is defined as the ratio of the pressure at the ED to the pressure at the EC entrance. We compared the cochlear integrated energy (ICE) values, when the TFEC2ED obtained from bare head (Case-1) and that measured using the ATF fitted with an earmuff (Case-2), respectively are used in the ICE-model, to estimate the effects of the difference between the TFEC2EDs on injury. The human BOP undermuff pressure data were used as model input. For Case-1, the input was at the EC. The ED pressure, reconstructed using the undermuff pressure and the TFEC2ED measured using the ATF, was used for Case-2. We found significant differences in the ICE-values as the result of the difference between the TFEC2EDs. We conclude that hearing protector models must correctly model the occlusion effects of HPDs by incorporating the change in the TFEC2ED.

Fit testing of a hearing protection device with integrated in-ear noise exposure monitoring

In-the-ear hearing protectors are often used in high-noise environments, such as in military operations and during weapons training. The fit of such a hearing protection device in the ear canal can cause significant variability in the noise dose experienced, an important factor in characterizing the auditory health risk. Our approach for improved dose estimates under these conditions involves a portable noise recorder used to capture in-the-ear noise behind a hearing protector, and on-body noise, while assessing hearing protection fit throughout recording. In this presentation we describe evaluation of this system using ANSI S12.42 testing using a shock tube and an acoustic test fixture, to evaluate impulse peak reduction and for measurement validation. Also explored were angle dependent effects on the peak insertion loss and measurement accuracy of the on-body recorder. An exploratory study was conducted with a small sample of experimenters during a recent Navy-sponsored noise survey conducted at Marine Corps Base Quantico. Results will be presented from a shooter’s ear and a nominal instructor’s position as well as from bystanders observing at a firing range. Our results show good correspondence between behavioral fit-testing of insertion loss and estimated protection from the in-ear and on-body microphones. [Work supported by ONR.]

Measurement of Earmuffs Attenuation at High Audible Frequencies

Abstract Standardised measurements of sound attenuation of hearing protectors are performed in the frequency range from 125 Hz to 8 kHz. However, noise present at many workplaces contains significant components at higher audible frequency. Therefore, the knowledge about noise attenuation with earmuffs in the audible frequency range above 8 kHz is also necessary for proper hearing protection. The aim of this study was to obtain values of the noise attenuation with 27 commonly-used earmuffs models in the 1/3 octave bands of 10, 12.5 and 16 kHz. The measurements were conducted with a real ear at threshold (REAT) method with participation of subjects. The study showed that attenuation of earmuffs ranged from 24.7 to 42.8 dB, depending on model of earmuffs and frequency band. Furthermore, the measurements were performed with the use of acoustic test fixture which is designed especially for testing hearing protectors. Results obtained with the use of acoustic test fixture indicated that this measurement method can lead to values close to attenuation measured with participation of subjects. On the other hand, values obtained with the use of acoustic test fixture may differ average up to 14 dB from REAT method.

Predicting sound-localization performance with hearing-protection devices using computational auditory models

Evaluation of the effect of hearing-protection devices (HPDs) on auditory tasks such as detection, localization, and speech intelligibility typically is done with human-subject testing. However, such data collections can be impractical due to the time-consuming processes of subject recruitment and the testing itself, particularly when multiple tasks and HPDs are included. An alternative, objective testing protocol involves the use of a binaural mannequin (a.k.a an acoustic test fixture) and computational models of the auditory system. For example, data collected at the eardrums of such a mannequin outfitted with an HPD can be fed into a binaural localization model. If the performance of the model with such input can be shown to be similar to that of human subjects, the model-based assessment may be sufficient to characterize the hearing protector and inform further design decisions. In this presentation we will describe the preliminary results of an effort to replicate human-subject localization performan...

On-body and in-ear noise exposure monitoring

Accurately estimating noise dosage can be challenging for personnel who move through multiple noise environments. Dose estimates can also be confounded by the requirement to wear hearing protection in some areas, but not others. One concept for improved dose estimates under these conditions is to capture noise in-the-ear and on-body simultaneously. An additional benefit of this setup is that hearing protection fit can be assessed in real time. To evaluate this dual-microphone approach, we prototyped a noise dosimetry device for military environments where loud impulse noise such as weapons fire drives the need for high dynamic range and a high sampling rate. In this presentation, we describe a system where the in-ear microphone is acoustically coupled to a disposable foam or flange hearing protection eartip. Initial laboratory tests with a shock tube and acoustic test fixture (ATF) show more than 30 dB noise reduction between the on-body and in-ear microphones. Furthermore, in-ear levels are consistent wi...

Evaluation of noise exposures and hearing loss at a hammer forge company

The NIOSH health hazard evaluation program evaluated employees’ exposures to high level continuous and impact noise at a hammer forge company. Personal dosimetry data were collected from 38 employees and noise exposure recordings were collected during two visits to the facility. Extensive audiometric records were reviewed and trends for hearing loss, threshold shifts and risk of hearing loss were assessed. The effectiveness of hearing protection devices for hammer forging was evaluated with an acoustic test fixture. A longitudinal analysis was conducted on the audiometric data set that included 4750 audiograms for 483 employees for the years 1981 to 2006. The analysis of the audiometric history for the employees showed that 82% had experienced a NIOSH-defined hearing threshold shift and 63% had experienced an OSHA-defined standard threshold shift. The mean number of years from a normal baseline audiogram to a threshold shift was about 5 years for a NIOSH threshold shift and was about 9 years for an OSHA t...

A finite element model to improve noise reduction based attenuation measurement of earmuffs in a directional sound field

The real attenuation of hearing protection devices (HPD) can be assessed in the field using a method based on continuous field microphone-in-real-ear (F-MIRE) measurements. The two-microphone method provides an indicator called the measured noise reduction (NR∗), defined as the difference between the measured exterior (outside the protector) and interior (under the protector) sound pressure levels (SPL). The HPD’s attenuation expressed in terms of the more common insertion loss (IL) can then be obtained from NR∗ using compensation factors. For earmuffs, NR∗ has been shown to vary of up to 20dB depending on the angle of incidence of the sound source. Therefore, there is a need to use sound incidence dependent compensation factors to relate NR∗ and IL. To evaluate these factors and more generally to improve the continuous F-MIRE method, a finite-element (FE) model of an earmuff on an ATF (acoustic test fixture) exposed to a directional sound field has been developed and its predictions compared with lab measurements for several incidence angles. Regarding the external microphone SPL and the NR∗, in one-third of octave bands, the model correlates very well with measurements for frequencies below 1250Hz whatever the sound incidence. Above 1250Hz, the FE model captures the trends, as a function of the incidence angle, but the agreement generally decreases with increasing frequency. A better correlation between the FE model and the experimental data is achieved for the variation of NR∗ (ΔNR∗) as a function of the sound incidence. Actions, such as (i) accounting for the headband in the model, (ii) refining the modeling of the sound source, (iii) improving the cushion modeling and (iv) better describing the backplate/cushion coupling conditions, are suggested to improve the model accuracy. To illustrate the potential of the modeling to improve the continuous F-MIRE measurement method, the FE model is used to determine an optimal position of the external microphone and to obtain estimates of exposure levels using the left and right ear exterior microphones.