Acoustic Test Fixture Research Articles

Parametry akustyczne ochronników słuchu i metody ich pomiaru

Acoustic parameters of hearing protectors and methods of their measurement The article presents how the basic parameter of hearing protectors, i.e. sound attenuation, is determined using a method based on the active participation of people (subjective method). In addition, objective methods for measuring acoustic parameters of hearing protectors, such as insertion loss, based on the use of miniature microphones and acoustic test fixture are described. The results of measurements of sound pressure level under hearing protectors carried out using an acoustic test fixture reproducing anatomical shape of a human head (including concha and ear canal), used as a standard for measuring acoustic parameters of hearing protectors in impulse noise situations are also presented. These measurements were carried out to verify the possibility of using this acoustic test fixture, to measure the sound pressure level reaching under the hearing protectors in the case of continuous noise. The tests showed that in the case of measurements with microphones that are standard equipment of the acoustic test fixture, dedicated to impulse noise, the use of these microphones is limited to high-level continuous signals. Microphones that are optional equipment on the acoustic test fixture were a more suitable solution for the intended purpose.

Effects of the earplug and external ear hardness of the direct transmissions induced by high-level impulse noises

The performances of Hearing Protection Devices (HPD) facing high-level impulse noises is limited by the transmission through the protector. This transmission can be evaluated by measuring the earplug's displacement along the ear canal axis. Previous measurements highlighted a displacement proportional to the peak acoustic pressure in the protected ear canal. To better understand the consequences on the HPD performances, this evaluation requires to be extended. Thus, various configurations are studied with Acoustic Test Fixture (ATF) external ears and earplugs of low and medium hardness. The earplug's peak displacements have been measured with a laser-vibrometer and the acoustic pressure with the ATF microphone. The impulse waves employed to excite the earplugs ranged from 150 dB to 180 dB peak. A medium hardness earplug inserted in a low hardness ear led to the worst performances with a plug displacement of 200 µm and a peak noise reduction of 17.8 dB(SPL) for an impulse noise of 177 dB(SPL). For the best case, a maximum displacement of 66 µm and peak noise reduction of 28.8 dB(SPL) have been measured. These results suggest new perspectives to improve the HPDs but also their evaluation through the choice of judicious ATF materials that conform to biological tissues.

Earmuff performance in the presence of high-level impulse from a recoilless weapon firing in a confined space.

A noise attenuation performance test was conducted on earmuffs using a recoilless weapon launch platform in a confined space, along with two acoustic test fixtures (ATFs). The overpressure at the ATF's effective tympanic membrane comprised direct sound at 185 dB sound pressure level (SPL) and reflected sound at 179 dB SPL. Wearing earmuffs reduced these peaks to 162 dB SPL and 169 dB SPL, respectively. The reflected sound from walls was defined as delayed sound. An analytical model for earmuff noise attenuation simulated their effectiveness. The simulation revealed that when the earmuffs attenuated delayed sound, the acoustic impedance of acoustic leakage and the acoustic impedance of the earmuff material decreased by 96% and 50%, respectively. The negative overpressure zone between direct and delayed sound decreased the earmuffs' fit against the ATF. Additionally, the enclosed volume between the earmuff and the ear canal decreased by 12%. After the installation of bandages on the earmuffs, the overpressure peak of delayed sound was reduced by 5 dB. Furthermore, the acoustic impedance of the earmuff's sound leakage path and the acoustic impedance of the earmuff material deformation path increased by 100% and 809%, respectively.

Influence of reference transducer location on hearing protector attenuation metrics for impulse noise

The ANSI/ASA S12.42-2010 standard can be used to estimate the attenuation for impulsive noises between 130 and 170 decibels peak sound pressure level (dB pSPL). The ANSI/ASA S12.42 method uses a complex transfer function between the signal received in the ear of an acoustic test fixture (ATF) and a reference transducer. The reference transducer is at the same radial distance from the impulse source and at less than 30 degrees from the ATF angle. The reference transducer is specified as a pencil-type pressure (> 40.6 cm long). The maximum angle and the minimum pencil probe dimensions could be incompatible near the source, or may introduce artifacts or hazards. In this study, several hearing protectors were evaluated with two rifles (A-Bolt .300 Winchester Magnum and Colt AR-15 5.56 caliber), two ATFs (GRAS 45CB), and five transducers (four ¼” microphones and one surface mount microphone). The ATFs were on opposite sides of the gun at distances for field levels between 180- and 140-dB pSPL. For each location and rifle, S12.42 metrics were calculated for all 20 combinations of ATF ear and reference transducer. Results indicated that S12.42 metrics were minimally affected by the locations of the reference transducers.

Evaluation of a multi-function in-ear device performance in the presence of impulse noise using acoustic test fixtures

Real-time noise exposure monitoring has the potential to assess risk before damage occurs. We present on the development of an in- ear device that (1) provides hearing protection; (2) measures the noise in the ear canal to estimate exposure dose; and (3) monitors distortion product otoacoustic emissions (DPOAE) over time. To validate the device’s ability to measure impulse noise under hearing protection, we measured the response in a field test. A single AR15 was shot multiple times, and four prototypes were used in multiple acoustic test fixtures (ATFs) at different relative distances from the exit of the gun. The ATFs included a G.R.A.S 45 CB in addition to a head simulator designed to detect both air- and bone-conducted sound. We evaluated the impulse peak insertion loss (IPIL) metric of the device inserted by itself or in combination with other hearing protection (“double protection” condition) to assess attenuation of impulsive noises. We also compared the device's microphone measurements to those obtained through the ATF microphones, and to the recordings of external microphones located at or near the ear of each ATF. Results demonstrate the value of testing in-ear devices in acoustic test fixtures during development phases.

Designing an acoustical test fixture to evaluate the objective occlusion effect

Earplugs are commonly used to prevent noise-induced hearing loss. However, their effectiveness is often hindered by the discomfort they cause, impacting consistent and correct use. An important acoustical discomfort, known as the occlusion effect, arises from an increased perception of bone-conducted physiological sounds (such as one's own voice, breathing, and chewing) when the ear canal is occluded. To objectively assess this discomfort, the study proposes the use of an acoustical test fixture (ATF) that avoids direct measurements on human participants. The ATF employs an anatomically realistic truncated outer ear, incorporating soft tissues, cartilage, and bone components to replicate the outer ear's bone conduction path, crucial for occlusion effect assessments. The study demonstrates that the proposed ATF can replicate key effects observed in objective ccclusion effect (OE) measurements on human participants, including significant OE at low frequencies diminishing with increasing frequency, reduction of OE with greater insertion depths, and distinctions among various earplug types—especially noticeable at deeper insertions. Furthermore, a computationally efficient finite element method-based virtual tester for the ATF is developed and validated.

Design and evaluation of a noise dosimeter integrated into an ear-muff hearing protector

In impulse-noise environments, such as during weapons training, it can be difficult to characterize an individual’s true cumulative exposure, and consequently their risk of hearing loss. Direct measurements of sound pressure in the ear canal underneath hearing protection are ideal for assessing the protected noise exposure because the effects of distance, orientation, protection, etc., are automatically included in the measurement. However, these types of measurements are not commonly collected outside the laboratory. In this research, we develop and evaluate a portable a noise dosimeter that is integrated into a commonly used muff-based hearing protector (3M™ PELTOR™ ComTac™ V). The design of the device aims to simultaneously measure both the external and the under-the-muff sound pressures in a portable, wearable form-factor. First, we demonstrate that the modifications do not affect the hearing protector attenuation using an acoustic test fixture and a fit-check system across 10 refits (p = 0.14). Next, we show through testing with a shock-tube and acoustic test fixture that it is possible to obtain valid pressure measurements through this system. Finally, we report on initial field testing during weapons training. Future application of this technology can be used to develop damage risk criteria based on in-ear measurements that will provide for more accurate individualized risk assessment.

Comparison of two acoustic test fixtures for measuring impulse level dependent attenuation

In the American National Standards Institute (ANSI) S12.42-2010 standard for measuring insertion loss of hearing protection devices (HPDs) in impulsive noise, the range of peak sound pressure levels for the impulses is 132 to 168 dB peak sound pressure level (dB pSPL). The specification of the acoustic test fixtures (ATFs) included an IEC 60318-4 ear simulator fitted with a microphone. The microphones typically used are a ¼” cartridge and have a dynamic range up to about 169 dB pSPL. For most small caliber firearms, the peak SPLs are less than 170 dB SPL at the location of the shooters ear (Schulz et al. 2013). For some military weapon systems that produce impulses greater than 170 dB pSPL, impulse insertion loss cannot be assessed without exceeding the microphone maximum dynamic range. Some condenser microphones have maximum dynamic ranges of 190 dB pSPL. This paper compares insertion losses for HPDs measured on two GRAS 45CB ATFs with microphones with maximum dynamic ranges of 169 and 193 dB pSPL. Waveform distortion, field probe to unoccluded transfer functions between the ATF and field probe will be examined for distortion and accuracy when estimating the impulse level dependent attenuation.

Measuring hearing protector attenuation of impulse noise on acoustic test fixtures using maximum A-weighted energy reduction

Hearing protection devices (HPDs) and firearm suppressors can help mitigate risk of hearing loss from small caliber firearms. ANSI S12.42 is the standard for measuring HPD impulse peak insertion loss (IPIL). No consensus standard exists for measuring firearm suppressor noise reduction, though the industry is working toward a standard using maximum accumulated A-weighted energy. A common attenuation metric for different impulse noise reduction technologies would inform the individual and combined impact of these technologies. The maximum A-weighted energy reduction for several HPDs was assessed using firearm impulses from two rifles (0.300 Winchester Magnum and 5.56x45) with two ATFs and five field microphones positioned on opposite sides of the gun at four locations (field levels between 180- and 140-dB pSPL). No adjustments were made to account for tissue/bone conduction. Maximum A-weighted energy reduction values for single protection ranged between 17 dB (earmuff with safety glasses) and 43 dB (preformed earplug). Similar to IPIL, maximum A-weighted energy reduction increased with increased impulse levels. Advantages, disadvantages, and technical issues associated with the procedure will be discussed.

Development of a biofidelic acoustic test fixture and evaluation in impulsive noise

Here, we describe the development and use of a novel biofiedlic head surrogate for evaluating head and hearing protection devices against impulsive, shock, and blast insults. The head surrogate, derived from biomedical imagery, includes biofidelic pinnea, ear canals, and skull. In addition to microphones positioned at the ear canal terminus, the surrogate includes accelerometers and a hydrophone to provide an estimate of sound propagation through the bone-conduction pathways. This allows use of the surrogate to estimate a device's ability to attenuate bone-conducted pressure waves. We discuss the tradeoffs between biofidelity, manufacturability, instrumentation, and cost. We will show example data from various test campaigns, including the estimating the attenuation of the bone-conduction pathway afforded by different helmet systems during a 178 dB SPL (peak) shock insult. We will conclude with a look towards the future, including potentially updating the underlying three dimensional models to align with modern biomedical models and databases.

Effect of shooting glasses on earmuff attenuation measured with an acoustic test fixture for gunfire impulses

The EPA requirement for hearing protector labels is based on the decrepit ANSI S3.19-1974 standard, which prohibits the use of eyewear during the REAT testing that leads to the NRR on the product label. Firearm users are recommended to wear protective eyewear while shooting. In this presentation, we review past work regarding the effects of eyewear on earmuff performance and present results from five samples of one product that includes both eyewear and an earmuff. Impulses ranging between 139 and 178 dB peak SPL were used. Impulse Insertion Loss was less than 10 dB below 400 Hz for earmuffs alone and negligible for the earmuff and glasses condition and attenuation in the high frequencies was reduced by approximately 15 to 25 dB. Impulse peak insertion loss values per ANSI S12.42 were reduced from 21 to 39 dB for the earmuff alone to 15 to 22 dB when eye protection was added. Hearing conservation programs need to account for the deleterious effect of protective eyewear on earmuffs, and additional studies with broad combinations of earmuffs and eyewear are needed to establish the range over which eyewear can be expected to compromise earmuff attenuation.

Protection exhibited by hearing protection devices using test fixtures under extended shock tube exposure ranges

Hearing protection devices (HPDs) are used in a variety of environments to mitigate the risk of hearing injury, particularly in environments with pervasive impulsive noise. Evaluation of HPD performance for these conditions can be performed with acoustic test fixtures to ensure human subjects are not exposed to injurious levels of noise. Impulse Peak Insertion Loss (IPIL) and Impulse Spectral Insertion Loss (ISIL), as defined in ASA/ANSI S12.42, are used to quantify and compare performance of HPDs in exposure environments created by different impulsive sources. To simulate a range of exposures relevant across industries, multiple shock tubes were used to generate waveforms with A-durations ranging from 0.1 to 6.0 ms and peak pressures ranging from 130 to 183 dB. Protection provided by each device varied depending on the source conditions with passive and active devices producing fundamentally different responses depending on source characteristics. Ultimately, evaluation of devices under a variety of impulsive noise conditions allows for a more representative evaluation of HPD performance, bridging the gap between standard testing protocols and real-world acoustic environments, thus enabling better informed HPD selection.

Enhanced finite-element model and acoustic test fixture to assess the objective occlusion effect induced by earplugs under bone-conducted stimulation

The occlusion effect refers to the increased perception, mainly at low frequencies, of bone-conducted sounds like physiological noises and one's own voice when one wears hearing protection devices. This phenomenon may lead to an acoustic discomfort and subsequently to less effective protection. It can be quantified by an objective indicator defined as the difference between the sound pressure level in the open and occluded earcanal. Several finite-element models based on truncated outer ears are available in the literature for predicting the objective occlusion effect but they suffer from several limitations mainly due to anatomical simplifications. In this paper, the occlusion effect of earplugs is assessed using an enhanced finite-element model and an augmented acoustic test fixture both based on the same real head geometry of a living participant. The finite-element head model is evaluated by comparing numerical results with various experimental data. The augmented acoustic test fixture is evaluated itself with respect to the participant. Using the finite-element model, the variability of the occlusion effect induced by the uncertainty on the stimulation position of the bone transducer, the position in the earcanal where the occlusion effect is assessed, and the uncertainty on the soft tissue Poisson's ratio is investigated.

Experimental evaluation of earplug behavior in front of high-level impulse noise using laser Doppler vibrometer.

Hearing protection devices facing high-level impulse noises provide an attenuation, generally, between 20 and 40 dB. One reason for this limitation is the direct interactions between the protection device and the impulse waves. In the case of earplugs, direct transmissions through the earplug occur. These direct transmissions combine with the already well-studied indirect transmissions arising from wave propagation in the external ear's tissues (skin, cartilage, and bone). To evaluate the transmission induced directly by the earplug, an experimental protocol using a laser Doppler vibrometer was developed. Thus, the earplug's outer lateral face (OLF) displacements and acoustic pressure at the eardrum were measured simultaneously. Two earplugs (polyurethane foam and acrylonitrile butadiene styrene) inserted in an acoustic test fixture were stimulated with impulses ranging from 137 to 180 dB-peak. A slight earplug OLF movement in the ear canal varying from 1 μm to 0.1 mm could be observed, which is likely related to ear canal longitudinal compression. The earplug's OLF displacement and acoustic pressure variation at the eardrum strongly depended on the earplug type. These direct transmissions and underlying consequences considerably alter the protection efficiency.

Effect of hearing protection attenuation on impulse insertion loss and kurtosis

Kurtosis (the fourth standardized moment of the sound pressure) has been used to assess the additional risk of hearing loss for complex or impulsive noise exposures. Murphy et al. [2012] reported the impulse peak insertion loss of five hearing protection devices (HPDs) for firearm impulse noise. Fackler et al. [2017] reported a spectral insertion loss for several HPDs assessed with firearm and shock tube impulse noise sources. Murphy [2019] previously reported the effect of five hearing protection devices on the kurtosis and insertion loss assessed with jackhammer noise. Unprotected Jackhammer noise exhibited kurtosis values between about 15 to 17, whereas protected exposures exhibited kurtosis between about 3 to 12. Anderson and Argo [2022] reported that insertion loss was unaffected by kurtosis level for seventeen HPDs measured on an acoustic test fixture under headphones. This paper will apply the complex transfer functions from Murphy et al. [2012] to the jackhammer noises measured by Murphy [2019]. The levels of the unoccluded noises transformed to the ear canal of the fixture will be compared to occluded jackhammer noises levels measured in the fixture.

Quantifying impacts of hearing protection devices on sound localization in azimuth and elevation: Refinement of acoustic predictors

Modern hearing protection devices (HPDs) mitigate the risk of noise-induced hearing loss when used as intended, but negative auditory perceptual side-effects continue to limit usability in critical settings. Dozens of studies have shown that HPDs lead to significant errors in sound source localization. Several studies, including previous studies by our group, have linked such errors to distortions of the spectra of transmitted signals. Here we provide an update on a multi-site effort to develop acoustic predictors and summary metrics to quantify HPD impacts on sound source localization. At two independent study sites, human listeners localized brief broadband signals with open ears and during use of passive and active earplug and earmuff-style devices. Sources spanning 360° in azimuth and −30° to + 60° in elevation enabled measurement of two-dimensional localization error across a broad range of source locations. An initial set of parallel acoustic measurements was obtained using a standardized acoustic test fixture. Present efforts are focused on evaluating prospective advantages of (1) incorporating binaural acoustic data in the prediction of errors and (2) accounting for individual HPD-impacted acoustic variability. The long-term goal of this work is to support application-specific selection of existing HPDs and inform the design of new HPDs.

Morphologic clustering of earcanals using deep learning algorithm to design artificial ears dedicated to earplug attenuation measurement.

Designing earplugs adapted for the widest number of earcanals requires acoustical test fixtures (ATFs) geometrically representative of the population. Most existing ATFs are equipped with unique sized straight cylindrical earcanals, considered representative of average human morphology, and are therefore unable to assess how earplugs can fit different earcanal morphologies. In this study, a methodology to cluster earcanals as a function of their morphologies with the objective of designing artificial ears dedicated to sound attenuation measurement is developed and applied to a sample of Canadian workers' earcanals. The earcanal morphologic indicators that correlate with the attenuations of six models of commercial earplugs are first identified. Three clusters of earcanals are then produced using statistical analysis and an artificial intelligence-based algorithm. In the sample of earcanals considered in this study, the identified clusters differ by the earcanal length and by the surface and ovality of the first bend cross section. The cluster that comprises earcanals with small girth and round first bend cross section shows that earplugs induced attenuation significantly higher than the cluster that includes earcanals with a bigger and more oval first bend cross section.

Measuring impulse insertion loss of hearing protection device using high-level impulse noise

The American National Standard, ANSI S12.42-2010, specifies the impulse peak insertion loss (IPIL) for a hearing protection device (HPD) using an acoustic test fixture (ATF) over a range of impulse levels nominally at 132, 150, and 168 dB peak sound pressure level (dB pSPL). The GRAS 45CB ATFs have ear simulators fitted with GRAS 40BP microphones (6.35 mm diameter) that have amplitude limits of about 169 dB SPL (172 dB pSPL). The open ear transfer function measured at the 168-dB level can cause the peak amplitude to exceed the design specification of the microphone. Accordingly, the standard directs the user to measure the 150-dB transfer function between the field microphone and unoccluded ear (HFF,TM) and apply it to the field microphone measurement at 168 dB. We have measured the HFF,TM with GRAS 45CB ATF and with the GRAS 45CB-S2 ATF equipped with GRAS 40BH microphones (193 dB/196 dB pSPL). Impulse peak insertion loss measured with both ATFs will be reported and compared.

A 3D Printed Human Ear Model for Standardized Testing of Hearing Protection Devices to Blast Exposure.

A 3D printed human temporal bone (TB) that is anatomically accurate would cost-effectively reproduce the responses observed in blast testing of human cadaveric TBs with and without passive hearing protection devices (HPDs). HPDs have become critical personal protection equipment against auditory damage for service members. Acoustic test fixtures and human TBs have been used to test and develop HPDs; however, the lack of a cost-effective, standardized model impedes the improvement of HPDs. In this study, the 3D printed TB model was printed with flexible and rigid polymers and consisted of the ear canal, tympanic membrane (TM), ossicular chain, middle ear suspensory ligaments/muscle tendons, and middle ear cavity. The TM movement under acoustic stimulation was measured with laser Doppler vibrometry. The TB model was then exposed to blasts with or without HPDs and pressures at the ear canal entrance (P0) and near the TM in the ear canal (P1) were recorded. All results were compared with that measured in human TBs. Results indicated that in the 3D printed TB, the attenuated peak pressures at P1 induced by HPDs ranged from 0.92 to 1.06 psi (170-171 dB) with blast peak pressures of 5.62-6.54 psi (186-187 dB) at P0, and measured results were within the mean and SD of published data. Vibrometry measurements also followed a similar trend as the published results. The 3D printed TB model accurately evaluated passive HPDs' protective function during blast and the potential for use as a model for acoustic transmission was investigated.

A comparison of methods to measure the sound attenuation of active noise reduction hearing protectors

Hearing protectors equipped with active noise reduction (ANR) functions have become popular and widespread in recent years. It is important to assess the sound attenuation performance of the ANR hearing protectors properly, which remains a problem. This work studies several typical measurement methods, including the real-ear attenuation at threshold (REAT), microphone in real-ear (MIRE), and the acoustic test fixture (ATF). In order to reduce the masking effect from the noise generated from the ANR electronic system when ANR is turned on during the REAT test, a modified tone sound instead of one-third octave band noise is used as the test signal. The results obtained from the different measurement methods on several ANR headphones and earphones are compared and the differences are discussed. It is found that the measurement differences among those methods are huge. The values of active insertion loss measured by the REAT method are much lower than those assessed by the MIRE procedure. This work contributes to the development of the test methods to assess the sound attenuation of the ANR hearing protectors.