Sound Pressure Level Reduction Research Articles

Complex-valued impedance tiles to reduce noise emanating through openings in mechanical systems

We present acoustic metasurfaces to achieve high noise reduction for an open structure, which refers to a structure with openings, by manipulating complex impedances along unit cells called impedance tiles. These acoustic metasurfaces are applied to inner walls of the open structure to suppress noise emanating through openings. Each impedance tile is made up of subwavelength Helmholtz resonators (HRs), and its effective impedance is tuned to minimize the noise level outside the open structure at an arbitrary target frequency by adjusting the geometrical parameters of those HRs. Rather than absorbing incident waves, the proposed metasurfaces significantly affect the magnitude and phase of scattered waves, causing the sound energy to be trapped near their surfaces and dissipated by visco-thermal losses in narrow orifices of the impedance tiles. As a result, the metasurfaces enable more effective noise reduction for the open structure compared to impedance-matching conditions, which absorb sound waves impinging on the surface. Experimental results show that the sound pressure level reduction exceeding 12.6 dB is verified for a single source at a target frequency of 360 Hz with λ/31-thick metasurfaces fabricated via three-dimensional printing. Furthermore, we demonstrate that the proposed metasurfaces are also useful in reducing noise when the number of target frequencies is more than one or when multiple sources are distributed in different locations, thereby offering a potential solution for noise reduction in a wide range of mechanical systems with open structures.

A new numerical model of gas leakage noise from elliptical sealing defects in pipelines

During the assembly of aviation conduits, various quasi-elliptical sealing anomalies often arise due to diverse factors. This paper presents a new numerical model for analyzing the noise distribution pattern of quasi-elliptical holes. Analogous to the velocity distribution law of an elliptical jet, the velocity distribution function of an elliptical defect boundary is constructed, and then the gas leakage flow field is solved by using the large eddy simulation method, and then the near-field noise distribution is calculated by using CFD. The model discerns that the rate of sound pressure level reduction along the minor axis surpasses the major axis as the aspect ratio increases. Within the azimuth angles plane, sound pressure levels gradually decrease from the minor to the major axis direction, and the discrepancy between sound pressure levels along these two axes diminishes with an increase in aspect ratio. Experimental validation underscores the model's precision, with accuracy exceeding 90% in approximating noise pressure levels near leakage sites.

Effects of blade tip thicknesses on hydrodynamic and noise performance of ducted propellers

Propeller noise plays an important role in underwater radiated noise. The radiated noise of a ducted propeller is closely related to the vorticity intensity in the gap between the blade tip and the inner wall surface of the duct. To investigate the influence of blade tip thickening on the hydrodynamic and noise performance of ducted propellers, six different models of ducted propellers with varying blade tip thicknesses are presented. The Reynolds-averaged Navier–Stokes (RANS) method is used to simulate the flow around the ducted propellers, and a combination of Detached Eddy Simulation (DES) and acoustic boundary element method (BEM) is employed to predict the noise. The results show that thickening the blade tips leads to a 4.15% efficiency loss at J = 0.6, but improves efficiency by 17.61% at J = 0.8. As the blade tip thickness increases, the length of the blade tip vortex gradually decreases, and the turbulent kinetic energy (TKE) in the chordwise middle region of the blade tip decreases. Additionally, the peak values of thrust and torque fluctuations of the blades primarily occur at the blade passing frequency (BPF) and its harmonics, with the minimum amplitudes of the BPF at the maximum blade tip thickness. Thickening the blade tips results in a maximum overall sound pressure level reduction of 0.50 dB for the ducted propeller, with a maximum sound pressure level (SPL) reduction of 4.24 dB at certain dominant frequencies.

Cavitation flow and noise reduction design of bionic hydrofoil based on orthogonal optimization

Hydrofoils hold considerable academic and practical significance in the realms of marine science, energy generation, and water-based engineering. They offer enhanced speed, efficiency, stability, and maneuverability. Bionic structures have emerged as potent tools for reducing energy losses and noise in hydraulic machinery, making bionic hydrofoils a hotbed of research activity. While prominent scholars have historically directed their bionic investigations toward airfoils, operating in compressible flow fields, recent research has shifted its focus to hydrofoils. The hydrofoil's operating environment is characterized by water instead of air, featuring incompressible flow, relatively low Reynolds and Mach numbers, and notably, cavitating flow. This study presents the bionic optimization design of a wavy leading edge for the hydrofoil, employing orthogonal experimental theory. The authors establish rankings for structural parameters of bionic hydrofoils and identify optimal parameter combinations, offering an optimization strategy for selecting bionic configurations. Subsequently, the authors conduct a numerical investigation into cavitating flow, integrating the FW–H (Ffowcs Williams and Hawkings) equation for the analysis of cavitation-induced noise. Notably, this research delves into the underlying mechanisms responsible for the efficacy of bionic structures in enhancing hydrodynamic performance, particularly in the reduction of cavitation-induced noise within cavitating flow, an area scarcely explored in formal publications. The results reveal that the amplitude of the wavy leading edge exerts the most significant influence on the lift-to-drag ratio, as well as the far-field sound pressure level, followed closely by the wavelength. When compared with a baseline hydrofoil, the optimized bionic hydrofoil demonstrates a substantial 45% reduction in maximum cavity volume and a noteworthy 1.3 dB reduction in far-field noise sound pressure level. These findings underscore the capacity of the optimized bionic hydrofoil to effectively suppress cavitation and its associated noise. The established optimization strategy, focused on cavitation suppression and noise reduction, lays a robust foundation for subsequent studies involving complex working conditions.

A new method for measurement of impulsive peak level attenuation of hearing protectors

ASA/ANSI S12.42-2010 contains the first standardized test method for measuring the performance of hearing protection devices (HPDs) with high-level impulsive noise. This standard defines the impulsive peak insertion loss (IPIL) as a time-domain metric that quantifies the reduction in peak sound pressure level provided by an HPD for impulsive noises. However, IPIL as defined in S12.42-2010 is dependent on the spectrum of the impulsive noise source used for measurements of HPDs. Recent studies of HPDs with impulsive noise have led to the investigation of frequency-domain metrics and calculation methods. Using frequency-domain calculations allows the impulsive peak level attenuation to be computed for a wide range of impulsive noises, not only those used for the measurement. Consequently, significant revisions to S12.42 are under consideration and a new standard, ISO 4869-7, is being developed in parallel with the ANSI revision. This presentation highlights key proposals and substantive changes including: measurement and calculation of the frequency-dependent impulsive insertion loss (IIL), incorporation of frequency-domain aspects such as real-ear attenuation at threshold (REAT) limits to attenuation, and a new impulsive peak level attenuation metric (IPLA) to be calculated from the REAT-bounded IIL.

Experimental investigations on aerodynamic and psychoacoustic characteristics of three-blade looprop propeller

Aeroacoustic noise in multiple rotor drones has been increasingly recognized as a crucial issue. This study focuses on addressing this challenge by introducing a novel low-noise looprop propeller design with three blades. Unlike traditional propellers, the looprop propeller utilizes three closed-loops to generate thrust. Through comprehensive experimental investigations conducted in an anechoic chamber using a hover stand test, we conducted an integrated study of the propeller's aerodynamic, aeroacoustic, and psychoacoustic characteristics. The results demonstrate that the three-blade looprop propeller achieves a notable reduction in the overall sound pressure level (OASPL), particularly in tonal noise at the blade passing frequency where the tone reduction amounts to approximately 15 dB. Additionally, we employed Zwicker psychoacoustic annoyance models to evaluate the propellers' psychoacoustic performance and discussed the environmental noise mask effect. The findings indicate that the three-blade looprop propeller exhibits an improved sound quality while maintaining aerodynamic performance comparable to that of conventional propellers. Overall, our study suggests that the looprop propeller design offers significant advancements in acoustic performance, particularly in terms of psychoacoustic characteristics. These findings hold promising implications for the field of drone technology and the potential for notable improvements in noise reduction.

Application of heat transfer at the combustor wall to attenuate self-excited thermoacoustic oscillations

Self-sustained thermoacoustic oscillations (also known as combustion instability) arise in combustors when pressure and heat release fluctuations couple to produce high-energy sound waves. The need for diverse strategies to control these oscillations will increase as future design moves towards highly efficient, lean combustors. In this work, the impact of heat transfer at the combustor wall on the generation of thermoacoustic oscillations is examined through computational fluid dynamics. Characteristics of the thermoacoustic oscillations generated across cases with a range of thermal boundary conditions are compared to an adiabatic reference case. The amplitude of limit cycle oscillations in a Rijke tube is seen to decrease as wall temperatures increase. A stable combustor with a negative growth rate is observed when the full outer wall is heated to 900 K, and a sound pressure level reduction of 75 dB is achieved. The most efficient stable configuration is achieved when only the inlet duct wall is heated to 1000 K, requiring a minimum of 430 W of input power. The present work introduces an alternative stable combustor design and demonstrates that optimizing the temperature distribution of the combustor wall can effectively dampen thermoacoustic oscillations.

Proper Orthogonal Decomposition analysis of mode switching in supersonic jets impinging on flat and corrugated plates

Abstract Understanding the occurrence of various feedback mechanisms of an under-expanded impinging supersonic jet is a crucial task in research. The presence of several jet modes is examined in this study for the flat and corrugated impinging plate geometries. The behavior of impinging plate configurations during mode switching is investigated by varying the flow state, such as the jet Mach number. The staging behavior at various jet Mach numbers is observed using acoustic spectral plots and schlieren flow visualization. To explore the presence of various types of modes during the jet impingement due to the modification of jet Mach number, ensemble averaging and Proper Orthogonal Decomposition of schlieren images are carried out. In the majority of situations, the corrugated design shows a reduction in tonal noise and overall sound pressure level. In exceptional cases, for the corrugated plates, the enhanced overall sound pressure level is caused by the existence of axisymmetric instability (A1, A2).

Experimental Study on Wind Turbine Airfoil Trailing Edge Noise Reduction Using Wavy Leading Edges

Aerodynamic noise produced by the rotating blade is an important hindrance for the rapid development of modern wind turbines. Among the various noise sources, the airfoil trailing edge noise contributes a lot to the wind turbine noise. The control of wind turbine airfoil trailing edge self-noise by bio-inspired sinusoidal wavy leading edges is experimentally studied in a semi-anechoic chamber. The noise radiated by the baseline NACA 0012 airfoil and various wavy airfoils is measured using a planar microphone array consisting of fifty-two microphones. The noise source identifications are achieved by using the CLEAN-SC method. The effects of velocity and angle of attack on noise radiation of the baseline airfoil are analyzed in detail. The noise control law of the wavy amplitude and wavelength on airfoil trailing edge noise is explored. Based on the acoustic beamforming results, the noise control effects of the wavy leading edges are intuitively demonstrated. In general, the wavy leading edge with a larger amplitude and smaller wavelength has a better effect on the airfoil trailing edge noise reduction. The maximum sound pressure level reduction can be up to 33.9 dB. The results of this study are expected to provide important information for wind turbine aerodynamic noise control.

A Virtual Sensing Active Noise Control System Based on a Functional Link Neural Network for an Aircraft Seat Headrest

Abstract Purpose The goal of this work is to propose a local Active Noise Control system for an aircraft seat’s headrest. This system should be able to extend the quiet zone beyond the physical microphones that can be placed behind the passenger’s head due to space constraints. Furthermore, the control algorithm should perform efficiently when non-linear phenomena exist during the sound propagation. Methods A functional link neural network and a computationally efficient Multiple Input Multiple Output approach are used for the system’s implementation. The quiet zone is extended by linearly estimating the acoustic pressure in front of the headrest surface. The main novelty of this paper is the combination of these methods in an attempt to improve acoustic pressure attenuation performance, while keeping computational complexity low. Results The proposed control algorithm has been evaluated through numerical simulations, including Finite Element Method and experimental tests at an aircraft cabin mock-up. The results show that for a real-world acoustic disturbance, a 10 dB reduction in sound pressure level was achieved 10 cm away from the headrest surface. In addition, the attenuation of some harmonics can reach 20 dB and in most cases is bigger than the linear FxLMS algorithm. Conclusions To summarize, it has been demonstrated that a multichannel functional link neural network using a simple virtual sensing technique can efficiently attenuate synthesized and real world acoustic disturbances captured in a tilt-rotor aircraft’s cabin. Finally, it can create an adequate quiet zone for gentle head movement while maintaining its stability.

Experimental investigation of propeller noise in ground effect

The aerodynamic propeller performance and aeroacoustic characteristics of a single two-bladed propeller subjected to Ground Effect are investigated using experiments in an anechoic chamber. A strong increase in thrust, torque and power coefficients when a propeller operates in Ground Effect is demonstrated, consistent with previous studies. A low-fidelity Blade Element Momentum Theory model adapted for Ground Effect is implemented to cross-validate the aerodynamic results. The near-field and far-field spectra and their coherence are studied for various propeller distances from the ground plane and compared with the isolated case. The far-field noise measurements indicate that additional tonal peaks appear with the introduction of the ground plane when compared to isolated configuration. While in-Ground Effect, additional high-frequency broadband humps are observed at some polar angles not seen in isolated configuration. Two discrete regions, a shielded and reflection zone, with respective reduction and enhancement of the Overall Sound Pressure levels are observed. The primary reason for noise increase within the reflection zone is a result of pure acoustic reflection from the ground surface. The near-field measurements from an array of embedded high-response microphones in the ground plane indicate that the main source of the propeller noise is in the tip region. Near-field measurements reconfirm flow results from literature with near-field and far-field acoustic results. The present study may find important practical applications within Urban Air Mobility (UAM), especially in the design of vertiport landing pads and propellers for effective noise mitigation, aerodynamic performance, and prediction during the vertical take-off and landing phases of flight.

Effect of Wavy Leading Edges on Airfoil Trailing-Edge Bluntness Noise

Among the several noise-generation mechanisms of airfoil self-noise, trailing-edge bluntness noise is an important noise source, which is caused by the vortex shedding at blunt trailing edges. A numerical study on airfoil trailing-edge bluntness noise control using the bio-inspired wavy leading edges is presented in this paper. The high-fidelity, improved, delayed, detached eddy simulation (IDDES) method was used to calculate the flow field, and the acoustic analogy method was used for noise prediction. For both the blunt-trailing-edge airfoils, a baseline airfoil with a straight leading edge and a bio-inspired airfoil with a wavy leading edge were used in this study. The chord-based Reynolds number was 400,000, and there was no angle of attack. The numerical results show that the trailing-edge bluntness noise of the baseline airfoil was significantly reduced by the wavy leading edges. The sound pressure level reduction was about 3.7 dB at the characteristic frequency, and the maximum sound pressure level reduction was as high as 35 dB. The trailing-edge bluntness noise was decreased at all directional angles. The maximum overall sound pressure level reduction was 6.3 dB at 0°. In addition, by analyzing the pressure fluctuations, wake characteristics, turbulent vortex structures and spanwise correlation and coherence of the flow field, the noise-reduction mechanisms of the bio-inspired airfoil are deeply revealed.

The Role of Traffic Volume on Sound Pressure Level Reduction before and during COVID-19 Lockdown Measures-A Case Study in Bochum, Germany.

During the SARS-CoV-2 pandemic, sound pressure levels (SPL) decreased because of lockdown measures all over the world. This study aims to describe SPL changes over varying lockdown measure timeframes and estimate the role of traffic on SPL variations. To account for different COVID-19 lockdown measures, the timeframe during the pandemic was segmented into four phases. To analyze the association between a-weighted decibels (dB(A)) and lockdown phases relative to the pre-lockdown timeframe, we calculated a linear mixed model, using 36,710 h of recording time. Regression coefficients depicting SPL changes were compared, while the model was subsequently adjusted for wind speed, rainfall, and traffic volume. The relative adjusted reduction of during pandemic phases to pre-pandemic levels ranged from -0.99 dB(A) (CI: -1.45; -0.53) to -0.25 dB(A) (CI: -0.96; 0.46). After controlling for traffic volume, we observed little to no reduction (-0.16 dB(A) (CI: -0.77; 0.45)) and even an increase of 0.75 dB(A) (CI: 0.18; 1.31) during the different lockdown phases. These results showcase the major role of traffic regarding the observed reduction. The findings can be useful in assessing measures to decrease noise pollution for necessary future population-based prevention.

Sustainable Sound Absorbers to Improve Acoustical Comfort in Atria: A Methodological Approach

Resource-efficient buildings are one of the most important challenges of the construction industry, which could be achieved by limiting the extraction of natural resources and by replacing them with industrial residues. The present work proposes innovative panels made from textile wastes as an efficient solution for making the semi-open atrium in the Polytechnic University of Bari, acoustically suitable for use as an area of study. Several measurements were carried out in the atrium under empty and occupied conditions in order to characterize the sound field of the space (i.e., the reverberation time and the sound pressure level) and the actual sound sources (i.e., the sound power level of the students occupying the space). The on-site measurement results were useful to calibrate a geometrical acoustic model implemented in CATT-Acoustic software and used to predict the effect of the proposed treatments. About 700 m2 of absorbent panels were distributed, leading to a predicted reduction in the reverberation time from 2.4 s to 1.4 s at 500 Hz, and consequently to an estimated 3 dB reduction in sound pressure level, resulting in a reduction in the background noise level due to speech. Furthermore, as a consequence of the “Lombard effect”, a decrease in the background noise level will likely reduce the sound power level of the speakers by 2 dB, with an expected overall reduction of 5 dB.

Experimentation, simulation, and statistical analysis of nanofillers reinforced bio‐based polyurethane foam for acoustical applications

AbstractThis research aims to develop bio‐based polyurethane foam (PU) with nanofillers to absorb sound energy across a broad frequency range and compare experimental results to those from simulation and optimization. PVDF (Polyvinylidene fluoride), MgO (Magnesium oxide), and Ni (Nickel) nanofillers were incorporated into bio‐based PU through absorption and hydrothermal reduction technique, which includes mechanical stirring, compressing, heating, and evaporating. To determine the weight percentages of nanofillers, the design of the experimental approach was utilized. As per ASTM standard E 1050‐12, the bio‐based PU foam composite's sound absorption coefficient (SAC) was assessed using an impedance tube setup. Response Surface Methodology was utilized to estimate optimum sample weight percentages using a central composite design (CCD) with weight percentages of the three nanofillers as input and the noise reduction coefficient (NRC) as response output. According to the CCD results, the combination of 15 wt% PVDF, 10 wt% MgO, and 5 wt% Ni shows the highest NRC value of 0.66. Then, a confirmation sample was prepared, and the NRC was calculated. The NRC achieved for the confirmation sample was 0.62. SAC simulation results are contrasted with experimental data using COMSOL Multiphysics 5.5. The confirmation sample's average sound pressure level (SPL) reduction ranges from 2 to 29 dB, according to the SPL plot. The conclusion shows that bio‐based composite foam may be an excellent material for absorbing sound in vehicles and aircraft and reducing industrial noise.

Multilayer structures for high-intensity sound energy absorption in low-frequency range

Although small-amplitude sound absorption problems have been comprehensively studied over the past decades, the high-intensity sound absorption problem in the low-frequency range remains unresolved. This paper is concerned with the design of a multilayer structure consisting of a main pore and hierarchical absorption layers to realize perfect absorption of high-intensity sound energy in the low-frequency range. The multilayer structure has enhanced energy dissipation ability in the high-intensity noise excitation environment due to vortex shedding in the sub-slits of the structure. An analytical model based on the equivalent fluid method is established for computing the nonlinear sound absorption coefficient of the structure. Effects of geometrical parameters on the sound absorption coefficient of the multilayer structure are investigated. It is found that under high-intensity sound excitation (e.g., 140 dB), the resistance of the multilayer structure grows along with the sound amplitude and renders a higher absorption coefficient, leading to great sound energy attenuation for high-intensity acoustic waves. The effective sound absorption coefficient of the designed structure is also studied by experiments based on an impedance tube. Noise reduction of a scaled-down payload fairing equipped with multilayer structures is investigated by experiments in a high-intensity sound pressure reverberation chamber in order to confirm the great noise control performance of the proposed structure. The results show that the sound pressure levels inside the fairing can be significantly suppressed by the multilayer structure in broadband, and the maximum reduction of the average sound pressure level is more than 17.5 dB.

Acoustic properties of commercial thermal insulators

Material manufacturers give very limited or no information of the acoustic properties of thermal insulators in broad perspective. The purpose of our study was to compare the acoustic performance of commercially available thermal insulators. Thirteen insulator types produced by several manufacturers were studied. The acoustic performance was determined holistically by measuring six acoustic quantities: sound reduction index of bare insulator, sound reduction index of encapsulated insulator (insulator between two boards), normal incidence sound absorption coefficient, airflow resistivity, dynamic stiffness per unit area, and reduction of impact sound pressure level in a floating floor. All measurements were conducted according to prevailing ISO standards in an experienced test laboratory. The acoustic performance differed significantly between insulator types for each acoustic quantity. The ranges were 1027 dB, 3352 dB, 0.200.78, 3.02700 kPas/m2, 1.5730 MN/m3, and 1536 dB, when the thickness of the insulator was 100 mm. Closed-pore insulator types usually carried worse acoustic properties than open-pore insulator types. Lower thermal conductivity was associated with worse acoustic performance of two acoustic quantities. Insulator manufacturers should consider to better declare the acoustic properties of insulator products, because this knowledge is needed in the acoustic design of building constructions and other applications.

An ultra-thin low-frequency broadband metasurface with near-zero suppression of aerodynamic acoustic pressure

A near-zero suppression mechanism of aerodynamic acoustic pressure is revealed by adopting the ultra-thin low-frequency broadband lotus-pods-neck Helmholtz resonator (LPNHR) metasurface presented in this paper. The LPNHR is designed by changing the single neck of a Helmholtz resonator (HR) to a lotus-pods multi-layer-hole neck and keeping the number and equivalent diameter of the holes in the upper layer greater than that in the lower layer, and the bandwidth of LPNHR could be much widened than that of HR since the reduced acoustic mass. During the incident fluid flow, compared with HR, greater pressure difference formed at the interface of each hole of LPNHR generates stronger multi-vortexes inside its neck. Larger multi-vortex areas with greater absorption area ratio significantly increase the average flow velocity at the neck interface of LPNHR, resulting in decreased impedance. Moreover, the stronger multi-vortexes weaken the influence of the main-flow on the fluid flow inside the neck, that is, the flow from the external flow field into the LPNHR neck is enhanced under the action of the strong vortexes. The impedance decreases further and the effective length of the neck and acoustic mass increase, the shift of the flow-influenced sound attenuation to higher frequencies is suppressed and turned to lower frequencies. When the impedance approaches zero, the incident and scattered acoustic pressure match in phase and the acoustic pressure fluctuation at the wall will be fundamentally suppressed. Which is the physical mechanism of LPNHR to achieve near-zero suppression of low-frequency aerodynamic acoustic pressure. Furthermore, by adjusting the multiple parameters of LPNHR, the near-zero suppression of lower-frequency and larger-bandwidth aerodynamic acoustic pressure at higher speed could be achieved. Finally, an average reduction of sound pressure level by 3.71 dB (A) in the range of 550 Hz–4150 Hz on the 1/4-scale Ahmed body surface at a speed of 50 m/s is experimentally verified through 26 mm thick LPNHR metasurface with a basic unit composed of six parallel cells. The near-zero aerodynamic acoustic pressure suppression mechanism with metasurface presented provides new approaches for low frequency aerodynamic noise control, showing great potential in engineering applications.

Numerical Study on the Aeroacoustic Performance of Different Diversion Strategies in the Pantograph Area of High-Speed Trains at 400 km/h

The speed increase in high-speed trains is a critical procedure in the promotion of high-speed railway technology. As an indispensable and complex structure of high-speed trains, the pantograph’s aerodynamic drag and noise is a significant limitation in the speed increase process of high-speed trains. In the present study, the hybrid method of large eddy simulation (LES) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy is applied to analyze the aerodynamic and aeroacoustic performances of pantograph installed in different ways, i.e., sinking platform and fairing. The results of simulation show that the application of pantograph fairing can reduce the aerodynamic drag greatly. In addition, compared with the pantographs installed alone on the train roof, the installation of the sinking platform brings about 2 dBA reduction in sound pressure level (SPL). Meanwhile, the utilization of the pantograph fairing mainly decreases the noise in the frequency band above 1000 Hz and the largest SPL reduction is up to 3 dBA among the monitoring points. Further analysis shows that the influence of different diversion strategies on the spectral characteristics actually attenuates the dominant frequency of the panhead. In the horizontal plane, the noise directivity of the pantograph installed with a fairing is similar to the pantograph installed alone on the train roof.

Proposed updates for ANSI/ASA S12.42 impulsive noise test methods

ANSI/ASA S12.42-2010 contains the first standardized test method for measuring the performance of hearing protection devices (HPDs) with high-level impulsive noise. This standard defines the impulsive peak insertion loss (IPIL) as a time-domain metric that quantifies the reduction in peak sound pressure level provided by an HPD for impulsive noises. However, IPIL as defined in S12.42-2010 is dependent on the spectrum of the impulsive noise source used for measurements of HPDs. Recent studies of HPDs with impulsive noise have led to the investigation of frequency-domain metrics and calculation methods. Using frequency-domain calculations allows IPIL to be computed for a wide range of impulsive noises, not only those used for the measurement. Consequently, significant revisions to S12.42 are under consideration. This presentation highlights select proposals including: definition and calculation of the frequency-dependent impulsive insertion loss, incorporation of frequency-domain aspects such as bone-conduction limits to attenuation, and redefinition of IPIL to be calculated from the impulsive insertion loss.