Acoustics Australia最新文献

The internal meanflow with nonuniform distributions of velocity and temperature is a major challenge for acoustic analysis of a muffler in the frequency domain. On the other hand, the three-dimensional time-domain numerical method is well suited for solving the influence of meanflow on the muffler, but it is time-consuming, especially for calculating the transfer matrix that requires two sets of boundary conditions. We proposed a more efficient time-domain method to calculate the scattering matrix (SM) of an actual engine muffler using a numerical model with only one set of boundary conditions. The reciprocity, as a basic property of waves, was for the first time demonstrated in such a complex muffler with hot nonuniform flow exhausted from the engine and used to reduce the procedures for calculating the SM. The reciprocal relationship was not only expressed in the modules of the transmission coefficients in the SM but also corrected in the phases using the time delay between the incident and transmitted waves observed with the time-domain method. At last, the SM was adopted to obtain the performance of the muffler, which was validated with the measurement. The proposed method shall make the time-domain method more efficient for calculating the characterizing matrix of a muffler without or with meanflow.

In 2018, the World Health Organization (WHO) stated that transport noise is the second biggest environmental problem affecting people’s health, after air pollution. The Australian Environmental Health Standing Committee (enHealth) also provides suggested health-based limits for transport noise exposure. To better understand the impact of transport noise in Australia, a strategic national transport noise model was developed, representative of the year 2018. The transport noise model presented included parameters for terrain, buildings, and noise barriers, with results verified against measured data. The model calculated the road, rail, and aircraft noise levels for the day, evening, and night-time periods, across all façades of all storeys for over 15 million buildings across Australia. The State of Victoria was chosen as a case study to document noise exposure levels to the community. Australian Census of Population and Housing data and planning zones allowed a population within each dwelling to be calculated and paired to the modelled noise levels. Based on noise levels at the most exposed façade, it is estimated that 48% of the Victorian population are exposed to road traffic noise levels that exceed the 2018 WHO recommendations. Additionally, 10% are estimated to be exposed to aircraft noise levels, and 11% are estimated to be exposed to rail noise levels, that exceed the 2018 WHO recommendations. These percentages are commensurate with higher affected European Member states based on 2017 noise mapping completed as part of the European Noise Directive. When compared against environmental noise exposure recommendations from enHealth (2018), it is estimated that 11% of the Victorian population are exposed to combined road, rail, and aircraft noise levels above the recommended day/evening 60 dB L_{Aeq 16 h} health-based limit, and 10% above the health-based night-time limit of 55 dB L_{Aeq 8 h}. This national transport noise model provides a base for further research into the impacts of transport noise on the community, particularly regarding health and property values. The model can also support government planning, complaints handling, and asset management in the planning of future noise abatement in Australia.

This work proposes and validates two computational tools for synthesizing distance-dependent head-related transfer function (HRTF), which is vital in spatial sound reproduction. HRTF is an anthropometric feature-dependent function that yields the direction-dependent gain of the auditory system. Even though it is subject to the distance of the auditory source, distance-dependent HRTF measurement is rare due to its high experimental cost. Numerical simulation tools can provide viable alternatives. The required computational resources and time increase exponentially with the frequencies and degree of freedom (DoF) of the simulations; still, it is faster than experimental procedures. This work proposes finite element computational solutions to measure distance-dependent HRTFs using domain truncation methods in association with frequency-dependent adaptive meshing. Two hybrid techniques to find HRTF in the entire region, employing infinite elements (IEs) and non-reflective boundary conditions (NRBCs) with near-field to far-field transformation techniques, have been implemented and analyzed. The proposed methods calculate distance-dependent HRTF in 0.2–20 kHz frequency band, with reduced computational cost and time. Additionally, the spatial resolution of the HRTF measurement has increased a 100-fold. Since locally connected finite elements are used, the near-field effects of HRTF are well incorporated, and the obtained HRTF matches well with the experimental results. The proposed tools can also calculate sufficiently accurate HRTFs even when the surface meshes are of reduced quality. The tools also possess the versatility in effortlessly integrating appropriate bioacoustic attributes (e.g., internal reflection of the middle ear walls) into HRTF numerical models, which is noteworthy.

The Helmholtz equation has been used for modeling the sound pressure field under a harmonic load. Computing harmonic sound pressure fields by means of solving Helmholtz equation can quickly become unfeasible if one wants to study many different geometries for ranges of frequencies. We propose a machine learning approach, namely a feedforward dense neural network, for computing the average sound pressure over a frequency range. The data are generated with finite elements, by numerically computing the response of the average sound pressure, by an eigenmode decomposition of the pressure. We analyze the accuracy of the approximation and determine how much training data is needed in order to reach a certain accuracy in the predictions of the average pressure response.

Wearing face masks has resulted in verbal communication being more challenging during the COVID-19 pandemic. This study aimed to investigate the effect of face masks on the speech comprehensibility of Persian nurses in healthcare settings. Twenty female nurses from the governmental hospitals randomly participated in an experiment on seven typical commercial face masks at two background noise levels. Nurses' speech intelligibility from a human talker when wearing each face mask was determined based on the speech discrimination score. The vocal effort of nurses wearing each face mask was determined based on the Borg CR10 scale. Based on the linear mixed model, the speech intelligibility of nurses from a human speaker wearing surgical masks, N95 masks, and a shield with face masks were approximately 10%, 20%, and 40–50% lower, respectively, than no-mask conditions (p < 0.01). The background noise decreased the speech intelligibility of nurses by approximately 22% (p < 0.01). The use of a face shield further decreased speech intelligibility up to 30% compared to using a face mask alone (p < 0.01). The vocal efforts of nurses when wearing surgical masks were not significant compared with the baseline vocal efforts (p > 0.05); however, vocal efforts of nurses when wearing N95 and N99 respirators were at an unacceptable level. The face masks had no considerable effect on the speech spectrum below 2.5 kHz; however, they reduced high frequencies by different values. Wearing face masks has a considerable impact on the verbal communication of nurses in Persian. The level of background noise in the healthcare setting can aggravate the effect sizes of face masks on speech comprehensibility.

The flow noise for the acoustic vector hydrophone in the flank array is studied in this paper. The hydrophones are usually mounted above the baffle and are protected by a dome. This paper simplifies the flank array to be an infinite dome placed above an infinite baffle model, and the acoustic vector hydrophone is located in the fluid layer between the dome and the baffle. The spectral reflection coefficient of the multilayer baffle is obtained by the transfer matrix and matched boundary conditions. The cross-spectral density matrix is derived by the wavenumber–frequency spectrum analysis method. In addition, the spectral transfer functions are verified by the finite element method. Numerical results are presented to illustrate the influence of the free-stream velocity, the dome parameters, the location of the acoustic vector hydrophone and the baffle on the auto-power spectra of each hydrophone. Besides, the cross-power spectra of each hydrophone and the spatial correlation are discussed in this paper. The particle velocity channels are more sensitive than the pressure channel to the flow noise below 4000 Hz if the hydrophone is near the dome. The cross-power spectra between the pressure and particle velocity are lower than the particle velocity power spectra in the whole frequency band, and that are lower than the pressure power spectra in the higher frequency. The spatial correlation radius of the pressure and the particle velocity of all directions is small.

In this paper, a micro-perforated panel (MPP) composite structure consisting of an inhomogeneous MPP (IMPP) backed with J-shaped cavities of different depths for low-frequency sound absorption is proposed. The goal is to increase the low-frequency (≤ 500 Hz) sound absorption performance of the IMPP. Sound absorption in a frequency range of 300–480 Hz was achieved with parallel-arranged IMPPs backed by J-shaped cavities, with average absorption of greater than 90%. A parametric analysis was used to optimize the structure's geometric parameters for the specified frequency range. The results show that when the length and volume of the back cavity depths increase, the low-frequency sound absorption peaks shift to a lower frequency. Similarly, the sound absorption curves are enhanced and move towards lower frequencies as the thickness of the IMPP increases. The structure was studied using an electro-acoustic equivalent circuit model (ECM) and finite element method (FEM) simulation. Model prototypes were then made using stereolithography (SLA) and verified by a square-shaped impedance tube-based experimental study to determine the normal absorption coefficient. The results revealed that the three types of curves, namely theoretical, FEM simulation, and experimental, were in good agreement.

Effective broadband duct sound propagation control is highly required in many practical engineering applications. In this study, a compact structure constituted by multiple detuned resonators is proposed for broadband duct noise transmission control. The coupling characteristics of two detuned resonators flush-mounted on the sidewall of a duct are firstly investigated. Results show that a coherent perfect absorption (CPA) is induced when these two resonators are precisely designed. Meanwhile, a nearly flat transmission forbidden band is formed, which is very beneficial for duct noise control. Furthermore, it is found that the appearance of the forbidden band is insensitive to the distance between resonators. On this basis, a customized broadband CPA-based structure constructed by detuned resonators is developed, in which the geometric parameters of each adjacent resonator satisfying the CPA condition and the resonators are closely placed. By overlapping the forbidden band of adjacent resonators, a broad duct sound transmission forbidden band is attained. The acoustic performance of the proposed compact design is demonstrated experimentally.