Influence of vehicle source directivity in Japanese and European outdoor sound prediction models under a semi-finite thick barrier configuration

The Japanese ASJ RTN-Model 2018, European Harmonoise and CNOSSOS-EU outdoor sound prediction models are respectively known to have symmetric, asymmetric and omnidirectional sound emission directivities along front-back direction of the source vehicle. However, the influence of such difference in directivities to final predicted sound levels has not been investigated much. In this study, the influence is investigated using the ASJ Model and the Harmonoise under a configuration of semi-finite thick barrier along a source road. The configuration is an idealization of Japanese roadside buildings that have gaps in between, unlike European buildings that continuously extend over a whole urban block. Under the configuration, distribution of A-weighted sound levels around the end face of the barrier are computed with and without source directivity taken into account by each model. It is found from the results that the source directivity of the ASJ model makes little difference in the noise level distribution. In contrast, the source directivity of Harmonoise is found to make differences of 0.5-0.8 dB at right behind the barrier depending on vehicle running direction. However, a combined effect of source directivity and reflection at the end face is found to be negligible.

Optimization of space-constrained micro-perforated absorbers by causality criteria

The problem of space-constrained absorbers in the low frequency range constitutes an area of continuous research. Micro-perforated panels are advantageous because they can be tuned by a proper selection of their constitutive physical parameters including the diameter of the perforations and their separation distance, their thickness and the length of the backing cavity. However, such optimal selection is not straightforward, especially when considering multi-layer partitions. Current optimization algorithms are based on the maximization of the total absorption coefficient averaged over a frequency band, that requires a compromise between the bandwidth and the thickness of the control device. In this work, the problem is analysed on the basis of a causality criterion. This principle is generalized from its formulation in the field of electromagnetism to obtain a relation that correlates the thickness-to-bandwidth performance of a micro-perforated absorber to its total absorption coefficient. Using this relation, an optimization procedure is presented for the sequential selection of the optimal physical parameters for single-layer partitions. An excellent agreement has been found between the optimal values obtained by the causality criterion and those achieved by critical coupling conditions.

Estimation and experimental test of the sound-absorption coefficient of a pin-holder structure (Case of sound waves incidence upon the side surfaces of a group of cylinders)

In this study, we conducted theoretical analyses and experiments related to the acoustic characteristics of the situation where sound waves are incident upon the side surfaces of a group of cylinders forming a pin-holder structure. The sound-absorption coefficient, entering its clearance between cylinders through the geometrical dimension of the clearance or the physical property of gas, was calculated. In the analytical model, the gap part of the pin-holder structure was divided into elements and approximated as a gap surrounded by two parallel planes. The characteristic impedance and propagation constant of the approximate gap were obtained and treated as one-dimensional transfer matrices; the sound-absorption coefficient was then calculated using the transfer-matrix method. The calculated value was compared to that obtained in an experiment with a sample prepared using a 3D printer; the sound-absorption coefficient was measured using a 2-microphone impedance-measuring tube. We attempted to make a simple yet accurate estimation of sound-absorption coefficient using these procedures. Our theoretical values displayed a similar tendency to that obtained by experiment.

Psycho-physiological evaluations of low-level impulsive sounds produced by air conditioners

Air conditioners are widely used in buildings to maintain thermal comfort for long time. Air conditioners produce sounds during operation, and air conditioners are regarded as one of the main noise sources in buildings. Most sounds produced by the air conditioner do not fluctuate over time and sound quality of the steady sounds produced by the air conditioner have been evaluated. However, air conditioners sometimes produce low-level and impulsive sounds. Customers criticizes such sounds are annoying when they sleep and they spend time quietly in the living room. The aim of this study was to determine the factors that significantly influence the psycho-physiological response to the low-level impulsive sounds produced by air conditioners. We assessed the A-weighted equivalent continuous sound pressure level (LAeq) and factors extracted from the autocorrelation function (ACF). Subjective loudness, sharpness, annoyance, and electroencephalography (EEG) were evaluated. Multiple regression analyses were performed using a linear combination of LAeq, the ACF factors, and their standard deviations. The results indicated that LAeq, the delay time of the first maximum peak, the width of the first decay of the ACF, and the magnitude and width of the IACF could predict psycho-physiological responses to air conditioner sounds.

A conjugate gradient least square based method for sound field control

In sound field control, a set of control sources is used to match the pressure field generated by noise sources but with opposite phase to reduce the total sound pressure level in a defined area commonly referred to as dark zone. This is usually an ill-posed problem. The approach presented here employs a subspace iterative method where the number of iterations acts as the regularization parameter and controls unwanted side radiation, i.e. side lobes. More iterations lead to less regularization and more side lobes. The number of iterations is controlled by problem-specific stopping criteria. Simulations show the increase of lobing with increased number of iterations. The solutions are analysed through projections on the basis provided by the source strength modes corresponding to the right singular vector of the transfer function matrix. These projections show how higher order pressure modes (left singular vectors) become dominant with larger number of iterations. Furthermore, an active-set type method provides the constraints on the amplitude of the solution which is not possible with the conjugate gradient least square algorithm alone.

Sound source modelling by nonnegative matrix factorization for virtual reality applications

Auralization is a suitable method for subjective noise evaluation of virtual prototypes. A basic requirement is the accurate modelling of the sound sources. This includes a dynamic and parametric description at multiple operating conditions. In the case of wave propagation including flow, such as aircraft or vehicle noise, aeroacoustics or fluid dynamics simulations are practically limited to the acoustic near field due to high computational costs. Especially challenging are simulations of rotating systems, such as fan noise radiation. For better applicability, the proposed method is based on in-situ recordings of flyovers. The processing chain compensates for source position and movement as well as atmospheric and soil damping effects on recorded data. The compensated source signal is decomposed into partial sources in spectro-temporal domain with nonnegative matrix factorization (NMF) and can optionally be enhanced by physically-based source information. The format of the source model obtained is ready to use for dynamic sound synthesis in real-time virtual reality applications.

Field study for the evaluation of the acoustic quality of open-plan offices

Even if the global health crisis is currently changing the work organisation in offices in the service industry, the problem of noise in open plan offices remains a major challenge with regard to occupational health and well-being. Since 2012, the French National Research and Safety Institute for the Prevention of Occupational Accidents and Diseases (INRS) has been carrying out acoustic surveys in French open-plan offices by measuring both some usual indicators of empty offices (Tr, D2S, Lp4m, rc, Lp) and also the ambient noise levels in activity. In addition, GABO questionnaires have been proposed to employees to assess their perception of the noise environment. So far, 50 open spaces were evaluated, with more or less data collected depending on the situation encountered. Approximately 1,400 employees have already answered the questionnaire. All of the sites visited cover the entire set of activities described by the ISO 22955 standard. An analysis of the links between the acoustic parameters and the perception of employees was carried out. This analysis provides additional information to the studies on the choice of acoustic descriptors and on the use of sound masking systems that aim to control background noise to reduce noise disturbance due to intelligible conversations.

Sound power and sound energy measurements using an ellipsoidal measurement surface

To support purchasing low noise products, sound power and sound energy measurements of sufficient quality need to be routinely made by consumers on a global scale. Sound power measurements using ISO 3744, 3745, and 3746 are conducted in a free field using an acoustic far-field approximation of the intensity integrated over an enveloping measurement surface. Sound power and sound energy measurements generally use a hemispherical, parallelepiped, or cylindrical measurement surface. Those measurement surfaces have limitations and assume that the measurement points lie on the measurement surface often in preferred positions. An alternative approach is to choose microphone positions that optimally satisfy the assumptions of the measurement. The measurement surface should then be fit to the chosen microphone positions. Regression methodologies are available for fitting ellipsoids. The number of microphone positions can be as few as three to fit an ellipsoid. An ellipsoidal measurement surfaces can abut zero, one, two, or three orthogonal reflecting planes. Correction equations for the microphone locations and the angle errors for the microphone orientation and wave propagation direction are shown. This paper will present simulations of sound power, sound energy, and corrections for environmental reflections for ISO 3745 and other measurement surfaces.

Regularized spherical harmonics-domain spatial active noise cancellation in a reverberant room

Active Noise Cancellation (ANC) at a target area in an open space, as opposed to cancellation in the ears through headphones, can lead to future applications. For instance, a personal acoustic environment in an airplane seat or inside a car, or a quiet zone in a noisy shared workspace can be possible using such open-space ANC without any uncomfortable on-body audio equipment. Recent advancements reinforce the practicality of such systems. However, regularization of the cancellation signal has been a crucial challenge in open-space ANC as it causes amplification of noise at locations away from the target area. This work presents a spherical harmonics-domain feed-forward spatial ANC method with a room-wide global cost function to address this issue. This room-wide global cost function is used for optimizing the set of regularization hyperparameters, while at run time only local information captured by a microphone array surrounding the target listening zone is required. Numerical experiments applying the proposed method in a simulated reverberant room show the effectiveness of the proposed method in creating a specific zone of silence with low to moderate noise amplification in the rest of the room.

Effects of the reduction of aircraft noise emission due to the travel restriction during the COVID-19 pandemic at residential areas around Tan Son Nhat Airport

The travel restrictions caused by the epidemic outbreak in early 2020 worldwide have caused many changes in all aspects of life, especially in the acoustic environment. This study examines the impact of this environmental change at Tan Son Nhat International Airport (TSN), the largest airport in Vietnam, by comparing the situations before and after the airport stopped operating all international flights in March 2020. The after-the-change survey was conducted in 2 phases, June and September 2020, three months and six months after the stop decision. The number of flights observed in August 2019 was 728; this number is 413 and 299 for the two surveys in 2020. The range of noise levels estimated for 12 sites around TSN decreased from 63-81 dB in 2019 to 32-67 dB in June 2020 and 33-69 dB in September 2020. At the same aircraft noise level, the percentage of highly annoyed (% HA) and the percentage of insomnia (%ISM) in the 2020 survey are higher than those in the 2019 survey. The comparison results of reaction to noise before and after the TSN's noise change indicated an increase in negative responses to noise might happen in the increased noise and reduced noise situation.