Acoustics Australia最新文献

This paper examines and compares methods of separating tonal and broadband components of the noise generated by small rotors as commonly found on small unmanned aerial systems (sUAS). Time synchronous analysis (TSA) methods with varying averaging algorithms, such as ensemble averaging (EA), exponential weighted moving averaging (EWMA) and Kalman filter averaging (KFA), are compared against themselves and against a cross-correlation-based method. The decomposition methods are used on noise measurements of a small isolated rotor under static operation and edgewise flight conditions at 10 m/s in the UNSW anechoic wind tunnel (UAT). The best method for isolating tones is TSA-EWMA and for isolating the broadband spectrum of noise is the cross-correlation method, based on the spectral reconstruction of the experimental data.

The bogie region is one of the most important aerodynamic noise sources of high-speed trains. A thorough understanding of the generation mechanism and characteristics of bogie aerodynamic noise is the prerequisite for effective implementation of noise control measures. In this study, a delayed detached eddy simulation (DDES) is performed to solve the unsteady flow field around the bogie region, and an aerodynamic noise source identification method based on the integral solution of the Ffowcs Williams-Hawkings (FW-H) equation is adopted to determine the dipole and quadrupole sources distribution in the bogie region. The identification results of the two types of sources provide different understandings of the noise generation mechanism of the bogie region but determine the same flow structures closely associated with the bogie aerodynamic noise, which are the shear vortex structures formed at the rear edge of the cowcatcher and the front side edges of the bogie cavity. The flow field data obtained by DDES simulation is also used as input for the FW-H solver to calculate far-field noise, and the source contribution, spectrum characteristics and directivity of the far-field noise are analyzed. The results show that at a speed of 350 km/h, the aerodynamic noise in the bogie region is still dominated by dipole sources, and the contributions of the bogie itself and the bogie cavity to far-field noise are equally important despite the significant differences in their radiation characteristics.

The leading-edge and trailing-edge serrated guide vanes, inspired by the silent flight of owl wings, were designed to reduce the aerodynamic noise produced by elbows in cruise. Using the acoustic finite element approach in conjunction with the large eddy simulation model, the aerodynamic noise produced by the elbow is calculated. Using an air piping test platform, this hybrid simulation technique is validated. Further simulation results showed that these two bionic guide vanes contributed to the decrease in the aerodynamic noise by streamlining the airflow and lowering the formation of laminar flow separation bubbles. In particular, the leading-edge serrated guide vane reduced noise by 4.6 dB, whereas the trailing-edge serrated guide vane reduced noise by 3.4 dB.

Light rail vehicles will often idle with their air conditioners running at terminus locations that may be located near sensitive receivers due to network constraints. This creates a pseudo-stationary noise source with similar level and characteristics to industrial air conditioners that can result in disturbance and complaints from nearby sensitive receivers. However, in practice in the state of New South Wales (NSW), Australia, this pseudo-stationary noise source is commonly assessed against airborne noise criteria for transportation noise. This is due to the Conditions of Approval on the Sydney Inner West Light Rail Extension project explicitly delineating noise produced by light rail vehicles from other sources. This interpretation has been applied on all subsequent light rail projects in NSW, which have assessed this noise source at termini against the Rail Infrastructure Noise Guideline (RING) requirements. The Noise Policy for Industry (NPfI) has been applied to other noise sources on these projects, specifically fixed equipment at stops and all noise sources at stabling facilities (including light rail vehicle air conditioning noise and traffic movements within the boundary of the facility). This paper examines the policy overlap between the RING and the NPfI that makes both documents potentially applicable to noise from light rail air conditioners when idling at termini, depending on interpretation and specific project conditions of approval. It also presents a hypothetical assessment of typical light rail activities near termini against both the RING and NPfI, to demonstrate the potential differences in project outcomes between the applications of the two documents. An example of a compromise that acknowledges the pseudo-stationary nature of the noise source as well as the benefits that public infrastructure provides relative to industrial facilities is also suggested, in lieu of a separate threshold or policy for this very specific circumstance.

The flow recirculation effect on the noise measurement of a multi-rotor unmanned aircraft system (UAS) hovering in a closed anechoic chamber is experimentally characterized in this work. The measured acoustic spectrogram reveals that the recirculation forms around 30 s after the UAS’s take-off, manifested as prominent fluctuations in blade passage frequency and its harmonics. However, the instantaneous overall sound pressure level shows no obvious increase with the development of the recirculation. The result indicates that the recirculation effect does not significantly change the total acoustic energy but increases the uncertainties in the spectral distribution, which can be quantified by spectral entropy. The quantitative analysis of different noise components shows that the recirculation has a minimal effect on the tonal noise levels but slightly increases the broadband noise level out of the rotors’ plane. The results from parametric tests suggest that this broadband noise increment has a positive correlation with the UAS’s hover height but a negative correlation with the UAS’s gross mass. The comparison with existing studies highlights the difference in the recirculation effect on the noise of isolated rotor(s) and free-flying multi-rotor UAS in confined spaces.

The impedance tube method is commonly employed to measure the acoustical properties of materials, but commercial versions are prohibitively expensive for researchers lacking access to well-funded acoustical laboratories. A significant expense in traditional impedance tube setups is the pressure field microphones. This study explores the feasibility of using low-cost consumer-grade electret microphones, which are substantially cheaper than their pressure field counterparts. Our impedance tube design was validated and tested with high-cost pressure field microphones, contrasting it with electret microphones priced under one US dollar (USD). The findings reveal that the sound absorption coefficient can still be effectively and accurately measured using inexpensive microphones, subject to suitable signal conditioning and accurate microphone calibration. The trade-off is a slight loss of accuracy for the low-end frequency range of < 250 Hz.

A method to distinguish the surface source and underwater source based on two-dimensional Fourier transform of interference pattern in deep-water environment with an incomplete sound channel is presented in this paper. Considering the modal characteristics of incomplete channel, the normal mode can be divided into three categories: trapped mode, bottom interacting mode and surface interacting-bottom interacting mode. Then, the interference spectrum can be obtained by performing a two-dimensional Fourier transform on the interference pattern. Due to the correlation between the interference structure and the source depth, the types and positions of interference spectral peaks vary at different source depths. Based on this, subspaces can be defined for the interference spectrum, and then the energy ratio of the different modal interference groups in the subspaces can be calculated for source depth discrimination. In this method, the identification of source depth is regarded as a binary classification problem, where the decision threshold is calculated from simulation results under a given false alarm probability. The source depth discrimination can be achieved through comparing the energy ratio with the given decision threshold. The effectiveness of the proposed method is verified using numerical simulations and experimental data.

The increasing application of DIN 4150-3 to above-ground structures such as commercial, residential and particularly heritage buildings in the preliminary planning stage of projects is problematic. DIN 4150-3 is often incorrectly interpreted when applied to Australian scenarios which has the potential for long-term consequences. Applying the DIN 4150-3 guide values for resonant vibration at the correct location (just below the roof) requires the consideration of potential amplification between the foundation and the roof level of the building, which does not appear to be common practice in Australia. A review of the literature found that roof vibration levels are typically 1.5 times higher than that at the foundation, but in practice can be up to four to six times higher, particularly in heritage structure applications. The correct application of DIN 4150-3 results in more stringent guide values at the foundation than those commonly applied in practice in Australia, the practical consequences of which are either an excessive number of pre-construction dilapidation surveys, or the restriction of vibration-intensive items of plant through increased buffer distances, which increases project costs and timelines. This paper proposes an alternative methodology to the application of DIN 4150-3 that, when complemented with the application of BS 7385-2, provides a sensible compromise for Australian scenarios between the competing requirements of asset owners and construction contractors that can be applied to all industrial, commercial and residential receivers, including those with “heritage” status.

University is an important stage of learning for students, so it is vital that higher education spaces are acoustically accessible to all and are places that promote equity and inclusion. The aim of this study was to measure the unoccupied noise levels and reverberation times of all of the classrooms in a typical Australian university to assess acoustic accessibility with a view to planning for a more accessible campus. A total of 166 classrooms were measured and categorised into good, ok, and poor classrooms according to the Macquarie University (MQU) Design Guidelines Review Performance Standards. Regarding unoccupied noise levels, 52% of classrooms were within the recommended < 35 dBA limit. Regarding reverberation times, 65% of classrooms were within the recommended 0.4–0.6 s limit. Finally, 40% of classrooms met both the noise level and reverberation time limit. The plans at the university to incorporate these findings to make the campus more acoustically accessible are discussed, as well as future research avenues so that all students and teachers can flourish.