Showing papers in "Acoustics Australia in 2018"

Health Effects Related to Wind Turbine Sound, Including Low-Frequency Sound and Infrasound

Abstract: A narrative review of observational and experimental studies was conducted to assess the association between exposure to wind turbine sound and its components and health effects in the general population. Literature databases Scopus, Medline and Embase and additional bibliographic sources such as reference sections of key publications and journal databases were systematically searched for peer-reviewed studies published from 2009 to 2017. For the period until early 2015 only reviews were included, while for the period between January 2015 and January 2017 all relevant publications were screened. Ten reviews and 22 studies met the inclusion criteria. Most studies examined subjective annoyance as the primary outcome, indicating an association between exposure levels and the percentage highly annoyed. Sound from wind turbines leads to a higher percentage of highly annoyed when compared to other sound sources. Annoyance due to aspects, like shadow flicker, the visual (in) appropriateness in the landscape and blinking lights, can add to the noise annoyance. There is no evidence of a specific effect of the low-frequency component nor of infrasound. There are indications that the rhythmic pressure pulses on a building can lead to additional annoyance indoors. Personal characteristics such as noise sensitivity, privacy issues and social acceptance, benefits and attitudes, the local situation and the conditions of planning a wind farm also play a role in reported annoyance. Less data are available to evaluate the effects of wind turbines on sleep and long-term health effects. Sleep disturbance as well as other health effects in the vicinity of wind turbines was found to be related to annoyance, rather than directly to exposure.

A review of the potential impacts of wind farm noise on sleep

Abstract: Adequate sleep is important for good health and well-being, and inadequate sleep leads to impaired attention and performance. Persistent poor sleep is also associated with cognitive and metabolic impairment, cardiovascular problems and diminished psychological well-being. Recent growth in wind farm developments has been associated with community complaints regarding sleep disturbance, annoyance and a range of health issues that some attribute to wind farms. Wind turbines create aerodynamic and mechanical noise that, if sufficiently loud, has the potential to disturb residents’ sleep, particularly for those living in close proximity. According to the World Health Organisation (WHO), noise effects on sleep are expected to occur with outside noise levels > 40 dB (A). On the other hand, the WHO guidelines also state that “when prominent low-frequency components are present, measures based on A-weighting are inappropriate”, so uncertainty remains regarding which alternative noise measures and noise limits are most appropriate to mitigate community impacts of wind farm noise on sleep. In Australia, dwellings are typically located > 1 km from the nearest wind turbine where wind farm noise becomes more biased towards lower frequencies ( $$\le $$ 200 Hz) at low sound pressure levels ( $$<\sim $$ 40 dB (A) outside) that may or may not be audible inside a dwelling. Nevertheless, as with any environmental noise, wind farm noise has the potential to disturb sleep, via frequent physiological activation responses and arousals affecting the micro-structure of sleep, and the overall macro-structure of sleep, including total sleep time potentially reduced by difficulty falling asleep and returning to sleep following awakenings for whatever reason. Over time, chronic insomnia could potentially develop in individuals with greater sensory acuity and/or those prone to annoyance from environmental noise. However, it is unclear if and how much sleep is disturbed by the relatively low sound pressure levels relevant to wind turbine noise. Good empirical evidence to investigate these plausible mechanisms is sparse. In this paper, we describe the psychophysiological mechanisms that underlie sleep disturbance in response to noise, review current evidence regarding the effects of wind farm noise on sleep, evaluate the quality of existing evidence and identify evolving research in this area.

Using Petrel II Glider to Analyze Underwater Noise Spectrogram in the South China Sea

Abstract: In this paper, a Petrel II autonomous underwater glider (AUG) was used to analyze underwater noise spectrogram in the South China Sea. For ocean observations, AUG has an excellent performance such as being efficient in all weather conditions, lasting for long time periods, being able to be remote controlled, which creates favorable conditions for analyzing underwater noise characteristics. In order to measure the underwater noise, a newly developed and customized underwater passive acoustic monitoring system was firstly installed on the Petrel II glider and tested in the South China Sea trial in August 2016. The observation data at a depth of 1000 meters range were taken as the research object and were used to analyze the noise levels over a long period through the method of the short-time Fourier transform. At the same time, the noise data have been compared in time to the actions of the glider during the sea trial. However, AUGs also have limitations such as undesired instrument and flow noise. By adopting the step-by-step method, these self-noise data of Petrel II glider were acquired and analyzed quantitatively, and the basic characteristics of self-noise under different working conditions are depicted. Removing the corresponding self-noise, the noise data at different depths measured by the Petrel II glider could be used to estimate underwater ambient noise levels across depth.

Acoustic Emissions of Semi-Permeable Trailing Edge Serrations

Abstract: The trailing edge of a NACA 0018 airfoil is modified through the attachment of serrations with different degrees of permeability Acoustic beamforming is used to inspect the turbulent boundary layer-trailing edge noise emissions from the unmodified and serrated trailing edges Different freestream velocities and angles of attack are investigated The serration permeability is prescribed by having slits cut into the solid surface of the serrations in two different configurations The results indicate that a certain benefit in noise reduction is obtained from a mixed solid/slitted configuration, while a fully slitted configuration loses most of the noise reduction performance

Matecho: An Open-Source Tool for Processing Fisheries Acoustics Data

Abstract: Matecho is an automated processing method to extract information and perform echo-integration and fish shoal extraction from various scientific echo-sounder sources providing digital acoustic data on fisheries and aquatic ecosystem. The open-source initiative helps foster collaboration and technological transfer. Matecho supports various formats, such as the international standard format for the exchange of fisheries acoustics raw and edited data. The procedure allows the semiautomatic cleaning of echogram data and the application of automatic data filters, i.e., transient noise, attenuated signal and impulsive noise removal and background noise reduction. Echo-integration processing is executed for each depth layer and integrates their characteristics per elementary sampling unit. Sound scattered layers are automatically detected by segmentation from the echo-integrated echogram, and shoals are extracted according to an iterative process of aggregation of filtered echogram echoes that allows, in both cases, the calculation of the ad hoc parameters describing morphological, spatial location and acoustic characteristics of sound scattered layers and shoals. Matecho is open-source software for researchers and provides end users with a user-friendly, free executable program.

A Review of Wind Turbine-Generated Infrasound: Source, Measurement and Effect on Health

Abstract: Some people who reside in proximity to wind turbines complain of a range of adverse health impacts. These include tinnitus, raised blood pressure, heart palpitations, tachycardia, stress, anxiety, vertigo, dizziness, nausea, blurred vision, fatigue, cognitive dysfunction, headaches, ear pressure, exacerbated migraine disorders, motion sensitivity, inner ear damage and sleep deprivation. This article begins with a historical review of prognoses such as Vibroacoustic Disease and Wind Turbine Syndrome which were proposed to explain the reported health symptoms and the hypothesised link to the emission of infrasound from wind turbines. A review of noise measurements at wind turbine sites conducted by various investigators shows that the level of infrasound is below the threshold of hearing. Notwithstanding, others postulate that stimulation by infrasound of the otolith organs causes nauseogenic symptoms or that stimulation of the outer hair cells, which are said to be particularly sensitive to infrasound frequencies, explains the symptoms. A review of social surveys is undertaken of self-reported health effects attributable to wind turbine noise, including the effects of sleep disturbance. A description is finally provided of physical exploration studies which subject participants to infrasound and measure their response.

Underwater Sound Source Localization by EMD-Based Maximum Likelihood Method

Abstract: The underwater object localization is important in defense, underwater biological and environmental applications. Localization using a passive sonar system is a challenging task. It is more challenging when the source and receivers are in the reverberant environment. Time delay estimation (TDE)-based localization is a well-known technique to localize source for last few decades. In this work, empirical mode decomposition maximum likelihood (EMD ML TDE) method is used to estimate the time delay in a reverberant environment. The sound source location is estimated by intersecting spherical surfaces from the time delay. The experimental results prove that EMD ML time delay estimation method is effective to localize a sound source in a reverberant environment.

Empirical Study of Room Acoustic Conditions and Neurophysiologic Strain in Staff Working in Special Open-Plan Bank Offices

Abstract: Noise in open-plan offices induces psychological stress and fatigue in staff Focusing on workstations and noise exposure, this study investigated acoustic conditions in special open-plan offices and their relationship with neurophysiologic strain Twenty banks and 104 participants were randomly selected Acoustic properties of banks and workstation partitions were assessed using the ISO criteria and speech transmission index (STI) Equivalent noise level (LAeq) of the staff was measured in three 30-min intervals, and skin conductance level (SCL) and respiratory rate (RR) of staff were assessed in three 5-min intervals at the beginning, in the middle, and at the end of the work hours The intelligibility of speech between staff workstations (SW) was better than that between staff and clients (SC however, with increasing STI between SW and with decreasing STI between S&C, the staff strain increased With increasing noise exposure, psychological stress of the staff increased too Because of the acoustic condition and partitions used in the banks, the acoustic comfort of staff was not desirable Since many other banks use such partitions, any improvement in partitions between workstations can enhance acoustic comfort in the banks

Comparison of Cepstral Peak Prominence Measures Using the ADSV, SpeechTool, and VoiceSauce Acoustic Analysis Programs in Vocally Healthy Female Speakers

Abstract: This study examined the correlation of, and agreement between, cepstral peak prominence (CPP) measures obtained from three acoustic analysis programs: Analysis of Dysphonia in Speech and Voice (ADSV), SpeechTool, and VoiceSauce. Voice data recorded from sustained /a/ vowel and connected speech of two cohorts of vocally healthy female participants were analysed using program default settings to measure smoothed CPP (CPPS) in ADSV, CPPS and CPP in SpeechTool, and CPP in VoiceSauce. Intraclass correlation coefficients, linear regression, and Bland–Altman plots were used for testing the correlation and agreement between these programs. There was good correlation between ADSV and SpeechTool with respect to vowel CPPS in both cohorts. Connected speech CPPS from these two programs showed moderate correlation in cohort 1 and good correlation in cohort 2. CPP values obtained from VoiceSauce were highly correlated with those from SpeechTool in both tasks. Bland–Altman plots showed that there were differences between programs in CPPS and CPP values. While CPPS and CPP values from these programs were correlated, they did not show absolute agreement. This implied possible different thresholds of detecting dysphonic severity across different acoustic analysis programs.

Clarifications on the Design and Interpretation of Conclusions from Health Canada’s Study on Wind Turbine Noise and Health

Abstract: It has been extensively communicated that Health Canada’s Community Noise and Health Study (CNHS) did not find positive associations between wind turbine noise (WTN) levels and any of the evaluated health outcomes, beyond an increase in the prevalence of high annoyance toward several wind turbine features. The authors emphasize that this general conclusion remains bound by the study strengths and limitations. Following the publication of the CNHS findings, there has been interest among some individuals to present alternative interpretations of the results originally reported by Michaud et al. (J Acoust Soc Am 139(3):1443–1454, 2016. https://doi.org/10.1121/1.4942391 ). While recognizing the importance of independent scientific re-evaluation and/or reinterpretation, this commentary serves to clarify and, where necessary, correct some of the information put forward by others. One factor that has been re-evaluated by external stakeholders is the subsample of participants that comprise the lowest WTN category. In their reanalysis, they have eliminated this category, or introduced alternative comparative data. This paper identifies substantial issues associated with the re-evaluation put forth. To thoroughly address these issues and to avoid further confusion or misinterpretation, the authors of the CNHS provide a comparison between the CNHS health condition prevalence data and nationally representative health-based surveys conducted in Canada during the same calendar year. In addition, this paper responds to comments received to date on the CNHS, including the study’s age range, the generalization of findings, the provision of raw data, and conclusions on the association between WTN level and health.

Comparison Analysis and Optimization of Composite Micro-perforated Absorbers in Sound Absorption Bandwidth

Abstract: Micro-perforated panel (MPP) absorber with a single uniform air cavity is regarded as a promising sound-absorbing structure; however, MPP absorption performance remains unfortunately limited due to the Helmholtz resonance mechanism, which results in more and more attention being placed on composite MPP absorbers. This study focuses on acoustic properties of different composite MPP absorbers, including three types of composite MPP absorbers which are coupled in serial, parallel and serial–parallel modes. Their mathematical models of the normal absorption coefficient are established by utilizing the acoustic electric analogy method. The single-cavity MPP absorber and three composite MPP absorbers are preliminarily designed to verify the equivalent circuit models and perform a pilot analysis of their sound absorption characteristics. Moreover, the particle swarm optimization algorithm is selected to optimize the absorbers so that optimal combination of structure parameters within a prescribed frequency range can be obtained. In addition, the absorbers are made based on the optimized parameters for experimental investigation. The results show that a wider absorption bandwidth may be achieved by composite MPP absorbers through introducing additional absorption peaks with reference to that for the conventional single-cavity MPP absorber, and there are more absorption peaks for the serial–parallel-coupled MPP absorber rather than the simply serial- or parallel-coupled MPP absorber so that better sound absorption effect may be achieved.

Two-Step Inversion of Geoacoustic Parameters with Bottom Reverberation and Transmission Loss in the Deep Ocean

Abstract: The parameters of deep ocean sediments are relevant for accurately predicting the sound field; however, it is difficult to measure the parameters in situ. Most inversion methods used in shallow water are inapplicable in the deep ocean because of the considerable differences in propagation characteristics. At present, no method for simultaneously obtaining sound speed, density, and attenuation that considers the sensitivity of sediment parameters is yet available. This study proposes a two-step inversion of geoacoustic parameters in the deep ocean. On the basis of the half-space model, the decline tendency of bottom reverberation level with travel time is used for the inversion of sound speed and density, whereas transmission loss is used for inversion of attenuation. Inversion results can be practical for acoustic applications when this method is used. Experimental data from the South China Sea in the summer of 2014 are processed during the inversion process. The sediment parameters obtained from the inversion process are close to the laboratory-measured sampling values and may be used to predict the sound field in various applications, such as in transmission loss in the deep ocean.

A Case Study of Recording Soundwalk of Miyajima and Itsukushima Shrine Using Smartphone

Abstract: In this paper, we present soundwalk of Miyajima and Itsukushima Shrine which was captured with a smartphone’s built-in microphone calibrated against a typical type 1 sound level meter using NoiseExplorer our in-house developed Android app. Itsukushima Shrine (Itsukushima-jinja) is a Shinto shrine on the island of Itsukushima (popularly known as Miyajima), best known for its “floating” or “half-submerged” torii gate. It is in the southern part of Hatsukaichi in Hiroshima Prefecture in Japan. The shrine complex was listed as a UNESCO World Heritage Site in 1996. NoiseExplorer app records the audio from microphone as WAV files and the current location of the smartphone to log files. The WAV files were used to analyze sound pressure level and generate frequency spectrums and spectrograms, whereas log files were used to generate noise maps. These two capabilities of smartphones make them cheap and versatile means of recording and describing soundwalk or soundscape of any site in the world. We discuss the unique soundscape of the cross-channel ferry, the shrine itself, the footpath going up to the nearby mountain and the soundwalk captured using a smartphone. The overall noise level of 64.1 dBA is well below the typical noise level of other Japanese tourist attractions.

Assessment of Acoustical Indicators in Multi-domed Historic Structures by Non-exponential Energy Decay Analysis

Abstract: The key concern of this study is to discuss the reliable acoustical metrics for analyzing particular sound fields within monumental multi-domed sacred spaces with an emphasis on multiple sound energy decay formation. The potential of such structures in featuring non-exponential sound energy decay characteristics necessitates new formulation of sound decay indicators in understanding of their sound fields. The early and late energy decay components within non-exponential sound energy decays can have a unique contribution to the interior acoustic quality. These acoustical features can also provide the functional and spiritual acoustical needs of monumental sacred spaces. Suleymaniye Mosque and Hagia Sophia, two major monuments of Istanbul World Heritage Site, are examined in this regard. Field measurements are the main method of data collection. Over collected room impulse responses, relevant acoustical predictors including decay rates and decay times are computed by applying Bayesian decay parameter estimation. Analysis results disclose double or triple decay formations in both structures. Authors argue that, the detection of multiple sound energy decays in analyzed structures sets out a new vision for room acoustics studies of sacred spaces and for multi-domed monuments. Thus, implementation of new metrics, namely multiple slope decay parameters to replace classical room acoustics indicators, is recommended and discussed in light of the outcomes of this study.

Absorption Performance of an Anechoic Layer with a Steel Plate Backing at Oblique Incidence

Abstract: The sound absorption of steel-plate-backed anechoic coatings at oblique incidence is investigated theoretically and numerically. An analytical expression for the absorption coefficient is derived using elasticity theory. A 2D numerical periodic model suitable for inhomogeneous coatings under oblique incidence is built using the finite-element method. To interpret the absorption mechanism, the dispersion curves for guided elastic waves in a uniform coating and a steel plate are calculated using the spectral method. The absorption coefficients are analyzed with respect to the incident angle and frequency using elastic plate resonance theory. The effects of the steel plate backing and the attenuation of the coating are investigated. For the Alberich anechoic coating with a steel plate backing, the absorption coefficients are calculated numerically, and the absorption mechanism is investigated by structural displacement vectors and deformations. The following absorption characteristics can be found. In the frequency-angle spectra for absorption coefficients, the bright areas resulting from low-order Lamb waves in the coating are recognizable at low frequencies. A broad, strong anechoic area appears at large incident angles and low frequencies; under these conditions, the displacements of the steel plate and the 2D cavity are the main components of structural vibration. The resonance moves to low frequency when the thickness of the steel plate is incorporated and the cavity height is increased. At relatively high frequencies, some new absorption areas result from the resonance of periodic cavities and cover the contribution from the high-order Lamb waves in the coating caused by attenuation. The corresponding absorption coefficients decrease with an increasing angle of incidence.

Analysis and Control for the Intake Noise of a Vehicle

Abstract: The sound quality of a vehicle equipped with an inline four-cylinder engine is poor at around 2250 rpm with characteristic of prominent pure tone. The experimental results indicate that the second (2nd)-order intake noise has a peak at this engine speed. To understand the mechanism and solve this problem, the finite element model of the entire intake system is established to calculate the acoustic mode. Meanwhile, the 1D computational fluid dynamics (CFD) model integrating the intake/exhaust system with the engine is constructed to predict the intake noise. The modal analysis results demonstrate that the natural frequency of the intake system is close to the frequency of the 2nd-order periodic pressure pulsation noise at around 2250 rpm. The resonance of the air column is the main cause of the above-mentioned problem. The results of 1D-CFD simulation agree well with the experiment. Based on the analysis results and considering the constrains of engine cabin space, three kinds of control measures are proposed, which include changing the length of intake dirty pipe, changing the length of the clean pipe and installing the Helmholtz resonator. The simulation results are as follows: (1) By increasing the length of intake dirty pipe appropriately, the 2nd-order noise is reduced at low engine speed, while increased at high engine speed. (2) The length of the intake clean pipe has obvious influence on the acoustic mode of the intake system, but little influence on the total noise, and the 2nd-order noise attenuation is not obvious. (3) Helmholtz resonator can attenuate the total noise and the 2nd-order noise in most of engine speed range. Therefore, the Helmholtz resonator is used for the solution. The acoustical characteristics of the intake noise and the vehicle sound quality are improved.

Effects of Different Lung Volume Conditions on Closed Quotient, Vocal Fundamental Frequency and Relative Intensity in Vocally Untrained Female Speakers

Abstract: The objective of this study was to determine the relationship between lung volume (LV) conditions and vocal fold vibratory patterns using measurements of closed quotient (CQ), fundamental frequency (F0) and relative vocal intensity. Forty-three healthy and vocally untrained females were asked to produce the vowel /a/ following breathing instructions that cued for higher, habitual, or lower LV conditions. Closed quotient was measured by electroglottography (EGG) and analyzed using criterion-level method of 25%. An average of CQ, F0 and relative vocal intensity were obtained. No significant difference was observed in CQ between cued LV conditions; however, there was a trend for CQ to increase in the cued high LV condition. Relative vocal intensity and F0 differed significantly across all conditions with higher F0 and relative vocal intensity observed at the high LV condition. These findings suggested that the use of different cued LVs did not have a significant impact on CQ. This may have been due to (1) the phonatory task, (2) variability in responses to the breathing instructions between individuals, and (3) the measurement of CQ. However, F0 and relative vocal intensity were significantly influenced by the LV. This offers a possible alternative approach in cueing pitch and loudness in singing and voice therapy.

Propagation of Underwater Noise from an Offshore Seismic Survey in Australia to Antarctica: Measurements and Modelling

Abstract: An offshore seismic survey was conducted over the western edge of the continental shelf in Bass Strait in 2006. Underwater noise from this survey was recorded on an autonomous sound recorder deployed in the Southern Ocean on the Antarctic continental slope. Sound emission and propagation models were verified by experimental measurements using parameters and position of the airgun array and characteristics of the underwater sound channel. A parabolic equation approximation method was used to calculate the sound field over the continental slope of Australia, and then, a normal mode model was employed to account for the transmission loss due to sound scattering by surface waves south of the polar front. The numerical predictions are consistent with the measurement results within a few dBs for the sound exposure and energy spectral density levels. It is also demonstrated by measurements and modelling that the best coupling of a near-surface sound source with the deep underwater sound channel takes place when the source is located over the continental slope at a sea depth of about half of the channel’s axis depth. The model can be used to predict masking effects of man-made underwater noise on the communication environment of marine mammals in Antarctica.

Study on the Optimization of the Distribution of Absorbing Material on a Noise Barrier

Abstract: Noise barriers have been widely used to decrease the transportation noise. How to utilize limited structure and material to achieve the environmental requirements for noise control is an important research topic. This research combines the use of boundary element method and SIMP to generate an acoustic topology optimization method and applies this optimization method to optimization distribution of absorbing material installed on the noise barrier’s edges. Optimality criteria method is used to update the design variables and look for the final optimal solution. A new material interpolation scheme for acoustic problems based on SIMP is given, where the interpolation variable is not real structural density used in conventional SIMP, but fictitious material density deciding the normalized surface admittance. It is noteworthy that gray elements exist in the acoustic optimization analysis based on SIMP. However, a modified method based on a smoothed Heaviside function is applied to eliminate the gray elements. In order to demonstrate the validity and efficiency of the proposed algorithm in this paper, vertical barrier and T-shaped barrier with two wells are used for the numerical analysis, respectively.

Comparison of the Effects on Memory Tasks of Babble and Broadband Noise

Abstract: The comparative effects on both working memory and recognition memory of the same A-weighted noise levels of background noise, typically of that present in many transportation work areas, were investigated (55 and 65 dBA). One noise was a babble, multi-talker incomprehensible speech and representative of the many work areas dealing with administrative tasks. The other noise was broadband and similar to services and machinery noise. Forty participants, half non-native English speakers, were asked to complete three different working memory tests (linguistics, grammatical reasoning and mathematics) and one recognition memory test (cued recall) in the presence of the two types of noise at the two different levels. Broadband noise at 65 dBA was found to adversely affect recall by as much as 15%. The native language advantage was only evident with the linguistic working memory task. The findings highlight the interplay between type of noise, level of noise, demand of task, and language background of the person completing the task, and also the limitations of the use of dBA alone for assessment of acceptability of a workspace.

Wind Farm Noise Uncertainty: Prediction, Measurement and Compliance Assessment

Abstract: In most jurisdictions containing wind farms, base noise limits have been set by local regulatory authorities, with the intention of protecting the amenity of surrounding communities. It is a standard requirement that during the planning process for a new wind farm, the developer demonstrates that the proposed wind farm will comply with the relevant limits. At present, results from noise prediction models are commonly presented without uncertainty values, despite the fact that simplifications and approximations have been made in the models. Therefore, when prediction models indicate that the wind farm will generate noise within 3 dB of base noise limits, it is likely that these limits will be exceeded. Despite the fact that regulatory authorities often require that compliance measurements are taken to validate predictions of wind farm noise, it is very difficult to make substantial changes to noise emissions, without a corresponding reduction in electrical power output. Current methods of compliance assessment do not provide an indication of the amount of time that wind farms exceed allowable noise limits as they focus on ‘average’ levels only. Therefore, it is possible for wind farms to exceed allowable limits on a regular basis, and by significant margins. Hence, a more conservative approach is warranted during wind farm noise prediction and it is proposed that the upper level of the uncertainty estimate of the prediction model should not be permitted to exceed the allowable level.

Noise Prediction in Industrial Workrooms Using Regression Modeling Methods Based on the Dominant Frequency Cutoff Point

Abstract: Noise pollution is one of the major problems in industrial environments. The physiological response to the noise in industrial environments depends on the characteristics of the noise and environment. This study aimed to develop an empirical model for predicting the level of noise in closed industrial spaces using regression modeling based on the dominant frequency cutoff point. After identifying and determining the effective input variables in the prediction of noise level, the relevant data were collected from 56 industrial workrooms and the model was developed using multiple regression technique. The two models were best fitted to estimate the noise level for workrooms with a dominant frequency of less or equal to and more than 250 Hz (R2 = 0.86, R2 = 0.85, respectively). Based on the results of this study, it is less costly and requires less equipment for noise evaluation and monitoring by the mentioned models during the design, implementation, and operation of industrial environments. These experimental models can be used as suitable measures for screening closed industrial spaces and ranking them in terms of the amount of noise pollution.

Turbojet Test Cell and Noise Impact Assessment in the Vicinity of Romanian Research and Development Institute for Gas Turbines COMOTI

Abstract: In this paper, the noise impact of a turbojet testing cell on the adjacent community is investigated. There are scenarios in which the noise from two different turbo engines is analysed. For the noise impact study, acoustic measurements were taken in the test cell to obtain the transmission loss of the three silencers of the test cell. To allow testing of high-power turbojets as Viper or R-11F-300 Tumansky, the test cell was restored and relocated to minimize the noise impact on the surrounding area. The study presents the affected buildings and the number of inhabitants exposed to noise that is assessed according to a German VBEB method based on noise level at facade of the buildings. The study presents the calculus methodologies of the acoustic propagation according ISO 9613-2 on the surrounding area and the noise limits according to the national legislation. To reduce the conflict areas, a technical solution for the intake silencer is proposed.

An Enhanced Sharpness on Acoustic Focus Via Artificial Iterative Phase Conjugation Processing

Abstract: For a low-frequency source in a homogenous medium, when the source is focused by phase conjugation (also referred to as the time reversal in the time domain), the focal spot is large because of the half-wavelength limit. In this paper, to reduce the focal spot size, artificial iterative phase-conjugated processing is proposed based on passive phase conjugation to focus a point-like source. As AIPCP operates in iteration mode using a computer, the iteration loops of each transducer are calculated independently, and then, AIPCP is achieved by summing the iteration outputs of all transducers together. Numerical simulations and experiments on an audible sound field are employed to illustrate that AIPCP generates a narrower focal spot than passive phase conjugation. Further analyses considered the theoretical change in focal spot size with the iteration number using Fraunhofer far-field approximation, and the result shows that the focal spot size decreases with increase in the iteration number. This finding is predicted in the near field and validated in the far field by simulations and experiments. Moreover, the half-wavelength limit is overcome at a distance from the sound source equal to the wavelength.