Sound Level Spectra Research Articles

Analytical model for sound of explosives and firearms.

An analytical model is presented for approximate prediction of the waveform and the sound level spectrum of blast waves generated by explosives and firearms. The model is based on numerical results for spherical blast waves, with overpressures between 10 kPa and 100 MPa, and a nonlinear propagation theory for blast waves. The matching of the numerical results and the nonlinear propagation theory is investigated, and compared with results from the literature and results of calculations with a nonlinear propagation model. The analytical model presented here assumes a Friedlander waveform, from which formulas are derived for the 1/3-octave band spectrum of the sound exposure level. The formulas take into account the nonlinear effect of pulse broadening, and the corresponding frequency shift. For firearms, the muzzle blast levels vary in general with the emission direction, and the model takes into account an empirical directivity correction. Apart from the directivity correction, the model has no empirical parameters. The waveform and the spectrum follow directly from the explosion energy or the explosive charge mass. Model predictions are in good agreement with measurement results of explosives and firearms, with the explosive charge mass ranging from 0.4 g to 4 kg TNT.

Tonal noise case study: Locating the source of tones on HVAC equipment

The overall noise level measured from chiller units is controlled by primarily tonal noises from (1) the fans, (2) the compressors, and (3) the associated pipework. Three large chiller units were measured using AHRI 370 methodology to quantify the overall sound level spectra and A-weighted sound level from different chiller units in a range of environmental conditions. Whilst this method can identify a general area of tonal emission to within a couple of meters across a unit’s surface that is emitting discrete tonal noises, the method is limited when there are multiple potential tonal sources. Sound intensity imagery was used on two units to quantify the overall sound power emanating from the source, but also to offer a more advanced method of localizing the source of discrete tones to aid in the application of mitigation treatments. Results presented include visually displaying and quantifying the location of noise emission from fan blade pass frequencies and associated harmonics, localization of problematic tones emanating from individual pipework junctions and valves, and the reductions in sound power emissions achieved from lagging compressors and pipework.

Subwoofer array design for optimal performance and minimal noise pollution in the Roman theatre of Italica, Spain

Subwoofer arrays are widely used in live sound events. However, the performance of sound systems and the generated environmental noise pollution in the vicinity of Roman theatres is not well researched and documented. The investigation aimed to determine the most suitable subwoofer array configuration for an outdoor Roman theatre according to their low frequency coverage across the audience area and overspill on nearby residential areas. Performance suitability was determined by measuring the overall sound pressure (dBA) and spectrum levels of the arrays within the audience area at several locations representative of nearby residential areas. The Array Performance Rating (APR) was calculated for each array configuration to complement performance assessments. Results showed a notorious difference between coupled and uncoupled arrays, as well as between ground-based and flown subwoofer configurations. It was proven that the flown point source array offered overall less sound pressure levels and spectral variability in the audience plane and it caused the least environmental noise pollution. The conclusions drawn from this study can provide valuable guidance applicable to future sound system deployments and reconstruction projects of ancient outdoor theatres of similar architectural and environmental characteristics.

A proper framework for studying noise from jets with non-compact sources

A quantitative assessment of the acoustic source field produced by a laboratory-scale heated jet with a gas dynamic Mach number of 1.55 and an acoustic Mach number of 2.41 is performed using arrays of microphones that are traversed across the axial and radial plane of the jet's acoustic field. The nozzle contour comprises a method of characteristics shape so that shock-related noise is minimal and the dominant sound production mechanism is from Mach waves. The spatial topography of the overall sound pressure level is shown to be dominated by a distinct lobe residing on the principal acoustic emission path, which is expected from flows of this kind with supersonic convective acoustic Mach numbers. The sound field is then analysed on a per-frequency basis in order to identify the location, strength, convection velocity and propagation angle of the various axially distributed noise sources. The analysis reveals a collection of unique data-informed polar patterns of the sound intensity for each frequency. It is shown how these polar patterns can be propagated to any point in the far field with extreme accuracy using the inverse square law. Doing so allows one to gauge the kinds of errors that are encountered using a nozzle-centred source to calculate sound pressure spectrum levels and acoustic power. It is proposed that the measurement strategy described here be used for situations where measurements are being used to compare different facilities, for extrapolating measurements to different geometric scales, for model validation or for developing noise control strategies.

Application of Large Eddy Simulation to Predict Underwater Noise of Marine Propulsors. Part 2: Noise Generation

Methods to predict underwater acoustics are gaining increased significance, as the propulsion industry is required to confirm noise spectrum limits, for instance in compliance with classification society rules. Propeller–ship interaction is a main contributing factor to the underwater noise emissions by a vessel, demanding improved methods for both hydrodynamic and high-quality noise prediction. Implicit large eddy simulation applying volume-of-fluid phase modeling with the Schnerr-Sauer cavitation model is confirmed to be a capable tool for propeller cavitation simulation in part 1. In this part, the near field sound pressure of the hydrodynamic solution of the finite volume method is examined. The sound level spectra for free-running propeller test cases and pressure pulses on the hull for propellers under behind ship conditions are compared with the experimental measurements. For a propeller-free running case with priory mesh refinement in regions of high vorticity to improve the tip vortex cavity representation, good agreement is reached with respect to the spectral signature. For behind ship cases without additional refinements, partial agreement was achieved for the incompressible hull pressure fluctuations. Thus, meshing strategies require improvements for this approach to be widely applicable in an industrial environment, especially for non-uniform propeller inflow.

Sound pressure level spectrum analysis by combination of 4D PTV and ANFIS method around automotive side-view mirror models

This paper proposes a data augmentation method based on artificial intelligence (AI) to obtain sound level spectrum as predicting the spatial and temporal data of time-resolved three-dimensional Particle Tracking Velocimetry (4D PTV) data. A 4D PTV has used to measure flow characteristics of three side mirror models adopting the Shake-The-Box (STB) algorithm with four high-speed cameras on a robotic arm for measuring industrial scale. Helium filled soap bubbles are used as tracers in the wind tunnel experiment to characterize flow structures around automobile side mirror models. Full volumetric velocity fields and evolution of vortex structures are obtained and analyzed. Instantaneous pressure fields are deduced by solving a Poisson equation based on the 4D PTV data. To predict spatial and temporal data of velocity field, artificial intelligence (AI)-based data prediction method has applied. Adaptive Neural Fuzzy Inference System (ANFIS) based machine learning algorithm works well to find 4D missing data behind the automobile side mirror model. Using the ANFIS model, power spectrum of velocity fluctuations and sound level spectrum of pressure fluctuations are successfully obtained to assess flow and noise characteristics of three different side mirror models.

Passive acoustic listening stations (PALS) show rapid onset of ecological effects of harmful algal blooms in real time

Monitoring ecological changes in marine ecosystems is expensive and time-consuming. Passive acoustic methods provide continuous monitoring of soniferous species, are relatively inexpensive, and can be integrated into a larger network to provide enhanced spatial and temporal coverage of ecological events. We demonstrate how these methods can be used to detect changes in fish populations in response to a Karenia brevis red tide harmful algal bloom by examining sound spectrum levels recorded by two land-based passive acoustic listening stations (PALS) deployed in Sarasota Bay, Florida, before and during a red tide event. Significant and temporally persistent decreases in sound spectrum levels were recorded in real time at both PALS in four frequency bands spanning 0.172–20 kHz after K. brevis cells were opportunistically sampled near the stations. The decrease in sound spectrum levels and increase in K. brevis cell concentrations also coincided with decreased catch per unit effort (CPUE) and species density per unit effort (SDPUE) data for non-clupeid fish and soniferous fish species, as well as increased reports of marine mammal mortalities in the region. These findings demonstrate how PALS can detect and report in real time ecological changes from episodic disturbances, such as harmful algal blooms.

Temperature effects on the noise source mechanisms in a realistic subsonic dual-stream jet

A detailed numerical investigation of temperature effects on both aerodynamics and acoustics of a dual-stream jet including a central plug is carried out. The geometry is representative of a realistic turbofan with a high by-pass-ratio (BPR) close to 9. Two take-off high-subsonic operating points are investigated numerically by compressible large-eddy simulation. For these two selected points, the secondary stream is exactly the same in terms of static temperature and velocity. Both jets have also the same primary velocity. The only difference lies in the static temperature of the primary jet. There is a ratio of two between the two jets considered in this study, namely Tj=400 K and Tj=800 K. More precisely, the primary jet temperature is reduced while keeping the acoustic Mach number constant, leading to an increase of the primary jet Mach number from Mj=0.65 in the heated case to Mj=0.89 in the cold case. Some experimental data are available for the hot jet while the cold jet is introduced for academic reasons. The heated jet compares reasonably well with the experimental data, taking into account the complexity of the geometry. Temperature effects have a limited impact on aerodynamic development and acoustic radiation. The influence of the core flow is found to be weak due to the high BPR considered and the radiated acoustic is mainly driven by the secondary flow. Further investigations are carried out in order to highlight the differences between the two cases. The acoustic production area are identified by the way of axial velocity skewness coefficient maps. Finally, a decrease of the primary stream temperature leads to the development of trapped acoustic waves inside the jet core. An increase of the overall sound spectrum level about 5 dB is thus observed in the upstream direction for the cold jet, in agreement with the vortex sheet theory.

Indoor sound level spectra of public entertainment premises for rating airborne sound insulation.

Public entertainment premises are significant sources of urban noise pollution. In this paper, indoor sound level spectra for different types of entertainment premises have been analyzed. While most of the spectra lie between two standardized curves proposed by the ISO 717-1:2013 (The International Organization for Standardization) standard, spectra recorded in discotheques show a pronounced spectral level below 160 Hz as a result of an intensive use of subwoofers. This indicates potentially inappropriate rating of sound insulation against such specific low-frequency noise by standardized spectrum adaptation terms. A modified spectrum adaptation term for a single-number rating of sound insulation under such circumstances has been proposed.

Thrust and Acoustic Performance of Small-Scale, Coaxial, Corotating Rotors in Hover

The findings from a test campaign aimed at understanding the thrust and acoustic performance of a small-scale, coaxial, corotating rotor in hover are discussed. Measurements of axial thrust are acquired alongside surveys of the near-field acoustics at eight observer positions above and below the tip path plane of the upper rotor. The variables of interest are the index angle and stacking distance between the upper and lower rotors, as well as rotor speed. Standard metrics like thrust coefficient, sound pressure spectrum level, and the sound pressure level of the various blade harmonics and subharmonics are studied. Human ear effects are also considered using the A-weighting standard, which is shown to have a profound influence on the perceived noise levels. The analysis demonstrates how rotor thrust and sound pressure levels are more dependent on rotor index angle than stacking distance. However, the index angle where the peak thrust coefficient resides is more sensitive to stacking distance at low rotation speeds than it is at high rotation speeds. Various rotor speed and index angle combinations of the stacked rotor are also shown to produce the same rotor thrust, all the while resulting in uniquely different sound levels of the various rotor harmonics.

Automated parsing of sound level meter data into artifacts, natural sources, and noise

The National Park Service (NPS) has nearly one million hours of 1/3rd octave band sound level measurements collected at hundreds of sites throughout the system. One of the principal purposes of these data is to measure the background or residual sound level against which all transient sounds are heard. Historically this analysis has depended upon trained listeners to identify which sound sources can be heard in segments of these data. These annotated records were then used to calculate an adjusted median sound level. To pursue a more efficient, automated method to deliver this result, matrix decomposition methods were tested to characterize their capacity to model time series of sound level spectra as low-rank decompositions, including options to account for anomalous events. These methods were combined with sound source identification based on component spectral properties and time weightings to yield promising approaches for automating NPS analyses. These results may also offer options for detecting and removing the effects of pseudonoise due to the turbulence generated by air flowing past the microphone.

Analysis of pressure fluctuation in transonic cavity flows using modal decomposition

Turbulent flows at a free stream Mach number of 1.19 over an open cavity with L/D ratio of 5.0 are numerically investigated using improved delayed detached-eddy simulation based on two-equation shear stress transport model. Modal decompositions including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are applied to analyze the pressure fluctuations. The extracted first six POD modes possess more than 75% of the total energy and contain multiple frequencies. Their spatial structures exhibit well regular and periodic behaviors along the cavity lip line. The DMD algorithm identifies the flow structures associated with single frequencies. Major distribution areas of high intensity pressure fluctuations move upstream as the mode frequency increases, and the structures with high frequencies are prone to break down near the trailing edge. The alternating pressure patterns convection process is clearly presented, and the propagation of acoustic waves validates that the acoustic waves share the same sound source but are radiated following two different paths, consistent with the feedback mechanism. In addition, the effects of free stream Reynolds number on sound pressure spectrum levels are investigated at a fixed pressure and temperature. Results show that free stream Reynolds number has no effect on the non-dimensional frequencies of the dominant modes, while the main recirculation area shrinks as Reynolds number increases. Furthermore, variations of sound pressure levels as a function of Reynolds numbers exhibit significant discrepancies for a fixed free stream pressure or temperature, indicating that the Reynolds number is not critical to the feedback tone amplitude. The enhancement of sound pressure levels mainly attributes to the increment of free flow pressure or the impingement of enlarged fluid velocity due to higher free stream temperature.

Gulf of Mexico low-frequency ocean soundscape impacted by airguns.

The ocean soundscape of the Gulf of Mexico (GOM) has not been well-studied, although it is an important habitat for marine mammals, including sperm and beaked whales, many dolphin species, and a potentially endangered baleen whale species. The GOM is also home to high levels of hydrocarbon exploration and extraction, heavily used commercial shipping ports, and significant fishery industry activity, all of which are known contributors to oceanic noise. From 2010-2013, the soundscape of three deep and two shallow water sites in the GOM were monitored over 10 - 1000 Hz. Average sound pressure spectrum levels were high, >90 dB re 1 μPa(2)/Hz at <40 Hz for the deep water sites and were associated with noise from seismic exploration airguns. More moderate sound pressure levels, <55 dB re 1 μPa(2)/Hz at >700 Hz, were present at a shallow water site in the northeastern Gulf, removed from the zone of industrial development and bathymetrically shielded from deep water anthropogenic sound sources. During passage of a high wind event (Hurricane Isaac, 2012), sound pressure levels above 200 Hz increased with wind speed, but at low frequencies (<100 Hz) sound pressure levels decreased owing to absence of noise from airguns.

Attenuation of peak sound pressure levels of shooting noise by hearing protective earmuffs.

Transmission losses (TL) to highly impulsive signals generated by three firearms have been measured for two ear muffs, using both a head and torso simulator and a miniature microphone located at the ear canal entrance (MIRE technique). Peak SPL TL have been found to be well approximated by 40 ms short-L eq TL. This has allowed the use of transmissibilities and correction factors for bone conduction and physiological masking appropriate for continuous noise, for the calculation of REAT-type peak insertion losses (IL). Results indicate that peak IL can be well predicted by estimates based on one-third octave band 40 ms short L eq and manufacturer-declared (nominal) IL measured for continuous noise according to test standards. Such predictions tend to be more accurate at the high end of the range, while they are less reliable when the attenuation is lower. A user-friendly simplified prediction algorithm has also been developed, which only requires nominal IL and one-third octave sound exposure level spectra. Separate predictions are possible for IL in direct and diffuse sound fields, albeit with higher uncertainties, due to the smaller number of experimental data comprising the two separate datasets on which such predictions are based.

Diagnosis and characterization of low frequency noise source for a cable car system

Noise emission generated by a cable car system in operation condition normally becomes a problem widely disturbing the residents living in extremely quiet environment. The noise source identification and the sound field simulation are discussed and addressed in this article. To identify the noise sources from the tower post of a cable car system, the spectral level of the structure-borne vibration, the near-field sound and the far-field sound are measured and analysed. Unexpectedly, it is found that there exists some special narrow band peak frequencies in the range 50~80 Hz and its multiples of all the measured vibration and sound level spectra. The fundamental peak frequency is identified as the frequency of the periodic regular uneven wire rope surface passing through the sheaves of cable wheels. Which depends on the running speed of the cable, the number of cable strands, the pitch of strands. Besides, the fundamental peak frequency appearing in the sound level spectra away from the surface of the tower post also depends on the vibraitonal mode of the post whether pertaining to radiation mode or not near the fundamental exciting frequency band. This can be clarified and illustrated by the sound field simulation analysis. Keywords: low frequency noise, cable car, structure-borne sound radiation

Sources and potential influences of uncertainty in ground impedance measurements and estimations

Knowledge of acoustic impedance of ground surface is essential for determining noise levels outdoors. At distances up to a few hundred metres from the source ground effect may be the dominant factor. The short-range measurement of sound level spectra from an omni-directional source has been used as a standard method to deduce the acoustic impedance spectrum of ground surfaces (e.g. ANSI S1.18, 1999). However, there remain a number of uncertainties in such measurement methods. For example, at low frequencies the difficulties in fitting theoretical or phenomenological models to short range data may result is a large uncertainty in predictions of sound levels at larger distances. Another source of uncertainty is the variability of the acoustic impedance of apparently uniform ground. Another potential influence on predictions of long term equivalent noise levels is seasonal variation. This paper discusses the extent and potential influences of such uncertainties in measured and deduced ground impedance spectra.

Improvement on acoustic performance of the accumulator of double-cylinder rotary compressors

Since the accumulator is one of main contributors to the overall noise level of the rotary compressor, research on the acoustic performance of accumulators is essential. Numerical analysis based on the computational fluid dynamic method shows that large pressure fluctuations are caused mainly by periodic rotating piston. The idea that the flow phase differences of two pipes could be utilized for suppressing fluctuations is proposed. The double standpipes of an accumulator are changed to the single pipe with two branches and each branch is connected to the inlet of the compressor. Flow structures and wall pressure fluctuations for the two configurations are computed. Computational results show that wall pressure pulsations of the new accumulator are obviously lower than that of the original. The acoustic experiments were carried out under the real working conditions. Compared with the original, the new accumulator shows that the overall noise level is reduced about 1.2 dB(A) and the sound spectrum levels are also lower in a wide frequency domain, which validate the numerical and theoretical analysis.

In-cab noise reduction on an air-rotary drill rig

The National Institute for Occupational Safety and Health (NIOSH) has investigated engineering noise controls to reduce sound levels in cabs on air-rotary drill rigs. A recent investigation revealed that some drillers are exposed to A-weighted sound levels exceeding 85 dB even though a cab is used. NIOSH studied the in-cab sound levels of one such rig. First, preliminary tests were conducted in a controlled environment using accelerometers and microphones with spectral analysis to identify the dominant noise sources for in-cab sound levels. The results indicate that vibration transmitted from multiple hydraulic pumps to the control panel produces a dominant spike in the sound level spectrum in the 400 Hz 1/3-octave band. Next, field tests were performed in a production environment to evaluate noise controls to reduce in-cab sound levels. It was found that utilizing hydraulic noise suppressors reduces the structure-borne noise transmitted to the control panel. Further, using hydraulic noise suppressors and enhancing soundproofing reduced the in-cab A-weighted sound levels by as much as 4 dB.

SOUND PROPAGATION CLOSE TO THE GROUND

▪ Abstract Some applications of the study of outdoor acoustics and sets of data for sound-level spectra obtained close to the ground are described. Measurements and models of ground effects arising from the interaction between sound traveling directly from source to receiver and sound reflected from the ground are emphasized. Details are given concerning the influences of porosity, layering, small-scale surface roughness, and tall vegetation. Areas of related current and future research are outlined.

Oceanic Rainfall Detection and Classification in Tropical and Subtropical Mesoscale Convective Systems Using Underwater Acoustic Methods

Abstract Measurements of the underwater sound produced by rain were made at three U.S. coastal sites in a study to determine the feasibility and limitations of the acoustic detection and classification of rainfall over water. In the analysis of the rain sound spectra, concurrent radar reflectivity observations were used to identify convective and stratiform regions of the precipitating clouds overhead. It was found that acoustic classifications of rainfall as to type, based on information in the 4–30-kHz frequency band, were in general agreement with radar-derived classifications. The classification technique is based on use of an acoustic discriminant, DR, defined as the difference in average spectral levels between the 10–30- and 4–10-kHz bands. A high correlation was found between sound spectrum levels (in decibels) in the 4–10-kHz frequency band and radar reflectivity, dBZ, suggesting the possible use of the 4–10-kHz band sound spectral level as a classification tool using spatially distributed hydrop...