Average Sound Pressure Research Articles

Wind Turbine Noise Propagation over Flat Ground: Measurements and Predictions

Noise from wind turbines is of concern in the planning process of new wind farms, and accurate estimations of immission noise levels at residents nearby are required. Sound propagation from wind turbine to receiver could be modelled by a simplified standard model assuming constant meteorological conditions, by an engineering method taking atmospheric and ground propagation conditions into account, or by a more exact model. Epidemiological studies have found a higher frequency of annoyance due to wind turbine noise than to other community noise sources at equal noise levels, indicating that the often used simplified model is not sufficient. This paper evaluates the variation of immission sound levels under the influence of meteorological variation and explores if the prediction of levels could be improved by taking the effect of wind speed on sound propagation into account. Long-term sound recordings and measurements at a distance of 530 m from a wind turbine show that the simplified standard model predicts the average sound pressure levels satisfactorily under downwind conditions, and that a more complex propagation model might not be needed for wind turbine noise at a relatively short distance. Large variations of sound immission levels at the same wind speed were however present. Statistical analysis revealed that these variations were influenced by meteorological parameters, such as temperature, static pressure and deviation from ideal downwind direction. The overall results indicate that meteorological factors influence the noise generated by the wind turbine rather than the sound propagation.

Sound transmission loss analysis through a multilayer lightweight concrete hollow brick wall by FEM and experimental validation

This research work tries to find the most efficient numerical procedure to predict the transmission loss (TL) through a multilayer wall for frequencies ranging from 100 to 5000 Hz. The wall is made of lightweight concrete hollow bricks joined by mortar, with gypsum lining on both faces. This study is motivated by the need to develop new products with better acoustic properties of airborne sound insulation. In order to achieve this objective a set of tests using source and receiving chambers have been modelled according to the basic requirements of the UNE-EN ISO 140-1 standard rule. The constructive element is located between both chambers in order to evaluate its acoustic behaviour according to the UNE-EN ISO 717-1 standard rule. Applying the finite element method (FEM), a two-dimensional model with fluid-structure interaction has been built in the case of the horizontal and vertical planes, with the purpose of including the different eigenfrequencies of the rooms. The values of the average sound pressure in both test chambers have been calculated for different excitation sources: uni and omnidirectional waves. The convergence and accuracy of the proposed method are then assessed by comparing FEM results with experimental measurements carried out by a certified laboratory. Finally, the numerical procedure implemented in this work may be useful to evaluate the acoustic behaviour for other structural building elements and reduce the manufacturing time to develop new products in construction.

Evaluation of noise exposures of miners operating highwall mining equipment.

Modern highwall mining systems are starting to replace surface coal augers in some highwall mining operations because of their ability to increase coal production and recovery rates. Another benefit of using highwall mining systems instead of surface coal augers is the reduction in noise exposures to the affected miners extracting the coal from the highwall. Several surface coal augers and highwall mining systems were evaluated for full shift noise exposures of the machine operator and helpers during this investigation. Engineering noise controls were also evaluated for each piece of highwall mining equipment. The operator and helper working on a modern highwall mining system were exposed to average sound pressure levels of 61.5 and 89.0 dBA, respectively, while the operator and helper working on a surface coal auger were exposed to higher average sound pressure levels of 95.4 and 94.0 dBA, respectively.

A Method for Estimation of Level and Fluctuation of Background Noises by Use of Distribution Function of Short Time Averaged Sound Pressure Levels

A Method for Estimation of Level and Fluctuation of Background Noises by Use of Distribution Function of Short Time Averaged Sound Pressure Levels

Acoustic intensity based data analysis and assessment in computational aeroacoustics

A methodology for assessing the quality and analysing the sound field of a numerical solution for an aeroacoustic problem is presented using an in-duct propagation problem as example. Namely an analysis of the acoustic energy conservation provides insight about the solution quality; sources of numerical error as well as physical source of sound are located by an analysis of the acoustic intensity; a mode analysis provides information about the sound field in the duct and a visualisation of the average pathway of sound by the acoustic intensity helps the interpretation of the modal amplitude data. The problem chosen for the presentation deals with the propagation of a single acoustic mode upstream through the tapering inlet duct of a centrifugal compressor. The propagation of tone noise is a typical problem for computational aeroacoustics. The numerical approach employs high-order finite-difference discretisation schemes with structured body-fitted meshes. An overset grid approach allows to overcome the geometrical complexity of the problem. The conservation of the acoustic energy is found to be violated by less than 3% in case of a potential base flow. In this case major sources of error are found in the interpolation between overset and background mesh. In case of a non-potential flow with boundary layers at the wall, the conservation of acoustic energy cannot be claimed. Consequently, it shows an increase of the acoustic energy, for which the accelerated flow in the nozzle is identified as source. It is shown that the filtering scheme affect the solution more than the spatial discretisation, if the grid resolution is relatively high. A strong scattering of energy into higher radial modes is detected. The fifth and sixth radial mode are found to be preferred in the current example. They are excited in the nozzle with a similar level. For these high radial modes, the transport of acoustic energy takes place further away from the duct wall. This explains the relatively large drop of about 20 dB for the average sound pressure level at the wall over the inlet nozzle.

50 kHz capacitive micromachined ultrasonic transducers for generation of highly directional sound with parametric arrays

In this study, we examine the use of capacitive micromachined ultrasonic transducers (CMUTs) with vacuum- sealed cavities for transmitting directional sound with parametric arrays. We used finite element modeling to design CMUTs with 40-microm- and 60-microm-thick membranes to have resonance frequencies of 46 kHz and 54 kHz, respectively. The wafer bonding approach used to fabricate the CMUTs provides good control over device properties and the capability to fabricate CMUTs with large diameter membranes and deep cavities. Each CMUT is 8 cm in diameter and consists of 284 circular membranes. Each membrane is 4 mm in diameter. Characterization of the fabricated CMUTs shows they have center frequencies of 46 kHz and 55 kHz and 3 dB bandwidths of 1.9 kHz and 5.3 kHz for the 40-microm- and 60-microm-thick membrane devices, respectively. With dc bias voltages of 380 V and 350 V and an ac excitation of 200 V peak-to-peak, the CMUTs generate average sound pressure levels, normalized to the device's surface, of 135 dB and 129 dB (re 20 microPa), respectively. When used to generate 5 kHz sound with a parametric array, we measured sound at 3 m with a 6 dB beamwidth of 8.7 degrees and a sound pressure level of 58 dB. To understand how detector nonlinearity (e.g., the nonlinearity of the microphone used to make the sound level measurements) affects the measured sound pressure level, we made measurements with and without an acoustic low-pass filter placed in front of the microphone; the measured sound levels agree with numerical simulations of the pressure field. The results presented in this paper demonstrate that large-area CMUTs, which produce high-intensity ultrasound, can be fabricated for transmitting directional sound with parametric arrays.

Students and Soundwaves: Five Strategies to Promote Good Classroom Acoustics

Students and Soundwaves: Five Strategies to Promote Good Classroom Acoustics

New findings on noise exposure in the construction trades

The degree to which employees on construction sites are exposed to noise often depends on a variety of individual activities. The calculation of the average noise exposure for different construction trades in this sector calls for an enhanced level of work analysis and measurement. Recommended here are personal measurements with simultaneous recording of the various activities and machines employed. With the subsequent assignment of average sound pressure levels and activities, it is then possible to draw conclusions about part exposures and mean exposure. At euronoise 2006 in Tampere, findings for a series of construction trades were presented. In the meantime, this series of measurements has been extended to include further trades. Furthermore subsequent measurements have been performed for individual trades, this was considered necessary because certain machines and working methods have changed in these fields in the last 20 years. The findings show that technical progress does not necessarily result in a reduction in noise exposure.

Indoor environment and acoustic conditions in two Finnish hospital wards

The aim of this study was to determine the acoustic conditions of two typical Finnish wards. The methods included long‐term noise measurements, building acoustical measurements and questionnaires for patients (N=33) and nurses (N=27). The average sound pressure levels were within 49 and 58 dBA in the corridors, office and patient rooms. Personal noise exposure levels among nurses were within 56 and 70 dBA. The noise was mainly caused by people. Ventilation noise level was 28 dBA. Building acoustic measurements showed reasonable agreement with national directions. Indoor environment was not a problem for most patients. Some disturbance from thermal conditions, dry air and noise was experienced. From daytime noise sources, other patients' snore and groans caused annoyance in some patients. Night‐time sleep was disturbed most by anxiety, pain, noise, thermal conditions and an uncomfortable bed. Patients were, on average, quite satisfied with the overall room conditions. Nurses were more bothered by environmental factors than patients, giving highest annoyance ratings to thermal conditions, air quality and noise. Noises were experienced to cause some distraction when concentrating to work tasks. Sounds of phones ringing were experienced as particularly detrimental. Lack of privacy was the most obvious problem in both staff and patient evaluations.

Road traffic noise and cardiovascular risk

Studies on the association between community noise and cardiovascular risk were subjected to a meta-analysis for deriving a common dose-effect curve. Peer-reviewed articles, objective assessment of exposure and outcome as well as control for confounding and multiple exposure categories were all necessary inclusion criteria. A distinction was made between descriptive (cross-sectional) and analytical (case-control, cohort) studies. Meta-analyses were carried out for two descriptive and five analytical studies for calculating a pooled dose-effect curve for the association between road traffic noise levels and the risk of myocardial infarction. No increase in risk was found below 60 dB(A) for the average A-weighted sound pressure levels during the day. An increase in risk was found with increasing noise levels above 60 dB(A) thus showing a dose-response relationship. A risk curve was estimated for the association using a polynomial fit of the data that can be used for risk assessment and the environmental burden of disease calculations.

Sound pressure minimization by adjusting the active source parameters with the use of a hybrid genetic algorithm

Results of computer experiments on the propagation and interaction of sound waves with the aim to minimize the sound pressure in a given zone of a rectangular region are presented. The sound pressure is minimized by the active action of additional sound sources positioned within the region of computation. The optimum parameters of active sources and their positions are determined by solving the minimization problem for the integral functional that expresses the average sound pressure in the given zone of the rectangular region. The problem is solved using a hybrid genetic algorithm.

A space domain complex envelope

Sound visualization tools, for example, beam forming and acoustic holography, exhibit the spatial look of sound in time or frequency domain. However, they normally require a significant amount of computation time to draw well the sound picture in space. The picture contains a great deal of information: Sound pressure distribution, intensity pattern, or energy information with respect to space. The information is often far more than what we need in practice. For example, when we want to know only the location of the sound sources and somewhat averaged sound pressure distribution, we need a means that can provide only what we need. The complex envelope in time domain can be a good starting idea to deal with the problems we have. A method to generate a spatial domain envelope has been theoretically developed. We found that the method not only has an advantage to show rather simple spatial sound distribution, but also reduce significantly the computation time. The latter makes it possible to see the sound picture faster than before: About ten times faster. This method has been applied to many practical examples: For example, sound from a musical instrument and sound from machinery.

Using sound pressure to estimate reaeration in streams

The reaeration coefficient (k) is an essential and sensitive component of the equations used to calculate whole-stream metabolism (WSM) for a stream reach by the open-channel method. However, the empirical methods used to estimate k (e.g., by propane or sulfur hexafluoride evasion rates) are time consuming and costly. We reasoned that the reaeration rate (the product of k and the dissolved O2 deficit or surplus) and sound level (noise) are related to turbulence in a stream and, thus, sound level should be related to reaeration. We used a simple and inexpensive sound level meter to measure sound levels at a fixed height (30 cm) above key geomorphic features (e.g., pools, riffles, cascades) in several streams in Alaska and Vermont. We calculated a feature-weighted average sound pressure for selected stream reaches based on the proportion of these geomorphic features within each reach. We calculated k in the Alaskan stream reaches based on propane evasion rates and found a strong linear relationship to the feature- weighted average sound pressure (n ¼ 11, r 2 ¼ 0.94, p , 0.001). We concluded that the sound pressure method, which requires ;0.5 h to complete and relies on inexpensive instrumentation, provides estimates of k that are comparable to estimates from the more resource-intensive volatile gas-evasion method. In the Vermont stream reaches, we were able to create a sound pressure-stage rating curve, similar to a hydrologic rating curve. Combining the k-sound pressure relationship with the sound pressure-stage rating curve holds promise as a way to estimate reaeration continuously, at modest cost and effort, and on a time scale similar to that on which dissolved O2, temperature, and light values are recorded to calculate WSM.

Drilling a vent often fails to give relief from occlusion

Drilling a vent often fails to give relief from occlusion

Vibro-acoustic analysis of a rectangular-like cavity with a tilted wall

In this paper, a fully coupled vibro-acoustic model is developed to characterize the structural and acoustic coupling of a flexible panel backed by a rectangular-like cavity with a slight geometrical distortion, which is introduced through a tilted wall. The combined integro-modal approach is used to handle the acoustic pressure inside the irregular-shaped cavity. Based on the model proposed, the distortion effect on the vibro-acoustic behavior of the coupled system is investigated using the averaged sound pressure level inside the enclosure and the averaged quadratic velocity of the vibrating plate. Simulations are conducted to examine the distortion effect on acoustic natural frequencies, acoustic pressures and structural responses. Effects of the wall inclination on coupling coefficients are also assessed, and an index is proposed to quantify the degree of variation of coupling strength.

Anthropogenic noise and baleen whales: A perspective from humpback whale acoustics

The effects of anthropogenic noise on baleen whales is a topic of intense interest not only to marine mammalogists but also to a variety of commercial interests, the military, oceanographers, and researchers that use sounds in the ocean. One of the major difficulties in setting standards for exposure levels and the effective use of anthropogenic sounds is a lack of knowledge of the auditory properties of whales. The auditory sensitivity and frequency range of hearing (audiogram) are not known, as well as how anthropogenic sounds physically affect the auditory system, the psychological well being, and the use of sounds by whales. One approach to gain some insight in this area is to study the acoustics of a baleen whale species, the humpback whale. The source levels and frequency range of humpback whale song units have been measured and can provide insights into what these whales expose each other to. The highest rms source level was 173 dB for a specific unit. The average sound pressure levels of chorusing have been measured and can provide insights into the long-term acoustic environment of humpback whales. The highest daily average of sound pressure level for chorusing sounds was 130 dB.

Transportation noise and cardiovascular risk: Updated Review and synthesis of epidemiological studies indicate that the evidence has increased

The review provides an overview of epidemiological studies that were carried out in the field of community noise and cardiovascular risk. The studies and their characteristics are listed in the tables. Risk estimates derived from the individual studies are given for 5 dB(A) categories of the average A-weighted sound pressure level during the day. The noise sources considered in the studies are road and aircraft noise. The health endpoints are mean blood pressure, hypertension and ischaemic heart disease, including myocardial infarction. Study subjects are children and adults. The evidence of an association between transportation noise and cardiovascular risk has increased since the previous review published in Noise and Health in the year 2000.

Sound Insulation Evaluation Using Transfer Function Measurements

The accepted methods for measurement of sound insulation between rooms, audiometric cabins and other adjacent closed spaces deal with average sound pressure measurements and random sound excitation. The acoustic field can be as diffuse as found in reverberation chambers or form well-determined stationary waves in rectangular rooms. The use of random noise excitation requires averages in time to reduce the expected inherent uncertainties. The use of techniques, such as MLS or swept sine excitation, can avoid time consuming averaging processes and reduce measurement. With the advent of new front-end devices and signal analyzers it is now relatively easy to obtain acoustic transfer functions between two points in space, which can be in adjacent enclosures. With these transfer functions, it is possible to obtain the level differences, required by the international standard, as suggested in a new draft ISO document. This paper presents results obtained for the insulation of a cabin, tested in a reverberation chamber, using random noise and swept sine excitation. An analysis is given of the viability of using fewer positions of sound sources when measuring the transfer function. Repeatability tests, for both methods, are also presented.

Field measurement of airborne sound insulation between rooms with non-diffuse sound fields at low frequencies

Procedures for the field measurement of airborne sound insulation between rooms with diffuse fields are described in International Standard ISO 140-4. However, many dwellings contain rooms with volumes less than 50m3, where low frequency measurements are less reliable; hence there is a need for a measurement procedure to improve the reliability of field measurements in rooms with non-diffuse fields. Procedures are proposed for sound pressure level and reverberation time measurements for the 50, 63 and 80Hz third octave bands. The sound pressure level measurement combines corner microphone positions with positions in the central region of each room. This provides a good estimate of the room average sound pressure level with significantly improved repeatability.

Guest Editorial: Noise and Health

Guest Editorial: Noise and Health