On the amplification of broad band jet noise by a pure tone excitation

It has been found experimentally that broad band jet noise can be amplified by a pure tone excitation as much as 6 to 7 dB. The jet noise amplification effect takes place at sound pressure levels which are present in real aircraft engines. The experimental investigation was restricted to a cold jet at high subsonic Mach numbers excited by a plane sound wave coming from inside the nozzle. Based on a simplified mathematical model an attenuator has been constructed which is able to reduce the jet noise amplification significantly.     

Ceiling baffles and reflectors for controlling lecture-room sound for speech intelligibility

Reinforcing speech levels and controlling noise and reverberation are the ultimate acoustical goals of lecture-room design to achieve high speech intelligibility. The effects of sound absorption on these factors have opposite consequences for speech intelligibility. Here, novel ceiling baffles and reflectors were evaluated as a sound-control measure, using computer and 1/8-scale models of a lecture room with hard surfaces and excessive reverberation. Parallel ceiling baffles running front to back were investigated. They were expected to absorb reverberation incident on the ceiling from many angles, while leaving speech signals, reflecting from the ceiling to the back of the room, unaffected. Various baffle spacings and absorptions, central and side speaker positions, and receiver positions throughout the room, were considered. Reflective baffles controlled reverberation, with a minimum decrease of sound levels. Absorptive baffles reduced reverberation, but reduced speech levels significantly. Ceiling reflectors, in the form of obstacles of semicircular cross section, suspended below the ceiling, were also tested. These were either 7 m long and in parallel, front-to-back lines, or 0.8 m long and randomly distributed, with flat side up or down, and reflective or absorptive top surfaces. The long reflectors with flat side down and no absorption were somewhat effective; the other configurations were not.     

Noise reduction in tunnels by hard rough surfaces

This paper examines the feasibility of using two-dimensional hard rough surfaces to reduce noise levels in traffic tunnels with perfectly reflecting boundaries. First, the Twersky boss model is used to estimate the acoustic impedance of a hard rough surface. Second, an image source model is then used to compute the propagation of sound in a long rectangular enclosure with finite impedance. The total sound fields are calculated by summing the contributions from all image sources coherently. Two model tunnels are built to validate the proposed model experimentally. Finally, a case study for a realistic geometrical configuration is presented to explore the use of hard rough surfaces for reducing traffic noise in a tunnel which is constructed with hard boundaries.     

A computer model for the prediction of factory noise

A computer program which calculates noise levels around a factory floor has been written. The collection of data for input to the program, formulae used in the calculation of noise levels and output from the program are described.The program has been tested using data collected in a variety of workshops and factories. The results of two of these case studies are discussed, together with overall results for all cases considered, which show that the program can predict sound levels with a high degree of accuracy.An interactive version of the program which enables a user to see immediately how certain changes to the data will affect noise levels is also described.     

Identification of noise sources in factory’s sound field by using genetic algorithm

Noise control is important and essential in factory, where the noise level is restricted by the Occupational Safety and Health Act. Before noise abatement being performed, the identification work in searching for the location and sound power level (SWL) of noisy sound sources is absolutely prerequisite. Several researches on new techniques of single noise control have been well addressed and developed; however, the research work on sound identification for the existing multi-noise plant is hardly found sufficient. Under the circumstance of unrecognized noises, the noise control work will expectedly be extravagant and fruitless. Therefore, the numerical approach in distinguishing noises from a multi-noise plant becomes crucial and obligatory.In this paper, the novel technique of genetic algorithm (GA) in conjunction with the method of minimized variation square will be adopted and used in the following numerical optimization. In addition, various sound monitoring systems in detecting the noise condition within the plant area will also be introduced. Before noises identification, the accuracy of mathematical model has then been proved to be in good agreements comparing to the simulated data of SoundPlan, a commercialized simulation package in sound field. Moreover, three kinds of multi-noise plants have been fully discussed and acknowledged by GA optimization. The results reveal that the relevant locations and sound power levels (SWLs) of noises can be precisely recognized. This paper surely provides a rapid methodology in the noise identification work for a multi-noise plant.     

The propagation of sound in narrow street canyons

This paper addresses an important problem of predicting sound propagation in narrow street canyons with width less than 10 m, which are commonly found in a built-up urban district. Major noise sources are, for example, air conditioners installed on building facades and powered mechanical equipment for repair and construction work. Interference effects due to multiple reflections from building facades and ground surfaces are important contributions in these complex environments. Although the studies of sound transmission in urban areas can be traced back to as early as the 1960s, the resulting mathematical and numerical models are still unable to predict sound fields accurately in city streets. This is understandable because sound propagation in city streets involves many intriguing phenomena such as reflections and scattering at the building facades, diffusion effects due to recessions and protrusions of building surfaces, geometric spreading, and atmospheric absorption. This paper describes the development of a numerical model for the prediction of sound fields in city streets. To simplify the problem, a typical city street is represented by two parallel reflecting walls and a flat impedance ground. The numerical model is based on a simple ray theory that takes account of multiple reflections from the building facades. The sound fields due to the point source and its images are summed coherently such that mutual interference effects between contributing rays can be included in the analysis. Indoor experiments are conducted in an anechoic chamber. Experimental data are compared with theoretical predictions to establish the validity and usefulness of this simple model. Outdoor experimental measurements have also been conducted to further validate the model.     

Modelling traffic noise mathematically

In this paper we describe a mathematical model for the prediction of traffic noise levels in an urban or suburban situation. The model is ultimately intended to provide an alternative to existing methods of prediction. At the present time, only noise levels produced by stationary sound sources have been considered.Any point in a chosen area is described by its grid co-ordinates. A detailed plan of the buildings or other structures in the area and the position(s) of the sound source(s) are needed as input to the model. Noise levels at all grid positions in the area are then calculated on the basis of the attenuation of sound due to direct propagation, diffraction and reflection.The results obtained so far are given and since the model is in an early stage of development, and has yet to be proved against measurements in real situations, possible refinements and future developments are discussed in some detail.     

Traffic noise reduction at balconies on a high-rise building façade

The performance of balconies with ceiling-mounted reflectors on a high-rise building fac?ade is examined using numerical analyses and scale-model experiments. The reflectors are designed to reflect direct and diffracted waves incident on the ceiling outside the balcony. The sound pressure reduction, provided by the reflectors, on a window surface adjacent to the balcony is evaluated at intermediate floors levels. In terms of A-weighted sound pressure levels, a balcony equipped with reflectors reduces road traffic noise by 7-10 dB(A), compared to an ordinary balcony, at incident angles of noise close to the angle for which the reflectors are designed. The efficiency is roughly the same as, or greater than, that of a balcony with an absorbent ceiling. However, it is also shown that when the vertical incident angle of the noise is smaller than the design angle of the reflectors, or the horizontal incident angle is large, efficiency is reduced.     

An inversion modelling method to obtain the acoustic power of the noise sources in a large factory

A common problem for large factories that wish to decrease their environmental acoustic impact on neighbouring locations is to find out the acoustic power of every noise source. As these factories cannot stop their activity in order to measure each source individually, a procedure is needed to obtain the acoustic powers with the factory under normal operating conditions. Their contribution to the overall sound pressure level at each neighbouring location can then be found and it is possible to calculate the improvements obtained after any modification of the sources. In this paper an inversion modelling method is used to do so. Acoustic powers are obtained by means of field sound pressure level measurements and with the use of a sound propagation software. A careful analysis of the solution has been carried out by simulating errors on the measured data in order to detect possible correlations between the acoustic power of different sources and avoid misleading interpretations of the results. The whole methodology has been applied to a liquid-gas production factory.     

Prediction of the sound field into high-rise building facades due to its balcony ceiling form

This study presents the acoustic performance off tall building facades closed to roadway due to one of balcony configurations, namely ceiling, with an inclined form in terms of traffic noise reduction. Three inclined angles are tested (5,10, and 15°) with different balcony depths by using a Pyramid Tracing model developed by A. Farina. The results in terms of A-weighted sound pressure level reduction are expressed in free field into the balcony back wall. The protection level, defined as the difference in noise levels before and after inserting the proposed balcony form, has been used to assess the reduction offered by that configuration. A maximum reduction due to using these forms is obtained at higher floors and at balcony of 2 m depths and more. As a consequence of simulation results, it is found that the prediction of protection levels from 10th to 15th floor can be calculated from an empirical equation.     

Optimization of constrained composite absorbers using simulated annealing

Sound reverberation is an important problem in some industrial environments. As indicated by the Occupational Safety and Health Act of 1970, noise is responsible for the psychological and physiological ills of workers. Therefore reduction of reverberation becomes essential. For maintenance and other reasons, the thickness of sound absorbers used for reverberation control may be constrained. Consequently there is interest in minimizing noise using sound absorbers with constrained thickness. Optimization of a composite absorber using a simulated annealing algorithm is presented. Simulated annealing is a stochastic relaxation technique based on analogy with the physical process of annealing metal. The algorithm requires a mathematical model for the acoustical properties of the absorber. Before optimization, the accuracy of the mathematical model was checked against experimental data. A program for optimizing in respect of broad band noise at a specified receiver has been created and run. Results prove that SA optimization provides a quick and efficient approach in designing constrained thickness composite sound absorbers.     

Possibilities and limits of sound power measurements with a real-time intensity analyzer

The sound power of a number of test objects was determined from spatially averaged intensity measurements. The results show that the influence of room acoustics is insignificant even for rooms of widely different room constants, if the measuring surfaces are exactly defined and if a good space-averaging technique is used. The intensity integrated over a closed surface defining a source-free space compared to the sound pressure integrated over the same surface gives a measure of the capability of a specific intensity measuring system to suppress external noise. For the test arrangements measured with broad band noise, this suppression was found to be 14–18 dB(A). A similar value of 15 dB was found from sound power measurements on a source with high external sound and an analysis of the results in one-third octave bands. From these measurements an analytical function was derived which describes the average error of the spatially averaged intensity as a function of the difference between the external sound level and the source sound level. For practical measurement situations a further analytical function was derived which gives this intensity error as a function of the difference between the measured (spatially averaged) pressure and intensity levels. Thus it is possible to estimate the error of intensity measurements directly from measured intensity and pressure data.     

On the prediction of impact noise,I: Acceleration noise

The introduction of legislation regarding the limits of noise in factories has led to the need for prediction of likely noise levels produced by a machine at its design stage. This paper, the first of a series, is concerned with the noise generated by impacting bodies due to the high surface accelerations during the contact period. An account is presented of the theoretical development and experimental validation of curves for the prediction of peak sound pressure and radiated energy for collisions of compact bodies which are incapable of flexural motions. It is shown that acceleration noise energy is of the same order of magnitude as that due to ringing, that it cannot be greater than 1·5 × 10^?4 times the kinetic energy input at impact, and that it falls off rapidly as the normalized contact time increases above a critical value.     

The influence of duration and level on human sound localization

The localization of sounds in the vertical plane (elevation) deteriorates for short-duration wideband sounds at moderate to high intensities. The effect is described by a systematic decrease of the elevation gain (slope of stimulus-response relation) at short sound durations. Two hypotheses have been proposed to explain this finding. Either the sound localization system integrates over a time window that is too short to accurately extract the spectral localization cues (neural integration hypothesis), or the effect results from cochlear saturation at high intensities (adaptation hypothesis). While the neural integration model predicts that elevation gain is independent of sound level, the adaptation hypothesis holds that low elevation gains for short-duration sounds are only obtained at high intensities. Here, these predictions are tested over a larger range of stimulus parameters than has been done so far. Subjects responded with rapid head movements to noise bursts in the two-dimensional frontal space. Stimulus durations ranged from 3 to 100 ms; sound levels from 26 to 73 dB SPL. Results show that the elevation gain decreases for short noise bursts at all sound levels, a finding that supports the integration model. On the other hand, the short-duration gain also decreases at high sound levels, which is in line with the adaptation hypothesis. The finding that elevation gain was a nonmonotonic function of sound level for all sound durations, however, is predicted by neither model. It is concluded that both mechanisms underlie the elevation gain effect and a conceptual model is proposed to reconcile these findings.     

Absorbing surfaces in ray-tracing programs for coupled spaces

Nowadays architects commonly use the ‘coupled space concept’; examples are mezzanines, half-open office spaces and exhibition rooms. There is a need to predict acoustical quantities for this category of spaces, since half-open spaces may be a cause of noise annoyance. The transmission of sound between coupled spaces depends on design decisions like position, shape and dimensions of the surfaces and on the reflection characteristics. This paper deals with some problems related to the application of absorbing surfaces in coupled rooms, especially when they are modelled in a ray-tracing program. Absorption coefficients from meausurements in reverberation chambers may exceed 1.0 and they do not bear any information about angle dependent behavior, so an extra conversion must be made into input values for the ray-tracing model. Therefore ray-tracing calculations have been performed in a computer model of a reverberation chamber. From a comparison study between measurements and calculations in three coupled rooms it is found that the accuracy is good, provided that the sound reflections on the walls are introduced as angle dependent. Care should be taken in choosing a diffusion factor of flat surfaces.     

Estimation of directivity and sound power levels emitted by aircrafts during taxiing, for outdoor noise prediction purpose

Integrated noise model (INM) is the most internationally used software to calculate noise levels near airports. Take off, landing or pass by operations can be modeled by INM, but it does not consider aircrafts taxiing, which, in some cases, can be important to accurately evaluate and reduce airports’ noise assessment.Aircraft taxiing noise emission can be predicted using other prediction tools based on standards that describe sound attenuation during propagation outdoors. But these tools require data inputs that are not known: directivity and sound power levels emitted by aircraft during taxiing.This paper describes methods used to calculate directivity indexes and sound power levels, based on field measurements made in Madrid-Barajas Airport (Spain). Obtained results can be used as inputs for general purpose outdoor sound prediction software, which will be able to evaluate noise at airports vicinity as industrial noise.Directivity and sound power levels have been estimated in octave and third octave band terms, for several aircraft families.     

Measurements of roughness noise

Experiments have been performed on the roughness noise produced by a two-dimensional turbulent wall jet boundary layer flowing over short fetches of sandpaper roughness. A range of rough surface sizes were studied from hydrodynamically smooth through fully rough. Velocity measurements were made to document the form of the wall jet boundary layer and the influence of the roughness upon it. Acoustic measurements showed background noise levels to be very low so that the sound produced by the rough surfaces could be clearly detected with signal to noise ratios as large as 20 dB. Even hydrodynamically smooth roughness was found to produce noise, conclusively indicating the presence of scattering as a source mechanism. Variations of the roughness noise spectra with flow speed and roughness size are found to be inconsistent with any simple parameter scaling. Boundary layer wall pressure fluctuation measurements made within the roughness fetches reveal a spectral form quite different than the roughness noise, and fluctuation levels some 50-70 dB higher. Despite these differences the wall pressure and roughness noise are found to be very simply related, at least at lower frequencies (<6 kHz). The roughness noise spectrum varies closely as the product of the wall pressure spectrum, the frequency squared, and the mean-square roughness height. This is the scaling predicted by scattering theory and implies a major simplification to the problem of roughness noise prediction for stochastic surfaces.     

Low frequency sound measurement in vehicles

Sound pressure level measurements in cars travelling at motorway speeds have shown that, in many cases, the overall level is very high in relation to the dB(A) and octave band levels, suggesting that much of the sound energy is in the low frequency and infrasonic regions. A technique has been developed to extend accurate octave band measurements down to the octave centred on 2 Hz. The system uses a calibrated sound level meter feeding a frequency modulation tape-recorder to record noise below 64 Hz, and an octave band analysis system to analyse the resultant tape recordings. Typical results are presented for a number of vehicles and it is found that sound pressure levels as high as 120 dB can be found in the octave bands between 2 and 16 Hz.     

Effect of woodwool shuttering in party floors on the flanking transmission of sound between maisonettes

The sound insulation of party walls in a four storey block of maisonettes was found to be considerably lower at first floor level than at other floor levels in the frequency range of approximately 600 to 3000 Hz. The rooms at first floor level lie immediately below the party floor which is built of dense concrete cast on woodwool shuttering, plastered beneath; the other floors are timber joist floors and this seems to be the only major difference in construction at different floor levels. Vibration measurements indicated that appreciable flanking transmission of sound was occurring through the ceiling in the frequency range 600 to 3000 Hz and it seems probable that the use of woodwool shuttering has led to a resonance effect producing these high vibration levels.     

Development of a highway traffic noise prediction model that considers various road surface types

A highway traffic noise prediction model has been developed for environmental assessment in South Korea. The model is based on an outdoor sound propagation method and is fully compliant with ISO 9613 and the sound power level (PWL) estimation for a road segment, as suggested in the ASJ Model-1998 that is based on PWLs. Due to that model’s selection of two pavement types, such as asphalt or concrete pavement, an unacceptable traffic noise prediction is made in cases where the road surface is different from that on which the model is based. In order to address this problem, several road surface types are categorized, and the PWL of each surface type is determined and modeled by measuring the noise levels obtained from newly developed methods. An evaluation of the traffic noise prediction model using field measurements finds good agreement between predicted and measured noise levels.