Experimental and numerical study on aeroacoustic sound of axial flow fan in room air conditioner

Fan is one of the main noise sources of the room air-conditioners. Axial flow fans are widely used in the outdoor unit of split type air-conditioners. The interaction between the fan and the heat exchanger should be taken into consideration. However, only a few researches have been carried out on predicting the aeroacoustic noise because of the difficulty in obtaining detailed information of the flow field. This paper is to understand the generation mechanism of sound and to develop a prediction method for the flow field and the acoustic pressure field of the outdoor unit. Acoustic measurement is performed in a semi-anechoic chamber. Effects of each components is analyzed. Based on commercial computational fluid dynamics (CFD) code, Fluent, Fukano’s model is used to predict the overall sound pressure level of broadband noise. The predicted sound pressure levels based on original Fukano’s model are 7.66 dB and 7.42 dB lower than measurement results at 780 rpm and 684 rpm, respectively. And the errors are about 13%. However, when wake width and relative velocity are both calculated by numerical simulations and the distance to blade trailing edge is taken into consideration, the difference of sound pressure level between measurement and prediction is less than 3.4 dB and errors less than 5.5% while the distance is less than 10 mm. Thus, the distance to blade trailing edge should also be an important parameter for Fukano’s model. In comparison with experimental results, it is clearly shown that the Fukano method based on numerical simulation can provide more accuracy than the original Fukano model and numerical results are in a reliable level.     

The effect of construction material, contents and room geometry on the sound field in dwellings at low frequencies

An experimentally validated finite element method is used to model the sound level in rooms at low frequencies. It is demonstrated that the dimensions of rectangular rooms strongly influence the sound pressure level difference. Additional factors were investigated which are not normally considered in the frequency range where diffuse sound field conditions can be assumed. Three effects were investigated: room damping due to wall vibrations, furniture, the effect of small deviations from simple rectangular shapes. It is confirmed by field measurements that the vibrations of masonry walls and floors introduce less damping than surfaces of lightweight construction. Assigning to the FE model a damping equivalent to a surface absorption of 0.02 reproduces the effect of walls of heavyweight construction. Damping equivalent to a surface absorption of 0.15 reproduces the effects of plastered timber-frame walls, floors and ceilings. The work was briefly extended to a room pair built with heavyweight and lightweight material of construction. The modification of the shape of the room frequency response highlights well the effect of material of construction. In-situ and laboratory measurements show that furniture has little effect on steady-state room response below 100 Hz. Modelling a wall recess smaller than 0.5 m improved the agreement between prediction and measurements but the assumption of a simple rectangular room remains appropriate.     

Sound radiation and sound quality characteristics of refrigerator noise in real living environments

The characteristics of various types of refrigerator noise were investigated in an anechoic chamber and in a real living environment - a 100 m² apartment which is a common size in Korea. It was found that the sound pressure level of the refrigerator noise in the real living room was about 10 dB higher than the level in the anechoic chamber at the same position (1 m in front of refrigerator). In addition, a tolerance level for refrigerator noise was determined by subjective evaluation experiments. Refrigerator noise was presented by a loudspeaker placed in the kitchen where the refrigerator is normally located. Level 2 responses to the subjective evaluation (“hardly perceivable”) corresponded to a sound pressure level of about 26 dB(A), for which 90% of participants were satisfied with the level of refrigerator noise. A semantic differential test using various adjectives was also conducted to evaluate the sound quality of refrigerator noise. With the semantic differential and the factor analysis, adjectives used in this experiment were grouped into three factors. From the results of correlation and multiple regression analyses on the psychoacoustical parameters and subjective evaluations of 30 kinds of refrigerators, sound quality index which predict the subjective rating score were proposed.     

Experimental study of sound propagation in a street

This paper presents an experimental study of the sound propagation in a street, performed during July 2002, in order to measure impulse responses at numerous locations in a street canyon, and, for several positions of the sound source. A specific attention has been paid on the processing of the experimental data, in order to obtain accurate values of the reverberation time and the steady-state sound pressure level, including also the compensation of the atmospheric attenuation. In total, 11 592 useful data have been collected during this experiment, and are available by contacting the corresponding author. Reverberation times and sound levels, in the narrow street, are then analyzed and compared, for the 4 source positions in the street. This experimental study gives interesting results on the behavior of the sound field in the street.     

Sound image method and sound energy vortex in rectangular room

1.TntroductionMostprob1emsofsolvingsoundficlds,suchassoundpressuredistributionsinroomsareclassiia1.lllIn10wfrequencyrange,wavemotionmethodisapplied.Inthemethod,thenorma1modesofroomaresolvedandthensummeduptocvaluatetheirunknownamplitudesbymatchingsoundsource.Inhighfrequencyrange,thenumberofnormalmodesneedcdisnumer0usandtogetfina1solutionwouldbeimpossiblebecauseofthetimeconsumptionincomputation.Hence,ther0omacousticsbasedongeometri-calacousticsandthestatisticswasdevcloped.Thewavemotionmethodcan…     

Experimental implementation of a low-frequency global sound equalization method based on free field propagation

An experimental implementation of a global sound equalization method in a rectangular room using active control is described in this paper. The main purpose of the work has been to provide experimental evidence that sound can be equalized in a continuous three-dimensional region, the listening zone, which occupies a considerable part of the complete volume of the room. The equalization method, based on the simulation of a progressive plane wave, was implemented in a room with inner dimensions of 2.70 m × 2.74 m × 2.40 m. With this method, the sound was reproduced by a matrix of 4 × 5 loudspeakers in one of the walls. After traveling through the room, the sound wave was absorbed on the opposite wall, which had a similar arrangement of loudspeakers, by means of active control. A set of 40 digital FIR filters was used to modify the original input signal before it was fed to the loudspeakers, one filter for each transducer. The optimal arrangement of the loudspeakers and the maximum frequency that can be equalized is analyzed theoretically in this paper. The presented experimental results show that sound equalization was possible from 10 Hz to approximately 425 Hz in the listening zone. A flat frequency response with deviations within ±5 decibels from the desired value was achieved. A higher demanding performance with deviations within ±1.5 decibels from a flat frequency response was attained in the interval between 20 Hz and 280 Hz. At the same time, the impulse response was quite well approximated to a delayed delta function in the listening zone. Examples of the spatial distribution of the sound field are also shown.     

Determination of the sound energy level of a gunshot and its applications in room acoustics

In some cases an impulsive noise source such as a gunshot can be a preferred alternative when investigating building acoustics, including sound insulation measurements, when compared to conventional steady state noise sources. A gun equipped with blank cartridges is an impulsive noise source that is lightweight and small enough to be easily transported. The differences in the noise characteristics between individual cartridges for the same gun are usually small, so the impulsive source can be replicated to a high degree. This paper is focused on the practical application of the sound exposure levels produced by a gunshot with a known sound energy level in the rooms under investigation. In this way, the equipment and methods required by the conventional method are simplified significantly. Furthermore, reverberation times need not be measured, since the equivalent absorption area can be directly obtained from the measured sound exposure levels. Using Green’s theorem, the roles of the sound source and measuring microphone were exchanged, which simplified the determination of sound insulation as it was easier to change the position of the gun than the microphone. The results obtained using the impulsive noise source were in good agreement with those obtained using the conventional method. Above 100 Hz, their difference in any frequency band of interest was less than 1 dB.     

A method for the low frequency qualification of reverberation test rooms using a validated finite element model

Low frequency behavior in small rooms is always a critical issue, but the recent extension of several standards to frequencies as low as 50 Hz opened an interesting debate in the scientific community as to which is the best (and most reliable) method to perform measurements. The present paper discusses the low frequency qualification of a typical reverberant test room in order to perform sound power measurements (carried out according to ISO 3741), by taking advantage of a finite element model of the room. Experimental measurements were first carried out in a standard reverberant chamber to demonstrate that the model provides accurate results in the range below 100 Hz. Statistical analysis of the results from measurements and simulations confirmed that, despite some small inaccuracies, the predicted results are in very good agreement with those measured both in terms of spectra and spatial distribution of the sound pressure level. Finally, the different steps of the low-frequency qualification of a reverberant test room are discussed. A selection procedure of the most suitable microphone positions is proposed, based on the results of the simulation, and, finally, on site measurements were carried out to validate the procedure.     

Characterisation of valves as sound sources: Fluid-borne sound

The sound pressure level in receiving rooms, caused by taps at the ends of pipe systems, is considered. The structure-borne sound power, from the pipes to the supporting wall, was obtained from intensity measurement of the fluid-borne sound power of the tap. The fluid-borne sound power is combined with a ratio of structure-borne sound power to fluid-borne sound power, obtained from laboratory measurements of similar pipe assemblies. Alternatively, a reception plate method is proposed, which avoids the necessity for intensity measurements. The structure-borne power into walls, to which the pipe work is attached, provides input to the standard building propagation model, which yields the predicted sound pressure level in the adjacent room.     

Reverberation time measurements in non-diffuse acoustic field by the modal reverberation time

The increasing presence of low frequency sources and the lack of acoustic standard measurement procedures make the extension of reverberation time measurements to frequencies below 100 Hz necessary. In typical ordinary rooms with volumes between 30 m³ and 200 m³ the sound field is non-diffuse at such low frequencies, entailing inhomogeneities in space and frequency domains. Presence of standing waves is also the main cause of bad quality of listening in terms of clarity and rumble effects. Since standard measurements according to ISO 3382 fail to achieve accurate and precise values in third octave bands due to non-linear decays caused by room modes, a new approach based on reverberation time measurements of single resonant frequencies (the modal reverberation time) has been introduced. From background theory, due to the intrinsic relation between modal decays and half bandwidth of resonant frequencies, two measurement methods have been proposed together with proper measurement procedures: a direct method based on interrupted source signal method, and an indirect method based on half bandwidth measurements. With microphones placed at corners of rectangular rooms in order to detect all modes and maximize SNRs, different source signals were tested. Anti-resonant sine waves and sweep signal turned out to be the most suitable for direct and indirect measurement methods respectively. From spatial measurements in an empty rectangular test room, comparison between direct and indirect methods showed good and significant agreements. This is the first experimental validation of the relation between resonant half bandwidth and modal reverberation time. Furthermore, comparisons between means and standard deviations of modal reverberation times and standard reverberation times in third octave bands confirm the inadequacy of standard procedure to get accurate and precise values at low frequencies with respect to the modal approach. Modal reverberation time measurements applied to furnished ordinary rooms confirm previous results in the limit of modal sound field: for highly damped modes due to furniture or acoustic treatment, the indirect method is not applicable due to strong suppression of modes and the consequent deviation of the acoustic field from a non-diffuse condition to a damped modal condition, while standard reverberation times align with direct method values. In the future, further investigations will be necessary in different rooms to improve uncertainty evaluation.     

Factors affecting sound exposure from firing an SA80 high-velocity rifle

The effect of distance on the peak sound pressure level and sound exposure level from an SA80 rifle has been investigated. Sound pressure waveforms were measured in two directions from the gun: downrange, from 50 m to 300 m, and to the left-hand side, from 0.3 m to 32 m. Some additional measurements were made to the right of the gun. Measurements made downrange showed three distinct features of the waveform; the shock wave from the supersonic bullet, the reflection from the ground, and the muzzle blast. The time elapsed between the shock wave and the muzzle blast increased with increasing distance: 94 ms for a distance of 50 m, and 507 ms for a distance of 300 m. The highest peak sound level downrange from a single round was between 151 dB(C) and 148 dB(C) at distances from 50 m to 300 m, and varied little if at all with distance. To the left of the gun, the peak sound pressure level of 161 dB(C) at 0.3 m reduced to 128 dB(C) at 32 m. The peak sound pressure level was estimated to be 137 dB(C) at a distance of approximately 20 m to the left-hand side. Hearing protection must therefore be worn by anyone closer than 20 m to a person firing. The peak sound pressure level was estimated to be 135 dB(C) at a distance of approximately 25 m and therefore hearing protection is recommended at distances of up to 25 m. The sound exposure level of 98 dB(A) at 20 m indicated that an observer at this distance could hear about 1440 rounds without hearing protection before the noise exposure reached the upper exposure action value specified in the Control of Noise at Work Regulations 2005. Peak sound pressure levels were on average 2.4 dB higher at the left ear compared with the right ear.     

Measurement of the sound velocity in fluids using the echo signals from scattering particles

With conventional methods the sound velocity c in fluids can be determined using the back wall echo. This paper proposes a novel technique, in which the signals reflected by scattering particles suspended in a fluid are analysed instead. The basic idea is that the particles generate the strongest echo signal when being located in the sound field maximum. Therefore the position of the echo signal maximum is a measure for the propagation time to the sound field maximum. Provided that calibration data or sound field simulations for the ultrasonic transducer are available, this propagation time suffices to determine both sound velocity and the location of the sound field maximum. The feasibility of the new approach is demonstrated by different kinds of experiments: (i) Measurements of the sound velocity c in four fluids covering the wide range between 1116 and 2740 m/s. The results show good agreement with values published elsewhere. (ii) Using the dependence of the sound velocity on temperature, it is possible to vary c over the comparatively small range between 1431 and 1555 m/s with increments of less than 10 m/s. The measured statistical variation of 1.4 m/s corresponds to a relative uncertainty not worse than 0.1%. (iii) The focus position, i.e. the distance of the maximum of the sound field from the transducer, was varied by time-shifted superposition of the receive signals belonging to the different elements of an annular array. The results indicate that the novel method is even capable of measuring profiles of the sound velocity along the ultrasonic beam non-invasively.     

A study of the accuracy of mobile technology for measuring urban noise pollution in large scale participatory sensing campaigns

The study reports on the relevancy and accuracy of using mobile phones in participatory noise pollution monitoring studies in an urban context. During one year, 60 participants used the same smartphone model to measure environmental noise at 28 different locations in Paris. All measurements were performed with the same calibrated application. The sound pressure level was recorded from the microphone every second during a 10-min period. The participants frequently measured the evolution of the sound level near two standard monitoring sound stations (in a square and near a boulevard), which enables the assessment of the accuracy and relevancy of collected acoustic measurements. The instantaneous A-weighting sound level, energy indicators such as L_A,eq, L_A10, L_A50 or L_A90 and event indicators such as the number of noise events exceeding a certain threshold L_α (NNEL ? L_α) were measured and compared with reference measurements. The results show that instantaneous sound levels measured with mobile phones correlate very well (r > 0.9, p < 0.05) with sound levels measured with a class 1 reference sound level meter with a root mean square error smaller than 3 dB(A). About 10% of the measurements for the boulevard location (respectively 20% for the square) were inaccurate (r < 0.3, p < 0.05). Nevertheless, mobile phone measurements are in agreement for the L_A50 and the L_A90 acoustic indicators with the fixed station (4-m high) measurements, with a median deviation smaller than 1.5 dB(A) for the boulevard (respectively 3 dB(A) for the square).     

Justification of standardized level differences in rating of airborne sound insulation between dwellings

It has long been recognized that single-number quantities R′_w, D_nT,w or D_n,w result in different conclusions in objective rating of airborne sound insulation between dwellings. The difference between the values of these single-number quantities (SNQ), however, does not prove which of them describes the sound transmission between rooms most correctly. The main object of this article was to study which SNQ correspond best with transmitted living sound levels in buildings when reverberation time, volume of receiving room and sound insulation are taken into account. Data of 100 field measurements of airborne sound insulation were collected as well as 207 reverberation times of furnished rooms. The transmitted sound levels of living sounds were evaluated on the basis of known living sound spectra and measured level differences D. The results show that the SNQs standardized to reference reverberation time of 0.5 s lead in all cases to best correlation between the SNQs and the sound levels of transmitted living sounds. It was also checked whether the rating by D_nT,w would lead to higher transmitted sound levels of living sounds in larger rooms, but this was not detected. The use of D_nT,w makes rooms of different volumes equal in regard to required sound insulation between them. It is thus justified to replace R′_w with D_nT,w as the SNQ for rating the airborne sound insulation. Widening the frequency range down to 50 Hz or up to 5000 Hz did not give noteworthy improvement in the correlation.     

Development of an optimised, standard-compliant procedure to calculate sound transmission loss: design of transmission rooms

A numerical procedure to estimate the transmission loss of sound insulating structures is proposed based upon the technology of acoustic measurements and standards. A virtual laboratory (VL), namely, a numerical representation of a real laboratory consisting of two reverberation rooms meeting certain sound field quality criteria is designed. VL is to be used for the numerical simulation of standardised measurements under predefined, controlled, acoustic conditions. In this paper, the design and optimisation of VL is investigated. The geometry of the transmission rooms is designed following first principles, in order for diffuse field conditions and sufficiently smooth primary mode distribution in the low frequency to be achieved. A finite element-based optimisation procedure, introduced by the author in previous work, is extended to arbitrarily shaped rooms. It is used to predict the appropriate local geometric modifications so as for improved mode distribution and smoother sound pressure fluctuations of the transmission rooms in the low-frequency range to be achieved and low-frequency measurement reproducibility and accuracy to be increased. Steady-state acoustic response analysis is performed in order to quantify the acoustic field quality of the virtual transmission rooms in the frequency range of measurements. A method to calculate the total absorption, A, of the receiving room is introduced by simulation of the reverberation time measurement procedure using Transient acoustic response analysis. The acoustic performance of VL is overall considered and is shown to meet in a sufficient degree, relative laboratory measurement standards in the frequency range of 100÷704 Hz.     

Sound fields near building facades - comparison of finite and semi-infinite reflectors on a rigid ground plane

The sound field in front of, and close to a building facade is relevant to the measurement and prediction of environmental noise and sound insulation. For simplicity it is often assumed that the facade can be treated as a semi-infinite reflector, however in the low-frequency range (50-200 Hz) this is no longer appropriate as the wavelengths are similar or larger than the facade dimensions. Scale model measurements and predictions using integral equation methods have been used to investigate the effect of diffraction on the sound field in front of finite size reflectors. For the situation that is commonly encountered in front of building facades, the results indicate that diffraction effects are only likely to be significant in the low-frequency range (50-200 Hz) when the façade dimensions are less than 5 m. This assumes that there is a point source close to the ground and microphones at a height of 1.2 or 1.5 m, at a distance between 1 and 2 m in front of the façade.     

Acoustic analysis of a liquefied petroleum gas-fired pulse combustor

Experimental investigation of acoustic characteristics of a Helmholtz type liquefied petroleum gas-fired pulse combustor is presented. In the experiments, the length of the tail pipe was changed from 1.9 m to 1.3 m by 10 cm intervals. Sound level measurements were taken from the exhaust side (outlet) and air flapper side (inlet) at a distance of 1 m from both sides. With decreasing lengths of the tail pipe, the sound pressure level increased. At the measurements related to the exhaust side, the maximum value of equivalent continuous sound pressure level, LEQ was 96.6 dB when the length of the tailpipe and fundamental frequency were 1.3 m and 63 Hz, respectively. Same kinds of measurements were performed at the air flapper side, but the LEQ value was stronger at the exhaust side than the one at the air flapper side. It was also observed that the effect of the type of gaseous fuel on the acoustic efficiency of the pulse combustor can be neglected when the results of the acoustic efficiencies were compared to those in the literature. In order to compare the accuracy of frequencies measured by the sound level meter, a suitable dynamic pressure transducer and a spectrum analyzer were used to perform amplitude and frequency measurements. The average deviation between the measurements performed by the sound level meter and dynamic pressure transducer was 2.4 Hz (3.8% errors) while the average deviation was 3.8 Hz (6% errors) between the sound level meter and spectrum analyzer.     

Study of vertical sound image control with parametric loudspeakers

A parametric loudspeaker utilizes nonlinearity of a medium and is known as a super-directive loudspeaker. In this paper, the sound localization in the vertical direction using the upper and lower parametric loudspeakers is confirmed by listening tests and physical measurements. The differences in levels between the upper and lower parametric loudspeakers are varied as a parameter. The direction of sound localization in the vertical plane can be controlled not only when the acoustical axis is set to the right ear but also when it is set to at 5 deg to the right of the right ear. The effect of the level difference between the upper and lower loudspeakers is weaker than the differences observed when using ordinary loudspeakers. We obtained interesting characteristics of the left-right sound localization in the horizontal plane with the upper and lower parametric loudspeakers in the vertical plane. It is found that by setting the parametric loudspeaker at the right ear (that is, the horizontal angle of a listener to it is only 3 deg to the right), the direction of sound localization in the horizontal plane moved approximately 10 deg to the right. Moreover, by setting the parametric loudspeaker 5 deg to the right, the direction of sound localization moves approximately 20 deg to the right. The ILD (Interaural Level Difference) using a dummy head is calculated from the measured left and right sound signals. It is determined that ILDs of the parametric loudspeaker are larger than those of the ordinary loudspeaker. A simple geometrical acoustic model is introduced and analyzed. The analysis helps to explain the measured characteristics.     

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.     

椰胡声功率级的半消声室测试

椰胡是具有地方特色的拉弦乐器之一,多用于演奏广东音乐和广东戏曲、曲艺的伴奏。但对其发声强度一直未进行过科学的测试。本文介绍在华南理工大学半消声室内参照ISO(GB)标准对椰胡声功率级的测量工作。由两位资深乐师分别用两把椰胡在pp,mp,f和ff力度下演奏单音、音阶和乐曲,对每一把椰胡的每一测试内容,由十通道测试设备同步测试中心频率为100～10000 Hz的1/3倍频带声压级谱,通过计算获得每把椰胡在演奏每项内容时的声功率级和动态范围。通过对两把椰胡的测试结果进行平均,获得该乐器在演奏上述内容时的典型声功率级数值及频谱。文中并将半消声室内的测试结果与混响室内的测试结果相对比,探讨测试环境对测试结果的影响。民族乐器发声强度及其频谱特性的获得是开展民族音乐厅堂音质研究的基础。