Sound Source Distribution Research Articles

The Sounds of Two Landscape Settings: Auditory concepts for physical planning and design

Sonic aspects relevant to landscape architects and planners are examined in order to show that soundscapes can be acknowledged and developed as an aesthetic resource for sustainable development. This auditory approach aims at an inter-sensory understanding of human perception and its significance for the design of physical environments. Two settings—a pasture landscape and a city garden—were studied by skilled listeners on site. They contributed to a preliminary terminology for the design of sonic environments, sonotopes. Clarity emerged as one cardinal principle for auditory refuges, and it was found to be related to the balance of particular sonic features and their background sounds. A figure/ground model described these proportions. The distances to sonic events and the tempo of sonotopes defined refuges. Sonic events mirrored the land use and constituted acoustic space according to the distribution of sound sources. Distant sources were suggested as a practical pointer and design tool for overall conditions together with tempo, which is described as the pace of interplay of figures that fade in and out of a background.

Padé approximations for boundary-layer losses in articulatory synthesis

To build an articulatory synthesizer it is necessary to model acoustic propagation in tubes with variable area. Acoustic propagation entails viscous and thermal losses, which are strongest at the vocal tract walls. In the standard boundary-layer approximation irrational frequency laws containing the square root of frequency best represent these losses. Our immediate goal is, given a sound source distribution in the vocal tract, to efficiently calculate the sound output from the mouth. In order to use digital filter theory in this process, a rational approximation to the square root of frequency is sought in the form of Padé approximations. One implementation is obtained by modification of the Kelly–Lochbaum algorithm for calculating wave propagation in a tube, using a high over-sampling rate. However, frequency-dependent loss means that both the reflection coefficients and the time delays through a tube section are non-constant functions of frequency—an assumption used in the Kelly–Lochbaum algorithm. The reflection coefficients are replaced by digital filters and the delay elements by filters with frequency-dependent group velocities. We discuss implementation of a Kelly–Lochbaum algorithm in a digital filter design using a Padé approximation of the viscous and thermal loss terms. [Work supported by NIH Grant No. DC-01247.]

A Study on Aerodynamic Sound Generated from Plates (2nd Report, Effects of Chord-to-Sound Wave Length Ratio on Characteristics of Radiated Sound)

In order to investigate the characteristics of aerodynamic sound generated from plates which have various chord-to-thickness ratios (=2.0∼37.5) and four types of edge shapes consisting of the semi-circular and square, wind tunnel experiments are conducted. Moreover, the sound source distributions of the aerodynamic sound generated from the plates with the square edges are calculated by the 2-dimensional numerical analysis based on the Lighthill's equation with the incompressible flow data. In this study, some basic characteristics of the aerodynamic sound such as (i) the Peak Strouhal number St, (ii) the increase in Peak SPL with the wind velocity U, (iii) the radiation directivity and (iv) the practical critical condition of acoustically compact for chord-to-sound wave length ratio c/λ, are investigated. As a result, it is found that the St depends on the leading edge shape and that the Peak SPL isn't proportional to U6 under the condition that c/λ becomes greater than about 0.3 (≒1/π). Moreover, the Lighthill's sound source near the trailing-edge is the main contributor for the sound field independently of the edge shape. This result leads to a simple model which consists of the dipole source and the rigid plate for the acoustic analysis. We can also derive the same critical condition of c/λ from the sound calculation results of the previous model by the 2-dimensional boundary element method.

Numerical analysis of dipole sound source around high speed trains.

As the maximum speed of high speed trains increases, the effect of aeroacoustic noise on the sound level on the ground becomes increasingly important. In this paper, the distribution of dipole sound sources at the bogie section of high speed trains is predicted numerically. The three-dimensional unsteady flow around a train is solved by the large eddy simulation technique. The time history of vortices shows that unstable shear layer separation at the leading edge of the bogie section sheds vortices periodically. These vortices travel downstream while growing to finally impinge upon the trailing edge of the section. The wavelength of sound produced by these vortices is large compared to the representative length of the bogie section, so that the source region can be regarded as acoustically compact. Thus a compact Green's function adapted to the shape can be used to determine the sound. By coupling the instantaneous flow properties with the compact Green's function, the distribution of dipole sources is obtained. The results reveal a strong dipole source at the trailing edge of the bogie section where the shape changes greatly and the variation of flow with time is also great. On the other hand, the bottom of the bogie section where the shape does not change, or the leading edge and boundary layer where the variation of flow with time is small, cannot generate a strong dipole source.

Measurement of Acoustic Intensity Distribution and Radiation Power of Flat-Plate Phased-Array Sound Source

The acoustic intensity distribution and radiation power of a flat-plate phased-array sound source consisting of Tonpilz-type transducers were measured. This study shows that the active acoustic intensity is skewed in the direction of wave propagation. In addition, it clarifies that if the measurement is carried out in the immediate vicinity of the sound source, the reactive acoustic intensity distribution is effective for identifying the positions of the individual sound source elements. Experimental values of active radiation power agree well with theoretical values. Conversely, experimental values of reactive radiation power do not agree with theoretical values; it is clear that they fluctuate significantly with distance from the radiating surface. The reason for this is explained in the case of a point sound source.

Calculation of diffuse sound reflections in predicting room acoustics using the image method

Numerous investigations have demonstrated that diffuse reflection is one of the most important factors in predicting room acoustics by computer simulation. Recent studies have suggested several computational algorithms in order to account for diffuse reflections in the ray-tracing or beam-tracing method. In this study, a computational algorithm for the calculation of diffuse sound reflections in the image method is suggested and a computer simulation system is developed based on the suggested algorithm. The methodology adopted in our computer simulation system is similar to the extended radiosity method, which was developed for computer graphics. Various descriptions of room acoustics, including spatial distribution of image sound sources, impulse responses and other commonly used room acoustical measures, can be obtained from the MLS based monaural room acoustics measurement system. The measured results in a midsized rectangular classroom with/without chairs on the floor were compared with the predicted results using the computer simulation in which diffuse reflection coefficients of the floor were varied. [Work supported by Korean Research Foundation Grant KRF-1999-1-310-004-3.]

Numerical Analysis of Dipole Sound Source at the Bogie Section of High Speed Trains.

In this paper, the distribution of dipole sound sources at the bogie section of high speed trains are predicted numerically. The three-dimensional unsteady flow around a train is solved by the large eddy simulation technique. The time history of vortices show that the unstable shear layer separated at the leading edge of the bogie section sheds vortices periodically. These vortices travel leeward while growing to finally impinge upon the trailing edge of the section. A compact Green's function adapted to the shape is also given numerically by solving Laplace equation. By coupling the instantaneous flow properties with the compact Green's function, the distribution of dipole sources is obtained. The results show that a strong dipole source is generated at the trailing edge of the bogie section where the shape changes greatly and the variation of flow with time is also great. On the other hand, the bottom of the bogie section where the shape does not change, or leading edge and boundary layer where a variation of flow in time is small, can not generate a strong dipole source.

K-1244 高速鉄道車両下部空力音に関する数値計算(J06-6 流体関連の騒音と振動(6))(J06 流体関連の騒音と振動)

In this paper, the distribution of dipole sound sources at the bogie section of high speed trains are predicted numerically. The three-dimensional unsteady flow around a train is solved by the large eddy simulation technique. The time history of vortices show that the unstable shear layer separated at the leading edge of the bogie section releases vortices periodically. A compact Green's function adapted to the shape is also given numerically by solving Laplace equation. By coupling the instantaneous flow properties with the compact Green's function, the distribution of dipole sources is obtained. The results show that a strong dipole source is generated at the trailing edge of the bogie section where the shape changes greatly and the variation of flow over time is also great.

Analysis of Aerodynamic Sound Source with Measurement of Static-Pressure Fluctuation.

The objective of this investigation is to develop a method for detecting the aerodynamic sound source in a low-Mach number turbulent wake. We therefore introduced coherent output power (COP) in terms of surface pressure and vorticity fluctuation to evaluate vorticity contribution to aerodynamic sound generation. Moreover, aerodynamic sound source was calculated by using a modified Lighthill-Curle's equation ; that is, surface pressure fluctuation was replaced by COP. It was found that the intensity of COP corresponds to the distribution of the aerodynamic sound source caused by unsteady vorticity fluctuation. It therefore more directily reflects the aerodynamic aspects of the sound source. And we measured the distribution of aerodynamic sound source in the wake of a circular cylinder with Reynolds number of 4.0×104. This measurements show that aerodynamic sound is generated near the formation region of Karman vortices. It is therefore concluded that the separated shear layers play an important role in generating aerodynamic sound. Moreover, our identification method by using COP can successfully evaluate aerodynamic properties of a sound source.

Analysis of Aerodynamic Sound Source with Measurement of Static-Pressure Fluctuation.

The objective of this investigation is to develop a method to detect the aerodynamic sound source in the wake of a circular cylinder at Reynolds number 4.0×104. We estimate the intensity of surface pressure which is related to vortex shedding in the wake of circular cylinder by using the coherent output power (COP) in terms of static-pressure and surface pressure on the circular cylinder. Then, we obtain vortices distribution which contribute dipole sound source on the cylinder by measuring COP at various positions in the wake of the circular cylinder. Moreover, we can calculate aerodynamic sound by using the Curle's equation with this estimated surface pressure. We also define the spatial distribution of aerodynamic sound source by means of this method. The result shows the aerodynamic sound generate near the formation region of Karman vortices, that is, X/D=1.5 to 2.0, Y/D=±0.4. Therefore, separated shear flow contributes strongly to the aerodynamic sound generation.

Spatial information in simultaneous multimicrophone recordings of thoracic sounds.

An algorithm for the reconstruction of the distribution of sound sources within the thorax was developed. For any point within the thorax, the algorithm tests the hypothesis that it contains the only relevant source to explain a given set of simultaneous measurements on the chest wall. A hypothetical source signal is calculated by a least‐squares estimation procedure for all points 1 cm apart on a equally spaced grid overlying the thoracic volume. The explained fraction of the total signal variance of all microphones is then displayed as a function of space in a 3‐dimensional grey‐scale coded image. The algorithm was applied to simultaneous lung sound recordings with eight microphones from two healthy male subjects. Preliminary findings suggest that inspiratory sounds are produced more peripherally in the thorax, while expiratory sounds originate more centrally. Some of the images depict features which correlate with anatomical structures such as the large airways, the heart, or during the process of swallowing, the esophagus. A mechanical lung model is being developed to investigate the significance and reproducibility of these images features. [Work supported by Swiss National Research Foundation, Roche Research Foundation, and NSF.]

Analysis of Aeroacoustic Source Distribution by Means of Numerical Simulation.

This study deals numerically with the aerodynamic sound radiated from the flow over a circular cylinder at a Reynolds number of l03. The standard acoustic analogy method is applied. The incompressible time-dependent two-dimensional flow around the cylinder is calculated by means of the finite difference method, then the variables are substituted into the approximate analytic solution of a nonhomogeneous wave equation. The result of far-field sound calculation using a solution which Howe expresses as a volume integral form is compared with that using another one which Curle expresses as a surface integral form. Good agreement is obtained between them. Howe's equation is used to investigate the sound source distribution in the flow field. The sign of the pressure fluctuation radiated from a small sound source region is opposite to those radiated from the neighboring ones. Most of the pressure fluctuations interfere and vanish as a consequence of volume integration. The remaider is observed as far-field sound. It is shown that there are some regions which contribute more significantly to the sound generation than the other regions on average in time.

Analysis of Sound Source Distribution by Means of Large Eddy Simulation.

Analysis of Sound Source Distribution by Means of Large Eddy Simulation.

Ear‐adequate spectral analysis for noise evaluation

In numerous sound situations, sound quality cannot be determined by means of conventional measuring techniques. The frequent discrepancy between subjective judgment of sound events yielded by standard measurement procedures, can be explained by the difference between a sound situation as received by a single omnidirectional microphone, on the one hand, and as received by the human hearing, on the other hand. A new procedure using a dummy head and a special binaural analyzer has been developed for spectral analysis that realizes frequency and time resolution in the entire audio spectral range. This is comparable to the properties of human hearing, which is able to perceive quickly changing time structures and minor frequency differences at the same time. New approaches for ear‐adequate noise evaluation in order to judge noise quality can be achieved by means of psychoacoustic properties of human heating and binaural signal processing. The results of these investigations show that human hearing, which has two auditory paths with a complex directional characteristic and a binaural pattern recognition, gives a different judgment of noise quality as compared to conventional measurement methods, especially in complex sound situations with a spatial distribution of several sound sources and a sound‐pressure level below 85 dB.

Controlled suppression or amplification of turbulent jet noise

As a result of acoustic excitation under very specific tuned conditions, both amplification and suppression of the broadband component of jet noise can be achieved in the same test rig. Associated with the amplification are discrete tones corresponding to the subharmonics of the excitation at the shear layer instability frequency. When jet noise is suppressed, flow visualization indicates that the turbulent mixing is less intense resulting in a general elongation of the jet flow in the axial direction. The observation of reduced jet spreading is confirmed by sound source distribution measurements using a polar correlation technique. Under these conditions, the axial source distribution is shifted downstream by approximately two diameters.

Machinery noise predictions at the design stage using acoustic modelling

This paper presents a simple method of predicting the L eq sound pressure levels in the operator's position for a machine, whilst the latter is still at the design stage. Simple acoustic models are used to estimate mean values for the transfer functions between vibrational response and sound pressure at a point. Given a force input to a structure (machine) and the subsequent vibration response, which can be calculated by finite element or other means, it provides a quick and easy method of accurately predicting overall rms sound levels. The method is applied and provided with data obtained from a full-scale industrial drop hammer. It is applicable to any machine which can be identified as a distribution of separate sound sources.

The Sound Radiated From Gears : 1st report, Behaviour by Means of Acoustical Holography

Acoustic behaviour of a pair of gears, of 156 teeth with 2mm module, is investigated by means of acoustical holography which is capable of indicating the location and the intensity distribution of sound sources. Several improvements on the system have been performed for the practical measurements, such as applying two hologram planes and a complex hologram. The sound of tooth contact frequency (7.88 kHz) is radiated strongly from the meshing point in the meshing direction with about 100 times greater power than that in the axial direction. Natural vibrations of the gear wheel occur in any conditions, and rotate together with the gear. The sound is produced according to the vibration from a spatially fixed viewpoint, thus the vibration causes two frequency sounds around the natural frequency instead of itself.

Further Studies of the Aeroacoustics of Jets Perturbed by Screens

It is observed that the placement of a screen across the exit plane of a jet nozzle results in a substantial reduction in noise intensity over what would be realized with an equivalent unperturbed jet. In the past, the general features of this phenomenon were analyzed experimentally utilizing both farfield pressure measurements and velocity (hot-wire) measurements. This investigation has been extended to include the measurement of nearfield pressure along with a more extensive program of hot-wire measurements. Nearfield pressure measurements were made with a static pressure probe developed previously. These data were used to determine the distribution of sound sources in the screen perturbed jet. The results obtained from nearfield pressure measurements gave comparable information about the effect of the screen on the acoustics sources as was obtained with hot-wire measurements and theory. Extensive hot wire data indicate that there is a major change in the characteristics of the flow field of a screen perturbed jet. The potential core in the mixing region extends farther downstream and turbulent intensities are reduced to a lower level. The strength of the quadrupole sources (assumed proportional to Reynolds stresses) has changed from a maximum value of 0.011 to less than 0.008 near the center of the mixing region. Theory indicates that the acoustic power is also proportional to the fifth power of the longitudinal scale of the flow. Through correlation techniques, a reduction in the length scale of the perturbed jet is observed. The results of the nearfield study are correlated with farfield observations.

Developments of a Generalized Theory of Jet Noise

Following a generalized theory of supersonic jet noise, explicit analytical expressions for identifying jet noise mechanisms and numerical calculations of high-speed jet noise characteristics have been obtained. This new theory is uniformly valid in the transonic and supersonic speed ranges, and it includes implicitly refraction and convection of sound through the jet flow. Given results include the transition from U-law to the U-law, differences in directivity for high and low frequencies, sound source distribution, noise spectrum, and the coupled effects of refraction and convection. The implications of these results and their correspondences with results in the existing literature are discussed.

Distribution of Sound Sources in a Jet Stream

It is shown that the axial distribution of sound sources in a turbulent jet stream can be found simply in terms of (1) the spectrum of the total radiated power and (2) the frequency of the sources as a function of location along the jet axis. With the use of existing data on the sound power spectrum and recently reported but preliminary data on the frequency dependence of the most probable source location, an approximate axial distribution is derived. The derived distribution is in agreement with calculations by Sanders based on turbulence measurements, and with calculations by Ribner based on Lighthill's theory.