Incoherent Monopole Research Articles

Analysis of source contribution to pass-by noise for a moving high-speed train based on microphone array measurement

Microphone array measurement processed using the delay-and-sum beam-forming and de-convolution techniques is commonly performed to identify and quantity noise sources on a moving train. Results from such a measurement may be employed to analyse contributions of the sources to pass-by noise of the train, proving directions for pass-by noise control and reduction. In order to carry out such an analysis, a source contribution model is developed in this paper. This model is established by assuming that noise sources on the train are incoherent monopole sources, and applying the frequency spectrum generated by a harmonic monopole source moving uniformly in a free-space. The model is validated against a set of measurements for a high-speed train running along a track on viaduct at different speeds.

High-resolution acoustic localization of changes in spatially-distributed coherent sources for structural health monitoring.

Localization of acoustic sources is a common remote sensing goal. When multiple sources are present and coherent, high-resolution localization typically becomes more challenging. The spectral estimation method with additive noise (SEMWAN) is an existing technique for high-resolution localization of incoherent monopole sources in low-signal-to-noise environments. SEMWAN utilizes a reference measurement to incoherently suppress background noise, but its performance suffers in applications involving spatially-distributed coherent sources, such as like a vibrating plate. However, by subtracting a reference measurement and using subarray averaging, SEMWAN can be extended to localization of small changes in distributed coherent sources. This revised approach, the spectral estimation method with coherent background removal (SEMCBR), permits remote acoustic localization of damage in a vibrating structure. A simple multi-source experiment using an 8-by-8 planar square microphone array with 6-cm spacing in both horizontal directions was performed to validate SEMCBR at a frequency of 5.0 kHz. Additional SEMCBR experimental results are reported for the same array placed 1.0 m above a 30 cm × 30 cm vibrating plate with and without damage. Cuts, boundary failures and delamination of a composite plate were successfully located with conventional spherical-wave beamforming and SEMCBR using a 0.1 to 6.0 kHz bandwidth. However, SEMCBR provides 4 to 6 times better resolution.

Coherence of radiation from incoherent sources: II. Ball and disk sources and coherence of sunlight.

Following the development of the companion paper [J. Opt. Soc. Am.36, 1433 (2019)JOAOD60740-323210.1364/JOSAA.36.00XXXX], additional models for the coherence of quasi-monochromatic light are constructed by placing incoherent monopole and dipole sources inside a spherical ball and on a circular plane disk. The paraxial approximation, which is valid for sunlight, reduces the number of different models from eight to three. Based on the solar images corresponding to these three models, the ball model is considered the most appropriate. The ball model can be modified to match the limb-darkening observations.

Coherence of radiation from incoherent sources: I. Sources on a sphere and far-field conditions.

Models for the coherence of quasi-monochromatic light from spherical incoherent sources are constructed by placing incoherent monopole and dipole sources on the surface of a sphere. All models allow for relatively simple numerical calculations of coherence functions for arbitrary source sizes and positions of observation points. We show analytically and confirm numerically that the far-field regime for transverse coherence is formed at distances larger than the source size, regardless of the wavelength.

Self-similarity of level-based wavepacket modeling of high-performance military aircraft noise

Construction of the equivalent acoustic source model for high-performance military aircraft noise presented in this paper begins with a decomposition of sound levels, from ground-based microphones 11.7 m from a high-performance military aircraft, into portions matching the similarity spectra associated with large-scale and fine-scale turbulent mixing noise (LSS and FSS). [Tam et al. AIAA Paper 96-1716 (1996)]. A Fourier transform of the spatial distribution of the decomposed levels yields frequency-dependent wavenumber spectra [Morris, Int. J. Aeroacoust. 8, 301-316 (2009)]. Each LSS-educed wavenumber spectrum corresponds to a wavepacket, consisting of a spatially varying amplitude distribution with a constant axial phase relationship. This wavepacket model produces a directional, coherent sound field. However, the asymmetry in the LSS-educed wavenumber spectra may indicate that a nonuniform phase relationship is a more physical choice that could be correlated to the axial variation in convective speed. The FSS component is modeled with a line of incoherent monopoles whose amplitude distribution is related to the FSS-educed wavenumber spectrum. While this level-based model is limited, the addition of the FSS-related source yields a better match to the noise environment of a high-performance military aircraft than is found using a single coherent wavepacket. [Work supported by ONR.]

Multimode radiation from an unflanged, semi-infinite circular duct with uniform flow

Multimode sound radiation from an unflanged, semi-infinite, rigid-walled circular duct with uniform subsonic mean flow everywhere is investigated theoretically. The multimode directivity depends on the amplitude and directivity function of each individual cut-on mode. The amplitude of each mode is expressed as a function of cut-on ratio for a uniform distribution of incoherent monopoles, a uniform distribution of incoherent axial dipoles, and for equal power per mode. The directivity function of each mode is obtained by applying a Lorentz transformation to the zero-flow directivity function, which is given by a Wiener-Hopf solution. This exact numerical result is compared to an analytic solution, valid in the high-frequency limit, for multimode directivity with uniform flow. The high-frequency asymptotic solution is derived assuming total transmission of power at the open end of the duct, and gives the multimode directivity function with flow in the forward arc for a general family of mode amplitude distribution functions. At high frequencies the agreement between the exact and asymptotic solutions is shown to be excellent.

Separating Turbofan Engine Noise Sources Using Auto- and Cross Spectra from Four Microphones

The study of core noise from turbofan engines has become more important as noise from other sources such as the fan and jet were reduced. A multiple-microphone and acoustic-source modeling method to separate correlated and uncorrelated sources is discussed. The auto- and cross spectra in the frequency range below 1000 Hz are fitted with a noise propagation model based on a source couplet consisting of a single incoherent monopole source with a single coherent monopole source or a source triplet consisting of a single incoherent monopole source with two coherent monopole point sources. Examples are presented using data from a Pratt& Whitney PW4098 turbofan engine. The method separates the low-frequency jet noise from the core noise at the nozzle exit. It is shown that at low power settings, the core noise is a major contributor to the noise. Even at higher power settings, it can be more important than jet noise. However, at low frequencies, uncorrelated broadband noise and jet noise become the important factors as the engine power setting is increased.

Beamforming for aircraft noise measurements

Phased array beamforming for aircraft noise source location has a long history, including early work on jet noise, wind tunnel measurements, and flyover testing. In the last 10 years, advancements in sparse 2-D and 3-D arrays, wind tunnel test techniques, and computer power have made phased array measurements almost common. Large aerospace companies and national research institutes have an advantage in access to major facilities and hundreds of measurement microphones, but universities and even consulting companies can perform tests with electret microphones and PC data acquisition systems. The type of testing remains a blend of science and art. A complex noise source is approximated by a mathematical model, and the microphones are deployed to evaluate the parameters of the model. For example, the simplest, but often the best, approach is to assume a distribution of mutually incoherent monopoles. This leads to an imaging process analogous to photography. Other models include coherent distributions of multipoles or duct modes. It is sometimes important to simulate the results that would have been obtained from single microphone measurements of part of the airplane in an ideal environment, had such measurements been feasible.

Multi-mode sound transmission in ducts with flow

Exhaust mufflers, large exhaust stacks, and turbofan engines are common examples of ducted noise. The most useful measure of the sound produced by these noise sources is the sound power transmitted along the duct. When airflow is present, sound power flow can no longer be uniquely determined from the usual measurements of acoustic pressure and particle velocity. One approach to sound power determination from in-duct pressure measurement, and the one discussed in this paper, is to predict the relationship between the sound power and pressure based upon an assumed mode amplitude distribution. This paper investigates the relationship between acoustic pressure and power for a family of idealized source distributions of arbitrary temporal and spatial order. Incoherent monopole and dipole sources uniformly distributed over a duct cross-section can be obtained as special cases. This paper covers the sensitivity of the pressure–power relationship to source multipole order, frequency and, in particular, flow speed. It is shown that the introduction of flow in a hard-walled duct can have a substantial effect on the behavior of the pressure–power relationship for certain source distributions. Preliminary experimental results in a no-flow facility are presented in order to verify some of the main results.

Visualization of multiple incoherent sources by the backward prediction of near-field acoustic holography.

When there are multiple noise sources which share the same frequency bands, a sound field measured or estimated by near-field acoustic holography (NAH) is obviously the combination of the sound fields generated by the multiple sources. The objective of this paper is to estimate the sound field generated by each source when the coherence functions between sources are zeros. This objective can be achieved by obtaining signals coherent to sources. This paper proposes a method to obtain the coherent signals by using the spatial information of NAH. The proposed method obtains the coherent signals from pressure signals at source positions estimated by the backward prediction of NAH. Thus it does not require any prior information on source positions, unlike the conventional method, which placed sensors close to sources in order to obtain the coherent signals. The proposed method was verified by a numerical simulation using two incoherent monopole sources.

Multimode radiation from an unflanged, semi-infinite circular duct

Theoretical expressions for sound radiation from a single incident duct mode, arriving at the open end of a semi-infinite circular unflanged duct with rigid walls, are used to obtain numerical results for (1) the single-mode sound power transmission coefficient, and (2) the multimode far-field directivity factor. For the multimode calculations the modes are assumed incoherent, and a weighting model is adopted which includes, as special cases, equal power per mode (above cutoff), and excitation by incoherent monopoles or axial dipoles uniformly distributed over a duct cross section. High-frequency asymptotic features of the results are explored in detail and analytical approximations are given. The findings have practical application to sound power measurement from tall exhaust stacks.

A rectangular plane sound source

This paper deals with the problem of calculation of the sound field generated by a rectangular plane sound source which can be substituted by an infinite set of incoherent monopole point sound sources, and with the possibility of applying the models derived under these assumptions to the prediction of sound fields radiated by building elements such as windows, etc. Verification of the published models was made by means of numerical integration. The discrepancy between the calculations according to these models and the results of integration led to the derivation of a new and more accurate model describing the sound field of this source by an analytical expression. The practical applicability of the derived model is illustrated by two examples.

Active sound intensity precision in a progressive and nonprogressive field

The work is mainly dedicated towards the precision of the measurement of active sound intensity. The object is to describe the basis used to understand what is happening when measuring the sound power via the active part of sound intensity whatever the field is. The effects of acoustic center variation, of the phase mismatch, of the proximity of the source, of finite approximations, and of angle of incidence of the incoming wave are all combined in a general formulation, valid for the progressive field. The nonprogressive field is studied by calculating the estimated active sound intensity due to two monopoles. The case of coherent and incoherent monopoles are examined. A general formulation of the resulting amplitude and phase is developed and emphasis is put on a systematic parametric study of the theoretical phase gradient. The parameters are mainly the sound power ratio between the two sources, distance ratio, propagation difference, angle of incidence, and frequency. A formulation which allows us to calculate the precision between the estimated active sound intensity and the exact one is proposed. A criteria for evaluating the degree of nonprogressivity of the field will be discussed. [Work supported by the “Institut de Recherche en Santé et Sécurité du Travail.”]