Acoustic Intensity Measurements Research Articles

Automatic measurement of spatial acoustic intensity.

A method automatically of measuring acoustic intensity is presented in this paper. This method uses measurements of the cross spectrum of the sound pressures at two closely spaced microphones. The two microphones used are arranged side by side and slid back and forth by a small distance in the direction of the microphone axes which are symmetric with respect to the holder axis. The holder can be turned on its axis by steps of a fixed angle for measuring the three directional components of an acoustic intensity vector. The acoustic intensity at a field point can be automatically measured by microphones arranged in this manner. This method is actually applied to the visualization of the flow pattern of sound energy emitted from a rectangular plate vibrating in resonance modes. As a result, an acoustic intensity probe composed of two microphones, as used in this study, has been proved to be an appropriate sensor for the automatic measurement of spatial acoustic intensity vectors.

Precision of sound power measurements with intensity technique

The precision of the sound power determination by measuring acoustic intensity depends on the precision of the measurement system, selection of the measurement area, and the number of measurement points. The decision on the optimum choice of these parameters depends on the properties of the radiated sound field and on the environmental effects including simultaneous sound radiation from other sources. To obtain qualifying information on the sound field properties, the measurement of the complex acoustic intensity may be necessary. To quantify the judgment on the measurement parameter selection, a series of indicators has been recently suggested. This presents an analysis of the mutual effects of the measurement parameters and their effect on the precision of the sound power determination.

CETIM schedules series of short courses on acoustic intensity and noise measurements

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Particle velocity detection using a thin membrane

Underwater acoustic intensity measurements require the detection of both the acoustic pressure and the fluid particle velocity. The measurement of the acoustic pressure is easily obtained using conventional (pressure‐type) hydrophones. The particle velocity may be obtained by measuring the flexural response of an appropriate membrane (constrained by an inertial structure) to the fluid particle velocity. Several thin (50–300 μm) membranes were constructed from materials which closely match (impedance, density) the acoustic medium. The use of low shear speed materials allowed for the construction of small (< 10 cm) membranes which are resonant (fundamental mode) to low‐frequency fluid oscillations. The response of such membranes to the acoustic particle velocity and pressure is reported in the 0.02 to 0.5 ka range.

Automatic measurement of spatial acoustic intensity.

A method of measuring acoustic intensity automatically is presented in this paper. The method uses measurements of the cross spectrum of the pressures at two closely spaced microphones. The two microphones used are arranged side by side with a short distance between their axes, and combined symmetrically with a holder axis. The holder can be turned on its axis by steps of a constant angle for measuring three directional components of an acoustic intensity vector. The acoustic intensity at a field point can be automatically measured by such composed microphones. This method is actually applied to a visualization technique of the flow pattern of sound energy emitted from a resonant modes. As a result, an acoustic intensity probe composed of two microphones, as used in this study, has been proved to be an apprepriate sensor for the automatic measurement of spatial acoustic intensity.

Jet noise source location via acoustic intensity

It is well known that sources of jet noise are described by a variety of models; thus there is no unique source distribution. If no assumptions are made about the nature of the sources, then one can infer virtual source distributions from certain farfield measurements. Here, however, the source locations are deduced directly from acoustic intensity measurements near an axisymmetric jet flow. The magnitude and direction of the acoustic intensity vector are found to be functions of position and source frequency. Acoustic intensity is measured along three directions in a plane of the jet axis; data are analyzed in 1/3 octave bands. The measurements are performed sufficiently far from the jet flow to avoid air flow over the probe. Clearly, the resolution of the source location scheme suffers with increasing probe-source separation: Thus a compromise is struck. The measured data permit the reconstruction of certain properties of the acoustic farfield and appear to demonstrate closure. [Work funded by Natural Sciences and Engineering Research Council of Canada.]

Acoustic intensity measurement apparatus and method including probe having ambient noise shield

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Effect of turbulent boundary layer flow on acoustic intensity measurements

This paper is based on one given at a previous Acoustical Society Meeting [G. C. Lauchle, J. Acoust. Soc. Am. Suppl. 1 75, S53 (1984)]. Comments raised during this presentation prompted the author to rework the analysis on intensity measurements. The conclusions are different from those stated previously; namely, acoustic intensity measurements are affected only slightly by the presence of a turbulent boundary layer flow. The analysis shows that if Δx > Λ, the intensity estimate should be unbiased. Here Δx is the probe separation in the streamwise direction and Λ is the streamwise correlation length of the pressure‐producing motions of the turbulent layer. It is pointed out also that Λ is inversely proportional to frequency, so a bias can exist in low‐frequency measurements. [Work supported by NAVSEA.]

Possible use of acoustic intensity measurement in standard tests of vehicle noise

Existing standard tests of motor vehicle noise, such as passby tests, are based on sound pressure measurements with single fixed‐location microphones. Recent developments in two‐microphone acoustic‐intensity techniques, and in instrumentation, permit the practical measurement of the sound power of vehicles and of components, such as engines. This can be performed in a normal test environment; a special acoustical facility such as an anechoic chamber or a reverberation room is not required. Sound power, which is a single‐number measure of the total output of a noise source, appears better suited to determining the overall effect of a vehicle on community noise. Also sound power is usually measured indoors, thereby avoiding scheduling delays due to bad weather and the inherent variability in the data associated with outdoor testing. Good repeatability (i.e., a standard deviation less than 5% or 0.2 dB) has been demonstrated in the measurement of the sound power of a medium‐duty truck, using acoustic intensity methods in an indoor test. Similar repeatability has been demonstrated in the measurement of the sound power of an engine for a heavy‐duty truck. These and other advantages and disadvantages of the use of acoustic intensity measurement methods for standard tests are discussed in the presentation.

Application of the fiber optic lever in surface vibration and acoustic intensity measurements

Renewed interest in optic motion sensors has been prompted by the potential application of such techniques to surface vibration and acoustic intensity measurements. Optical sensors intrinsically possess one highly desirable attribute, that is, no loading of the structure, and possibly several others including high resolution and wide bandwidth. Of the available optic sensors, the fiber optic lever is perhaps the simplest practical approach. The characteristics of the fiber optic lever are such that the principal parameters, i.e, linear range, resolution, bandwidth, and working distance can be optimized to meet the constraints imposed by most surface vibration problems of interest. The key strategy in optimizing the lever is to obtain sufficient linear range to encompass low‐frequency, high‐amplitude vibrations while retaining sufficient resolution for small amplitude high‐frequency motions. When sufficient reflected light is available to meet these requirements, wide bandwidth and excellent phase characteristics can be achieved by presently available electro‐optic devices. The potential for a device permitting hand held “sweeps” of complicated noise radiators exists but is critically dependent on achieving extended linear range. The extent to which these requirements have been achieved in instruments optimized for acoustic intensity measurements close to the vibrating surface is described.

Acoustic intensity measurements at the surface

A surface acoustic intensity meter is described for measuring acoustic intensity at a vibrating surface using a fiber optic lever as a displacement transducer and a condenser microphone to measure surface pressure. Digital signal processing using a two‐channel FFT analyzer allows the calculation of surface intensity as a function of frequency. The technique has been tested in an anechoic chamber on a point monopole and a steel plate clamped at the edges. Calculated sound power levels are compared with results obtained by traditional techniques.

Effect of turbulent boundary layer flow on measurement of acoustic pressure and intensity

Acoustic measurements performed with pressure sensors (microphones and hydrophones) can be subject to error if the sensors are in contact with a turbulent boundary layer flow. These errors are a result of the random pressure fields (flow noise) generated by the turbulent motions of the fluid. In this paper, a simple analysis is given whereby the flow noise response of a pressure sensor placed in a turbulent boundary layer flow can be estimated. If the purpose of the sensor is to measure the sound emitted from a source outside of the turbulent boundary layer, then a bias error for the measurement can be calculated. The error formula is equally applicable to intensity measurements made with the two‐sensor technique, providing the mean flow Mach number is small. Example calculations are presented. [Work supported by NAVSEA 63R‐31.]

Measurement of the complex acoustic intensity, the kinetic energy, and the potential energy in a reflective environment

A four microphone intensity probe array has been designed and used to measure two components of the active and reactive acoustic intensity vector. The probe is also used to measure two components of the kinetic energy and the potential energy. Measurements of a point source above a reflecting plane and near the edge of two large reflecting planes were made. These results will be presented and the effects of sensor spacing, probe rotation, proximity of probe to source and boundary, and sensor phase error will be shown and the results will be compared to theoretically predicted values.

A review of the in‐duct method for measuring acoustic intensity and acoustic properties of materials and systems

The decomposition theory [Seybert and Ross, J. Acoust. Soc. Am. 61, 1362–1370 (1977)] is reviewed and some new results applicable to problems in measuring acoustic intensity and acoustic properties in plane‐wave ducts are presented. Specifically, it is shown that formulations for the net acoustic intensity and for the so‐called transfer function approach of measuring acoustical impedance proceed directly from the general decomposition theory. Potential sources of signal processing and instrument errors are discussed, as are methods for instrumentation phase calibration. Experimental data showing the application of the decomposition technique to acoustic intensity and property measurement are presented.

The use of radiation efficiencies in the diagnosis of noise sources

Using the recently developed two‐microphone technique of acoustic intensity measurement, radiation efficiencies of a single panel and individual panels of a complete vehicle structure have been made under controlled, laboratory conditions. The data obtained from the single panel were used to develop measurement techniques and study the sound field near the panel. Radiation efficiencies of the vehicle structure were used along with measured vibration response obtained under normal operation conditions to determine the sound power produced by each individual panel. Results describing the technique development and the application to the vehicle structure are presented in this paper.

Acoustic intensity from manganese nodule deposits

Abundant information on manganese nodules in the deep ocean can be obtained from remote acoustic intensity measurements. The previously suggested low‐frequency reflectivity measurements, neglecting the incoherent part of the backscattered acoustic signals, are quite good in predicting the presence of these nodules. However, in order to judge the effect of the size distribution on the acoustic response and to get the mean nodule size information, a wider frequency range is required. The contribution of the incoherent part of the acoustic response to the backscattered intensity becomes increasingly important as the sounding frequency gets higher. One may actually under‐, or over‐, estimate the amount of, and the average size of, the manganese nodules by carelessly neglecting the incoherent acoustic intensity. The intensity calculation based on the energy principle for nonabsorbing scatterers will be discussed, and the results will be presented for different concentrations of nodules with uniform as well as Rayleigh size distributions. All results presented shall be for sparsely distributed nodules made using the single scattering theory. The future extension to highly concentrated nodule fields by using the multiple scattering theory will also be discussed.

Effect of Turbulent Boundary Layer Flow on Measurement of Acoustic Pressure and Intensity

Acoustic measurements performed with pressure sensors (microphones and hydrophones) can be subject to error if the sensors are in contact with a turbulent boundary layer flow. These errors are a result of the random pressure fields (flow noise) generated by the turbulent motions of the fluid. In this paper, a simple analysis is given whereby the flow noise response of a pressure sensor placed in a turbulent boundary layer flow can be estimated. If the purpose of the sensor is to measure the sound emitted from a source outside of the turbulent boundary layer, then a bias error for the measurement can be calculated. The error formula is equally applicable to intensity measurements made with the two‐sensor technique, providing the mean flow Mach number is small. Example calculations are presented. [Work supported by NAVSEA 63R‐31.]

Effect of the sound field structure on the measurement of acoustic intensity

The effect of instrumentation phase and magnitude error on the measurement of the active and reactive acoustic intensity for different sound fields will be discussed. We will show experimental results of measurements made for the active and reactive intensity in some semireverberant fields. An easy method to calculate the effects of phase bias will be presented and demonstrated for the experimentally measured data. The measured results will be compared to theoretically predicted values and the measuring system will be described.

Acoustic intensity measurements in the reflective environment

The two-microphone intensity method was used to investigate the intensity distribution and radiated power of several sources. The sources used were two open pipes of 30-mm and 95-mm diameter and an aluminum circular plate of 100-mm diameter. The pipes and plate were mounted with baffles. The investigations were confined to the low-frequency range of plane-wave propagation inside a pipe. The experiment was performed in the nearfield and farfield of the sources in the reflective environment. The theoretical and experimental results will be discussed for both intensity and radiated power. The influence of the source impedance will be analyzed for the intensity measurements, particularly at points located close to the source. The intensity error that can occur due to the partial absorption of the reflected waves by the source will be discussed.

Measurement of acoustic intensity in reverberant sound fields

The analysis of acoustic intensity measurement in propagating unbounded acoustic fields has been widely investigated in the last five years. There has, however, been very little published on the measurement of acoustic intensity in reverberant environments. It is the object of this talk to present some basic theoretical and experimental results that pertain to the nature of understanding sound energy flow in reverberant and specialized active acoustic fields. Measurements of a source in a reverberation room will be shown for distances of up to four times the critical distance from the source. The effects of bias and random errors in context to reverberant fields will be discussed and related to the experimental results presented.