Acoustic Intensity Measurements Research Articles

Application of the Boundary Element Method and Modal Analysis to Tire Acoustics Problems

Abstract Road traffic noise needs to be reduced, because traffic volume is increasing every year. The noise generated from a tire is becoming one of the dominant sources in the total traffic noise because the engine noise is constantly being reduced by the vehicle manufacturers. Although the acoustic intensity measurement technology has been enhanced by the recent developments in digital measurement techniques, repetitive measurements are necessary to find effective ways for noise control. Hence, a simulation method to predict generated noise is required to replace the time-consuming experiments. The boundary element method (BEM) is applied to predict the acoustic radiation caused by the vibration of a tire sidewall and a tire noise prediction system is developed. The BEM requires the geometry and the modal characteristics of a tire which are provided by an experiment or the finite element method (FEM). Since the finite element procedure is applied to the prediction of modal characteristics in a tire noise prediction system, the acoustic pressure can be predicted without any measurements. Furthermore, the acoustic contribution analysis obtained from the post-processing of the predicted results is very helpful to know where and how the design change affects the acoustic radiation. The predictability of this system is verified by measurements and the acoustic contribution analysis is applied to tire noise control.

Flame radiation and acoustic intensity measurements in acoustically excited diffusion flames

This article describes results of an investigation of the mechanisms responsible for the driving of axial instabilities in solid propellant rocket motors. Specifically, the results of two different experimental methods for measuring the driving of axial acoustic fields by sidewall stabilized diffusion flames are compared. These methods utilize laser Doppler velocimetry (LDV) and CH flame radiation measurements, respectively. The results reveal that the interaction between the acoustic field and the flame produces a space-dependent, oscillatory normal velocity component in the flame region, and oscillatory reaction-and heat-release rates, which depend upon the excited acoustic field. The results suggest that the flame either drives or damps the acoustic fields, depending upon the amplitude and frequency of the excited acoustic wave. In addition, these studies show that the flame driving processes vary within the flame region. The results obtained from the flame radiation measurements are in excellent agreement with those obtained using the velocity measurements.

Case History: Acoustic Diagnostics of an Automotive HVAC System

Abstract : The heating, ventilation, and air conditioning (HVAC) system of an automobile can be a source of acoustic annoyance, particularly when it is operated under maximum airflow conditions. In order to suggest possible design changes for the purpose of noise control, a systematic characterization of the acoustic sources and mechanisms must be conducted initially. This paper addresses such a characterization for a typical automotive system. The approach is based on acoustic intensity measurements of the stand-alone HVAC system operating under maximum airflow conditions in an anechoic chamber. The experimental data indicate that the centrifugal blower is the dominant source of noise, particularly at low frequencies, while separation zones, the flow over sharp edges within the HVAC system, louver flows, and flow-induced casing vibration are secondary sources of noise that become increasingly important as the frequency approaches 3 kHz. Qualification of these identified sources of sound is aided by detailed flow visualizations conducted underwater on an optically clear HVAC system utilizing similitude.

Acoustic intensity measurement in the presence of mean flow with application to noise measurements in a wind tunnel

Wind tunnels are frequently used in the experimental study of moving sound sources. Although a number of techniques are currently used to determine the radiation characteristics of a sound source in a wind tunnel, they are usually limited by a lack of explicit directional information and/or a requirement that measurements be made in a free field. The multiple microphone acoustic intensity technique provides explicit directional information and can be used to measure sound fields in the near field of a source and, therefore, represents a possible alternative to the current measurement techniques used in wind tunnels. The formulations currently used to measure acoustic intensity are limited to either zero flow or plane waves in a one-dimensional mean flow. A new formulation has been developed that extends the method to the case of a spherically spreading wave in a uniform one-dimensional flow. Using an analytical simulation, the intensity distributions for a monopole and dipole source are studied. Initial results suggest that the formulation is suitable for the analysis of compact sources. Although a more detailed study will be required to develop a practical measurement tool, this study provides additional insight into the problem of intensity measurement in the presence of mean flow.

Near-field identification of vibration sources, resonant cavities, and diffraction using acoustic intensity measurements

Near-field acoustic intensity measurements are a powerful analysis and diagnostic tool when all available measured quantities are utilized. The advantage of considering the reactive intensity and complex specific acoustic impedance along with the active intensity is stressed. An added feature to the data presentation is the pressure phase that is reconstructed from the time-independent measured data. The active intensity plots are thus enhanced by superimposing the lines of constant phase so that the wave-front propagation is visualized. In the first measurement configuration, an active source absorbing power is differentiated from a resonant cavity absorbing power and diffraction is localized. In the second configuration, the mode shape of a vibrating plate is identified.

Acoustic diagnostics of a HVAC system

The heating ventilation, and air conditioning (HVAC) system of an automobile can be a source of acoustic annoyance, particularly when it is operated under maximum airflow conditions. In order to suggest possible design changes for the purpose of noise control, a systematic characterization of the acoustic sources and mechanisms must be conducted initially. This presentation addresses such a characterization for a typical system. The approach is based on acoustic intensity measurements of the stand‐alone HVAC system operating under maximum airflow conditions in a flow‐through anechoic chamber. The experimental data indicate that the centrifugal blower is the dominant low‐frequency source of noise, while separation zones and the flow over sharp edges within the HVAC system ducting are secondary sources of noise that become increasingly dominant as the frequency increases into the kHz range. Qualification of these identified sources of sound is aided by detailed flow visualizations of the subject system. [Work supported by Ford Motor Co.]

Validation of acoustic intensity measurements for power transformers

The purpose of this study is to demonstrate and deploy acoustic intensity for in-situ sound power measurements of power transformers and to establish the technical basis for using acoustic intensity techniques for measuring transformer noise. Sound intensity is a new technology using two closely-spaced and phase-matched pressure-sensitive microphones to approximate one component of the intensity vector. The limitations as they apply to power transformer noise measurements are described. Both the laboratory and the field measurements support the claim that sound intensity has applications in the power distribution industry as a tool for quantifying noise, both on the factory floor and in the field. >

Measurement of the complex reflectivity of submerged thin plates at oblique incident angles using the two‐sensor intensity technique

Abstract : This study has involved the adaptation of the two-sensor acoustic intensity measurement method to an underwater environment. The intensity technique involves measuring the acoustic pressure at two closely spaced points. The average of the pressures approximates the pressure at the midpoint of the two sensors while the difference between the pressures and the application of the Euler's equation leads to an approximation for the acoustic particle velocity at the midpoint between the two sensors. These two quantities, the acoustic pressure and the acoustic particle velocity, are all that is needed to approximate the acoustic intensity and the acoustic impedance. The objective of this research was to develop the system and use it to measure the complex reflectivity of submerged thin plates at non-normal angles of incidence. The two-sensor technique has advantages over more conventional methods of measuring the reflectivity in that the sensors need only be positioned once during the test. This leads to a savings in effort and time for the experimenter. The source of acoustic energy used to ensonify the plates took form of a parametric or virtual and fire array. The parametric array is formed by exciting a transducer with a high frequency carrier signal which has been amplitude modulated with a low frequency signal. (jhd)

Stationarity of acoustic intensity measurements on steel plates

The problem of identifying reproducible frequency bands useful for the nondestructive evaluation of steel plates using the dynamic acoustical intensity scanning concept (DAIS) is addressed. Reproducible frequency bands are essential so that subsequent changes in sound intensity patterns can confidently be attributed to structural changes rather than to random fluctuations. A number of evaluation methods are tried. Stationary frequency bands are identified by all methods in the range of 100–4000 Hz. The most sensitive and practical evaluation method is deemed to be the comparison of intensity versus time data collected separate from the plate scans. This method is accurate, convenient, and can be automated to exclude all unwanted frequency bands during testing. The remanent intensity factor technique, while useful in identifying stationarity, is less sensitive and more difficult to implement.

Errors in the automatic measurement of spatial acoustic intensity in the direction of sound energy propagation with a rotating probe

For the automatic measurement of spatial acoustic intensity the authors have devised a rotating probe comprising two microphones, which are arranged in parallel with the axis of the probe's holder and shifted by a minute distance from each other in the direction of the microphone axis. If measurements are carried out, once before any rotation of the probe, and twice at two different predetermined angles of its rotation around the holder axis, then it is possible to determine easily the three components of an acoustic intensity vector in the corresponding three co-ordinate axis directions. Theoretical and experimental investigations are presented of the possible error in automatic measurement of spatial acoustic intensity in the direction of sound energy propagation with such a probe. On the basis of the theory of measurement with a rotating probe and the fundamental error theory of the acoustic intensity method, it is possible to calculate the theoretical error in the direction of sound energy propagation. The results of calculations of this error agree well with those of measurements, and it is shown that the measurement error in the direction of sound energy propagation depends upon the frequency of the sound wave as well as on the incidence angle on the probe. These results have made it possible to determine the measurement error limit in the direction of sound energy propagation.

An acoustic intensity measurement simulation for evaluating sound power determination standards

Work has been done for several years on the standardization of sound power determination using acoustic intensity measurements. The validation of the standards through testing is difficult because of the large number of measurement conditions to be tested. To help overcome this problem, a computer code simulating the measurement has been developed. Whereas past attempts have been limited to special cases, the developed code is intended to handle cases that integrate more of the industrial reality. The measured noise sources as well as the extraneous noise sources are modeled by any number of monopoles for which the user specifies their quantity, source strength, location, relative phase, and degree of coherence. This approach offers computational simplicity as well as the possibility to model realistic radiation patterns such as plate radiation. The proposed ISO standard (ISO/DP 9614) is evaluated for a plate radiating in the presence of background noise. Recommended corrective actions were found appropriate for farfield measurements and inapropriate for nearfield measurements.

Broadband acoustic holography reconstruction from acoustic intensity measurements. I: Principle of the method

A method, broadband acoustic holography from intensity measurements (BAHIM), is developed for the reconstruction of sources using nearfield acoustic intensity measurement. Nearfield acoustic holography (NAH) is used, but data concerning complex pressure on the hologram are not acquired directly. These data are obtained by processing measurements of active intensity and potential energy density (quadratic pressure). These quadratic values are measured independently at each point on the hologram and therefore do not require a phase reference signal corresponding to the source. This method is thus applicable to sources emitting a broadband spectrum and, more generally, to industrial sources, for which the excitation is not known. The principle of the method is described and the advantages of this type of processing are illustrated by examples.

Avoiding pitfalls in mastering sound intensity measurements

Sound or acoustic intensity measurements yield results that were previously impractical or, in some cases, not feasible to obtain from measurements of sound‐pressure level. Unfortunately, these techniques are also more sophisticated and present more opportunities to get erroneous results. Errors of 10 dB and more are not uncommon, nor is it rare to obtain irrelevant data. Because intensity is measured indirectly, these errors are often not obvious. To get results sufficiently close for engineering purposes, a considerable investment in instrumentation, education, and practice is required. This paper suggests how to begin the process of mastering intensity technology, including suggestions for initial reference materials. A series of experiments is suggested not only to develop proficiency, but to verify validity of the results determined. Examples are given to illustrate limitations of intensity techniques.

Accurate estimation of amplitude, frequency, and phase of harmonic signal components using an FFT

The signal analyzed using an FFT is seldom identical to the true signal. This can result in inaccurate estimates of amplitude, frequency, and phase of harmonic signals. Using an extension of a method described earlier [J. C. Burgess, “On digital spectral analysis of periodic signals,” J. Acoust. Soc. Am. 58, 556–567 (1975)], it is possible to estimate the true amplitude, frequency, and phase of a harmonic signal with good accuracy. For example, acoustic intensity measurements may require determining the phase difference between two harmonic signals of the same amplitude and frequency. Using the method described, this phase difference often can be estimated with an error less than 0.01%. The method requires use of an optimum data window that can be expressed as a Fourier series having only a few nonzero coefficients.

Application of diffraction tomography to acoustic intensity measurement

Diffraction tomography is applied to measure acoustic intensity in three‐dimensional space. With Rytov's approximation, the homogeneous Helmholtz equation is obtained. The backpropagation algorithm is used to reconstruct the scattered field. Taking the special case of incident plane waves, the image kernel is expressed in convolution form, and reconstruction becomes a deconvolution process. An experimental setup is proposed. The object is a vibrating plate driven at its center. The acoustic pressure is measured from various angles. The pressure measurement acts as a diffraction tomogram. The particle velocity is reconstructed. The structural intensity (SI) is then computed using the finite difference method in terms of particle velocity. A cross‐section image of the SI is obtained, which is an advantage over the nearfield acoustic holography method of J. D. Maynard, E.G. Williams, and Y. Lee [J. Acoust. Soc. Am. 78, 1395–1413 (1985)], which can only give the acoustic intensity as contours and three‐dimensional plots. From the cross‐section image, one will be able to clarify the outstanding theoretical problems of active intensity and reactive intensity.

A computerized system for measuring the acoustic output from diagnostic ultrasound equipment

The measurement arrangement, which complies with the requirements of the US Food and Drug Administration, consists of a positioning system with a full range of degrees-of-freedom and a digital oscilloscope, both under complete computer control. The acoustic pressure-time waveform is recorded using membrane-type and needle-type polyvinylidene fluoride (PVDF) hydrophone probes. The overall bandwidth of the system depends on the hydrophone probe used and can range up to 100 MHz. A complete description of the system and the measurement procedures is given, along with a brief discussion of the various factors which affect measurement uncertainty. The largest overall uncertainty of the same associated with acoustic intensity measurements was determined to be no greater than 20% for I(sppa) and 25% for I(spta) (spatial peak pulse average intensity and spatial-peak temporal-average intensity, respectively). Other applications of the system include transducer characterization and research work in ultrasound dosimetry and bioeffects.

The prediction and measurement of acoustic intensity in the near field of a vibrating surface

Measurements were made of the acoustic intensity radiated by a piston mounted in a large baffle. A conventional two-microphone probe was used and both the near field and far field regions were explored. The vibrational pattern of the piston was examined using 133 measurement points on the surface. At 1000 Hz the motion was essentially uniform while at 5000 Hz there was significant non-uniformity. Predictions were made of the acoustic intensities at these two frequencies by employing numerical integration techniques and using the measured velocity as a boundary condition. There was generally good agreement between the predicted and measured values of intensity.

Fourier Transform Applications to Tire Life Testing

Abstract The Fast Fourier Transform (FFT) has been used increasingly in such tire tests as uniformity, ride vibration analysis, acoustic intensity measurement, and modal analysis of tire and vehicle structures. A new application for the FFT is in monitoring tire endurance and high speed tests to identify significant changes in tire structural integrity and to provide advanced warning of impending tire failure so that the test may be terminated prior to ultimate failure of the tire. Greater information concerning the initiation of failure can then be obtained than is possible by trying to reconstruct the remains of a failed tire. Continuous display of FFT analysis of tire carriage vibrations reveals one to fifteen harmonics of tire rotation frequency on a new tire, with higher harmonics growing with test time. Just prior to failure, these higher harmonics grow in unison to very high levels with a “white-harmonic” spectrum appearing. Since the tire would then fail within 5 to 20 seconds, the test should be terminated. This paper details the techniques applied and the results obtained to date in creating a test monitoring program that will notify the test operator of significant test developments, and in automatic shutdown of tires nearing failure.

Two‐microphone measurement of acoustic intensity in the ear canal as a calibration of high‐frequency hearing

As an alternative to measures of ear canal pressure in the vicinity of the tympanic membrane, we propose an acoustic intensity measurement slightly downstream from the point of sound delivery as a convenient and meaningful calibration of high‐frequency hearing. If sound absorption in the air medium and at the walls of the ear canal is neglected, the power flow through any cross section of the ear canal would be equal to the power flow into the middle ear. A sound delivery and intensity measurement system has been built using a two‐port earmold. The tube of an Etymotic Research ER‐2 sound delivery system passes through one port and a pair of probe tubes (outer diameters = 1 mm, tip separation ≈ 3 mm) pass through the other. The latter are connected to ER‐7 microphones and used for acoustic intensity measurements based on a cross‐spectrum algorithm. Data are given on the sensitivity of measurement results with respect to the relative positions of the sound delivery point and probe tube tips. [Work supported by a grant from the Deafness Research Foundation.]

Measurement of coherence with a vector acoustic intensity probe

Complex coherence is usually defined in the acoustics literature as the (ensemble-averaged) cross spectrum between pressure measurements made simultaneously at two points and normalized with respect to the square root of the product of the autospectra at the same two points. If the two points are sufficiently close together, a considerable simplification occurs which allows the complex coherence to be modeled as a function of one position with two parts. The real part corresponds to the classical definition of plane-wave intensity. However, it is scaler in character (omnidirectional). The imaginary part corresponds to the definition of intensity assumed by acoustic intensity measurement methods of more recent interest. This part exhibits a vector character. Vector acoustic intensity probes consisting of at least four elements arranged in closely spaced pairs along three orthogonal directions, can completely characterize complex coherence fields of the form described above. Ambient noise fields (and fields containing both signals and noise) can, in turn, be characterized by complex coherence measurements. These characterizations are of practical importance in estimating the accuracy and array gain of acoustic intensity measurements.