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May 1988

Volume 83, Issue S1, pp. S1-S122

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back to top Session QQ. Noise V: Noise Measurement and Propagation
Contributed Papers
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Scattering/diffraction effects of a sound intensity probe incorporating direct particle velocity measurements (A)

G. Krishnappa

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S92-S92 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Experimental studies were carried out to examine the scattering/diffraction effects of a sound intensity probe that determines particle velocity directly by using two pairs of ultrasonic transmitters and receivers. The measurement techniques employed were to detect the change in the separation distance acoustically of two closely spaced microphones placed in the immediate vicinity of the velocity measuring probe and to determine the accuracy of sound intensity measurements by comparing with sound‐pressure level measurements made in the farfield of an acoustic source inside an anechoic room. The results show that there are noticeable changes in the acoustic field in the frequency range 3000–6000 Hz due to the insertion of the probe, and the scattering/diffraction effects tend to increase with increase in the angle of incidence of the sound waves, but do not affect in any significant way the accuracy of sound intensity measurements. Measurement accuracies of the probe are within 1 dB up to 5000 Hz, which includes other sources of errors associated with signal processing such as errors inherent with FFT techniques.
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Use of sound intensity for determination of air‐moving device noise emission (A)

A. C. Balant and George C. Maling, Jr.

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S92-S92 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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American National Standards on sound power determination via sound intensity and on measurement of noise from air‐moving devices are at an advanced stage of development. The latter proposed standard requires a special test facility such as a reverberation room or a hemi‐anechoic room, and does not permit the use of sound intensity for determination of noise emission. In this paper, the results of sound power determinations on air‐moving devices in a reverberation room, a hemi‐anechoic room, and in an ordinary laboratory environment are presented. The data in the ordinary room were obtained using sound intensity methods. The accuracy of the data in all three environments is discussed. Many of the “field indicators” which are recommended in the proposed sound intensity standard have been calculated in order to determine acceptable ranges for these indicators. The results indicate that determination of air‐moving device sound power in ordinary rooms is adequate for many engineering purposes. Use of sound intensity for determination of air‐moving device sound power should increase the amount of data available on these sources.
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Investigation into the use of the filtered‐x LMS algorithm for noise reduction in a duct (A)

Peter L. Schuck

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S92-S92 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Burgess [J. Acoust. Soc. Am. 70, 715–726 (1981)] simulated an active noise reduction system in a duct. This system utilized a version of the filtered‐x LMS algorithm to obtain noise reduction over a wide bandwidth. A similar system has been implemented using the TMS‐32020 signal processing chip on a PC expansion board. In the filtered‐x LMS algorithm, the reference signal must be filtered by the transfer function between the transversal filter output and the error signal input in order to maintain stability of the LMS weight update. In our system, this transfer function is calculated from the measured response to a maximum length sequence generated by the noise reduction system itself. Performance results are given for this system in a test setup. Some issues regarding practical implementation of this algorithm in fixed point arithmetic will also be discussed.
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Accurate estimation of amplitude, frequency, and phase of harmonic signal components using an FFT (A)

John C. Burgess

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S92-S92 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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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.
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The measurement of sound by the simultaneous use of several time constants (A)

Svein Arne Nordby, Steiner Bohn, and Richard J. Peppin

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S92-S92 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Nonstationary or transient sounds measured by a sound level meter will produce different values of sound level, depending on the time constant, or lack thereof, used. The technique of determining exposure, the so‐called “doubling rate” will also result in significant differences in readings. Most investigations of the effects of the different time constants and averaging rates on nonstationary signals required several sound level meters or a predefined signal that was repeated for each different measurement method so that the differences could be compared. This paper presents the results of tests using a sound level meter that can simultaneously measure with various time constants and with several “averaging” techniques allowing the operator to measure Lmax and Lmin using “fast,” “slow,” “impulse” time constants and “peak” detection using a doubling rate of 3, 4, 5, 6, and impulse, all on the same arbitrary source.
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Computational studies of the diffraction integral occurring in the MAE theory of sound propagation over hills and valleys (A)

James A. Kearns, Ji‐xun Zhou, Yves H. Berthelot, and Allan D. Pierce

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S93-S93 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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An important class of problems pertaining to outdoor sound propagation is that of the diffraction that occurs when the ground is neither perfectly flat nor perfectly rigid. Such problems are encountered in the study of long‐range propagation of sound over hills and valleys. It has been shown previously [J. Acoust. Soc. Am. Suppl. 1 79, S30‐S31 (1986)] that the theory of matched asymptotic expansions allows one to express the diffracted field in terms of a complex integral involving Airy functions of complex argument. In some limiting cases, the diffraction integral reduces to some computationally very efficient forms: an equation based on geometrical acoustics in the illuminated region, a creeping wave series in the shadow zone, and a knife‐edge Fresnel diffraction integral far behind the ridge. In the present paper, the transition between these different regimes is investigated numerically by computing the general integral, and particular attention is given to the matching with the creeping waves series solution in the penumbra region. Computational results are compared with data obtained in laboratory scaled experiments. [Work supported by NASA Langley Research Center.]
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Sound attenuation in air over water on Puget Sound during temperature inversion and noninversion conditions (A)

Jan H. Hauge

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S93-S93 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Excess attenuation of sound over a 7500‐ft distance was measured on 6 days to determine effects of temperature inversions and wind direction. The tests were conducted using one‐third octaves of pink noise at center frequencies of 250, 500, and 1000 Hz. During temperature inversions, excess attenuation downwind was found to be significantly reduced and negative. Results of the tests were used to predict sound levels that would be received at island residences and a bird sanctuary under a proposed industrial use of the mainland shoreline.
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Model scale testing of outdoor noise propagation from a power plant (A)

Robert A. Putnam

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S93-S93 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A proposed cogenerating power plant of 70‐MW capacity is to be located in an urban setting and will be subject to stringent environmental noise emission criteria. A four‐cell forced draft cooling tower is included in the plant design. Computer modeling of cooling tower noise propagation to affected neighboring residences involved significant uncertainties due to the complex shapes of power plant building elements and existing surrounding buildings. Without a finished facility in which to perform full‐scale field tests, the next best thing is a scale model test. This paper will discuss the specifics of the 1/20th scale test setup, materials, selection, instrumentation, test execution, some problems and some lessons learned. The results of the test will be given in terms of the differences in octave band sound‐pressure levels between an unobstructed free‐field value and the modeled in situ value, at several receptor positions. The test enabled the specification of more precise sound attenuating elements for the cooling tower fans and inlets in order to comply with the governing regulations.
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