Volume 79, Issue S1, May 1986
Index of content:
- PROGRAM OF THE 111TH MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Session A. Noise I: Miscellaneous Topics
- Contributed Papers
Field experience in measuring and evaluating acoustical parameters for motion picture theatre environments79(1986); http://dx.doi.org/10.1121/1.2023099View Description Hide Description
Since early 1983, the General Cinema Corporation has been engaged in a major expansion and renovation program of its movie theatre complexes throughout the United States. Furthermore, recent developments in motion picture presentation technology involving films with extended dynamic and frequency ranges have led to reconsideration of and, in some cases, adaptation of conventional criteria for evaluating the acceptability of theatres for viewing and listening to films. The results of field measurements (octave‐band reverberation times, room‐to‐room NR'S, and background sound‐pressure levels) are reported for a large number of facilities including data for the often disregarded 31.5‐, 63‐, and 8 K ‐Hz bands. The considerations in obtaining adequate field data and in developing criteria, as well as practical cost effective acoustical control measures developed for new generation theatres, are reviewed.
79(1986); http://dx.doi.org/10.1121/1.2023100View Description Hide Description
The building environmental control system and computer/business equipment are two major noise sources in modern office buildings. This paper focuses on the technical challenges that must be addressed by the acoustical engineers in the design of office acoustics in the computer age. In several important areas (such as standards, design tools, technology development, etc.), integration is examined in some detail. The state‐of‐the‐art of building environmental control systems and computer cooling systems will be reviewed. Technology integration potentials will be presented. A case study of integration is demonstrated. Current and future research needs are identified.
79(1986); http://dx.doi.org/10.1121/1.2023101View Description Hide Description
Noise control of three kinds of business equipment, namely, document printer, currency dispenser, and endorser/encoder, was investigated. These machines which use microprocessors as the control devices were developed recently for use in banking. During the development or improvement stage, an acoustic improvement program for each kind of equipment was initiated with sound power level determination, followed by noisediagnostic tests using the coherence function technique. One of the common noise sources in the equipment was the broadband printing noise which was radiated directly from the printing head and through machine structures. The currency dispenser had gear noise which resonated with the machine structure at the gear mesh frequency. Recommendations for noise reduction included utilization of sound absorptionfoam inside cabinets, increase of sound transmission loss of machine cases, sound isolation for structure‐bornenoise, and treatments of openings. Test results of some of these improvements are presented.
79(1986); http://dx.doi.org/10.1121/1.2023102View Description Hide Description
The noise produced by motor‐generator sets in Army field situations causes detectability and speech intelligibility problems. A program for computing the aural detectability distance of Army material was developed and used to guide a noise reduction effort for this generator. A field expedient solution was pursued which was developed through the testing of a number of increasingly complicated barriers and enclosures. This report describes the procedure used and shows the 1/3‐octave band level, the insertion loss, and the detectability contour obtained for each configuration. The insertion loss values are compared to theory, and cooling problems associated with noise reduction of generator sets are discussed. The final enclosure configuration attenuated energy in those low frequencies which cause detection, by up to 20 dB, and reduced computed detectability from 1000 to 200 m. The use of such a program can be of assistance, for the development of Army material, and for directing noise control efforts to those frequencies which control aural detectability.
79(1986); http://dx.doi.org/10.1121/1.2023103View Description Hide Description
Jet aircraft and engine test facilities are being designed to operate using air cooling only in the exhaust system. Many of these facilities are experiencing low‐frequency noise due to aerodynamic instability. The presentation will include a discussion of low‐frequency noise generation in jet engine test facilities and measured data. Data are measured down to 2 Hz since much of the noise energy is in the infrasonic frequency range.
Sound transmission loss in a rectangular attenuator symmetrically lined with continuously variable flow resistance79(1986); http://dx.doi.org/10.1121/1.2023155View Description Hide Description
The sound attenuation in a free‐path rectangular duct of constant cross‐sectional area was theoretically studied for the case of a symmetrically decreasing wall resistance at its both ends. The change of the wall conductance from either side was thereby taken to be parabolic. In addition to the high transmission loss it possesses, this attenuator was found to be wideband and to have small reflection coefficients at both ends, if it is inserted between two similar hard ducts. The solution of the obtained Bessel differential equation consists of a combination of modified Hankel functions of the order 1/3. The attenuator characteristic at a certain frequency was found to depend only on both the duct length and an attenuator characteristic area (A) defined as A = S/g, where S is the actual attenuator cross‐sectional area and g is a normalized wall conductance at a distance of 1 cm from attenuator ends. Compared to the case of the homogeneous lined attenuator, the insertion of the proposed attenuator between two hard ducts was found to give a much smaller reflection coefficient at comparable values of sound transmission loss. This is due to its good matching characteristics at both ends. As an example of calculations, a duct of 1‐m length having a characteristic area of 500 cm2 was found to have an average transmission loss of about 17 dB for the frequency range up to 600 Hz. Meanwhile, the reflection coefficient was found to decrease with frequency and to have a value less than 0.1 for frequencies above 350 Hz. Experimental results were found to be in fair agreement with the theoretical results. It is worth mentioning that parabolic change of the wall conductance can be realized by means of a homogeneous sound absorbing flow resistance together with a slit which changes parabolically along the length of the attenuator. By the experimental implementation of this slit, a variable perforation area having the same dependence on length and perforation ratio of more than 35% was used. Due to the small reflection coefficient it possesses at both ends, this attenuator can be used as an exhaust silencer.
Comparisons between A‐weighted sound‐pressure levels in the field and those measured on people or manikins79(1986); http://dx.doi.org/10.1121/1.2023156View Description Hide Description
The A‐weighted sound‐pressure levels for 75 different industrial noise spectra were determined. Previously published sound‐pressure level transformations by Kuhn and Guernsey [J. Acoust. Soc. Am. 73, 95–105 (1983)] from the field, either progressive or diffuse, to the body or head surface were applied to these noise spectra and the resulting A‐weighted sound‐pressure levels were calculated. For the most part, a microphone at the torso or head measures a larger A‐weighted sound‐pressure level than a microphone in the unobstructed sound field. The range of the resulting measurement error, due to the presence of the body or the head, is approximately − 1 dB(A) to + 5 dB(A). The magnitude of the error is sensitive to the type of sound field, the subject's orientation relative to the angle of incidence of the sound, the noise spectrum, and the microphone location. Other factors, for example, the microphone size and directivity, the distance between the microphone and the body, and the absorption of the sound by different types of clothing will also affect the measurement errors. Although these factors are implicit in some of the reported transformations, their individual, explicit effect on the measurement errors remains to be determined.
79(1986); http://dx.doi.org/10.1121/1.2023157View Description Hide Description
Personal and handheld computers are used in the acoustical calibration laboratory of the MineSafety and Health Administration (MSHA), Pittsburgh Health Technology Center. The laboratory was established to calibrate noise dosimeters used by MSHA coal and metal/nonmetal mine inspectors. Equipment was selected to set up new systems and to make existing systems more efficient. Analog signals from some dosimeters are measured by the computer systems and converted to useful information, while other dosimeters come equipped with standard computer interfaces. Programming of the systems was done in‐house. This has two advantages. First, the programmer has constant communication with the eventual end users of the system, which results in better planning. Second, the system can be adapted immediately by anyone familiar with the program, when changes in operating procedures become desirable or necessary. In general, the systems are accurate, prevent time‐consuming mistakes, and can be made to guide the user through the calibration procedure. The installation of computer controlled calibration systems has proven to be successful.
79(1986); http://dx.doi.org/10.1121/1.2023158View Description Hide Description
Spreadsheet programs, once used as a financial/business tool, can be effectively used for solving acoustics and noise control problems using a personal computer. The spreadsheet allows one to write a pseudoprogram in terms of algebraic expressions which follows algorithms similar to our thinking process. The spreadsheet is essentially a two‐dimensional matrix where entries can be made both in rows and columns. The entries, called cells, contain different types of information, such as numbers and formulas. Numbers are like expressions with fixed values. Formulas are algebraic expressions where references can be made to other parts (cells) of the program. These expressions are computed in a similar fashion as is done in a calculator but at a much faster speed. Also, the spreadsheet computer program has plenty of flexibility and versatility in many areas compared to a conventional digital computer program. One of these is the automatic updating of all results (algebraic expressions) that are affected due to the change of a value in the spreadsheet. This paper discusses the VISI‐CALC® computer program spreadsheet. A worker noise exposure prediction problem is demonstrated using the VISI‐CALC® program with a Hewlett‐Packard desktop computer.
- Session B. Physical Acoustics I: High‐Temperature Acoustics
- Invited Papers
79(1986); http://dx.doi.org/10.1121/1.2023214View Description Hide Description
For reasons which are, at present, poorly understood, the sun and presumably stars like the sun undergo continuous oscillations. The most prominant of these have been identified as normal modes which are gravity‐modified acoustic resonances involving the whole sun. The spatial structure of the modes is described as the product of spherical harmonics Yl m (θ,φ) and a radial eigenfunction. The value of l determines the depth of mode penetration for each value of n. The identification of the modes with l and n permits a comparison of the observed frequencies with theoretical ones and tests the adequacy of our solar models. The frequency degeneracy in m is broken by rotation, and the measurement of the fine structure of the frequencies permits a determination of the depth dependence of the solar rotation rate. Present results indicate that the sun does not deviate substantially from a condition of uniform rotation. These results appear to rule out the hypothesis that the solar gravitational potential could have a large enough quadrapole term due to solar internal rotation to invalidate tests of general relativity.
79(1986); http://dx.doi.org/10.1121/1.2023215View Description Hide Description
It is often necessary to measure the acoustic pressure in a high‐temperature environment as, for example, in the study of combustionnoise. High‐temperature microphones have been developed, and techniques have been devised for adapting “room temperature”microphones for use in the high‐temperature environment. The unique features of this measurement problem are identified, and state‐of‐the‐art methods for its solution are presented and discussed. Particular attention is paid to the acoustic waveguide probe system, in which a common room temperaturemicrophone is thermally isolated from the high‐temperature environment through a pressure transmitting tube. Results of a recent analysis of such a system are presented and compared with new measurements. It is suggested that the probe system, when used with the model to correct for axial temperature gradients along the probe, may be the optimum solution to this difficult measurement problem.
79(1986); http://dx.doi.org/10.1121/1.2023216View Description Hide Description
A liquid metalthermoacoustic engine is studied both theoretically and experimentally. This type of engine promises to produce large quantities of electrical energy from heat at modest efficiency with no moving parts except the acoustic oscillations in the liquid metal. In the engine, heat flow from a high‐temperature source to a low‐temperature sink amplifies a standing acoustic wave in liquid sodium. This acoustic power is simply converted to electric power by means of a magnetohydrodynamiceffect at the acoustic oscillation frequency. A detailed thermoacoustic theory applicable to this engine is developed, and it is found that a reasonably designed liquid sodium engine operating between 700 °C and 100 °C should generate about 60 W/cm2 of acoustic power at about 1/3 of Carnot's efficiency. Construction of a 3000‐W thermal laboratory model engine is almost complete. A 1‐kW, 1‐kHz liquid sodiummagnetohydrodynamic transducer has also been designed and built. It is now very well characterized both experimentally and theoretically. The first generator of its kind, it already converts acoustic power to electric power with 40% efficiency.
- Contributed Papers
79(1986); http://dx.doi.org/10.1121/1.2023217View Description Hide Description
The velocity of sound and the nonlinearity parameters of HY80 3.25% NiCrMoV steel samples which were heat treated at 538 °C for periods ranging from 0–100 h were measured.Elastic constants (c 11 and c 111) associated with these measurements were reported, and they were compared with values obtained from other steels. It was found that the difference between measured values among the samples is larger than one would expect. Possible reasons for the discrepancies are discussed.
Resonant cavity techniques for accurate measurements of the ratio of the speed of sound to the speed of light79(1986); http://dx.doi.org/10.1121/1.2023218View Description Hide Description
In principle, measurements of the resonance frequencies of both the acoustic and the microwave modes of a single cavity can determine the ratio of the speed of soundu of a monatomic gas to the speed of lightc. Such measurements, carried out with high accuracy, could determine the universal gas constant R and the thermodynamictemperatureT with unprecedented accuracy. The realization of these possibilities can be greatly facilitated by judicious choices of cavity geometry and resonance modes. The present state of the art suggests that the ratio u/c can be measured to parts per million accuracy using cavities whose geometry is known only to parts per thousand. Recent experimental and theoretical results will be presented.
79(1986); http://dx.doi.org/10.1121/1.2023256View Description Hide Description
Intense acoustic fields can be used to position objects without mechanical contact. This phenomenon finds application in high‐temperature materials research by enabling a specimen to be heated, melted, reacted, cooled, and solidified in a containerless state. The acoustic force vectors capable of levitating an object are given by the gradient of the acoustic potential energy density. By suitable shaping of the acoustic field, a closed energy well can be created and a small specimen captured therein. Some recent experiments were carried out in microgravity aboard the Space Shuttle in order to reduce the requirements for intense acoustic fields, typically from 145 to 165 dB, at 15 kHz. Preliminary results are presented showing successful containerless processing of glass specimens of density 5 g/cm3 at 1550 °C. Ground‐based experiments have levitated densities of 20 g/cm3 at STP and densities up to 4 g/cm3 at 1000 °C. Various results from these experiments will be presented along with a discussion of the effects of acoustic cooling, thermal perturbations of the acoustic field, and the presence of harmonics. [Work supported by NASA.]
79(1986); http://dx.doi.org/10.1121/1.2023257View Description Hide Description
An expression for the acoustic radiation force on a small spherical particle of radius R in a standing wave field is derived. The particle is inside a long tube chamber with a temperature gradient along the axis of symmetry. Assuming R < λ, and neglecting convection,acoustic streaming,heat conduction, and viscosityeffects, an expression for the force that consists of a “local” version of Gor'kov's result [L. P. Gor'kov, Sov. Phys. Dokl. 6, 773 (1962)] as well as correction terms of order βλ, where β = T′/T, is obtained. Also the effect of various temperature gradients on the acoustic positioning (levitation) properties of the system is investigated numerically. The results of this analysis will be compared to the uniform temperature case. [Work supported by NASA.]
79(1986); http://dx.doi.org/10.1121/1.2023258View Description Hide Description
The acoustic radiation force was measured for a dual‐temperature resonant chamber. This rectangular chamber has its long dimension approximately 8.5 times the square cross‐sectional dimension and the opposite ends are at widely different temperatures. Force profiles were obtained for two hot‐end temperatures of 500 °C and 750 °C, while the cool end remained at approximately room temperature. The lateral force was measured as a function of the long dimension of the chamber along the temperature gradient, and as a function of the drive voltage. The highest force per unit mass measured at 500 °C was 103.8 dyn/g for a frequency of 4450 Hz, and at 750 °C the highest force was 82.8 dyn/g for a frequency of 5500 Hz. Qualitatively, the measured force profiles correspond well with the theoretical curves; however, correlation of absolute magnitude is yet to be determined. [Work supported by NASA.]
79(1986); http://dx.doi.org/10.1121/1.2023259View Description Hide Description
Ultrasonic levitators have been used in ground‐based laboratories to observe melting and freezing phenomena characteristic of organic materials and metals. The behavior of freely suspended substances undergoing a first‐order phase transition and the influence of a high‐intensity acoustic field have been of primary interest. The undercooling ability of the materials and the acoustic field‐melt interaction are related subjects, and some experimental observations have been obtained in order to elucidate this relationship. The size of the levitated samples is on the order of 1 mm and the temperature range studied is between − 25° and 400 °C. [Work supported by NASA.]
- Session C. Physiological Acoustics I and Psychological Acoustics I: Otoacoustic Emissions and Evoked Response
79(1986); http://dx.doi.org/10.1121/1.2023260View Description Hide Description
A specialized microphone for use in measuring cochlear emissions contains four modified Knowles microphones mounted in mechanical opposition for minimum vibration sensitivity in their summed output. That output is electrically equalized to provide a flat frequency response from 100 Hz to 12 kHz when measured with a disposable foam eartip containing 10 mm of 3.8‐mm‐i.d. probe tubing. To facilitate measurement of stimulated emissions, and especially cochlear distortion products, the construction includes two 1.35‐mm‐i.d. earphone coupling tubes mounted to pass through the 3.8‐mm‐i.d. probe tubing, permitting the delivery of stimuli from two independent ER‐2 insert earphones, each providing a flat eardrum‐pressure frequency response to 12 kHz. The complete microphone exhibits a typical noise spectrum level of − 20 dB SPL at 1 kHz decreasing to − 26 dB at 5 kHz, 8 to 10 dB below that of a single Knowles EA‐1954 microphone and 5 to 10 dB below the apparent noise level of good young ears based on the estimate of Killion [J. Acoust. Soc. Am. 59, 424–433 (1976)].
79(1986); http://dx.doi.org/10.1121/1.2023302View Description Hide Description
Spontaneous otoacoustic emissions (SOAE's) can be synchronized by external tones within a narrow frequency band characterized by a synchronization tuning curve [E. Zwicker and E. Schloth, J. Acoust. Soc. Am. 75, 1148–1154 (1984)]. Surrounding this range of synchronization is a region of unstable partial synchronization leading to amplitude fluctuations of the stimuli. Psychophysical synchronization threshold curves [D. T. Kemp, Scand. Audiol. Suppl. 9, 35–47 (1979)], based on an abrupt change in the percept of low‐level stimuli near threshold microstructure, show similar tuning. These two types of synchronization tuning curves are compared in subjects with strong SOAE's and associated threshold microstructure. Simple models for these tuning curves are discussed in terms of driven limit‐cycle oscillators. [Work supported by NIH.]