Volume 37, Issue 6, June 1965
Index of content:
- PROGRAM OF THE SIXTY‐NINTH MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Session A. Noise‐Abatement Symposium I
- Invited Papers
37(1965); http://dx.doi.org/10.1121/1.1909564View Description Hide Description
Basic noise‐generation mechanisms are discussed in terms of simple physical models. Examples of these and more‐complex mechanisms are illustrated with pressure‐time histories, frequency spectra, sound‐pressure levels, directionality, etc. The outline reads: (1.) Introduction. (2.) Simple Source (Pulsation)—power emission; gas bubbles in water; other examples; pulsed flow (auto engine, pulse jet); heat release (sparks,combustion); vibration of a general surface; moving source (sonic boom). (3.) Dipole Model—power and directionality; oscillating force (Aeolian tones); steady force on moving member (propeller noise). (4.) Quadrupole Model—power and directionality; oscillating force‐pair (stress); jet noise. (5.) Boundary Layer and Duct Flow—flow noise;noise from excitation of wall vibration; grills and horn diffusers as airfoil noise generators.
37(1965); http://dx.doi.org/10.1121/1.1939369View Description Hide Description
A wide variety of equipment of commercial manufacture is available for the measurement of noise. The problems that occur in making such measurements for engineering and legal purposes arise from the ability or inability of the user to establish correctly the calibration of the equipment and to understand the process of measurement. The user must ask himself whether his instruments are capable of performing the required measurement in terms of bandwidth, resolution, waveform, meter characteristics, and, in many cases, the statistics of the noise‐generating process itself. The application of commercially available equipment including techniques for measuring, recording, and analysis of noise are reviewed and specific examples related to the measurement of machine, airflow, and vehicle noise are presented.
37(1965); http://dx.doi.org/10.1121/1.1939371View Description Hide Description
This presentation is intended to supply information primarily to the individual who has recently become involved with the many problems of “what are acceptable noise levels” and who has not necessarily been involved in the day‐to‐day business of noise control. Guidelines and criteria describing acceptable noise levels are presented in terms of the following areas of interest: (1) industrial hearing loss, (2) airport levels, (3) industrial levels, (4) residential levels, (5) office levels, (6) auditorium levels, and (7) speech. All terms such as perceived noise level, loudness, noisiness, SIL, etc., that are involved in the interpretation of guidelines and criteria are defined.
37(1965); http://dx.doi.org/10.1121/1.1939372View Description Hide Description
This area of noise control is illustrated by a consideration of the motor‐vehicle noise problem. There is a welter of existing legislation in this country and abroad. It is mostly subjective in nature and therefore generally ineffective in controlling motor‐vehicle noise. The objective legislation that does exist is expressed in several different measures, such as dBA, dBB, and dBC, and various measurement distances. Suggestions for objective limits have also included loudness in sones and PNdB. These various limits are compared with one another, with existing and proposed standards, and with limits that appear economically and technically feasible as a result of recent studies. The nature and effectiveness of existing legislation and law enforcement practices are also reviewed.
- Session B. Psychological and Physiological Acoustics I: Mainly Loudness
- Contributed Papers
37(1965); http://dx.doi.org/10.1121/1.1939373View Description Hide Description
The psychophysicaltesting and evaluation of conventional and communication‐type hearing protectors continues to present a major problem in industry, engineering, and Government procurement. Testing is time‐consuming, costly, impractical, and not always reliable for routine engineering applications where outcome of such tests cannot be awaited. TESTEAR provides an objective testing method by which the attenuation characteristic of hearing protectors or microphonenoise shields can be plotted automatically in a few minutes from a high intensity sound source generating pure tones, bands of noise, or even from actual noise in any given environment. Because the test signals are at high sound‐pressure levels, TESTEAR does not require expensive anechoic test rooms. For the same reason, the recorded data are more representative of the performance of hearing protectors under actual field conditions. Comparative tests conducted with 30 subjects with the threshold‐shift method and with TESTEAR physical method produced attenuation curves showing close correlation between methods. TESTEAR does not require instrumentation other than that normally found in the acoustics laboratory.
37(1965); http://dx.doi.org/10.1121/1.1939374View Description Hide Description
Real‐ear‐response curves of representative circumaural and supra‐aural earphones have been measured with a probe‐tube microphone, the orifice of which is located 1 mm beyond the edge of the tragus. At each sitting, hearing thresholds have also been determined at 9 accurately tuned audiometric frequencies from 125 to 8000 cps. Preliminary data for 3 subjects indicate that the SPL at threshold is virtually independent of the earphone used. However, the earphone response at constant input voltage varies greatly from subject to subject, particularly at high frequencies. Typical earphones show intersubject differences of 10–20 dB in the 8‐kc/sec region. Furthermore, a 5% change in frequency may change the response for a given subject by 10 dB. With some subjects, the effort of ear‐canal resonance is clearly evident in the response curves.
37(1965); http://dx.doi.org/10.1121/1.1939375View Description Hide Description
This program was intended to evaluate the effectiveness of the Air Force Hearing Conservation Program in light of several years experience. More than 82 000 audiometric tests on over 58 000 people were transferred to punched cards, processed by a large digital computer, and analyzed. In spite of a large amount of missing, illegible, or otherwise unusable data, it was possible to derive much useful information from the Hearing Data Repository. Benefits from the use of ear protection were not apparent, which leads one to question if people really use ear protection when they should or if they use it properly. The information in the repository on noise exposure, past and present, was not in a form permitting more meaningful interpretations. Further, it was found that the sample used had poorer hearing than a sample of the general population used as a reference, and one would want to derive an Air Force reference. [This program was sponsored by the School of Aerospace Medicine.]
37(1965); http://dx.doi.org/10.1121/1.1939376View Description Hide Description
Fletcher's definition of loudness units (LU) implies a direct relationship between the sensation of loudness and the difference limen for perception of a change in sound intensity. This property of the ear can be derived from the behavior of a mechanistic model. The equations describing the model applied to experimental data on the difference limen for intensity perception in the ear permit calculation of equal‐loudness contours and the diffuse‐field threshold for the ear, which, in turn, can be compared with experimental data.
37(1965); http://dx.doi.org/10.1121/1.1939377View Description Hide Description
The methods for computing loudness developed by K. E. Zwicker and S. S. Stevens were applied to several complex sounds encountered in our work on architectural acoustics. The loudnesses computed on the basis of Stevens' method did not agree closely with the loudnesses computed by Zwicker's method, and the results obtained by using the two methods were not related to one another in any consistent way. Further, studies with subjects showed that both sets of computations gave results at variance with the responses of the subjects. Investigation of the loudness vs frequency contours for our subjects showed closer conformity to the Fletcher and Munson data than to the more recent equal‐loudness contours reported by the National Physical Laboratory, the functions reported by Zwicker, or the band‐pressure levels that form the basis for the loudness weighting Stevens' method. However, this feature does not suffice to account for the discrepancies observed.
37(1965); http://dx.doi.org/10.1121/1.1939378View Description Hide Description
Loudness is measured for 500‐, 300‐, 100‐, 30‐, and 10‐msec tones at 1000 cps in 5 subjects with clinically normal hearing to determine the nature of temporal summation above the threshold of audibility. The results from numerical‐magnitude‐balance experiments showed that the form and slope of the loudness function at 1000 cps does not change as a function of duration, at least between 500 and 10 msec. The absolute difference among the numerical‐magnitude‐balance functions was determined by magnitude production. These results showed that the intensity increase required for equal loudness as a function of duration is the same as that required for the detection of identical tones at the threshold of audibility. The combined results of the foregoing experiments indicate that the growth of loudness as a function of duration is constant for sensation levels greater than 40 dB; and, at sensation levels less than 40 dB, there is a progressive increase in the rate of growth of loudness with a decrease in sensation level. [This research was performed at the Laboratory of Sensory Communication, Syracuse University, Syracuse, New York 13210.]
37(1965); http://dx.doi.org/10.1121/1.1939379View Description Hide Description
In the investigation of loudness as a function of duration, the slope of the loudness function was 0.43, considerably different from the slope of 0.54 obtained by numerous other investigations of loudness at 1000 cps. To explain this difference in slope, the hypothesis was made that, when loudness is measured in the absence of a standard presented at a constant sensation level and assigned a fixed number, an implicit loudness‐assigned number combination is used as the standard, and that this standard is the comfortable listening level. The comfortable listening level for the subjects used previously was determined as well as the number assigned to this level. By applying appropriate transformations to the results of prior experiments concerned with the slope of the loudness function as a function of the relation between the sensation level at which the standard is presented and the number assigned to that level, it is possible to show that when a standard is not presented the most likely candidate for an implicit reference loudness is the comfortable listening level. [This research was performed at the Laboratory of Sensory Communication, Syracuse University, Syracuse, New York 13210.]
37(1965); http://dx.doi.org/10.1121/1.1939380View Description Hide Description
What is the effect on hearing sensitivity of stress on the eardrum from differential air pressure? A hard earphone casing was fitted to provide small static pressure (up to 10 in. ) and measures were made of sensitivity to pure tones at various frequencies. Sensitivity was impaired up to 10 dB at low frequencies by pressure, and the amount of loss varied directly with amount of pressure. High frequencies (presumably above the resonant frequency of the middle ear) showed no loss of sensitivity. There was no reliable difference between effects of positive and negative pressure. The results suggest indirectly that the tensor tympani muscle of the middle ear might serve as a low‐frequency noise suppressor. A forced‐choice procedure was developed for later tests, which enables rapid estimation of the stimulus energy necessary to achieve a fixed predetermined level of performance. Some characteristics of the procedure are described.
37(1965); http://dx.doi.org/10.1121/1.1939381View Description Hide Description
The experiments reported were designed to obtain further data on the perceived noisiness complex sounds consisting of a steady‐state pure tone imbedded in a background of random noise. From these data, a method for including a “pure‐tone correction factor” in calculation of perceived noisiness in PNdB is derived. The proposed procedure should have practical application for the evaluation of sounds from modern‐day jet aircraft or other broad‐band sounds that may contain relatively intense, audible pure‐tone components. Various problems involved in the measurement and interpretation of band spectra for the location of steady‐state pure‐tone components in broad‐band random noise are discussed.
37(1965); http://dx.doi.org/10.1121/1.1939382View Description Hide Description
In exploring the possibility that people can ascribe an absolute scale of acceptability for aircraft noise, subjects from airport neighborhoods judged, in separate tests, the relative and the absolute acceptability of noise produced by actual aircraft flyovers and by recorded flyover signals. The flyover noise judgments were made indoors and outdoors at two locations near the Los Angeles International Airport. Most subjects judged both approach noise and takeoff noise (produced mainly by jet aircraft). Judgments were compared with the maximum perceived noise level occurring during the flyovers. Little difference was observed in acceptability rating scores for approach and takeoff noise or for actual and recorded noise signals. However, a shift in ratings between outdoor and indoor judgments occurred similar in magnitude to that observed in earlier British tests. The relative judgment tests also showed little difference between ratings of takeoff and approach noise or live and recorded signals. However, the results indicated that a larger change in perceived noise level was required for a doubling (or halving) of the acceptability rating than originally assumed in developing the perceived noise level scale. [Work supported by the Systems Research and Development Service, Federal Aviation Agency.]
- Session C. Scattering and Shell Vibrations
37(1965); http://dx.doi.org/10.1121/1.1939383View Description Hide Description
The propagation of elastic waves along cylindrical shells is one of the very few wave problems that can be solved exactly. If the wavelength is Λ, the radius (r), and the thickness (h), we might expect on physical grounds that when Λ≫r the shell behaves like a rod and when r≫Λ≫h it behaves like a plate. For solid cylinders, the highest group velocity is , whereas for plates it is about 1.05 C 0. Physical arguments indicate that for r≫Λ≫h the higher group velocity also applies to cylindrical shells. This prediction has been tested theoretically and experimentally. First: the exact frequency equation was shown to reduce to Lamb's symmetrical and asymmetrical plate waves.Second: a computation for r/h=32 established the region of wavelengths where such plate waves are propagated. Third: experiments on pulse propagation in thin steel cylindrical shells showed a precursor wave traveling faster than C 0. This did not produce circumferential strains and would appear to be the predicted plate wave.
37(1965); http://dx.doi.org/10.1121/1.1939384View Description Hide Description
An experimental study has been made of the sound field in the interior of a thin circular cylindrical shell exposed to a diffuse wide‐band sound field. The sound field was explored point by point and noise reduction, defined as the dB difference between the SPL at a point and the SPL in the diffuse field exterior to the shell, has been investigated as a function of shell geometry.
37(1965); http://dx.doi.org/10.1121/1.1939385View Description Hide Description
The free‐vibration problem of a thin, isotropic, oblate spheroidal shell was solved by Galerkin's method. Membrane theory and harmonic axisymmetric motion were assumed to derive the differential equations of motion. The equations of motion lead to two ordinary differential equations with variable coefficients. The two differential equations can be reduced to one ordinary second‐order differential equation with variable coefficients. This eigenvalue problem is solved by Galerkin's method. It was shown that Galerkin's solution for the oblate spheroid yields the exact solution for the sphere as the eccentricity of the oblate spheroid goes to zero. It is shown that two sets of frequencies exist for the oblate spheroidal shell. Galerkin's method converges rapidly for those of the lower set. After the frequencies are determined, the tangential displacements are obtained as solutions of a homogeneous system of equations. For numerical illustration, an oblate spheroidal shell with eccentricity equal to 0.688 was considered.
37(1965); http://dx.doi.org/10.1121/1.1939386View Description Hide Description
A complete solution for the transient response produced by time‐dependent surface loads on a thin elastic shell of arbitrary shape is derived by means of the classical method of spectral representation. The solution is expanded in terms of the normal modes of free vibration, and their orthogonality is proved for an arbitrary shell. This method of solution is applicable to any shell for which the free‐vibration characteristics (natural frequencies and mode shapes) are known. For application, the response of a hemispherical and a shallow spherical shell to a suddenly applied uniform pressure is calculated and the convergence of the solution is studied in detail. By introducing mode‐participation factors, the contribution of each mode to the solution is determined. It is shown that for the shallow shell about four modes are necessary in the solution, while for the hemispherical shell the number of required modes is about fifteen. In either case, when coupled with some available free‐vibration analysis, the calculation of the transient response by the method of this paper is well within the practical range of a digital computer.
37(1965); http://dx.doi.org/10.1121/1.1939387View Description Hide Description
The transmission of an acoustic wave through an infinite, homogeneous, isotropic cylinder has been derived to be a strong function of frequency and the angle of incidence between the axis of the cylinder and the incoming wave. From this analysis, two important characteristics are found: the ring frequency, where the wavelength of a longitudinal wave in the material is equal to the circumference, and the coincidence frequency, where the trace wavelength of the incident wave is equal to the bending wavelength in the shell wall. The transmission loss will take on minimum values at these two points. Measurements with noise excitation of a closed cylindrical shell with l/d=1.5 show the same gross features to be present and with additional effects of low‐frequency resonances, but with very little angular dependence. The presence of stiffening corrugations and irregularities in the shell leads to a random vibration field and allows methods developed for the transmission of random sound through flat panels to be successfully used for noise‐reduction estimates.