Index of content:
Volume 58, Issue 1, July 1975
58(1975); http://dx.doi.org/10.1121/1.380660View Description Hide Description
Data obtained from many sources are used to describe the physical characteristics of traffic noise, its individual components, and some of the means for reducing motor vehiclenoise. The noisecharacteristics of trucks,automobiles, and motorcycles are discussed separately. The composite of all motor vehiclenoise (traffic noise) is described in terms of the operating parameters of interest, including vehicle speed, engine speed, throttle position, engine type, tire‐tread type, and road surface smoothness.
Subject Classification: 10.60; 50.25, 50.50.
58(1975); http://dx.doi.org/10.1121/1.380640View Description Hide Description
The surfaces of an internal‐combustion engine can be expected to radiate sound resulting from time‐varying gas and mechanical forces developed during the operating cycle. The rapid pressure rise accompanying spontaneous ignition of the fuel within the combustion chamber leads to the characteristic ’’knocking’’ noise of diesel engines. In contrast, the rate of change of pressure within the cylinders of spark‐ignition engines is comparatively slower, which results in less intense gas forces. Further, impacts between the piston and cylinder walls, or deformation of the crank mechanism, may be responsible for the noise of some engines. In both cases vibrational energy is radiated primarily by the crankcase and by nonstructural parts, such as the oil pan and valve covers. For air‐cooled engines there is significant radiation from the cylinder‐cooling fins. Engine‐radiated noise may dominate the overall noise spectrum of diesel‐powered trucks at speeds near 30 mph.
Subject Classification: 10.60; 50.30, 50.50.
58(1975); http://dx.doi.org/10.1121/1.380651View Description Hide Description
In this paper the state of the art of characterizing and controlling intake and exhaust system noise of internal‐combustion engines is reviewed. Various types of internal‐combustion engines are discussed as sources of noise and in terms of their interaction with intake and exhaust systems. A review of muffler technology shows that linear resistive and reactive effects are well understood, as are certain nonlinear effects. However, effects of high sound pressure levels, flow, duct perforations, and temperature gradients all acting together require study. The paper also shows that mufflers providing 30–35 dBA of insertion loss are practical and that insertion loss can be predicted, but only roughly.
Subject Classification: 10.60; 50.30, 50.50.
58(1975); http://dx.doi.org/10.1121/1.380661View Description Hide Description
In motor vehicles there are two primary sources of aerodynamic sound: the engine‐cooling fan and the interaction of either the entire vehicle or some of its components with the airstream created by vehicle movement. The mechanisms for sound generated by these sources are discussed in terms of the associated physical parameters (fan size, fan‐blade design, cooling‐air velocity and path, etc.; body configuration, interior design, etc.).
Subject Classification: 10.60; 50.50, 50.55.
58(1975); http://dx.doi.org/10.1121/1.380662View Description Hide Description
The relative importance of tire noise to overall vehicle noise is established. A general description is then given of the tire engineering process and of tire structures. The important parameters influencing tire noise are discussed, based on presently available data, followed by an identification of unknown and contradictory areas. The basic mechanisms of tire‐noise generation, although not well understood, are investigated largely from a theoretical viewpoint. Areas for future research and development are identified based on gaps in the existing physical data base and a rather primitive level of understanding of noise‐generating mechanisms.
Subject Classification: 10.60; 50.50, 50.30.
Deep‐scattering‐layer observations off New Zealand and comparison with other volume scattering measurements58(1975); http://dx.doi.org/10.1121/1.380663View Description Hide Description
Acoustic volume scatteringmeasurements using 5 and 12 kHz pulses and explosive signals were made during the 1971 winter season at 18 deep‐water stations north and east of Auckland, New Zealand. Strong scattering layers (maximum volume scattering strengths −58 to −70 dB r e 1 yd) were observed at depths of about 500 yd during daytime measurements at both 5 and 12 kHz. The number, location and depth of minor scattering layers are the principal station differences. C o l u m n strengths (column length about 1000 yd) obtained from pulse measurements do not vary greatly from station to station and show good agreement with c o l u m n strengths from explosive measurements. Night values of c o l u m n strength are higher, with one exception, than day values. The explosive results covering 1/3‐octave bands from 1 to 20 kHz have overall c o l u m n strengths ranging from −70 to −40 dB. A characteristic frequency pattern, in which a sharp rise in c o l u m n strength from 1 to 5 kHz is followed by a fairly constant level thereafter, is exhibited on all stations. Comparison of the New Zealand data with that from other ocean areas (Indian Ocean,Coral Sea, Hawaii, Atlantic, Gulf of Mexico, Southern California) shows that New Zealand scattering strengths are relatively high and that the characteristic frequency pattern is of widespread occurrence. The prevalence of this pattern suggests a mechanism common to all these locations. Midwater fishes differing in species from area to area but having a common range of swimbladder sizes are believed the cause of this characteristic frequency pattern.
Subject Classification: 30.30,30.40.
58(1975); http://dx.doi.org/10.1121/1.380664View Description Hide Description
A multielement cylindrical lens fabricated from polystyrene and an unfilled silicone polymer has been developed with angular resolution in the order of 0.2 deg at 1000 kHz. Theoretical and experimental work shows the lens maintains good performance over a 20‐deg field of view with insignificant temperature‐induced defocusing in the 10 ° to 25 °C temperature range. Tolerance analyses completed during the lens design resulted in reasonable mechanical tolerances being required during fabrication.
Subject Classification: 30.85; 35.80.
58(1975); http://dx.doi.org/10.1121/1.380636View Description Hide Description
This is the first of a sequence of papers, which develop ray‐theory expressions for velocity profiles with depth dependence only where the velocity is expressed as a generalized power series in depth. The method treats nonanalytic as well as all analytic (i.e., expandable in a Taylor series in depth) profiles. This first paper is limited to profile expansions about a velocity minimum and to the case of rays which turn around in a layer. Here the ray‐theory results may be expressed as generalized power series in the phase velocity. Ray‐theory range, travel time, range derivatives, phase integral function, and group velocity are treated. The method treats certain profiles, which were heretofore regarded as insolvable in closed form. Even when alternative approaches are tractable, the method has certain advantages. These include simpler and more accurate computing techniques, ease of range derivative evaluation, and improved understanding of theoretical results. Complete derivations are presented and results are verified by comparing the new method solutions with classical results for the linear, hyperbolic cosine, and exponential profiles.
Subject Classification: 30.20, 30.25; 20.20.
58(1975); http://dx.doi.org/10.1121/1.380635View Description Hide Description
The acoustic pressure due to a point harmonic source in a horizontally stratified ocean is represented as a multipath expansion of the integral representation. Generalized Wentzel–Kramers–Brillouin–Jeffreys solutions are used to solve the depth‐dependent wave equation. By applying numerical integration in conjunction with stationary phase, the problem of caustics is eliminated. Computed predictions are compared with theoretical results and another computer model.
Subject Classification: 30.20; 20.20; 85.84.
58(1975); http://dx.doi.org/10.1121/1.380638View Description Hide Description
The attenuation of a 1 MHz ultrasonic signal has been measured in 78%N2/22%O2 at temperatures of 302 °K, 467 °K, and 689 °K and pressures from 1 atm to 1/60 atm. The measuredattenuation was corrected for classical effects to obtain the attenuation due to rotational relaxation. In addition, the total absorption has been compared to computed values based on a linear combination of viscous,thermal conduction,diffusion, and relaxation losses to check the constants used for each of these terms. The rotational relaxation time of the mixture was found to be 7×10−10, 10×10−10, and 21×10−10 sec atm at 302 °K, 467 °K, and 689 °K, respectively.
Subject Classification: 35.20, 35.35.
58(1975); http://dx.doi.org/10.1121/1.380639View Description Hide Description
An analysis is presented of the superharmonic, subharmonic, and combination resonances in a multi‐degree‐of‐freedom system which has cubic nonlinearity and modal viscous damping and is subject to harmonic excitation. It is shown that in the absence of internal resonances, the steady‐state response contains only the modes which are directly excited. It is shown that in the presence of internal resonances, modes other than those that are directly excited can appear in the response. The strong influence of internal resonances is exhibites in numerical examples involving hinged–clamped beams. It is shown that when a multimode solution exists the lowest mode can dominate the response, even when it is not directly excited.
Subject Classification: 40.30, 40.20.
58(1975); http://dx.doi.org/10.1121/1.380641View Description Hide Description
The paper summarizes noise‐reduction research and development programs of The Boeing Company from the early 1950s to the present time. It discusses the background for each program and presents the end results obtained or progress made. Acoustic treatments are described that permit all current Boeing turbofan†powered commercial aircrafdt models to comply with applicable U.S. Government and international noise rules. Benefits attainable through noise‐abatement operating procedures are included. The paper concludes with a discussion of unsolved problems that need attention by the scientific community to permit continuing reductions in aircraft noise.
Subject Classification: 50.40, 50.50, 50.55, 50.75.
58(1975); http://dx.doi.org/10.1121/1.380642View Description Hide Description
Flyover noisecharacteristics are presented for the Boeing 707‐300B/C equipped with standard production nacelles and with experimental quiet nacelles modified by the addition of sound‐absorbingmaterials. Acoustic‐treatment theory and technology required to develop the quiet nacelles are discussed. Results from, and techniques for, noise‐certification flight testing to meet FAA regulations are presented. Comparisons of noise levels of a modified 707 aircraft,noise floors, and noise from other operational aircraft are provided.
Subject Classification: 50.40, 50.50, 50.55, 50.75.
58(1975); http://dx.doi.org/10.1121/1.380643View Description Hide Description
A review is given of various noise‐reduction concepts pertinent to JT8D‐powered 727 and 737 airplanes, with emphasis on acoustic technology. Acoustic treatment for engine fan noise reduction is shown to result in production airplane configurations that comply with existing FAR Part 36 regulations for new airplanes. Two jet noise suppression concepts aimed at further 727 noise reduction are discussed. An ejector/suppressor concept that has been demonstrated in an FAA‐sponsored ground and flight test program is shown to achieve 6‐ to 8‐EPNdB suppression of jet noise at takeoff power. Airplane performance penalties, however, have precluded production application of this configuration. The refan concept, currently under development on a NASA contract, involves JT8D engine modifications to incorporate a higher work‐extraction fan. This modification realizes lower jet noise that, when coupled with extensive acoustic treatment, results in significant noise reductions with a minimum impact on airplane performance.
Subject Classification: 50.40, 50.50, 50.55, 50.75.
58(1975); http://dx.doi.org/10.1121/1.380644View Description Hide Description
A summary review is presented of the results of current and recently completed noise‐reduction programs for DC‐8 and DC‐9 airplanes. The relationship of these programs to other efforts by Douglas to develop quieter CTOL airplanes is briefly outlined. The engine/nacelle concepts studied were (1) application of nacelle acoustic treatment, (2) variable‐area exhaust nozzles, (3) jet exhaust noise suppressors, and (4) engines with larger diameter new front fans. Acoustic analyses leading to the selection of certain engine/nacelle configurations are described, as well as design constraints which limit the noise‐reduction capability of certain designs. Acoustic design features of the modified engine/nacelles are discussed along with estimates of the changes in aircraft community noise levels, as well as estimates of the changes in aircraft weight, performance, cost, and operational restrictions. The noise‐reduction estimates for the DC‐8 and DC‐9 are based on ground runup and flyover noise tests. Comments are presented on the potential noise‐reduction benefits, and associated aircraft operational problems, resulting from noise‐abatement operating procedures.
Subject Classification: 50.40, 50.50, 50.30, 50.55, 50.75.
58(1975); http://dx.doi.org/10.1121/1.380637View Description Hide Description
An analysis of noise measurements taken at various distances from a number of aircraft during noise flight testing is presented. Data scatter is shown to increase with distance and with decreasing aircraft/observer angle. Data normalization techniques, utilizing current standards, are inadequate at large distances and low airplane/observer angles. Aircraft noise measurements taken under uncontrolled situations are of doubtful value in predicting community noise.Noise measurements obtained directly beneath the airplane flightpath, under carefully controlled conditions, are highly repeatable and adjustable for moderate distances (within 1 to 2 EPNdB); however, when adjusted to sideline distances, these data can lead to variations on the order of 5 EPNdB. The accuracy of these sideline noise estimates is of prime concern since they are used to determine aircraft noise ’’footprints’’ in noise‐exposure studies. Because a 3‐EPNdB change in EPNL can lead to area halving or doubling, it is concluded, based on current technology, that the use of footprints in community noise‐impact studies can be extremely misleading.
Subject Classification; 50.20, 50.75, 50.80.
58(1975); http://dx.doi.org/10.1121/1.380645View Description Hide Description
In a field investigation of noise levels in Cincinnati, Ohio, 20‐min noise recordings were made at 19 sites during 19 weekdays, at ten different times, according to an incomplete Latin‐square statistical plan. L 10, L 50, and L 90 values were determined for each sample. An analysis of variance was carried out separately for each of the three parameters and the significance of sampling hours and days analyzed. L 10 and L 90 values were found highly correlated to L 50 values (r = 0.96 and 0.93). However, the L 10 − L 90 difference which is the basis of the Traffic Noise Index was found to be poorly correlated to L 50 levels (r = 0.44).
Subject Classification: 50.70; 50.75.
58(1975); http://dx.doi.org/10.1121/1.380646View Description Hide Description
Optimum signal processing for passive sonar target range and bearing estimation is discussed for the case where the sonar array consists of an M‐element linear array of hydrophone point detectors whole individual outputs are corrupted by sensor‐to‐sensor‐independent self‐noises of arbitrary power spectra. By comparing the measurement error covariance matrix to the Cramér–Rao matrix bound the system performance, relative to the theoretical optimum, is determined. Further, the Cramér–Rao bound is used to determine an optimum signal processor. The optimum processor is configured as a set of M (M‐1)/2 cross‐correlator delay estimators (one for each hydrophone pair), followed by a Gauss–Markov estimation of the array delay vector (target steering vector), which in turn is followed by a linear weighting of the estimated delay vector elements to determine a bearing estimate and a range estimate. The processor is shown to have the Cramér–Rao matrix bound for its measurement error covariance matrix.
Subject Classification: 60.30; 30.82.
Comparison between the growth of the averaged electroencephalic response and direct loudness estimations58(1975); http://dx.doi.org/10.1121/1.380647View Description Hide Description
The question of the validity of the auditory‐averaged electroencephalic responses (auditory AER) in auditory research and in audiometry is a very important one. In the present experiments, auditory AER and direct estimates of loudness magnitude were conducted concurrently on the same subjects, in the hope of reducing the consequences of the great within‐subject and between‐subject variability always encountered in such experiments, and of the differences in the degree of alertness. The peak‐to‐peak amplitude of the N 1 − P 2 waves and the N 1 and P 2 latencies were used as characteristics of the auditory AER. Even in these conditions, no clear relations appeared between the AER and the loudness estimations. The discussion shows that they are probably not directly connected.
Subject Classification: 65.59, 65.50.
58(1975); http://dx.doi.org/10.1121/1.380648View Description Hide Description
A manikin for hearing aid and related acoustic research was designed with median human adult dimensions. Ear simulation matches the acoustic response with an auricle, an ear canal, and an eardrum that equal the median ear in dimensions, acoustic impedance and modes. Dimensions of torso and head are based on published anthropometric data, but the auricle is based on data obtained for this development. The ear canal and eardrum are adapted from the earlike coupler by Zwislocki. The ear entrance sound pressure was found to be relatively insensitive to surface or skin impedance of the head. Validating measurements show the manikin, designated KEMAR, to be like a median human in acoustic response to free fields.
Subject Classification: 65.22, 65.80, 65.35,65.82.