Index of content:
Volume 25, Issue 3, May 1953
25(1953); http://dx.doi.org/10.1121/1.1907048View Description Hide Description
Aircraft noise presents a system problem which to date has been attacked mainly at the level of individual components. The system includes: (a) aircraft as noise sources; (b) atmosphere and terrain as influences on sound propagation; (c) people, under several classes and conditions, as responders to noise; (d) physical components for controllingnoise; (e) operating procedures for reducing noise exposure in communities; (f) public relations; (g) aviation planning policies and economics; and (h) many organizations concerned with characteristics and consequences of aircraft noise. The nature of these components is reviewed in a general way, with emphasis on their inherent interrelations. This discussion provides a framework for unifying the several subjects included in the present Aircraft Noise Symposium.
25(1953); http://dx.doi.org/10.1121/1.1907049View Description Hide Description
Available basic characteristics of different aircraft noise sources under the condition of zero forward speed are summarized: Total acoustic power; acoustic mechanical efficiency; directivity and frequency characteristics are given for the rotation and vortexnoise of propellers, for the exhaust noise of reciprocating engines, and for different types of jet engines. The physical mechanisms underlying the different noise sources are discussed and the influence of changes in the parameters such as tip speed or pitch of the propeller and diameter or velocity of the gas jet are shown. In cases where measurements on the actual propulsion systems are incomplete, the basic physical conclusions are drawn from experiments on model airscrews and small air jets. The changes in the characteristics of the noisegenerators during flight are discussed briefly.
25(1953); http://dx.doi.org/10.1121/1.1907051View Description Hide Description
The mixing region of a jet is observed to be a complex noise generator. The noise produced is highly directional and is affected by various geometric and flow parameters as well as by conditions in the settling chamber upstream of the nozzle. Noise measurements for a family of circular model air jets ranging in diameter from to 12 inches are consistent with available data for a turbojet engine. The intensity of the fluctuating pressure field near the jet is greatest at an axial distance of approximately two diameters downstream from the nozzle exit and decrease generally with increasing distance. The frequency spectrums recorded near the jet boundary are usually peaked, the peak frequencies being higher near the jet exit than at points farther downstream. These noise frequencies generally increase with increasing jet fluid velocity and decrease with increasing jet size. Hot wire surveys of turbulence (axial velocity fluctuation) in the jet stream indicated spectrums which were very similar in quality to the noise spectrums recorded just outside the jet boundary and at the same axial stations.
25(1953); http://dx.doi.org/10.1121/1.1907052View Description Hide Description
The noise of a jet changes character after the pressure ratio exceeds the critical value appropriate to sonic exit velocity, the general roar being dominated by a loud “whistling” or “screeching.” Schlieren photographs show that sound waves of ultrasonic frequency are caused by the transition of the initially laminar boundary layer to turbulence and also by this turbulence interacting with the shock waves of the flow. Larger disturbances have also been noted, involving both the jet stream and some of the air external to the jet, and these also give rise to sound waves which have been photographed: it is these which are held responsible for the audible effects. A two‐dimensional study has shown the latter phenomenon to be enhanced, and it is shown how the system of disturbances is self‐maintained by virtue of sound waves creating initially small disturbances at the jet exit. The directionality of the sound field has been predicted and found in agreement with experiment, and the dimensions of the motion are compatible with the suggested mechanism. The relation to edge tones is pointed out and the mechanism indicated, a photograph of this phenomenon also being shown. Finally mention is made of how the characteristic noise of jets working above the critical pressure might be reduced, the suggested methods having been found successful in practice.
25(1953); http://dx.doi.org/10.1121/1.1907053View Description Hide Description
An attempt is made in this paper to classify noises of various aerodynamic origin by means of an efficiency of conversion from mechanical to acoustical energy, and also by means of representative spectra associated with corresponding characteristic frequencies. The classification has been tried successfully on measurements of wind tunnelnoises, turbojet noises, and air jet noises.
25(1953); http://dx.doi.org/10.1121/1.1907054View Description Hide Description
The basic concepts concerning the mechanism of propeller noise generation are reviewed, and equations for calculating the noise field both near and far from the propeller are given. The problem of noise due to non‐steady airloads on the blades, and differences in noise from the approaching and retreating blades are discussed. Effect of tip speed, number of blades, blade thickness, and blade width are considered, and it is shown that reducing the tip speed and increasing the number of blades are probably the most effective means of reducing the efficiency of noise generation and alleviating the noise problem. Noise characteristics of such special types as the supersonic, dual‐rotating, tandem, and shrouded propeller are also presented. Studies of propeller weight show that substantial noise reduction on transport type propellers would result in a considerable weight penalty. For the propeller‐driven aircraft in the 400 to 500 mph speed range, the slower turning quieter propeller is likely to have better propulsive efficiency than the noisier high speed propeller. Hence, in this speed range, the weight penalty of the quieter propeller may be offset by its higher propulsive efficiency.
25(1953); http://dx.doi.org/10.1121/1.1907055View Description Hide Description
The study of the different atmospheric effects indicates that in short‐range sound propagation the attenuation by irregularities in the wind structure (gustiness) often is of major importance in comparison with humidity, fog, and rain, and ordinary temperature and wind refraction. However, the ground attenuation can be of equal importance to the gustiness, in particular, when the sound source and the receiver are sufficiently close to the ground. The effect on the attenuation of the height of the source and the receiver off the ground is presented as a function of frequency for a typical ground impedance. The attenuation curve exhibits a maximum which in most cases lies at a frequency between 200 and 500 cps.
25(1953); http://dx.doi.org/10.1121/1.1907056View Description Hide Description
The aircraft engine manufacturer has been continually confronted with the factor of noise control in conjunction with the development and production testing of engines. The scope of this control activity has increased many fold during recent years as the result of the rapid growth in types and size of military power plants. Presented in this paper is a review of the over‐all control procedure including typical noise sources, control objectives, recent techniques used in the control and attenuation of aircraft powerplant noise, problems associated with the design of noise control provisions, and limitations in the existing methods. This review includes data on reciprocating, turbojet, turboprop, and ram jet powerplants along with a description of installations used for controlling the noise both in ducted exhaust systems and in propeller‐type test cells. This discussion cites cost, space, and high tolerance in design factors as being the major design problems. Areas of continued endeavor are outlined for the engine manufacturer, acoustical consultant, and the noise control equipment manufacturer.
25(1953); http://dx.doi.org/10.1121/1.1907057View Description Hide Description
A noise control project is described which includes a noise muffler for the ground testing of the F‐89 air plane which is powered by two turbojet engines. Included are the prediction of performance, acoustical design considerations, correlation of acoustical and thermodynamic requirements, general construction, acoustical tests during construction, noise survey around the exposed airplane, and neighborhood survey around the completed installation. This noise suppressor is comparatively simple and inexpensive, requires no water cooling even for afterburner operation, and has proved adequate in terms of neighborhood relief and protection of operating personnel.
25(1953); http://dx.doi.org/10.1121/1.1907058View Description Hide Description
The various schemes currently used for controlling the noise of aircraft engine test stands and warm‐up operations on the ground are briefly surveyed. The first step is to evaluate a design goal for acceptable levels in the neighborhood for the installation. The size of cross section required to handle the intake and exhaust gases is determined from the amount of air consumed, the temperature, acceptable gas velocities, and the aerodynamic pressure drops. The modern trend in size of engines is such as to require cross sections of 200 sq ft or more. Several different styles of structures have been found successful. The various types used—steel mufflers, duct splitters, and plenums and 180‐degree bends—are discussed, and typical data are given for each type. Economic considerations are emphasized. The use of scale model acoustic tests for untried acoustic designs is strongly recommended.
25(1953); http://dx.doi.org/10.1121/1.1907059View Description Hide Description
An acoustic model study was made of the silencer for a propeller engine test cell in which the acoustical treatment consisted of conventional splitters for high frequency silencing and thick splitters designed to give low frequency reduction. The scale chosen for the model study was , and absorption coefficients of silencing treatment were simulated at frequencies eight times those of interest in the full‐scale structure. After construction of the test cell, field measurements were made of the noise reduction provided by the silencing treatment, using as a sound source both a propeller engine and loudspeakers driven by a thermal noise source. Measurement of the noise reduction provided in the scale model permitted prediction of reductions in the full structure which showed reasonably good agreement with field measurements. On the basis of the model study and field measurements on high frequency splitters, an empirical relationship between noise reduction and absorption coefficient was determined.
25(1953); http://dx.doi.org/10.1121/1.1907060View Description Hide Description
Radiation patterns for gas turbine exhaust stacks have been calculated for the case of wide‐band noise, assuming negligibly low gas velocities and temperatures. Both shape of stack and band width of analyzing equipment are considered. Theoretical results are compared with data measured on models and on a prototype exhaust system.
25(1953); http://dx.doi.org/10.1121/1.1907061View Description Hide Description
The materials and structures problems associated with the construction of aircraft noise silencing systems often make classical solutions to the acoustic problems extremely difficult. The associated problems of fluid mechanics and physical construction must be carefully considered along with acoustical requirements at the time of initial design. A review of some of these problems specifically associated with various types of aircraft engines will be useful. Similarly, the various types of acoustical material suitable for broad band attenuation will be considered from their structural standpoint.
Ultrasonic Waves and Electrochemistry. I. A Survey of the Electrochemical Applications of Ultrasonic Waves25(1953); http://dx.doi.org/10.1121/1.1907062View Description Hide Description
The applications of ultrasonic waves in electrochemistry are discussed in terms of (1) the effects of ultrasonic waves on electrode processes, (2) electrokinetic phenomena involving ultrasonic waves, and (3) the use of ultrasonic waves as a tool in the study of the structure of electrolyticsolutions.
25(1953); http://dx.doi.org/10.1121/1.1907063View Description Hide Description
When ultrasonic waves are introduced into an ionic solution or a suspension of charged colloidal particles, alternating potential differences are generated between points separated by a finite distance in the direction of propagation (other than an integral multiple of the wavelength). Pulse‐modulated ultrasonic waves have been used to study these effects at a carrier frequency of 200 kc/sec. The colloidal vibration potentials have been measured in a number of colloidalsilicasuspensions of various concentrations, particle sizes, and conductivities. The experimental results are in general agreement with the mathematical treatment of Enderby. Measurements of ionic vibration potentials in potassium chloride solutions indicate a value of 6μv per cm/sec for a 10−3‐molar solution. This value corresponds to a difference of 80 in the apparent gram‐ionic‐masses of the hydrated potassium and chloride ions.
25(1953); http://dx.doi.org/10.1121/1.1907064View Description Hide Description
If a wire covered with a fiber or a porous coating is submerged in a dilute electrolytic solution and irradiated with ultrasonicwaves, an alternating potential of the same frequency is produced on the wire relative to the bulk of the surrounding solution. The effect has been studied with pulse‐modulated ultrasonicwaves at a carrier frequency of 200 kc/sec. For a double cotton‐covered copper wire in 0.0001‐molar sodium chloride solution, the open circuit response has been found to be approximately 10−7 volt per dyne/cm2. In general, the acoustical response is dependent on the nature and concentration of the electrolyte, the pH, and the type of covering but independent of the metal. The effect is explained on the basis of the interaction of the ultrasonicwaves with the diffuse double layer of ions surrounding the fibers of the covering. Hydrophones involving this effect offer particular advantage at frequencies above 100 kc/sec because of the small dimensions of the acoustically sensitive elements.
Ultrasonic Waves and Electrochemistry. IV. The Production of Alternating Components in the Potential of a Polarized Hydrogen Electrode with Ultrasonic Waves25(1953); http://dx.doi.org/10.1121/1.1907065View Description Hide Description
If sound wavesinteract with an electrode upon which a gas is being liberated by electrolysis, an alternating potential of the same frequency is developed on the electrode relative to the bulk of the surrounding solution. The effect has been studied in the case of a polarized hydrogen electrode with pulse‐modulated ultrasonicwaves at a frequency of 200 kc/sec. Under favorable conditions the amplitude of the alternating potentials is of the order of 10−3 volt per dyne/cm2. The effect has been found to depend on the formation of gas bubbles at the electrode surface. Although the amplitude of the effect is a function of the conductance of the solution and the current density, it is essentially independent of the hydrogen ion concentration of the solution in the range 10−4 to 10−8 molar and is also independent of the specific metal upon which the hydrogen gas is being liberated. On the basis of these results, the effect is attributed primarily to the modulation of the i‐r drop in the solution immediately adjacent to the electrode surface by periodically expanding and contracting gas bubbles.
The Compressibility of Solutions. II. An Ultrasonic Study of Aqueous Solutions of Some Simple Amino Acids and Their Uncharged Isomers at 25°C25(1953); http://dx.doi.org/10.1121/1.1907066View Description Hide Description
Some time ago, the first author showed that the apparent molal compressibility of the solute in a dilute aqueous solution is a linear function of the square root of the concentration for electrolytes, and of the first power of the concentration for nonelectrolytes. Both of these linear relationships may hold up to fairly concentrated solutions and afford convenient interpolation formulas for the compressibility as a function of concentration. The conclusion about electrolytes has been confirmed by several others, but little has been done with nonelectrolytes.
This article describes a 4‐Mc interferometer used to determine adiabatic compressibilities of solutions to about ±0.01 percent. Studies of several amino acids and their uncharged isomers, including glycine and glycolamide; α‐ and β‐alanine and lactamide, from about 0.05 M to near saturation, show linearity with concentration in dilute solutions and a general increase in the limiting slope and decrease in the limiting value of the apparent molal compressibility with increasing dipole moment of the solute molecules. These results are interpreted qualitatively by the theories of dipolar interaction developed by Fuoss and Kirkwood, and of dipolar charging energy developed by Kirkwood, but systematic deviations are observed for the apparent molal compressibilities and the volumes and heat capacities studied previously.