Volume 29, Issue 6, June 1957
Index of content:
- PROGRAM OF THE FIFTY‐THIRD MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
- Session A. General Acoustics
- Contributed Papers
29(1957); http://dx.doi.org/10.1121/1.1918709View Description Hide Description
Comparisons show good agreement between empirical and theoretical relations describing finite amplitude distortion for both plane and spherical waves. Magnitudes of the harmonic components of pressure in an originally sinusoidal spherical wave can be described in terms of a parametre called distortion factor which is a simple function of wavelength distance from the source and the source intensity. Evidence for the existence of a limiting acoustic pressure both for plane and spherical waves will be reviewed and useful interpretations of this limiting pressure will be offered.
Some Experiments in Unsteady Aerodynamics Leading to the Development of an Acoustic Drag Coefficient29(1957); http://dx.doi.org/10.1121/1.1918711View Description Hide Description
In the course of some supersonic wind‐tunnel experiments, an unsteady flow condition was encountered. A significant increase in drag accompanied the instability. It has been possible to account approximately for the drag increase by means of an acoustic drag coefficient based on the acoustic power withdrawn from the flow. The experiments were carried out at Mach numbers 3.5 and 4.3 for a cylindrical model, closed at its ends, with a flow‐separation spike extending from the upstream end. Several steady and unsteady flow regimes were observed. For spike lengths of about 2‐cylinder diameters a stable conical shock was attached at the spike shoulder. For somewhat shorter lengths, however, a strong oscillation of the shock took place, accompanied by a pronounced increase in drag. A simplified, one‐dimensional model is treated analytically, and an expression for the acoustic disturbance downstream of an unsteady shock is developed. (Some differences from published work are noted.) On the basis of this expression the acoustic power radiated by the oscillating shock is estimated. The energy removed from the flow as acoustic power represents work done on the model by the flow. Thus an acoustic drag coefficient can be specified, which is found to account approximately for the observed increase in drag.
29(1957); http://dx.doi.org/10.1121/1.1918713View Description Hide Description
This paper describes measurements of noise produced at the surface of a high‐speed rotor which is magnetically suspended and accelerated in a test chamber with controlled atmospheric conditions. Measurements of air drag and temperature rise at the high‐speed surface also will be described. [This work has been supported under Air Force Contract No. AF‐33(616)‐2331.]
29(1957); http://dx.doi.org/10.1121/1.1918715View Description Hide Description
The speed of sound in water at 5 temperatures near room has been determined from measurements of variation of phase with distance on the axis of a progressive wave emitted by a small piston‐like radiator. The minimum distance from source to receiver was large enough to render negligible the Fresnel‐type interference errors; errors arising from the finite diameter of the receiver were also negligible. Reverberation effects were reduced partly by use of absorbing baffles and partly by pulsing the sound at a rate such that the reverberation died out between pulses. Readings of the screw were taken each half‐wavelength, as indicated by Lissajous figures, for 30 wavelengths (total distance of 18 mm for the frequency of 2.6 Mc). The variation of distance with phase was accurately linear; the standard deviations of the points were about 1 micron, which is also the least count of the screw. The standard deviations of the speed of sound were in the range 0.04 to 0.12 m/sec. The results agree with our earlier ones obtained by an L/T method to within 0.04 m/see (27 ppm) and disagree with those measured with ultrasonicinterferometers.
Effect of Small Additions of Cobalt on the Dynamic Magnetostrictive Properties of Nickel Ferrous Ferrite29(1957); http://dx.doi.org/10.1121/1.1918717View Description Hide Description
The dynamic magnetostrictive properties of nickel ferrous ferrite were investigated to determine its suitability for use in electromechanical transducers. A previous investigation [C. van der Burgt, J. Acoust. Soc. Am. 28, 1020–1032 (1956)] showed that small additions of cobalt increase the value of the electromechanical coupling coefficient. Recent work by the authors showed that the improvement resulting from cobalt additions is much larger when 3% to 6% of the NiO is replaced by FeO and the specimens are quenched from above 1100°C to room temperature. In this manner a reproducible material having values of electromechanical coupling coefficient equal to or greater than that of nickel is obtained. The maximum value of the coefficient (0.36) occurred for sintered at 1400°C and quenched to room temperature.
29(1957); http://dx.doi.org/10.1121/1.1918719View Description Hide Description
In the past, sonic processes operating discontinuously at levels of a few hundred watts did not require that the engineer design for optimum utilization of energy or avoid apparatus subject to rapid deterioration through fatigue. This paper discusses some of the problems introduced by the use of continuous power levels of hundreds of kilowatts.
- Session C. Architectural Acoustics
29(1957); http://dx.doi.org/10.1121/1.1918721View Description Hide Description
Two large auditoria have just been built by the Province of Alberta in the cities of Calgary and Edmonton. Each hall is intended for a variety of uses, who conflicting requirements result in an interesting architectural and acoustical problem. The main auditorium, which seats 2700 persons, is designed for both concert use and large‐scale stage presentations. Surrounding it are facilities for smaller‐scale meetings, exhibitions, conventions, and social gatherings. The acoustical design was done by the Alberta Department of Public Works in consultation with the National Research Council of Canada. This paper will report on the performance of the auditoria as indicated by physical tests and by a test concert. In the design stages extensive sound absorption tests were made in the reverberation chamber on such auditorium components as wall panels, carpets, and occupied and unoccupied theater chairs. Reverberation measurements were also made in the auditoria, as the components were installed. The relation between the two sets of data will be discussed.
29(1957); http://dx.doi.org/10.1121/1.1918723View Description Hide Description
During the last six months the ART Acoustics Section has been moved to new quarters. In these new facilities a reverberation room has been built for use in the acoustic research of that group. The room has a volume of approximately 3000 cu ft and a reverberation time of about 5 seconds. The four walls and ceiling of the room have been splayed. Measurements indicate that the room has a relatively uniform reverberation with respect to frequency. Data will be presented to show the ambient noise level in the room, the steady‐state sound energy distribution within the room, and the reverberation time as a function of frequency and location within the room.
29(1957); http://dx.doi.org/10.1121/1.1918725View Description Hide Description
With the recent completion of a new 3000 cubic foot reverberation room, Riverbank Acoustical Laboratories are now able to meet explicitly all the requirements of the American Standards Association and the American Society for TestingMaterials for the measurement of airborne‐sound transmission loss of walls and partitions. The measuring method is explained and the several checks for accuracy are discussed. Some of these checks have been: the influence of microphone position; measurement error and repeatability; insurance against sound leaks and flanking transmission paths; interchanging transmitting and receiving rooms; and agreement of measured results with those which can be calculated.
29(1957); http://dx.doi.org/10.1121/1.1918728View Description Hide Description
Three reverberation time equations and three sound power equations are scrutinized for their utility in the calculation of sound power from measurements of the average steady‐state sound pressure level and reverberation time in a reverberant room. It is noted that the Sabine reverberation time equation is theoretically plausible for this use, even in a moderately absorbing room, providing it can be established that the average sound energy density decays exponentially with time. The sound power calculation may also be performed in terms of the “Sabine absorption” which is simply the absorption computed (by the Sabine equation) from the observed reverberation time and room volume. The “Sabine absorption coefficient” may exceed unity; indeed, it approaches infinity as the walls of the room become completely absorbing. An experiment was performed in a concrete room of volume 1350 ft3, to which three amounts of sound absorbing material were added. Under these four conditions the appropriate measurements were made with two independent systems (one having half‐octave bands, the other third‐octave bands), from which were calculated the sound power spectra of a printing calculator and a unit heater. In addition to simplicity, both theory and experiment support use of the “Sabine absorption” instead of the “room constant” in the calculation of sound power.
29(1957); http://dx.doi.org/10.1121/1.1918730View Description Hide Description
This paper is a report on continuing research on acceptable noise levels in office spaces. A new questionnaire has been tried with different adjectival cues and more categories on the rating scales. The results with this questionnaire show a nonlinear relation to those obtained from the previous study. The maximum permissible noise levels are in agreement with those published earlier. More difference between the stenographic and the executive personnel's requirements were obtained during this study than was the case for the previous study.
29(1957); http://dx.doi.org/10.1121/1.1918731View Description Hide Description
The prediction of noise levels in living and working spaces is of considerable importance in the architectural design of buildings. Two charts have been developed which simplify the tedious process of calculating the level of noise entering a room through walls, floor, and ceiling from both interior and exterior sources. The use of published transmission loss and sound‐absorption data with the charts and some simple data forms allow the rapid estimation of interior noise levels without resort to slide rule or log tables. Accuracy is commensurate with one's ability to determine criteria for background noise in the spaces under consideration.
29(1957); http://dx.doi.org/10.1121/1.1918733View Description Hide Description
Because of the architectural and mechanical advantages of constructing office partitions only to the height of an acoustic tile ceiling, the problem of noise transmission above the partition and the resulting loss of privacy is occurring increasingly frequently. This paper discusses a method of predicting the noise reduction along this path in terms of the conventional room factors of the source and receiving rooms as well as factors such as the acoustic tile material and configuration, the height of the space above the ceiling, the width of the rooms, etc. A tentative method of evaluating the “degree of speech privacy” afforded by a given NR in the presence of a given background noise spectrum is also shown.
29(1957); http://dx.doi.org/10.1121/1.1918735View Description Hide Description
Hollow, lightweight aggregate concrete masonry blocks are widely used for the walls of offices, studios, dormitories, and the like. It has been found that the transmission loss provided by such walls in the field does not compare favorably with the loss which one would predict from the weight of the wall. Further, the transmission loss values which we measure in the field are often found to differ appreciably from test data reported by the National Bureau of Standards and other testing laboratories. This paper will present field data and discuss some of the reasons for the discrepancies between the theoretical mass law, transmission loss, the values measured in the laboratory, and the values actually obtained in the field.
29(1957); http://dx.doi.org/10.1121/1.1918737View Description Hide Description
In an effort to determine optimum materials for use as a comfort measure under carpeting in airplanes, an experimental study was made of the static and dynamic characteristics of seven types of isolation materials and various combinations of these materials. Tests of wear under simulated use conditions were made on those materials most effective for isolation. Results (which are presented in some detail) indicate that, of the materials tested, a foam Neoprene has the best combination of isolation and durability characteristics from the viewpoint of physical measurements. Attempts to develop an approach for predicting dynamic characteristics from static measurements were partially successful. Various nonlinear effects, reported by previous observers of elastomeric material behavior, were noted.
29(1957); http://dx.doi.org/10.1121/1.1918739View Description Hide Description
While in the past it has been difficult to give structural openings a sound transmission loss of the same order as that of the walls, recent work on doors, windows, and ventilation ducts has achieved this goal. Slides will be shown of the basic design of such structures and of typical installations. Measurements at the National Bureau of Standards will be cited.
29(1957); http://dx.doi.org/10.1121/1.1918741View Description Hide Description
An engineering office is located beneath a printing shop in a particular all‐concrete industrial building. The engineers in the office have complained about the high noise levels caused by the printing machines, most of which were mounted directly on the concrete floor overhead or on wood skids. This problem led to the design and evaluation of vibration isolation pads for a representative printing press such that the noise levels in the engineering office may be reduced to reasonable acceptable values. This paper summarizes pertinent airborne and structure‐borne noise levels before and after installation of the vibration mounting arrangements for one printing press.
- Session D. Psychoacoustics I
29(1957); http://dx.doi.org/10.1121/1.1918743View Description Hide Description
Certain difficulties occur in weak signal studies which can be avoided if large signal studies can be utilized to study the same phenomena. Weak signals are those which are employed in detection experiments. They are those which are commonly considered to be in the “threshold” range for the noise level employed in the experiment. Large signals are those well above the noise level, so large that a detection experiment would be impractical. The problems encountered in weak signal experiments involve the observer's ability to remember parameters of the signal such as frequency and amplitude. Uncertain memory has effects on detection rates which are functions of the signal level. These effects are difficult to isolate from those arising from the properties of the auditory mechanism, and these appear to be inseparable if weak signal studies alone are employed. One would expect some of these memory effects to disappear in large signal recognition experiments. With large signals, frequency memory has relatively small effect on the observation, signal itself carrying the necessary information. Amplitude memory can be eliminated by presenting two signals, the observer being asked to state which is the louder. (This experiment in its operation is identical with one employing a comparison tone and a standard.) Performance in experiments of these types theoretically depends on the energy of the difference signal, and optimum performance is indicated by the ratio 2E/N 0, where E is this energy. Using the variable d′, an observer efficiency can be determined, thus permitting an estimate of the memory effects. Data are presented for weak and large signal experiments first estimating the effect of frequency memory and then estimating the effect of amplitude memory. The method of using large signal studies as a substitute for weak signal studies in investigating the hearing mechanism is described. (20 min.)
29(1957); http://dx.doi.org/10.1121/1.1918745View Description Hide Description
The question whether the measured threshold of audibility improves with experience has widespread implications. It refers to the nature of the threshold itself, but it also affects the psychophysical experiments involving comparisons of thresholds of audibility. If practice has no effect, then the amount of experience of the listeners can be disregarded and results obtained on groups of listeners with unequal sophistication may be compared with each other. If the opposite is true, experiments using threshold comparisons would require either pretaining of the listeners or careful counterbalancing. In several experiments, inexperienced listeners showed statistically significant improvement in their threshold of audibility with practice. Depending on the experimental conditions, the average improvement amounted to from 4 to 10 db. Experiments performed on several groups of listeners under similar conditions, yielded practically identical results.
29(1957); http://dx.doi.org/10.1121/1.1918747View Description Hide Description
Measurements have been made of residual masking in a “free‐field,” produced by a 250 cps pure tone. Data have been obtained for the shift in threshold, 150 and 200 milliseconds following the cessation of the masking tone, employing the ABX Technique described by W. A. Munson and M. B. Gardner [J. Acoust. Soc. Am. 22, 177 (1950)]. Such data have been obtained at sound pressure levels of 110 and 90 db. The threshold shift at 250 cps produced by thermal noise has been measured as a function of the mel band intensity level of the noise. From these data so‐called “loudness density patterns” for the 250‐cps masking tone have been computed. These are plots of the loudness contribution in millisones per mel vs frequency 200 milliseconds following exposure to the masking tone. [This work has been supported under U. S. Air Force Contract AF 33(616)2454.]