Index of content:
Volume 24, Issue 5, September 1952
24(1952); http://dx.doi.org/10.1121/1.1906918View Description Hide Description
If a system comprising a water column bounded by a steel cylindrical shell is driven at a fixed frequency, the pressure amplitude has been found to vary along the axis in a complex manner. On the hypothesis that this phenomenon results from the excitation of two or more modes of vibration having different speeds of phase propagation, an investigation has been made of waves in liquid‐filled cylindrical shells. It is shown that, in addition to the modes associated with boundaries of high impedance, there is also a possible mode associated with each type of free wave in the shell. The phase velocities of several symmetrical free waves are calculated by an extension of the method employed by Love for solid cylinders. A comparison with measurements appears to confirm the hypothesis.
24(1952); http://dx.doi.org/10.1121/1.1906919View Description Hide Description
Some frequencies for the symmetrical normal nodes of a finite, isotropic disk are calculated by satisfying the boundary conditions exactly at the flat surfaces and approximately at the curved surface. It is found that two sets of frequencies occur for each vibration pattern. One set approaches the odd order thickness compressional frequencies, whereas the other approaches the even order thickness shear frequencies, as the radius of the disk increases indefinitely. As a test of the validity of the approximations, the maximum residual stresses at the curved surface are compared with the compressional component of the stress at the center of the disk.
24(1952); http://dx.doi.org/10.1121/1.1906920View Description Hide Description
The forces due to acoustic radiation in a beam of finite cross section in a nonviscous medium striking a plane reflector at oblique incidence are derived from simple mechanical considerations. The formulas are applied to a wedge‐shaped vane. For a vane, the wings of which include an angle of 90°, the force turns out to be quite independent of the coefficient of reflection at the boundary between vane and medium.
24(1952); http://dx.doi.org/10.1121/1.1906921View Description Hide Description
A new method for the observation of ultrasonic field distributions described, utilizing a starch plate in a dilute solution of iodine in a manner analogous to the use of photographic emulsions. Near‐field diffraction patterns are shown as illustrative of results, which appear to be superior to those of other methods, and the advantages of the new method over previous techniques are described.
24(1952); http://dx.doi.org/10.1121/1.1906922View Description Hide Description
The velocity,dispersion, and absorption of ultrasonic waves in dry, ‐free air have been measured at 32°C, and at two and three Mc/sec, and at pressures ranging from 0.020 to one atmos. Dispersion of the velocity has been found beginning at 30 Mc/atmos, increasing by five percent at 100 Mc/atmos, accompanied by a large increase in absorption such that at the higher limits of f/p, reached measurements became nearly impossible with the equipment used. The ratio αexp/αclass decreased from about 2.4 to 1.3 and Cv/R from 2.5 to nearly 1.5 as f/p increased. The changes in velocity, absorption, and internal specific heat are interpreted as the result of the slowing of energy exchange between translational and rotational states. Assuming that relations for relaxation of translational‐vibrationai exchange also hold for this case, the relaxation time for translational‐rotational exchange as derived from the dispersionmeasurements has been found to be 2.29×10−9 sec. This corresponds to a frequency of the midpoint of the dispersion curve of 116 Mc/atmos, and to 16 as the number per molecule of collisions required for an energy exchange between translational and rotational states. Absorption results were more difficult to secure; using low frequency values a relaxation time of about 3×10−9 sec is indicated, giving 87 Mc/atmos as the f/p value of the midpoint of the dispersion curve, and 21 as the number of collisions required for the energy exchange.
24(1952); http://dx.doi.org/10.1121/1.1906924View Description Hide Description
The criteria for an “optimum” hydrophone receiving system for underwater acoustic signals are considered, and the equilibrium thermal‐noise pressurespectrum expression is shown.
Experimental ambient sea‐noise spectrum levels are in excess of the thermal noise at all frequencies below 25 kc. Extrapolation of experimental curves (−5 db/octave) shows intersection with thermal‐noise spectrum (+6 db/octave) between 50 and 200 kc, depending on sea state. (For a definition of “sea state,” see Instructions to Marine Meteorological Observers (U. S. Department of Commerce Weather Bureau, U. S. Government Printing Office, Washington, D. C., June, 1941), seventh edition, circular M.) The ambient‐noise level is expressed as a function of the equivalent temperature.
Formulas are then derived for the required plane‐wave signal level for which electrical signal‐to‐noise ratio is unity. The operating noise factor is defined in terms of the electroacoustical efficiency of the linear reversible hydrophone, the amplifiernoise figure, and the equivalent ambient‐noise temperature for the sea condition and frequency under consideration.
24(1952); http://dx.doi.org/10.1121/1.1906925View Description Hide Description
The high noise level associated with aircraft gas turbine operation creates two noise problems: (1) The reduction of the noise in the neighborhood of the installation to an acceptable level, and (2) the protection of operating personnel from excessive noise. This paper reports work done by the Aircraft Gas Turbine Division of the General Electric Company at their plants in Lockland, Ohio, and in Lynn, Massachusetts. Permissible sound levels were established for surrounding residential areas. The attenuation due to spherical divergence, and also the probable deviation due to varying atmospheric conditions, was determined. Thus, the sound level permissible at the exhaust stacks of the test cells was obtained. Knowing the noise level inside the test cell, the required sound attenuation of the stacks was calculated. Various acoustical treatments were investigated. An acoustical design is proposed which makes use of a combination on various duct treatments, and which meets the established design objectives. The permissible noise level inside the control room is discussed, and the required attenuation through the barrier between test cell and control room is calculated. A design for a control room meeting the established noise‐level criterion is proposed.
24(1952); http://dx.doi.org/10.1121/1.1906926View Description Hide Description
In a previous paper by Fletcher and Galt [J. Acoust. Soc. Am. 22, 89 (1950)] a method was described for measuring experimentally and also for calculating the interpretation aspect of the perception of speech. The listeners were considered to have normal hearing. In the present paper the same principles are applied to persons having abnormal hearing.
24(1952); http://dx.doi.org/10.1121/1.1906927View Description Hide Description
The unexplained difference in sound pressure in the ear canal which appears to exist when equally loud low frequency tones are presented alternately from an earphone and from a loudspeaker has bedeviled acousticians for many years and, unfortunately, still continues to do so. There are presented here the results of some of the measurements carried out at the Bell Telephone Laboratories which show the magnitude of the effect and various attempts at explaining it. While no satisfactory explanation has been found, it is hoped that publication of these results will stimulate interest in the problem.
The Space‐Time Pattern of the Cochlear Microphonics (Guinea Pig), as Recorded by Differential Electrodes24(1952); http://dx.doi.org/10.1121/1.1906928View Description Hide Description
Pairs of very small electrodes were placed in two or more turns of the cochlea of the guinea pig. The cochlear microphonic from a short segment (about 1 mm) of the cochlear partition can thus be recorded, and without contamination by action potentials. The outputs of the second, third, and fourth turns were compared with that of the first turn with respect to both amplitude and phase as a function of frequency. The space‐time pattern thus revealed is a traveling wave which passes up the cochlea to a distance that depends on the frequency. The pattern agrees well with that of mechanical movement (Békésy) except that the cochlear microphonic shows relatively greater amplitude (voltage) in the basal turn. Low frequencies travel farther up the cochlea than do high. The velocity (for a 750 cps tone) is about 100 m/sec in the basal turn and about 2 m/sec in the fourth turn. Phase differences of as much as 5π were observed, by means of Lissajous patterns, between the responses of the first and the third turn. Certain discontinuities in the input‐output relationships and in phase relationships were found to be a function of frequency and of position along the cochlear partition. These discontinuities occurred at or near the frequencies that showed a phase difference of 2π from the basal end of the cochlea. The relation of these critical frequencies to position along the cochlea constitutes a new “frequency‐map” of the cochlea.
The space‐time pattern of the cochlear microphonic proved to be very little affected by removal of portions of the bony wall of the cochlea or delivering acoustic energy through a hole near the apex. It is concluded that, in addition to “direct driving” of the cochlear partition by pressure differences between the two scalae and to “surface waves” arising from such drying, the transmission of transverse waves along the solid structures of the cochlear partition must also be included for a satisfactory interpretation of all of the available data.
24(1952); http://dx.doi.org/10.1121/1.1906929View Description Hide Description
Measurements have been made under the microscope of many structures in the cochlea of the guinea pig and the results plotted graphically as a function of position along the cochlea. The structures measured are the spiral lamina, Rosenthal's canal, the basilar membrane (length, width and thickness), the channels and windows of the cochlea (cross‐section area) and the hair cells (angles of orientation).
The Masking of Tones by White Noise as a Function of the Interaural Phases of Both Components. I. 500 Cycles24(1952); http://dx.doi.org/10.1121/1.1906930View Description Hide Description
Work by Hawkins, Hirsh, Licklider, Stevens, Webster, and others has shown the considerable reduction in masking which occurs when the interaural phase of either the signal or the noise is reversed, or when the interaural phase of the signal is shifted by various amounts. The present paper extends this work by shifting the interaural phases of the masking component of the noise and of the tone by various amounts between +180° and −180°, and also by shifting the noise timewise by amounts up to 4.0 milliseconds. The interaural phases of both the noise and the 500∼ tonal signal are therefore parameters of this study.
The results are in agreement with those already published, but show in addition large reductions in threshold even when the noise and the tone differ in their interaural phase positions by amounts less than 180°. They also show a periodic fall and rise of the masked threshold as the interaural time difference for the noise is varied. Finally, they show the importance of interaural correlation in determining the extent to which the binaural threshold will be lower than the monaural.
24(1952); http://dx.doi.org/10.1121/1.1906931View Description Hide Description
It is shown that Brownian motion at the eardrum is only partly due to noise of the air. (This part has been calculated by Sivian and White.) The main part is due to Brownian motion of the eardrum itself. The apparent flow of energy is close to the absolute threshold of the ear; it is reduced to one half when the ear is provided with an exponential horn.
The discussion of the Brownian noise in the inner ear reveals a serious discrepancy; considering one individual cell it proves to be very difficult to reconcile the low threshold with the thermal agitation. A mechanism is suggested for the excitation of the sense cells by which the disturbances by Brownian motion are appreciably suppressed; in this model the microphonic activity plays an important part. The model is based on some recent measurements of the microphonic activity of the lateral line organs of fishes, which are shortly summarized. It follows from these measurements that the microphonic potentials are related with the tension of the hairs on the sense cells.
24(1952); http://dx.doi.org/10.1121/1.1906932View Description Hide Description
The contribution to the loudness of noise made by various frequency bands was studied over a wide range of noise levels. Paralleling results with pure tones, the contribution of noise of the middle frequency range to loudness is greater than that of low and high frequency noise (for bands 250 to 300 mels in width). In order to produce maximal loudness with a fixed over‐all sound level, it was found, in most cases, more economical to spread the noise spectrum over a wide frequency range than to concentrate the noise spectrum in a restricted range of frequencies.
24(1952); http://dx.doi.org/10.1121/1.1906933View Description Hide Description
The effect upon the intelligibility of speech in noise of the interaction of sharp frequency limiting and severe peak clipping was studied. The results are compared with previously reported results of similar tests with frequency‐limited speech signals that were not subjected to amplitude distortion. The intelligibility of unclipped speech, relative to that of the peak‐clipped signal under corresponding experimental conditions, is a function of the signal‐to‐noise (S/N) ratio under test and is, to a rough approximation, independent of the frequency range of the speech signal passed. At high S/N ratios, the intelligibility of the unclipped speech signal is higher than that of the severely peak‐clipped signal. Under low S/N ratios, however, the intelligibility of the latter is considerably higher than that of the unclipped signal.
- LETTERS TO THE EDITOR
Natural Vibrations of Rectangular Quartz Plates and Determination of the Dispersion of Sound in Ethyl Ether24(1952); http://dx.doi.org/10.1121/1.1906935View Description Hide Description
An acoustic interferometer is used to measure the modes of vibration of rectangular quartz plates in air. The same instrument is used to study the dispersion of sound in ethyl ether vapor over a range of frequencies from 163.5 kc to 2482 kc and a range of pressures from 15 mm of mercury to 340 mm of mercury.
24(1952); http://dx.doi.org/10.1121/1.1906936View Description Hide Description