Volume 21, Issue 5, September 1949
Index of content:
21(1949); http://dx.doi.org/10.1121/1.1906536View Description Hide Description
Structures are described which refract and focus sound waves. They are similar in principle to certain recently developed electromagnetic wave lenses in that they consist of arrays of obstacles which are small compared to the wave‐length. These obstacles increase the effective density of the medium and thus effect a reduced propagation velocity of sound waves passing through the array. This reduced velocity is synonymous with refractive power so that lenses and prisms can be designed. When the obstacles approach a half wave‐length in size, the refractive index varies with wave‐length and prisms then cause a dispersion of the waves (sound spectrum analyzer). Path length delay type lenses for focusing sound waves are also described. A diverging lens is discussed which produces a more uniform angular distribution of high frequencies from a loud speaker.
21(1949); http://dx.doi.org/10.1121/1.1906537View Description Hide Description
The analysis of the propagation of sound waves in narrow tubes has usually been restricted to shapes yielding tractable mathematical expressions. A great number of practical applications do not fall within these categories and await a solution. An approximate solution of sufficient accuracy for narrow tubes of arbitrary shapes developed in this paper has been applied to a wire‐filled tube. The theoretical predictions check satisfactorily with the experimental results. It is believed that this study will be useful in other similar applications.
21(1949); http://dx.doi.org/10.1121/1.1906538View Description Hide Description
An improved method for automatically extracting the pitch information of speech sounds has been devised. It employs a combination of gain control, double detection, voiced sound selection, unvoiced sound exclusion, and a means for counting the fundamental vibrations in the voiced sound intervals. Reliable indications of pitch have been obtained over a range corresponding to frequencies from 100 to 600 cycles for a wide variety of voices. The pitch‐indicating signals have been applied, for study purposes, to a number of visual portrayal means for showing pitch changes, and descriptions of several of these systems are included.
21(1949); http://dx.doi.org/10.1121/1.1906539View Description Hide Description
It has recently been shown that the binaural threshold for a pure tone presented against a background of noise depends upon the interaural phase differences of the tone and the noise. For example, the threshold of a low frequency tone, in phase at the two ears, is low when it is heard against noise that is out of phase at the two ears, but is high when it is heard against noise that is in phase.
The masked threshold of a 250‐cycle tone presented against a background of intense noise is in fact about 15 db lower under antiphasic conditions than it is under homophasic conditions. (Homophasic refers to the condition in which the tone and the noise have the same interaural phase difference. Antiphasic refers to the condition in which either tone or noise, but not both, is out of phase at the two ears.) When the tone is masked by another tone, fairly close in frequency but not so close that beats are detected, the dependence of masking on interaural phase does not appear. The present experiment attempts to determine what characteristics masking sound (“masker”) must have if it is to produce interaural phase effects.
A 250‐cycle tone was presented against four different kinds of background. The masking sounds were (1) pure tones, (2) regular pulses (125 p.p.s.), (3) random pulses (average 125 p.p.s.) and (4) random noise. Combinations of filters permitted the presentation of these maskers in frequency bands that were varied in width and center frequency.
The results indicate that the threshold for the tone in the presence of a regular, periodic masker does not depend to any significant degree upon interaural phase relations. A necessary characteristic of the masker that produces interaural phase effects is randomness or irregularity. Both masking and the interaural phase effects increase as the frequency band of a random masker approaches the frequency of the tone. Band width alone does not contribute to the interaural phase effects except as widening a band tends to bring one cut‐off nearer the frequency of the tone. Wide and narrow band maskers, both of which contain the frequency of the tone, produce the same effects.
21(1949); http://dx.doi.org/10.1121/1.1906540View Description Hide Description
Electro‐acoustic equipment and methods for the study of aural microphonics and action potentials of laboratory animals are described. A new feature is the placement of electrodes not only on the round window but also in one or more very small holes drilled into the cochlea of the guinea pig.
Analysis of the aural microphonics so recorded confirm the conclusions of Wever and Lawrence, of Békésy, and others. The microphonics generated at the apex by low tones or near the round window by high tones axe conducted electrically to the other end of the cochlea with an attenuation of at least 10 db. The effects of the local injury caused by the small holes are described, and their relation to anatomical position confirms the earlier data of Stevens, Davis, and Lurie. Detailed comparisons of phase relations, wave form, etc., at different positions did not prove feasible because of electrical leakage of the aural microphonic from one turn to the next through the bony partitions.
At high intensities the microphonic for a tone of 3000 c.p.s. or higher may show a very strong “sub‐harmonic” at the apex although the sub‐harmonic component is very small at the round window. The threshold for appearance of the sub‐harmonic is very sharp.
21(1949); http://dx.doi.org/10.1121/1.1906541View Description Hide Description
From transmitting frequency responses in water, admittance measurements in air and water, and radiation patterns in water, calculations were made of the electro‐acoustic efficiencies at 1250 kc of four one‐inch square X‐cut quartz crystals of practically equal thickness having radii of curvature of ∞, 25, 7, and 4 centimeters, respectively. Efficiency values calculated from acoustic measurements do not check as well with the calculated potential efficiencies as do those calculated from admittance measurements. The latter values indicate that the radiation resistance the same for the four crystals. Focusing measurements show agreement with A. O. Williams' prediction that the point of maximum acoustic intensity in the radiation pattern is not necessarily at the center of curvature of the crystal. Resonance frequencies obtained from admittance measurements in air make plausible the deduction that the effective mass of a crystal decreases with increasing curvature.
21(1949); http://dx.doi.org/10.1121/1.1906542View Description Hide Description
An approximate theory has been derived describing part of the sound field due to a concave spherical radiator, vibrating with uniform normal velocity; the radius a of the circular boundary is assumed to be large relative to the wave‐length and large relative to the depth of the concave surface. The theory describes the distribution of pressure, particle velocity, and intensity along the axis of symmetry and in the vicinity of the focal plane, perpendicular to the axis at the center of curvature. It is shown that the ratio of the intensity at the center of curvature to the average intensity at the radiating surface is nearly equal to (2πh/λ)2 where h is the depth of the concave surface and λ is the wave‐length. This ratio can be made very large by suitable choice of dimensions, and the focusing is then very sharp. The point of greatest intensity is not at the center of curvature but approaches it with increasing kh = 2πh/λ, and the greatest intensity is not much greater than the intensity at the center of curvature except when kh is small. In the central part of the focal plane, at angle θ from the axis, the pressure is approximately proportional to (2/ka sinθ)J 1(ka sinθ), which is equivalent to the directivity function of a flat circular piston of radius a, for the region at large distance from the piston. The calculations are in reasonable agreement with G. W. Willard's experimental data for a 5‐mc concave quartz crystal, when allowance is made for the non‐uniform normal velocity of the crystal.
21(1949); http://dx.doi.org/10.1121/1.1906543View Description Hide Description
This device provides for rapid analysis of short samples of speech and other sounds. It permits direct viewing of the energy‐frequency distribution of the sound at instants of time in a two‐dimensional pattern, and also over intervals of time as a three‐dimensional pattern. Magnetically recorded on a disk at slow speed, and speeded up 200 times on playback, the sample of sound is analyzed rapidly by a broad band high frequency system.
The three‐dimensional portrayal (in time, frequency and amplitude) shows the whole sound sample, including an interval of about , on one cathode‐ray tube, together with a movable indication of the point in time at which the “instantaneous” or two‐dimensional frequency‐amplitude section, appearing on another tube, is taken. This two‐dimensional pattern displays amplitude on a decibel scale and covers about a 40 db range. The complete pattern is made up of 190 amplitude values for the 100 to 4000 cycle frequency range so that the pattern is formed by 190 vertical lines. The effective band width of the resolving filter is 45 cycles. The patterns are scanned at a rate of 2/sec. so that a slow phosphor screen is used for viewing. Successive sections approximately , 5, 10, 20, or 40 milliseconds apart may be established and photographed automatically. Or for direct viewing manual selection of any desired point of analysis is provided.
21(1949); http://dx.doi.org/10.1121/1.1906544View Description Hide Description
A unified theory of the reciprocity calibration of electromechanical transducers is presented in a form which facilitates the derivation of particular calibration theories. The theories of two types of calibration procedure are treated as special cases. One of the theories is shown to reduce, when lumped standards are used, to that of the original technique of Trent; the other is descriptive of a new procedure.
The application of calibration theories is discussed—particular attention being devoted to the use of distributed‐constant standards and the consequent difficulties involved.
21(1949); http://dx.doi.org/10.1121/1.1906545View Description Hide Description
The physical conditions for a valid reciprocity free‐field calibration are examined in considerable detail. The expression for the absolute value of the reciprocity parameter J is derived in a general manner. The result shows that J is actually a transfer admittance relating the volume current Q of the linear, reversible transducer to the pressure P this unit causes at a certain point in space. The usefulness of this concept in other than free‐field conditions is pointed out. The formula for the sensitivity of the unknown microphone is derived by a method which emphasizes the significance of the physical arrangement, and the limiting conditions are stated. Practical methods for determining the validity of the calibration are suggested.
21(1949); http://dx.doi.org/10.1121/1.1906546View Description Hide Description
This paper deals with the theory of a class of relaxation mechanisms which may in part account for the absorption of sound in certain solids. The basic postulates of Eyring's theory of absolute reaction rates are applied to the development of general differential equations describing structural‐chemical state changes. The macroscopic effects of transitions between the activated and unactivated states of complexes of atoms are discussed, and the results are applied to the problem of sound absorption and mechanical relaxation due to structural‐chemical changes in soda‐silica glasses. It appears that the transition of alkali ions from holes in the glass lattice into the interstitial space, as in electrolyticconduction, is associated with the acquisition of a certain activation energy, so that the transition probabilities are finite. The process is thus of the nature of a relaxation mechanism which is capable of causing sound absorption. The functional dependence of absorption on frequency predicted by the theory and the predicted functional dependence of the relaxation time on temperature are found to agree with experimental results.