Volume 33, Issue 12, December 1961
Index of content:
33(1961); http://dx.doi.org/10.1121/1.1908542View Description Hide Description
Measurements have been carried through about the angular distribution of sound traveling in a reverberation chamber in which a certain amount of absorbing material is installed. It was measured during the decay phase of sound. Two methods were used, the first one with a directional sound source and a nondirectional microphone, the second one using a nondirectional source and a directional microphone developed in the PTB. In the latter case, the differences of angular distribution of sound, impinging on the absorbing sample, could be shown for two different states of diffuseness accomplished in the reververation chamber by installing different numbers of diffusing elements in the room. The angular distributions were investigated at frequencies of 1.2, and 4 kc.
33(1961); http://dx.doi.org/10.1121/1.1908544View Description Hide Description
The 3200‐seat Congress Hall in Jerusalem, Israel was completed in the spring of 1960 after a construction period of some 10 years. The original primary design goal was to provide a place for large assemblies the most important of which was the biennial meeting of the World Zionist Congress. During construction, the repeated use of the partially completed “unroofed” structure for out door concerts led to the reconsideration of the original design goals to give primary emphasis to music. The hall now has become the Jerusalem home of the Israeli Philharmonic while still serving its function for congresses and other uses. This paper traces the development of the acoustical design goals as well as discusses the important acoustical design features, the results of measurements made in the completed hall, and the reactions of musicians who have played in the hall.
33(1961); http://dx.doi.org/10.1121/1.1908546View Description Hide Description
The probability distribution of pressure amplitudes in a room is studied theoretically and experimentally. A point source is assumed to emit a Poisson sequence of pulses. The method of cumulant generating functions is used to obtain the distribution of response in the direct field, the reverberant field, and the transition region. In the direct field, the source statistics are reproduced. For sufficiently reverberant spaces, the reverberant field tends to normality independently of the source distribution. For sufficiently short pulses (or short autocorrelation time of the source) the direct and reverberant fields are statistically independent. When these simplifying conditions are not met, we indicate how the results may be generalized. Implications for calculations of random fatigue and component malfunction are discussed briefly.
33(1961); http://dx.doi.org/10.1121/1.1908548View Description Hide Description
It was established that the loudness of a 1000‐cps pure tone in the presence of random noise is given by , where I is the pure tone intensity and I 0 the threshold intensity of the pure tone in the presence of masking noise. The exponent n is approximately 0.27, and the value of k depends on the choice of units. This relation also holds when the only masking noise present is the physiological noise generated in the ears, and it thus describes the conventional loudness function.
33(1961); http://dx.doi.org/10.1121/1.1908550View Description Hide Description
By use of a high‐power, corona type loudspeaker we have been able to demonstrate behavioral and endocrine responses of laboratory rodents to measured high sound‐pressure levels at the frequencies at which their hearing is most sensitive, namely, 10 to 40 kc. The corona type loudspeaker behaves in the fashion predicted by the theoretical analysis reviewed in this paper. It is limited by extremely low efficiency (2.5%) and high production of ozone. At high frequencies it is limited by the volume of the corona, and at low frequencies by the failure of the quasi‐adiabatic condition within the corona. Despite these limitations its performance has not been equaled by other types of loudspeakers; it is a unique tool for studying the responses of laboratory rodents to sound fields.
33(1961); http://dx.doi.org/10.1121/1.1908552View Description Hide Description
A method of directly evaluating the impulse response of a reciprocal electroacoustic transducer is presented. The method is essentially the time‐domain analog of the conventional (frequency‐domain) reciprocity method. The transient response of a coupled pair of identical transducers is used to compute the impulse response of either of the pair. A numerical method of obtaining solution is presented and is shown to be equivalent to the numerical solution of a real convolution integral equation. First, an approximate solution for one member of the pair of identical transducers is obtained. Then, a more precise solution is generated by minimizing the squared error between the actual response of the coupled pair and the one obtained by convolving the approximate impulse response with itself.
The method was tried on two pairs of condensermicrophones, the microphones within each pair being very nearly identical. A pair of Western Electric 640‐AA microphones were tested with the grids both on and off, and a pair of Bruel and Kjaer type 4131 microphones were tested with the grids on an off and with grids equivalent to those on the W.E. 640‐AA both on and off.
Two unsatisfactory solutions resulted when W.E. 640‐AA grids were used on the transducers. Three solutions were satisfactory, even though each contained slight negative drift for large values of time. One solution contained no appreciable error in its entire time course.
33(1961); http://dx.doi.org/10.1121/1.1908554View Description Hide Description
Relative resonant displacement of bar magnetostrictors as a function of polarization at atmospheric stress and at a compressional, axial stress of 230 atm is measured. Laminated bars of nickel and 2V‐permendur are driven at frequencies both at and well below the resonant frequency of the bars, about 23 kc. Measurements of the Q and the reversible permeability enable one to compare the magnetostrictive constants and the coupling at the two stress levels for both materials.
The resonant displacement was found to change only slightly upon application of the compressive stress, for polarizing fields up to 100 oe. The compressive stress increased the maximum coupling for permendur by about 10%, the stress decreased the maximum coupling for nickel by about 10%.
33(1961); http://dx.doi.org/10.1121/1.1908556View Description Hide Description
Procedures are described for reducing the speech wave to a specification in terms of the time‐varying vocal‐tract resonances and source characteristics. The basic method, which has been called analysis by synthesis, involves the comparison of speech spectra with a series of spectra that are synthesized within the analyzer. Each comparison spectrum is generated according to a set of rules based on an acoustical theory of speech production. The result of the analysis of each input spectrum is a set of parameters that describes the synthesized spectrum providing the best match. In one version of the method convergence, towards the best match is controlled by the experimenter; in another version convergence to a match is accomplished automatically without the intervention of the experimenter. All the operations have been programmed on a general‐purpose digital computer and have been applied to the analysis of vowels and some consonants. The advantages of the analysis techniques are discussed.
33(1961); http://dx.doi.org/10.1121/1.1908558View Description Hide Description
The damping constants of vowelformants, as expressed in the half‐power bandwidths, have been studied from the reports of several investigators. The accuracies of methods of measurement used have been estimated through application of the same methods to an electrical analog of the vocal tract. Two new sets of measurements are presented, one on spectrograms of real speech, the other making use of an artificial larynx, applied to real subjects. It is concluded that, while wide individual variations occur, the average bandwidths for male voices probably lie in the lower range of those which have been reported.
Diffraction of Wide and Narrow Light Beams by Distorted Finite‐Amplitude Progressive Ultrasonic Waves in Water33(1961); http://dx.doi.org/10.1121/1.1908560View Description Hide Description
Results of an investigation of diffraction of light passing through a distorted finite amplitude ultrasonic wave are given. A method is developed and used to determine the second harmonic component of the distorted wave at various distances from the transducer while maintaining a constant local fundamental component. It is shown that the observed discrete diffraction order intensities (wide light beam) and the observed continuous light distribution (narrow light beam) are in good agreement with theoretically predicted values. Measurements were made at 3.0 Mc in water with approximately local fundamental pressure amplitude.
33(1961); http://dx.doi.org/10.1121/1.1908562View Description Hide Description
In this paper we present a detailed study of the theory of free, axisymmetric vibrations of thin elastic spherical shell and demonstrate by experiment that the normal modes of vibration predicted by theory do exist. The theory, which is an expansion of an ancient study by Lamb, predicts the existence of two infinite sets of normal modes, one of which is bounded in frequency and the other unbounded. The first four modes in each set are identified by experiments on a small steel shell.
33(1961); http://dx.doi.org/10.1121/1.1908564View Description Hide Description
In two‐dimensional linear acoustic theory Huygens' principle fails; the failure of Huygens' principle being manifested by the existence of a tail to the acoustic pulse. A two‐dimensional problem of importance is the radiation of acoustic pulses of various shapes by an infinitely long circular cylinder. The problem is attacked directly via the use of the Laplace transform. In the inversion of the Laplace transform, relatively new function called the Nielsen W 0 function is encountered. Exponential, sine, and rectangular pulses are studied and their shape determined at various distances from the cylinder. Finally the passage to the steady state of a semi infinite sine wave train is outlined.
33(1961); http://dx.doi.org/10.1121/1.1908566View Description Hide Description
It is noted that an electromagnetic wave incident upon the boundary of an ionized medium in the presence of a transverse magnetic field will excite, in addition to the reflected and transmitted electromagnetic waves, a reflected sound wave and a transmitted modified sound wave. It is shown that the power coupled into these acoustic waves is proportional to the magnetohydrodynamic coupling parameter and to the ratio of soundvelocity to light velocity. Finally it is pointed out that electromagnetic transmission through a thin ionized layer may take place by means of this modified sound wave even though the layer thickness is large compared to the electromagnetic skin depth, although the efficiency of transmission will be very low for weak magnetohydrodynamic coupling.
33(1961); http://dx.doi.org/10.1121/1.1908568View Description Hide Description
An examination of the scattering process in the vicinity of cutoff is made using rigid surfaces with sinusoidal corrugations which have a maximum slope near one. Measurements of the specular amplitude for several angles of incidence are presented with small increments in frequency. The ratio of the pressure amplitude at the peaks and troughs of the corrugations as a function of frequency is also observed for several angles of incidence.
33(1961); http://dx.doi.org/10.1121/1.1908570View Description Hide Description
The Rayleigh solution for reflection from a corrugated surface, and some possible modifications, are examined. For small slopes these methods produce similar analytical expressions and almost identical numerical results. When the first nonradiating term is included in the computations, the magnitude of the scattered amplitudes is essentially the same, but an estimate of the phase change as a function of frequency is provided. For larger slopes, differences appear in the analytical form as well as in the numerical results. The Rayleigh expression has advantages above cutoff, while a new approach seems better before and just after cutoff.
33(1961); http://dx.doi.org/10.1121/1.1908572View Description Hide Description
The differential equation for radial vibrations of a thin cylindrical shell is derived by Hamilton's principle for the case of constant internal and transient external pressures. Both pressures are assumed symmetrical about the cylinder's axis of symmetry, and the natural frequencies and mode shapes are obtained. The general solution to the transient forced‐vibration problem is presented using integral transform methods. The general solution is then carried out in detail for the specific case where the external pressure is a step function moving over the cylinder in the axial direction. It is pointed out that such a solution may be considered as a crude model of what happens to a thin‐skinned missile during launching from a silo. Several other problems are mentioned which have the same type of governing differential equations and for which the solutions presented would be useful.
33(1961); http://dx.doi.org/10.1121/1.1908574View Description Hide Description
A theory of linear systems with randomly varying parameters is developed under the assumption that the parameter processes are Gaussian processes obtained by linear filtering of white noise. By this device, the restriction to white‐noise parameter processes of previous studies has been removed.
By a method related somewhat to a technique used in the theory of the multiple scattering of light, a system of linear integral equations for the determination of various second‐order moments of the system output is derived. This system can be solved explicitly in some interesting cases.
A theory of stability in mean and in mean square is given for random systems. It was found that mean square stability depended only on the values of the auto‐ and cross‐correlation functions of the parameter processes at the origin and not on the detailed structure of these functions.
The stability theory is applied to an RLC circuit with randomly varying capacitance. The possibility of stabalizing unstable deterministic systems with random noise is discussed.
33(1961); http://dx.doi.org/10.1121/1.1908576View Description Hide Description
Approximate formulas are derived for the lowest natural frequencies of hinged elastic arcs having the center lines in the form of a circle, of a cycloid, of a catenary, and of a parabola in the case of vibrations occurring either in the plane of the initial curvature or in the plane perpendicular to the plane of the initial curvature of the arcs. Numerical results for the frequencies are presented in tabular and graphical form. The paper supplements previous published papers by the present authors in which the same problems were solved for the case of elastic built‐in arcs.
33(1961); http://dx.doi.org/10.1121/1.1908578View Description Hide Description
Shots were fired at a fixed location and constant shallow depth at intervals over a period of 15 hr. Shots fired in this way produced a highly repeatable signal. The signals were received at a fixed location 63 miles away on hydrophones at 3000‐ and 9100‐ft depths. The signals exhibit about 40% correlation over a period of and about 20% correlation over a period of 1 hr.
33(1961); http://dx.doi.org/10.1121/1.1908580View Description Hide Description
A simple derivation is given of the phase autocorrelation function of a monochromatic signal during transmission through a medium with random fluctuations of refractive index. The resulting phase coherence function is then applied to the theory of the linear array antenna to yield a formula for the degraded directivity pattern.
The analysis developed here was conceived primarily with reference to acoustic signals and sonar ranging systems, although it may be applied, mutatis mutandis, to a wider variety of acoustic and even electromagnetic problems. For concreteness, however, we shall employ a vocabulary appropriate to the case of sonar signals and antennas.
The problem to be considered is the following: Given a source of monochromatic radiation situated far enough from the receiving antenna to be considered a point source, what will be the response, as a function of bearing angle, of a linear array antenna to the signal if the transmission medium has small random irregularities of its refractive index?