Index of content:
Volume 49, Issue 5A, May 1971
49(1971); http://dx.doi.org/10.1121/1.1912505View Description Hide Description
A simple model consisting of a random array of parallel elastic fibers in air is considered for the purpose of deducing the acoustic behavior of flexible porous media. A scattering analysis is employed and the mechanism of internal dissipation is thus interpreted as the irreversible mode conversion of incident dilatational waves into viscous and thermal waves by scattering at the fiber boundaries. The analysis is initially carried out for a dilute suspension such that the presence of the fiber scatterers is considered only to increase the attenuation of the imbedding fluid. This treatment is subsequently refined to include multiple scattering effects. Expressions for propagation constant, specific characteristic impedance, and surface normal impedance are deduced as functions of fiber radius, fiber concentration, and various physical constants of the fiber solid and air. The phenomenological parameters required in other theories are thus avoided. Furthermore, for a typical glass‐fiber material the multiple scatteringtheory is found to be in reasonable agreement with measurement.
49(1971); http://dx.doi.org/10.1121/1.1912506View Description Hide Description
The articulation index (AI) has been widely accepted as an objective measure of environments for speech communication and/or privacy. It quantifies speech intelligibility in terms of the signal‐to‐noise ratio on a scale extending from 0 to 1. While more experimentation is required to verify the subjective equivalent of any given AI, the AI itself can be calculated and hence predicted. It has been found that in buildings without complete floor‐to‐ceiling partitions the AI changes drastically with the following variables: speech effort, speaker orientation, background noise, speaker‐to‐listener distance, and barrier attenuation (if present and unflanked). This paper discusses the relative effect of these variables on the AI, and thus demonstrates the importance of each parameter in the design of open‐plan facilities.
49(1971); http://dx.doi.org/10.1121/1.1912507View Description Hide Description
Some measurements performed to determine the rise times of two condensermicrophones are described. The difficulties related to the measurements and their interpretation are explained. At peak levels of about 140 dB re 2×10−5 N/m2 the rise time of the B&K 4136 without protective grid and the B&K 4138 with protective grid are determined to 7±1 μsec.
49(1971); http://dx.doi.org/10.1121/1.1912508View Description Hide Description
Finite‐element method was originally developed for analyzing structures such as beams and shells. A flexure‐type composite vibrator has been used as an example to illustrate the application of this method to vibration problems involving coupled mechanical and electrical systems [Y. Kagawa and G. M. L. Gladwell, IEEE Trans. Sonics UltrasonicsSU‐17, No. 1, 41–49 (1970); J. Acoust. Soc. Japan 26, No. 3, 117–128 (1970)]. This paper develops and adapts this method for the analysis of electromechanical filters of complex shape and construction. The transmission characteristics of a filter are calculated directly without employing the usual method of an equivalent electrical circuit. Two examples of analysis are introduced for presentation. In the first one, the input and output electrodes are set up on the transducers of a composite vibrator, while in the second, two vibrators are coupled by a pair of bars.
49(1971); http://dx.doi.org/10.1121/1.1912509View Description Hide Description
A conjugate gradient optimization technique was successfully applied to the synthesis of reactive mufflers. Those parameters that minimize the differences between a muffler transfer function and a desired transfer function are automatically located, eliminating intuitive parameter selection and analysis. Although a two‐parameter search is presented, the procedure can be extended to n‐parameter space.
Improvement of Low‐Frequency Response in Small Loudspeaker Systems by Means of the Stabilized Negative‐Spring Principle49(1971); http://dx.doi.org/10.1121/1.1912510View Description Hide Description
As a practicable means for extending the low‐frequency response of small loudspeaker systems, schemes employing stabilized passive negative springs have been investigated. Experimental loudspeaker systems have been constructed and are described. A typical negative‐spring loudspeaker system, consisting of a 4.3‐in.‐diam diaphragm in a sealed enclosure of 0.105 ft3 provides a piston displacement of 0.200 in. peak‐to‐peak at 65 Hz, when driven by an input power of 2.3 W into a nominal voice‐coil impedance of 11 Ω.
49(1971); http://dx.doi.org/10.1121/1.1912511View Description Hide Description
Effect of ultrasonic vibration on coefficient of friction between a rigid steel die and plastically deformed metal has been studied. The coefficient of friction was measured during a strip‐drawing operation with and without application of ultrasonic vibration. The application of ultrasonic vibration during the drawing process substantially reduces the coefficient of friction and die stresses.
Effect of Boundary Layer on the Transmission and Attenuation of Sound in an Acoustically Treated Circular Duct49(1971); http://dx.doi.org/10.1121/1.1912512View Description Hide Description
The transmission of sound in an acoustically treated circular duct containing sheared flow is treated by reduction of the governing equations to a two‐point boundary‐value problem. The pressure modes and transmission wavenumbers are obtained by an iterative procedure for a wide range of driving frequencies, duct‐flow Mach numbers, and boundary‐layer thicknesses with a given lining impedance model. Computations are made for both inlet and exhaust flows. For inlet flows, the direction of sound propagation is opposite to the flow direction, while in the exhaust flow the sound is propagated in the same direction as the flow. The numerical results show a major reduction of achievable attenuation of the least attenuated mode in the inlet flow, owing to the presence of the boundary layer, particularly at the higher Mach numbers. The attenuation in the exhaust flow shows only minor effects due to the presence of a boundary layer. The reduction in attenuation in the inlet flow indicates that the design of acoustic linings for inlet applications must include careful consideration of boundary‐layer effects.
49(1971); http://dx.doi.org/10.1121/1.1912513View Description Hide Description
This paper presents techniques for determining nonperiodic spacing of events for the purpose of reshaping noise‐frequency spectra. The object of the application is to provide reduced noise levels and the redistribution of the frequencies at which there is noise energy so as to generate fewer perceptible sounds. Three analytical techniques for predicting the noise spectrum resulting from nonuniform spacing are presented. Emphasis is focused on a semigraphical design technique using Bessel functions, which has proven to be of great value in the actual selection of modulated event spacing. Fourier analysis of an impulse approximation and a sinusoidal wave approximation are two other alternative techniques presented. The techniques outlined are applied to the problem of reshaping the noise spectrum of a 22‐blade fan in a 5‐hp induction motor. A comparison of predicted results provides an evaluation of the alternative approaches.
49(1971); http://dx.doi.org/10.1121/1.1912514View Description Hide Description
Acoustical waves induced by sonic booms in a room with an open window are investigated by using an electrical analog. The good agreement between the results from the electrical analog and those of Vaidya show that the electrical analog is a suitable device for investigating the room response to sonic booms.
49(1971); http://dx.doi.org/10.1121/1.1912515View Description Hide Description
The spectra of soundgenerated during sodium‐water reactions have been observed at room temperature and 315°C. The audible sound radiated from hydrogen bubbles and the frying noisegenerated at the interface of sodium and water have peaks in the acoustic spectra at frequencies below 1.5 kHz and near 2 kHz, respectively. The experimental results of the oscillation frequency of a hydrogen bubble and its logarithmic decrement agree with theoretical predictions. The frying noise with peaks near 2 kHz results from a pressure that is generated owing to expansion caused by the heat liberated from the chemical reaction of water and sodium.