Volume 30, Issue 1, January 1958
Index of content:
30(1958); http://dx.doi.org/10.1121/1.1909368View Description Hide Description
The Monte Carlo method of numerical analysis is applied to the determination of mean free paths and acoustic weighting factors relating probability of collision with walls in rectangular parallelopipeds. The method of calculation is discussed and its extension to determination of reverberation times in coupled rooms and auditoria is evaluated. Tables of relative wall collision probabilities are given for rectangular parallelopipeds of various proportions.
30(1958); http://dx.doi.org/10.1121/1.1909380View Description Hide Description
It is important to know the output of a source as a function of position in a reverberation chamber. Expressions are given for the sound power output of simple monopole and dipole sources as functions of source position in various reflecting environments. They are obtained by the use of the method of images and a general formula due to Rayleigh for the output of a number of simple sources. The cases of a dipole source near a reflecting plane and a simple source near a reflecting edge and corner are treated, and the effect of the band width of the source is considered. The results apply when the reflectors enclose the source, as in a reverberation chamber, unless the distance in wavelengths between parallel walls is small and the absorption in the enclosure is low. Some experimental data are given, and the reverberation chamber method of measuring the power output of sources is discussed. In the general case of an extended source emitting nonspherical waves near reflecting surfaces, it may be more convenient to find the variation of power output with position by experiment than by calculation.
30(1958); http://dx.doi.org/10.1121/1.1909366View Description Hide Description
In recent years there has been a growing concern about the adequacy of the present American reference for normal hearing of pure tones. Results of earlier studies indicate that survey data tend to confirm the present zero reference, but data from laboratory studies have provided some basis for the contention that the present reference is too high. The purpose of the present study was to provide more definitive data on the threshold of hearing for pure tones through an extended laboratory investigation. Audiometric measurements were made on three groups of subjects ranging in age from 18 to 24 years inclusive. All subjects were otologically normal and showed a life history of minimal exposure to high intensity noise. Two groups were tested with ANB‐HIA earphones and a 5‐db step testing procedure in the method of limits; the other group was tested with Permoflux PDR‐8 earphones and a 2‐db step testing procedure. The results of the study indicate significant differences between the groups tested with the two types of earphones, as well as between male and female subjects. In general, excellent agreement was found with other laboratory studies. The threshold curves of the present study expressed in terms of coupler pressures are offered for consideration as a proposed laboratory standard for normal hearing.
30(1958); http://dx.doi.org/10.1121/1.1909370View Description Hide Description
Circumaural ear defenders require a cushion which is highly adaptable to the irregular contours of the head so as to provide a good air seal, and yet behaves like a stiff spring so as to minimize low‐frequency cup vibration. These apparently contradictory requirements are met by a cushion consisting of an annulus of highly flexible yet rather inextensible material partially filled with an incompressible fluid. A theory of cushion behavior is given and is supported by experimental data. Experiments with real heads show that the attainable low‐frequency attenuation is limited by the viscoelastic properties of the flesh. Values of circumaural flesh impedance derived from the experimental data are compared with the mastoid data of other workers, and the relationship between sound transmission and threshold shift data is discussed. The design of a rigid cup and flexible frame suitable for use with the cushion is also discussed.
30(1958); http://dx.doi.org/10.1121/1.1909372View Description Hide Description
For more than a decade it has been known, in a qualitative way, that the long‐time average power of a speech signal in an amplitude‐limited communications system can be increased materially by clipping the peaks of the speechwave and amplifying the remainder until the new peaks have the maximum allowable amplitude. This power increase can be computed from the statistical distribution for the instantaneous amplitudes of speech. Some of the available distributions have been collected and compared, and, upon their showing good agreement, one of them has been used for the computation. Simple formulas show that the power increase can be neither greater than the amount of clipping nor greater than the peak factor of speech. For 24 db of peak clipping, the power gain is about 12 db. The exact value depends upon the choice of peak factor for unclipped speech. In practice, the actual gain in signal‐to‐noise ratio on peak‐limited communications systems (e.g., AM and DSB radio) will be less than that computed here.
30(1958); http://dx.doi.org/10.1121/1.1909374View Description Hide Description
Present methods for accelerometer calibration include small‐amplitude sinusoidal drive, large‐amplitude sinusoidal drive, and pulse‐integration (ballistic pendulum) methods. The first of these is of limited use because it fails to reveal nonlinear effects and pulse saturation. The second is difficult to accomplish except at a limited number of frequencies, while the third gives very little high‐frequency information. A method of direct calibration, using bonded wire strain gauges as a standard, is described. The method is valid over a very wide frequency range, up to 40 kc with present instrumentation. Several examples of the use of the method are discussed. Results show that the method is an easy one to use and that it quickly reveals any deficiencies of accelerometers and their amplifiers, if present, e.g., “droop” resulting from insufficient low‐frequency response, overloading of cathode follower, loss of high‐frequency detail caused by too low cutoff of accelerometer filter, hash caused by accelerometer resonance, etc. Limitations of the method and evaluation of accuracy also are discussed. This method has been found to give a complete picture of the actual operation of the accelerometer and associated circuits quickly and accurately, and the results are easily interpreted.
30(1958); http://dx.doi.org/10.1121/1.1909376View Description Hide Description
A new frequency analyzer using a set of ADP crystal resonators is described. The signal to be analyzed is applied to the electrodes of the crystals, which have different resonance frequencies. The crystal is placed between the crossed polarizers, and the existence of the signal component is observed as the brightness of the beam coming through the crystal with the corresponding resonance frequency.
30(1958); http://dx.doi.org/10.1121/1.1909378View Description Hide Description
Large displacement behavior of the variable‐reluctance, and the mathematically equivalent electrostatic, transducing mechanism was analyzed in order to evaluate the importance of mechanical instability as a limiting factor in high‐power sound projectors. Previous work by Hunt et al. had shown the existence of static instability. This paper first considers the effect of transients on static stability and then shows that instability can also occur under steady‐state dynamic conditions even when the system is statically stable. When the excitation is increased in order to increase the amplitude of the oscillations, the average displacement also increases, which leads to collapse of the gap. This interdependence of the oscillatory motion and the average displacement, intentionally excluded from linearized treatments, is the cause of dynamic instability. Detailed calculations show the conditions for instability for typical values of mechanical Q, polarization, and frequency. Under steady‐state dynamic conditions the average displacement can exceed the static limit of the gap length, and the relative oscillatory displacement amplitude can reach or more without collapse.
30(1958); http://dx.doi.org/10.1121/1.1909382View Description Hide Description
A new sound synthesizing system controlled by an optical program pattern (similar to the Sonagram) has been devised. The system employs ADP crystals with different resonance frequencies as the mechanical filter bank with electrical input and optical output. As the source signal, any electrical wave form can be used, and the pitch of voiced sounds can be controlled by a separate pitch channel. Possibilities of development of some variations for the visible speech playback and a Vocoder system without any electrical filter bank are described.
30(1958); http://dx.doi.org/10.1121/1.1909384View Description Hide Description
The principle of a speech annunicator as a warning signal indicator was evaluated in three experiments against a master warning signal. With respect to two important criteria of performance (warning signal identification time and performance upon a concurrent visual‐motor tracking test), the speech annunciator was dearly superior to the master warning indicator. With respect to two other criteria of performance (initial warning detection and performance upon a concurrent message‐reproduction task), the speech annunciator was equivalent to the master warning indicator. The results suggest that a speech annunciator, perhaps coupled with a master indicator, may be a useful warning indicator system.
30(1958); http://dx.doi.org/10.1121/1.1909386View Description Hide Description
The reproduction of messages, selected from defined information sources, was studied in a multichannel listening task. Standardization of message procedures was controlled independently of standardization of message nomenclature. It is shown that the standardization of procedures or nomenclature is an effective determinant of message reproduction. And, the joint standardization of both nomenclature and procedure is more effective than either alone.
30(1958); http://dx.doi.org/10.1121/1.1909388View Description Hide Description
An approximate expression is derived for the disturbance in a one‐dimensional wave guide resulting from an expanding spherical wave which is loosely coupled to the wave guide at all points along its length. For the case of a slender rod immersed in a fluid, curves are presented showing axial strain for three ratios of rod velocity to fluid velocity.
30(1958); http://dx.doi.org/10.1121/1.1909390View Description Hide Description
A simple derivation of the force which acts on a body placed in sound field is given. The tensor properties of the force are exhibited, but it is also shown that for a rigid body the force may be derived from the scalar pressure alone. It is shown how the time average force is determined by the momentum lost by the sound wave during the collision with the body. Using this result, the force is related to the scattering amplitude, and a simple expression in terms of the phase shifts is given.
The connection between the force and the scattering cross section is examined. A force factor is defined. It is shown that κ = 1 for plane scatterers perpendicular incidence. A simple general formula for the force in the limit of short wavelengths is derived. It is shown that neither for very long wavelengths nor for very short ones is κ in general equal to 1.
The case of a rigid sphere is treated in detail. Curves are shown for the force and for the force factor κ.
30(1958); http://dx.doi.org/10.1121/1.1909392View Description Hide Description
The paper presents a résumé of a theoretical study of the problem of flexural vibrations of the wails of orthogonally stiffened cylindrical shells. Frequency equations are given for the stiffened shells. In addition an experimental method of verifying the theory is described. Experimentally determined frequencies for an isotropic as well as two different designs of orthogonally stiffened shells are given in tables. The experimental and corresponding theoretical results are discussed.
A peculiar dip in the frequency spectrum which was discovered by Arnold and Warburton for uniform isotropic shells is shown to exist theoretically and is verified experimentally for stiffened shells. In addition, there is shown to be a bias in the excitability of the modes of vibration as determined experimentally. The reason for this phenomenon is discussed.
The principal axes of stiffness are in the longitudinal and transverse directions for the shells under study. In one case the axis of maximum stiffness is in the longitudinal direction and in the other case it is in the transverse direction. As the stiffness tensor is the same for the two cases, except for orientation, an opportunity is provided for examining the effect of rotating the stiffness axes through 90°.