Volume 46, Issue 4A, October 1969
Index of content:
46(1969); http://dx.doi.org/10.1121/1.1911780View Description Hide Description
This paper describes a new method of approximating diffuse sound fields such as are needed, for example, in the measurement of sound insulation between rooms. The method is found to be a substantial improvement over conventional procedures, especially for field measurements.
46(1969); http://dx.doi.org/10.1121/1.1911781View Description Hide Description
The effects of sound‐pressure amplitude and static pressure are studied, primarily from experimental text results. Oscillograph records of the transient response of both Helmholtz and pipe resonators are analyzed to determine the damping, decay decrement, and relaxation time of the free oscillations. Damping is determined to be inversely proportional to the static pressure. Damping is found to be a function of amplitude and to decrease as the oscillating pressure amplitude decreases, although at a lesser rate. Since the pressure amplification at resonance is proportional to the reciprocal of the damping, very high amplifications are realized for systems with light damping and low pressure inputs. It is shown that seashells are pipe resonators that respond continuously in the audible range to inaudible inputs. Experimental data and theoretical analyses are used to make a plot of amplification at resonance versus input sound‐pressure level.
46(1969); http://dx.doi.org/10.1121/1.1911782View Description Hide Description
A system for sorting and representing all possible waveforms has been developed for those classed as finite duration, balanced, constant amplitude, and frequency modulated. They are first categorized according to their frequency‐time schedules into subclasses that are ranked in terms of the ratio between average and midrange frequencies. By representing successive intervals between time‐axis crossings as elements of isosum integer partitions, it is then possible through primitively recursive relations to enumerate and generate in a well‐ordered manner all possible waveforms in unambiguously defined subclassses. The waveforms can then be examined in sequence for optimal attainment of desired signal‐processing properties. Furthermore, the subclasses themselves have a natural order that permits the entire parent class to be surveyed. The technique is applied to the problem of measuring an unknown clustered multipath distribution by replica cross‐correlation. In each category of waveforms, an optimal subset is found which occupies a limited area of the map of isosum integer partitions. Relationships are developed among multipath distribution extent, waveform autocovariance features, and the computation time required for the resolution process. One example is to seek waveforms whose autocovariance has maximum lag in which it does not vary more than a specified amount. In a particular class, this yields waveforms whose autocovariance has essentially only one isolated sidelobe in addition to the central peak. This permits an explicit resolution of the multipath structure using Rank 3 sets of linear equations. As measured by the number of digital computer cycles required in the calculation, these optimal waveforms are 10 times more effective than linear FM or pseudorandom noise pulses of equal time‐bandwidth product.
- LETTERS TO THE EDITOR
46(1969); http://dx.doi.org/10.1121/1.1911783View Description Hide Description
When flexible polyurethane foam is considered as an effective fluid, the density calculated with the model is much less than the physical density of the absorbent. The inference is that the structure plays little part in the absorptive behaviour. This is consumed by considering the more elaborate two‐fluid model.
46(1969); http://dx.doi.org/10.1121/1.1911784View Description Hide Description
The nearfield pressures of a rigid piston (ka = 0.906 at f = 7.55 kHz) and a flexing piston (ka = 2.4 at f = 20.0 kHz) in an infinite soft baffle were measured in a tank by means of a toneburst technique. The resulting pressure profiles are presented as isobaric contours, comparable to those for pistons in a rigid baffle derived analytically by H. Stenzel [Handbook for the Calculation of Sound Propagation Phenomena, A. R. Stickney, Trans. (Springer‐Verlag, Berlin, 1939)].