Volume 50, Issue 3A, September 1971
Index of content:
50(1971); http://dx.doi.org/10.1121/1.1912690View Description Hide Description
Among the factors that could affect the intelligibility of helium speech at deep submergence are an upward shift of formant frequencies in accordance with the velocity of sound, a disproportionate shift of first formant frequencies as a result of increased pressure, attenuation of high frequencies of the voice just when they become essential, deterioration of microphone response with depth, change of the diver's hearing above 3 kHz with depth, occupational deafness above 3 kHz, and acoustic reaction of a mask cavity as opposed to the larger space of a habitat or diving bell. Of these, the first and the last two are the most important. The key to the problem is a microphone of satisfactory design. In conjunction with such a microphone, a helium translator can be made to restore intelligibility even when the listener has a severe high‐frequency hearing loss.
50(1971); http://dx.doi.org/10.1121/1.1912691View Description Hide Description
In many applications, broad‐band ultrasonic transducers capable of producing short video pulses are required. Previously, plane‐wave analysis with equivalent circuits has proven successful in predicting pulse shape in the time and frequency domains. The present approach is to recognize that piston sources radiate nonplanar waves, and that the frequency spectrum of a broad‐band piston source can be measured experimentally. With the spectrum as a weighing function for the field profiles of a monofrequency piston source, a superposition is performed to find the pressure and phase profiles in the radiation field of a broad‐band transducer. Experimental measurements are presented that take advantage of the broad‐band pulse technique combined with spectrum analysis. These include thickness gauging of thin materials and interface layers, and relative viscosity measurements.
50(1971); http://dx.doi.org/10.1121/1.1912692View Description Hide Description
An investigation is made of the effect of the beam pattern of a source exciting an acoustic surfacescatter channel using the Fresnel corrected physical optics approximation. Results are presented for monochromatic and impulsive excitation using both deterministic and random boundaries. In the random case, non‐Gaussian statistics are permitted for the surface deformations. This study concentrates on the limits of very narrow and very broad frequency‐invariant beam patterns and specular geometry at moderate grazing angles. The results for the broad beam pattern are found to be simpler than the corresponding ones for narrow and intermediate beam patterns. The theory is applied towards the calculation of impulse response convolutions and bifrequency spreading functions. An attempt is made to relate current and past physical results in a nomenclature suitable for application in signal processing and communication theory.
- LETTER TO THE EDITOR