Variation Of Sound Level Research Articles

Underwater Acoustic Transmission Variations Caused by Thermal Layers—Theoretical

A perturbation calculation on the eikonal equations by Bergmann [Phys. Rev. 70, 486 (1946)] is extended to apply to acoustic transmission in a medium with a single horizontal layer in which the deviation of the index of refraction from unity is very small. Numerical calculations for the sound level deviation for transmission in such a medium are presented. A temperature layer about 2 yd thick with a maximum temperature deviation from the surrounding water of 0.02°C can produce sound level variations of 2.2 db at 500-yd range. Excellent agreement is obtained in a comparison between these results and direct acoustic transmission measurements in an underwater medium closely approximating these assumptions. (This work was supported by the Bureau of Ordnance, U. S. Navy, under Contract NOrd 12937.)

Techniques for Determining Acoustics of Amphitheaters by Scale Model Studies with High Frequency Sounds

Techniques are described for high frequency tests used in small scale models of architectural structures. The two models tested simulated large outdoor amphitheaters with open-sided roof structures. One project, sponsored by the Chicago Park District, was an amphitheater seating 25,000 with roofed area seating approximately 4000. The other was a covered amphitheater for 3000 listeners now under construction at Ravinia Park near Chicago. The scales for the models were 16 to 1 and 20 to 1, respectively. Wave-length similitude was maintained by using proportionally higher frequencies. Steady-state measurements at frequencies between 2 and 30 kc were made to determine sound levels in the stage area and distribution of sound in the audience area. The effect of an audience was simulated by use of acoustical absorbents. Studies of oscillograph patterns for sound pulses gave information on reverberation times and smoothness of decay curves for various microphone positions. Changes in stage design were needed for both amphitheaters to reduce variations in sound level in the seating area. Observed decay slopes corresponded to reverberation times between 1.4 and 2 sec. in the roofed areas of the prototypes. For the larger amphitheater, the output requirements for the sound amplifying system were determined. The cost of such model studies is found to be under two percent of the construction cost. The results enable the designer to eliminate many costly mistakes.