In this research work, it is found that the impulsive noise, accompanied by some oscillatorily damping pressure change, is generated by the ascending bubbles, which are blown into the water through a porous plate made by sintering of AS resin, and through a needle tube. This impulsive noise is detected by a small hydrophone, inserted into the ascending bubbles, and is supposed to be generated at the instant of rupture of the ascending bubbles or coagulation of them. However, the generation of the impulsive noise is not continuous, but intermittent. From our experimental results of the high-speed photography and the flash photography, the oscillatorily damping pressure change, following the impulsive noise, is concluded to be caused by the pulsative oscillation of the tiny bubbles, separated from the relatively large ascending bubbles. The frequency of the oscillatorily damping sound pressure change, following that impulsive noise, is usually the order of several kHz, but sometimes it is over than 20 kHz and included in the so-called ultrasound range. Moreover, the order of the peak to peak sound pressure amplitude of that impulsive noise is sometimes over than 100 mbar, when the air flow rate for the formation of the small air bubbles in water is high enough.
The acoustic field around a disc or a strip in diffuse sound field has been studied based on wave theory in our previous papers. But calculation of sound pressure level around the plate was very complex, and was difficult to pursue for high frequencies. To eliminate these defects approximation by geometrical acoustics is introduced jointly. By this method distributions of sound pressure level around a disc and a strip are calculated. The results are in good agreement with the experimental values, which are measured in a reverberation chamber, for wave length sufficientry shorter than the size of the plate. As a very simple and handy method another approximation is also investigated considering only the energy flow in the field. The results can predict the gross aspect of the field around the plate.
Subjective rating of speech loudness is carried out by determining a loudness balance between reference and test speech. The rating involves (1) the ability to discriminate variations in sound intensity when voices are the same. Since there is generally a timbre difference between two voices, the rating also entails (2) within-subject variations caused by fluctuations in judgement in regard to loudness balance for different measuring times and (3) between-subject variations caused by individually different loudness sensations. In this paper, these variations are investigated considering the influence of the speaker, the transmission frequency characteristics of the test system, the framework of the experiment, and the training given to the subject. Within-subject and between-subject variations are derived through the use of a sufficiently trained subject. Variation magnitudes are σ=1. 1dB and σ=1. 4dB, respectively, for a telephone transmission system.
This paper presents a theoretical study of tunnel effect of virtual modes propagating in a surface duct. The ocean model used to describe a stratified duct assumes the smooth surface and the bilinear sound velocity profile. Using Labianca's virtual mode treatment (J. Acoust. Soc. Am. , 53, 1137 (1973)), an exact representation of virtual field excited by a point source are derived. Introducing the tunnel effect of sound field from the surface duct to the lower duct, relation between the tunnel effect and the leakage coefficient is derived. The leakage attenuation is negligible for usual ocean parameters in comparison with the surface reflection loss, but the tunnel effect of virtual modes in the vicinity of maximum mode number is important and the effect of tunnel field below the duct can not be neglected. Some design formulas are derived and calculated for actual ocean parameters.