Rigid body radiation by half sine transient acceleration of a sphere was studied. The results were applied to the estimation of radiated energy due to collision of two spheres. Radiation efficiency and Fourier transform of motion velocity were used for the energy spectrum formulation. Total acoustic energy was evaluated from the spectrum as a function of non-dimensional parameter β=k_da, where k_d is the wave number for transient motion and a is the sphere radius. For impact between two identical spheres, estimates of total radiated energy and peak frequency in the sound pressure spectrum agree well with experimental values. Experimental data were obtained for steel and polymer balls. Value ranges for β, radiated energy and the peak frequency in sound pressure spectrum for a practical situation are shown.
A parametric receiver uses the nonlinear interaction of pump wave and incoming signal wave, and it can dispense with phasing sensors along the array. Directivity of the receiver is the same form with the line array of pump transducer-detector distance. In the present paper, SNR of the receiver is measured using 40kHz pump wave and 97cm array length in air, and causes of noise generation are discussed. Harmonic components of the pump wave are phase-modulated by signal wave in the same fasion with the pump. The fact that the harmonic parametric receiving array really exists suggests an effective way for SNR improvement. The fluctuations of parametric signals caused by a weak airflow are also investigated. It is made clear that the amplitude fluctuation suppression for the output signal of parametric receiver are very important problems in order to develop the receiver for practical use.
Until now, there are many approaches on a prediction of traffic noise and ground vibration from various contrastive point of view, such as evaluation or control sides, theoretical or experimental sides, deterministic or statistical sides and descriptive or constructive sides. In this paper, differing from the previous methods, a new trial of prediction of hybrid type between the above contrastive view points has been proposed in a unified but simplified prediction form with the flexible applicability to arbitrary types of the vehicles situation and the propagation environment, without introducing any kinds of model on the above traffic flow pattern and propagation. Finally, the effectiveness of the present prediction method has been experimentally confirmed too by applying it to the actually observed data on the road traffic noise and ground vibration under typically complicated traffic flow situations which can not be predicted by the well-known standard prediction method.
Since materials with a continuous variation of refraction index are not available for the construction of spherical Luneburg lens, the desired continuous variation of index with radius is approximated by a number of constant-index spherical layers. There are several methods for determining the index difference between adjacent layers and their radii. The common method for step-index division is to choose equal index increments in which the outer part of the lens is more precisely divided than the inner part but here most of the acoustic power passes the inner part of the lens. This paper describes a step-index method which equally distributes the acoustic power incident on the lens. A wave solution has been obtained for the acoustic field in spherical multi-layered lens. The relative pressure and the relative intensity were computed along the axis of the lens. The pressure focal point and the intensity focal point were obtained and they were used for the comparison of the focusing properties. In the case of 3, 4, and 6 layered lens, the focusing properties of the new lenses were compared with those of the Morgan lenses which have equal index increments. Although the focal points of pressure and intensity did not coincide for the equal-index-increment lens, they almost coincided for the new lens of 3 layers with K_0a 30-40 (k_0: wave number, a : radius of lens). It is shown that increasing the number of steps does not improve the focusing properties but choosing the thickness and the specific acoustic impedance of the outermost layer is effective in improving of the focusing properties of the multi-layered lens.
An active noise attenuation system, which is a compact low frequency noise silencer, was developed. The system in this paper controls repetitive low frequency noise in a pipe, such as the intake noise of air compressors. The conventional low frequency noise absorbers like resonance type became very large, because of the wavelength of low frequency sound. But active technique made a low frequency attenuator compact. In this system, a synchronous signal of a machine is fetched to measure the fundamental frequency of a noise source. And the sound waveform at the pipe end is analyzed with discrete Fourier expansion by a micro-computer. Then the system drives an additional loudspeaker which produces a canceling waveform. Within one second after the system started operating it reduced the sound generated by a primary loudspeaker up to 15 dB. And it finally achieved over 20 dB of attenuation. The system also attenuated an intake noise of a real mini-compressor by 18 dB in experiment.