Most of the previous studies on the sound insulation system were introduced from the deterministic viewpoint in a frequency region. So these idealized studies with no finiteness of geometric scale seem to be too simple to adapt to the complicated situation of actual sound systems with random excitation. In this paper, especially paying our attention to the energy fluctuation in a time series model rather than the usual spectral density in a frequency region, a simplified identification method for an arbitrary sound insulation system and then a simple prediction method for the output probability distribution are proposed. Concretely, based on a preexperiment with a test input of white noise, the order of time series model and its parameters for system are identified by use of the well-known least square method. Next, by use of the result of this indentification, the probability distribution of output energy is predicted for the present system in an actual case when the noise input excitation of arbitrary distribution type passes through the system. Finally, the present method is applied to the room acoustical experiment in several types of actual cases containing double wall with sound-bridge. Thus, the effectiveness of our identification and prediction methods is experimentally confirmed.
In this research work, it is clarified that mechanism of impulsive noise generation in a bubbly water flow by a solid wall or by something in it is caused by the separation of large bubbles and that the damping oscillation waveform of impulsive noise is caused by the pulsating oscillation of the separated small bubbles. In the bubbly water flow of up to 4m/s, the frequency of impulsive generation and the peak value of the noise amplitude become larger, as the flow velocity increases. The peak value of the impulsive noise, however, is at most 30kPa, and the value is very small in comparison with that of cavitation noise. The frequency of impulsive noise generation changes greately with the bubbly flow velocity. In case of the velocity of about 1. 5m/s, it is about one hundredth of the value at the velocity of 4m/s. When the bubbly water flow velocity is under 2. 5m/s, the occurrence of impulsive noise whose peak value is above 10kPa bacomes less than once per second, and it becomes negligible in comparison with that of impulsive noise whose peak value is around 3kPa.