Different types of noise are being generated around us, and many researches have been done for a long time to reduce noise. Noise is reduced through source control, path control and receiver control methods, and path control method is mainly used. For example, silencers are installed in the intake and exhaust systems to reduce the noise caused by engine combustion of vehicles. To control noise effectively, accurate sound source analysis is required. It is very difficult to analyze sound source characteristics by using theories. Therefore, experimental methods are mainly used to obtain detailed characteristics of a sound source. The operating conditions of vehicles such as rpm, change over time. In this case, the sound source characteristics should be measured in a transient state. However, most previous studies focused on measuring sound source characteristics in a steady state because the measurement was complicated in a transient state.
In this study, the characteristics of sound sources were measured in steady and transient conditions for a speaker and a blower. Especially, a methodology for measuring the characteristics of sound sources in transient states was presented. Compared to the case where white noise by speaker was used as a sound source, the sound source strength measured when a sine wave was used as the sound source was about 35 dB higher, but the sound source impedance was the same a trend. When transient sine wave by speaker was used as a sound source, the small shift of the peak frequency was found. As a result of using the blower as a sound source, different magnitudes of sound source strength were observed depending on the rotational speed, but the sound source impedance showed the same value even if the rotational speed was changed. The characteristics of the sound source measured in the steady and transient state were compared. When the change in rotational speed was slow, the sound source characteristics of the steady and transient state were similar. In the case of rapid change in rotational speed, it was observed that the sound source characteristics in the transient state is estimated due to the inertia of the air. Using the measured sound source characteristics, the silencer performance and radiated noise were predicted and compared with the measurement results. The two results showed similar trends overall but differed from the measured values due to the background noise in some frequencies.
This study presented a methodology to measure the characteristics of sound sources in transient state and control or reduce sound in duct systems. It has been confirmed that the silencer performance and radiated noise can be predicted by applying the characteristics of the measured transient sound source.