A new method for distinguishing the resonant of shell

The resonant of the ship shell is the important aspect for reflecting the natural characteristic frequency of radiated noise of the ship shell. To master the frequency spectrum characteristics of the ship radiated noise and analyze its changing regularity with ship speeds, there is great significance for distinguishing the line spectra of acoustic field which is stimulated by mechanical equipment or shell resonance. When the natural frequency of the mechanical equipment is equal to the shell of the ship, it might stimulate the resonance of the shell. It is difficult to distinguish the characteristic frequency of the resonant frequency and the equipment when the spectrum resolution is 1 Hz, since the line spectrum is only reflected as f Hz. However, with zoom spectrum analysis, it can be distinguished as two different line spectra when the resolution of spectrum is 0.1 Hz or higher. They are f1 Hz and f2 Hz respectively. Using a line spectrum extraction technique, we extract three line spectra: f, f1 and f2 under the conditions of different frequency resolution; and, then their real-time phases are received, which are represented as ψ, ψ1 and ψ. Finally, the differences of ψ and ψ1, ψ and ψ2 are calculated. The changing regularity of the above two differences with time are entirely opposite. In order to verify the validity of this method, a test is carried out with the Beidiao 991 ship. In the pump chamber, a high-power motor is installed on the shell, and its working frequency is the same as the natural frequency of the shell. The test results show that the method can effectively judge whether or not the shell of ship is resonant.