Experimental Investigation of Underwater Noise Produced by Ships by Means of Sound Intensity Method

Abstract

Ship noise is a one of most important part of the field of the acoustic environment. Its reduction and monitoring is a very important aspect in the performance of underwater acoustic systems and in the habitability of the vessel for the crew and passengers. In naval operations the noise radiated by a ship is also dominant source of information for underwater sonar weapons systems. Sound intensity is the vector quantity which informs us about the sound power flux, the direction of the sound source. The probe tracks the position of source and at the same time, measures the component of sound-intensity vector. Generally only one component of vector sound intensity was measured, perpendicular to a given measurement surface. The sound intensity of a moving source includes eects such as flow noise that are not present when the source is stationary. The accuracy of measuring acoustic intensity using two closely spaced hydrophones by correcting the phase mismatch error by measuring the transfer function between the two detector systems at the same time was examined. The two measuring hydrophone instrumentation systems were earlier carefully calibrated. Our results have been obtained using digital methods, where was used PULSE B&K measuring system. PACS numbers: 43.30.Nb 1. Intorduction Very little data is available on sound intensity and even sound pressure level (SPL) measurements and noise spectra of small boats or big vessels. Noise from warships and merchant vessels with keel aspect source levels ranging between 140 and 192 dB re 1 „Pa at 1 m. Hostile naval forces also use the underwater noise signatures radiated by surface ships and submarines for detection and classification of ships as targets. At low speeds up to 8 kn, the underwater noise signature of the ship is dominated almost totally by tones from the ship’s Diesel generator set. Above this speed, the ship’s signature is dominated by three sources: main engine diesel firing rate harmonics, blade harmonics, and than wideband cavitation noise. The eect of the noise aspect of maritime modern civilization on sea ecosystems is frequently ignored, but it is beginning to be recognized as a trouble worthy of further research. A small number of available measurements have been made of the full sound intensity vector, and little has been done so far to explore its use. The sound intensity hydrophone probe was designed to measure sound intensity together with the unit direction of flow as a vector quantity. This was achieved by setting up two hydrophone probe to measure the sound energy flow. The distance between them was 120 mm, it allows to determine the sound intensity with accuracy §1 dB in the frequency