HYDRODYNAMIC SIREN AS A BROAD-BAND UNDERWATER SOUND PROJECTOR

The siren mechanism is adapted to underwater uses for sound production . Such a hydrodynamic siren works as a powerful, broadband mixed sound projector as well as a less powerful, more noiselike sound projector. The broadband mixed sound consists of the harmonics due to the rotor rotation rate and the hydrodynamic cavitation noise. The cavitation no ise level at rotor rotation is almost the same as that when the rotor is not rotated. The maximum acoustic pressure of a model siren is 189 dB re: 1 mu Pa at 1 m when integrating between 0 and 10 kHz, and 179 d B re: 1 mu Pa at 1 m as to one peak line spectrum. The siren gain atta ined by the rotation rate harmonics is approximated at about 20 dB. Th e maximum overall electro-acoustic conversion efficiency is estimated as 1.2% for the rotor-stator clearance of 0.15 mm and the applied wate r pressure of 8.2 kg/cm(2), that is, 0.80 MPa. Since the siren has a d ual hole configuration, a third set of harmonics is virtually derived from the greatest common divisor obtained from the numbers of holes in the two sets of holes (i.e., 8 holes and 20 holes give an effective v irtual 4 holes). The radiation directionality and depth dependence of the model siren are also examined. The experimental results on the hyd rodynamic cavitation noise at nonrotation of the rotor almost match th ose revealed in previous literature on the noise from cavitating water jets and hydrofoils. However, the results show a strong dependence of the spectrum level on the receiving position. More detailed knowledge on the cavitating jet length, the bubble concentration along it, the vibration of the surviving bubbles, etc., are needed to understand the hydrodynamic cavitation noise more intensively. Although the siren ca nnot avoid the harmonic generation due to the rotation rate, a siren a s a noise maker is proposed by considering a random configuration of m any holes and random occurrence of hole opening in one rotation period . (C) 1995 Acoustical Society of America.