Hydrodynamic siren as a broadband underwater sound projector

Abstract

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 noise 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 μPa at 1 m when integrating between 0 and 10 kHz, and 179 dB re: 1 μPa at 1 m as to one peak line spectrum. The siren gain attained by the rotation rate harmonics is approximated at about 20 dB. The maximum overall electro-acoustic conversion efficiency is estimated as 1.2% for the rotor–stator clearance of 0.15 mm and the applied water pressure of 8.2 kg/cm2, that is, 0.80 MPa. Since the siren has a dual 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 virtual 4 holes). The radiation directionality and depth dependence of the model siren are also examined. The experimental results on the hydrodynamic cavitation noise at nonrotation of the rotor almost match those 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 cannot avoid the harmonic generation due to the rotation rate, a siren as a noise maker is proposed by considering a random configuration of many holes and random occurrence of hole opening in one rotation period.