Basic problems caused by depth and size constraints in low-frequency underwater transducers

Abstract

Scaling of a conventional transducer design to very low frequencies usually leads to greater power than needed and greater size than is considered tolerable. Normally, therefore, scaling guidelines are abandoned, and very low frequency transducers are miniaturized, that is, made with dimensions much smaller than the sound wavelength. The resulting low radiation resistance leads to high mechanical Q, poor power-to-weight ratio, and poorly behaved driving-point impedance. By employing electrical equalizers and acoustoelectrical feedback, the designer can achieve broad bandwidth in spite of the high Q, but an oversized driving amplifier is required. Transducers that operate at full ocean depths are usually liquid-filled. The stiffness of the interior liquid increases the transducer size and tends to raise its Q and lower its coupling factor. A Helmholtz resonator configuration is often advantageous in this situation. For more moderate depths, compliant tubes or compressed gas may be used to reduce the stiffness of the interior. However, depth capability is always obtained at the expense of acoustic performance.