Flextensional transducers applied to long range hydroacoustic communication

This paper presents a concept for long distance hydroacoustic communication employing a flextensional transducer with giant magnetostrictive driving element (Terfenol-D). Bandwidth and power requirements together with a candidate signalling scheme are presented for a 50 km transmission link aiming towards a bit rate between 20 and 50 bits/s. This capacity makes the link suitable as control link for a semiautonomous long range ROV. A magnetostrictive flextensional transducer for the task is specified and its advantages over conventional transducers are pointed out. Since the attenuation of hydroacoustic signals is increasing with frequency, long range hydroacoustic data transmission calls for high power sources operating at low frequencies. At 50 km transmission distance there is a 10 dB gain to collect by shifting the operating frequency from 5 kHz to 1 kHz. The required power to meet a satisfactory signal to noise ratio (>5dB) over a 100 Hz frequency band is still about 600 W when assuming a noisy receiver environment as e.g. an offshore oil field. In addition to the power requirement a large bandwidth is a desirable feature of a transducer applied to hydroacoustic communication. The generation of high power acoustic signals at low frequencies requires a transducer which can produce a very high rate of volume displacement. One of the best ways to achieve this, is by using the so-called flextensional design using a shell shaped as an elliptical cylinder with an active element along the major axis. The shell acts essentially as a mechanical transformer where a relatively small displacement imparted along the major axis is converted to a larger displacement along the minor axis. This principle applies regardless of the type of driving element, and flextensional transducers with piezo-electric ceramics as driving element are available on the market. However, the flextensional design becomes really interesting when Terfenol-D is used as the driving element. Terfenol-D is a new magnetostrictive rare earth alloy composed of terbium, dysprosium and iron. With this new alloy it has been possible to design a transducer with a low resonance frequency and with a bandwidth which permit us to achieve relatively high bit rates over long distances. The combination of low frequency and high output of acoustic energy makes it possible to establish a telemetry link for distances exceeding 50 km. The size of the new type of transducer would be smaller than 0.2 m in diameter around the major axis.