L.S. Wang

Potential performance of parametric communications

From a designer viewpoint, a parametric array should not appear different from any other type of acoustic transducer and should be described by a limited set of design equations together with their range of validity. In this paper, these design equations are stated and discussed. They are used to optimize the acoustic parameters of an underwater communication system using parametric transduction and to evaluate its performance in terms of signal-to-noise ratio and data-rate limits as a function of transmission range. It turns out that, for a maximum data transmission rate at a given range, there is a set of optimum design parameters which is a function of the array size only. This means that, once given an operational range, the primary frequency, the electrical power, the maximum acoustic source level, and the directionality of the transducer can be deduced directly from the array diameter.

Measured channel sounding characteristics and their relationship with the performance of a parametric communication system

This paper presents the results of experiments carried out in the Mediterranean Sea using parametric transduction to effect acoustic communication. Short duration, pulsed carrier signals were used to estimate the impulse response of the channel and these observations were related to the performance of a parametric communication system operating under the same conditions. The statistical characteristics and spectra of the amplitude and phase fluctuations were used to analyse the pulsed response over short observation periods. Typically, the amplitude fluctuations experienced by the main path were Ricean in nature with a spectral content in the sub-Hertz range whilst the phase fluctuations tended towards more Gaussian-like behaviour with a comparable frequency spectra. Under these conditions a high data rate differential phase shift keyed (DPSK) telemetry link was established over a 1.7 km path. The ensuing performance of the communication system, characterised by the bit error rate for various symbol rates, is presented and demonstrates the viability of a parametric telemetry system.

Underwater acoustic communication utilising parametric transduction with M-ary DPSK modulation

A real-time M-ary differential phase-shift keying (MDPSK) communication system utilising parametric transduction has been constructed. The system employs a 50 kHz primary frequency and a 5 kHz difference frequency. It has been tested in the Gulf of Lion and at Cap Ferrat in the Mediterranean Sea. Experimental results indicate that the system can be used in shallow water to realise real-time acoustic communications at ranges of tens of kilometres and can achieve data rates of 1, 2, and 3 kb s/sup -1/ for 2-, 4-, and 8-DPSK respectively.

Combined finite element-boundary element analysis of a viscoelastic anechoic panel for underwater applications

In this paper the combined finite element-boundary element method is applied to the analysis of the sound-structure interaction around a multi-layered anechoic baffle. The baffle is made of three layers: an aluminium backing supports a second layer of highly butyl rubber. The third layer, loaded polyurethane, presents a regular grid of steep shaped cones with a fluid-matching function. The viscoelastic materials are fully modelled into the finite element method using linear integral constitutive relationships. The combined FE-BE algorithm is based on a partial application of the Burton and Miller Helmholtz gradient formulation to overcome non-uniqueness problems. Examples of analysis of acoustic scattering in a range of frequencies typical of underwater communications are given together with a practical application of the structure.

Effects of water surface on the field of a parametric source

The effects of a flat sea surface on the secondary wave field for a parametric acoustic source with a piston type transmit transducer have been assessed both theoretically and experimentally. When the virtual array of a parametric source is truncated by the sea surface, the amplitude of the secondary signal is reduced. In addition to the normal cancellation between the direct and surface reflection paths, there are two more mechanisms which cause the reduction of the signal level with a rough sea surface. The first one is the destructive summation of the secondary field before and after the intersection with the surface, the second one is the loss of the coherence of the primary signals after reflection. The Westervelt model is used to predict the secondary field. Experiments in an indoor laboratory tank have been carried out to measure the primary and secondary fields. It is found that, with a flat water surface, the reduction in the signal level depends on the characteristics of the parametric source and the geometry of the problem. Both the theory and experiments show that there is a maximum loss caused by the truncation. The experimental results indicate that the maximum loss is almost 10 dB in signal level for the problem concerned, while the theory predicts an even higher value.