The propagation of sound through a one-dimensional periodic array of water and perspex plates is studied theoretically and experimentally. It is shown that the passbands and stop bands of a scatterer with a finite number of layers correspond to the bands and bandgaps of an infinite "sonic bandgap crystal." The transmission coefficient of various finite structures is computed and measured as a function of frequency. The analogy with the electronic bandstructure of crystals, and the photonic bandstructure of macroscopic periodic dielectric structures, is found to be a close one. It is shown that the position and width of passbands can easily be engineered. Results are included for a finite "crystal" with a vacancy defect, in which a narrow passband appears in each of the stop bands.

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During the past five years data on tidal currents have been collected in Start Bay by several cooperating teams using a variety of methods. This paper reduces the data to a standard format, and then uses algebraic interpolation to construct contour maps. These show the variation of the peak current speeds and of the residual tidal currents over the Bay. The maps show that the residual currents are negligible along the spine of the Skerries Bank and it is proposed that this is an important factor in understanding the permanence of the Bank.