It is frequently necessary to inspect structures operating at temperatures above 3000C, or in high radiation environments. In these cases it becomes more difficult to use conventional ultrasonic transducers. A radical alternative would be to use an acoustic waveguide made from a radiation and temperature resistant material (e.g. stainless steel or ceramic) to couple the energy into the test structure from a transducer and instrumentation located in an area where they can be operated easily and repaired/replaced as necessary. This idea has been used over fairly short ranges (tens of mm) but it would be attractive to extend the range to ten metres or more. The end of the waveguide would be attached directly to the critical structure at the point of interest. Effectively this approach uses an 'acoustic cable' to interrogate the test area instead of the conventional approach which uses an electrical cable, so that all the electrical parts of the system are outside the harsh environment. This makes both continuous health monitoring and periodic inspection much easier.