For a gas turbine engine (10), a method and apparatus for determining temperature of combustion gas in a flow path (17) of the engine. In one embodiment a waveguide (30) includes a cavity (36) with a first waveguide end (32) including a barrier layer (58) which may be a membrane. With the first end positioned to inject an acoustic signal into the flow path, the barrier layer (58) isolates the cavity from combustion gas. Acoustic instrumentation positioned along the surface (38i) of a wall (38) of the waveguide provides generation and propagation of acoustic signals into the flow path for detection and measurement after reflection back into the cavity. The wall may include sections of two different materials, one material having a higher melting temperature than the other, or one material having greater electrically isolating properties than the other section.