Abstract: Apparatus for monitoring the escape of steam from the tubes of a power station boiler or the like employs a microphone located externally to the combustion space to sense an increase in combustion space noise level. At least one long narrow listening tube leads from the combustion space and has at its end an elbow to which is sealed a microphone assembly that senses the noise in the combustion space. Preferably a diaphragm is interposed between the microphone assembly and listening tube that seals the end of the listening tube while transmitting sound substantially unattenuated. The diaphragm is a plastics sheet compressed between washers that have an undulating line of contact. Noise from escaping steam at a frequency of 4-8 KHz may be detected with a gain of typically -20 db. The signal from the microphone assembly is amplified and processed by means to indicate the level of sound in the combustion space.