The sensors are used to receive the acoustic-frequency signal in the furnace. When the boiler operator in normal condition, the received signal is from background noise, of which frequency is mainly concentrated on the low-frequency region and the intensity is little. When the tubes leak, the intensity of the noise strengthens to large extent, and the frequency is mainly concentrated on the moderate- or high-frequency region. At this time the sensors can change the intensity and frequency spectrum of the noise to the electrical signal, and transmit it to the monitoring system in the control room far away. The sensors are fixed on the tail unit of the conductive tubes. Each sensor consists of an enhanced sound-wave sensor and self-test noise part, which are sealed in a stainless steel shell. Generally the detecting range is in a 10~15m hemisphere space, controlled by a gain knob. The value of the knob changes with the fluctuation of the background noise.