Abstract: An acoustic detector that includes active noise cancellation is presented. An acoustic detector converts sound waves, which include sound waves of interest as well as noise, to an electronic signal. An operator listens for a characteristic sound, for example the sound characteristic of fluid leaking from a pipe, as she varies the position of the transducer. At least one other acoustic detector is positioned in order to monitor noise. The electronic signal from the noise monitoring acoustic detector and the electronic signal from the acoustic detector are combined in a processor in order to cancel the noise. In some embodiments, the operator may adjust a cancellation band so that some frequencies are not cancelled. For example, the operator may adjust the cancellation band so that a co-worker's voice may be heard.