as well the detector 22 detects the changes in received light intensity and produces an analog output signal for example over the range of 1 to 5 mv this output signal is then amplified by the preamp 24 which amplifies the incoming signal to an approximate one volt level the preamp also serves to isolate the filter 26 from the detector 22 otherwise the signal bandwidth of the detector 22 may be restricted by direct connection to the filter 26 the amplified analog signal is then processed through the filter 24 having a band pass for example of 800 hz to 1250 hz indicative of the normal operating range of engine noise the filter of course rejects all other undesirable signals not relevant to normally operating engine parameters and the band pass may be selectively changed so as to accommodate various aircraft engine noise frequencies the filtered output signal is then amplified by audio amp 28 so as to provide sufficient energy to drive the headphones 30 which may alternately be a loudspeaker or similar acoustic transducer as the situation may require the landing signal officer or similar personnel on board the carrier may then easily monitor the output of the headphones 30 so as to hear the engine noise generated by the approaching aircraft after having given a particular radio transmitted command to the pilot to correct.