Abstract: A field sprayer nozzle monitor comprises a sensor head in the form of an L-shaped horn device which is clamped onto the nozzle cap by way of a slot which receives the fin of the nozzle cap. The horn includes a mouth which receives sound from the spray nozzle and transmits it to a microphone protected with the horn. A central processor unit receives signals from each of the sensors in turn and processes those to compare with a predetermined calibration signal to determine which nozzles are blocked or faulty. The selection between two different frequencies can be made for different nozzle types. A dynamic averaging system is used to accommodate changes which take place at all nozzles which are independent of fault at the nozzle.