Ensure S1 is in the off position and insert the two batteries with their positive terminals upper most. Turn on the switch S1 and rotate the potentiometer. If all goes well you should hear a range of noises as the potentiometer is varied which are centred around the main frequency of 4kHz. The sound is deliberately not too loud so as not to irritate. If you wish to hear it louder you can insert a pair of earphones into the 3.5mm jack socket or connect the system to a stereo amplifier.