The computer sends out a 5 V electrical pulse of 80 s duration to an audio amplifier. A horn driver loudspeaker attached to the output then produces an acoustic pulse (audible click) which travels down the air column of a cylindrical source tube of internal radius 5 mm and then enters the object to be measured. The complicated object reflections are then picked up by a microphone in the side wall of the source tube and sampled by the computer at a sample rate of Hz. This experiment is repeated 1000 times and the result averaged to increase the signal to noise ratio. Length is chosen so that the input pulse has completely passed the microphone before the first reflections return from the object. The length is ideally chosen so that the round trip time is sufficient to record the full object reflections without interference from reflections from the source. This is of course dependent on the length of the instrument; longer instruments have reflections which carry on for more time meaning that a longer distance is required for . The reflectometer used in this chapter has lengths m and m.