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Apparatus

A schematic diagram of the apparatus used is shown in figure 5.2 and a photograph of the apparatus is shown in figure 5.3.

Figure 5.2: Schematic diagram of the apparatus
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Figure 5.3: The acoustic pulse reflectometer
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The computer sends out a 5 V electrical pulse of 80 $\mu$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 $F_s=50,000$ Hz. This experiment is repeated 1000 times and the result averaged to increase the signal to noise ratio. Length $l_2$ is chosen so that the input pulse has completely passed the microphone before the first reflections return from the object. The length $l_1$ is ideally chosen so that the round trip time $t=c/(2 l_1)$ 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 $l_1$. The reflectometer used in this chapter has lengths $l_1 = 7.37$ m and $l_2 = 3.10$ m.

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Next: Experimental measurement of the Up: Acoustic pulse reflectometry Previous: Losses   Contents
Jonathan Kemp 2003-03-24