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Acoustic pulse reflectometry was introduced in section 1.5.2. Here we describe the apparatus and theory used in applying the technique for the measurement of musical wind instruments. The apparatus used was developed by Sharp et al. [6,19,20]. An acoustic pulse or audible click is produced by a loudspeaker and is directed down a cylindrical source tube and into the object to be measured. The resulting reflections are then measured and analysed to find the internal profile and input impedance.

Since we are sampling digitally, we receive information on the reflections from the instrument once every sample (ie. once every $T=1/F_s$ seconds where $F_s$ is the same rate). The result is that information is obtained on the change in internal profile at discrete points along the instrument's bore. The reconstructed bore will therefore be approximated by a series of cylinders whose length is such that the primary reflections from successive cylinders occur at the sample rate.

Jonathan Kemp 2003-03-24