White noise
The ARS is essentially set up to create a fingerprint for different samples by constructive and destructive interferences. Figure 1 is a schematic of the quartz rod ARS which illustrates the path of the sound through the quartz rod. A function generator is the source though any device that is capable of outputting sound in voltage form could be used (i.e. CD player, MP3 player or sound card). White noise is generated and the voltage is converted into a sound wave by a piezoelectric disc coupled to the quartz rod. The sound resonates down the quartz rod which is shown as a blue sinusoidal wave and two key interactions occur. A portion of the energy (red) is introduced into the sample and interacts in a specific manner dependent of the sample and another portion of the energy (blue) continues unaltered through the quartz rod. The two energies will still have the same frequency though they will have changes in their phase and possibly amplitude. The two waves recombine after the sample and constructive or destructive interference occurs depending on the phase shift and amplitude change due to the sample. The altered combined energy is converted to an electrical voltage by another piezoelectric disc at the end of the quartz rod. The voltage is then recorded onto a computer by a sound card. The sample is coupled to the quartz rod at constant pressure which is monitored by a pressure transducer which also acts as the sample holder. Rubber grommets are used to secure the quartz rod to a stable stand minimizing coupling of the rod to the surroundings. Broadband white noise is used to obtain a full spectrum; however, most sound cards only pick up between 20 and 22,050 Hz. The waveform that is sent to the computer is a time-based signal of the interactions of white noise with the sample. Fast Fourier transform (FFT) is performed on the waveform to transform the time-based signal into the more useful frequency spectrum.