Thermoacoutrimetry

 

There are two basic forms of thermoacoutrimetry. The first measures the spontaneous generation of acoustic noise during the heating of a sample and measures the intensity of that noise and the temperature(s) at which it occurs. The second measures the velocity of sound through the sample by an acoustic generator attached to the sample and an acoustic sensor subsequent to it. The apparatus used for measuring noise generation is shown in figure 22.

 

The sample is situated in an oven and an acoustic conduit (a sort of stethoscope) is situated near it so that any sound generated is led to an acoustic sensor, the output from which is amplified and the final signal acquired by a computer. The results are normally presented as the amplitude of the generated noise against the temperature of the sample on the computer monitor or printer.

 

Figure 22. The Basic Instrument for Measuring Noise Generation During Heating

 

The acoustic signals are generated in the sample in bursts and usually occur at a specific temperature. The noise usually results from the reorientation of the structure or a phase change as a result of the thermal energy acquired by the sample. When a substance in an amorphous form crystallizes, for example, the structure is radically changed and the reorientation of the molecules causes the crystals to rub against one another and generate the noise. The noise generation might be considered as resulting from a sort of ‘molecular earthquake’. The results obtained from a thermocoustic measurement is shown in figure 23.

Figure 23. Graph Relating Amplitude of Acoustic Generation against Sample Temperature

 

The curves obtained were obtained from a sample of clay.