Sound velocity measurements by the use of dynamic light scattering: data reduction by the application of a Fourier transformation.
By the use of photon correlation spectroscopy in a heterodyne technique, the ultrasonic velocity of transparent fluids can be determined. The quantity measured is the intensity correlation function that forms a damped oscillation. The oscillation frequency value can be found easily by the application of a numerical Fourier transformation. However, a dependence on the sample time of the applied correlator and the oscillation decay time must be taken into account. An estimation for the accuracy of the deduced frequency is given on the basis of a Fourier transformation of noisy synthetic data. For example, we find that a sampling interval of 100 ns, the sound frequency from scattering 514.5-nm light at an angle of 3° would be determined to ±0.3% for a typical near-critical fluid with a sound velocity of 100 m/s. For larger sound velocities, and for fluids far from the critical point, the uncertainty would decrease rapidly.