ABSTRACT
Early measurements of reflection coefficients of outdoor ground surfaces were made at normal incidence and used adaptations of the standing wave or impedance tube technique which is a standard method to obtain the acoustical properties of building materials. A modification of the impedance tube method measures the complex transfer function between two microphones positioned near to the sample surface. Tone bursts or other impulse waveforms may be used as an alternative to continuous sound sources for impedance measurements. The normal surface impedance of the ground has been obtained from direct measurements of pressure and volume velocity at the ground surface by means of a mechanically driven cylindrical Helmholtz resonator chamber, which is pounded into the ground. Typically, soils have flow resistivities that are too high for any significant change in reflection coefficient with increasing acoustic pressure. Model parameter values have been deduced by using both the template method and by fitting impedance spectra resulting from the direct impedance deduction method.
