A method and apparatus for determining properties of at least one of a surface or materials adjacent to a portable device. The method includes windowing a segment of the received signal to remove an edge transients, computing the FFT power spectral density of the signal, determining a peak in the spectral energy at a frequency, finding local peaks by determining the difference in the signal amplitude is relation to a pre-determined threshold, and computing harmonic energy according to the local peaks and the difference and determining at least one property of the surface or material.