Effects of Low-Pass Filtering on Acoustic Analysis of Voice

Abstract

Low-pass filtering is often applied to eliminate effects of environmental noise when preparing voice recordings for acoustic analysis. This study tested the effects of low-pass filter cutoff frequency on the results of acoustic voice analysis, with a particular interest in the effects of low cutoff frequencies on nonlinear dynamic parameters. A crossover randomized controlled trial was performed using voice recordings of sustained vowel phonation obtained from the Disordered Voice Database. A second-order Butterworth filter was applied to the voices at cutoff frequencies ranging from 5000 to 40 Hz. Percent jitter, percent shimmer, fundamental frequency (F(0)), signal-to-noise ratio (SNR), correlation dimension (D(2)), and second-order entropy (K(2)) were calculated for each signal. Traditional acoustic parameters were validly measured at cutoff frequencies as low as 300 Hz. The SNR and percent shimmer were improved by cutoff frequencies of 300 Hz or higher; F(0) and percent jitter were unaffected by filtering at these frequencies. D(2) and K(2) were measured stably for signals filtered at cutoff frequencies as low as 100 Hz. To ensure accuracy in acoustic voice analysis, setting the cutoff frequency of a low-pass filter at least one octave above the F(0) (minimum of 300 Hz) is recommended. Nonlinear dynamic measures of D(2) and K(2) proved more robust and maintained accuracy at lower frequencies.