VFR transmission of LPC vocoder coefficients is a technique developed to reduce the average transmission rate without appreciable loss of quality. The technique transmits parameters at a variable rate in accordance with the changing characteristics of the speech signal. To assess the effectiveness of VFR transmission, an experiment was performed to compare it with 2 other methods for reducing the bit rate: reducing the number of poles and increasing the quantization step size of the LPC coefficients (log-area ratios). Stimulus sentences (32) were prepared by passing 4 utterances (2 sentences .times. 2 speakers) through 8 vocoder systems in a 2 .times. 2 .times. 2 factorial design; 2 values were assigned to each of the 3 parameters: average frame rate, number of poles and quantization step size. Eight listeners made 7-point category ratings of quality degradation. Of the three methods studied, the VFR technique produced the highest quality at any given transmission rate (or, equivalently, yielded the lowest bit rate for a fixed level of speech quality).