To develop quantitative measures of motor unit action potential (MUAP) sounds, we correlated Fourier domain features of isolated, individual MUAPs with classic time domain measurements. There were moderate correlations between amplitude or duration measurements, and absolute low-frequency (39 to 234 Hz) energy and amplitude of the peak frequency, and high correlations between the composite time domain feature of amplitude x duration, and total energy, amplitude of the peak frequency, and absolute low-frequency energy. Polyphasic potentials have multiple peaks in the magnitude component of the Fourier transform. Phase information appears to convey the "crisp" sound of MUAPs close to the recording electrode. The clinical description of "large" or "small" MUAPs by sound is likely based on absolute low-frequency energy, and incorporates both amplitude and duration information. We conclude that features of isolated MUAPs may be analyzed in the Fourier domain, and that they correlate closely with traditionally used measures of known diagnostic significance. The sound of the EMG used by clinical electromyographers is amenable to quantitative analysis.