This paper explores the relationship between the glottal pulse amplitude (Up) and the amplitude of the first harmonic (H1), as well as the combined effects of Up, the open quotient (O-q) and degree of pulse asymmetry/skew (R-k) on the low end of the source spectrum. This serves to elucidate their relationship to the H1-H2 estimate, widely used to make inferences on changes in O-q and voice quality. It has been suggested that HI is mainly determined by Up and that the pulse shape has a relatively small impact. To investigate this, a series of glottal pulses were generated using the LF model, where U-p was kept constant, while O-q and R-k were systematically varied. The resulting harmonic amplitudes of these pulses show that U-p is not the sole determinant of H1. Rather, H1 is highly dependent on O-q and to a certain degree also on R-k. Although the effects of these parameters on the lowest harmonics is rather complex, we find that the H1-H2 measure is broadly correlated with O-q. However, there is also a strong effect of differences in glottal skew, particularly at high O-q values, which could invalidate inferences on O-q and voice quality from estimates of H1-H2.