Upper and lower bounds on first-order rate-distortion functions are derived for some selected speech probability models and two different fidelity criteria. The bounds are computed numerically when the analytical procedures are intractable. The models employed involve the Laplacian density, the gamma density, and first- and second-order modified Bessel functions

and

. For both the absolute difference and the mean-square difference distortion measures, the

model yields the tightest bounds on

functions.