In this paper we continue the study of signed double Roman dominating functions in graphs. A signed double Roman dominating function (SDRDF) on a graph G = (V,E) is a function f : V(G) → {−1, 1, 2, 3} having the property that for each v ∈ V(G), f [v] ≥ 1, and if f (v) = −1, then vertex v has at least two neighbors assigned 2 under f or one neighbor w with f (w) = 3, and if f (v) = 1, then vertex v must have at leat one neighbor w with f (w) ≥ 2. The weight of a SDRDF is the sum of its function values over all vertices. The signed double Roman domination number γsdR(G) is the minimum weight of a SDRDF on G. We present several lower bounds on the signed double Roman domination number of a graph in terms of various graph invariants. In particular, we show that if G is a graph of order n and size m with no isolated vertex, then γsdR(G) ≥ 19n−24m9 and γsdR(G) ≥ 4 √ n 3 − n. Moreover, we characterize the graphs attaining equality in these two bounds