Second-order section representation, specified as an nrow-by-6 matrix, whose rows contain the numerator and denominator coefficients of the second-order sections:
sos <- rbind(cbind(B1, A1), cbind(...), cbind(Bn, An)), where B1 <- c(b0, b1, b2), and A1 <- c(a0, a1, a2) for section 1, etc. The b0 entry must be nonzero for each section.

Overall gain factor that effectively scales the output b vector (or any one of the input Bi vectors). Default: 1.