Cloud-averaged spectra of all sources with detections in HCO⁺ 4–3 and C¹⁸O 3–2. Non-detections are omitted from averaging. The HCO⁺ 4–3 data are shown in black, the C¹⁸O 3–2 in red, and Gaussian fits in cyan. The sum of two 1D Gaussian fits is shown for the Perseus cloud. Perseus shows a clear secondary feature while Taurus is well fit by single Gaussians in both lines. Note that the y-axis scale is 0.25 × the figures presented in Appendix B.