If you're happy with the sound of the recording, apply the compression across the entire frequency range. Where to start: try a ratio of 2:1 at a threshold of -6dB. Then apply gain to get your peak levels back up to around 0dB. Listen, and repeat: trial and error. If the peaks sound okay (not squished or distorted) but the quiet sections aren't loud enough, raise the ratio and/or lower the threshold. If the peaks sound squished or distorted, lower the ratio and/or raise the threshold. If you go too high on the ratio and/or too low on the threshold, it'll sound like ass, so just listen to different settings to see which sounds best to your ears.