Low-pass filter for cleaning vocal speech before performing onset detection
The problem is that for speech, noise and speech may well occupy the same frequency bands, so removing an arbitrary band by simple filtering can remove speech as well, which will prevent your detector from triggering, as well as not removing out-of-band noise, which will produce false positives.

So, what you are looking for might not be just a low pass filter, but a Voice Activity Detection algorithm, which is a more sophisticated bit of DSP technology.  There are many research papers on this topic, so it's not a short answer question.