Using the DSP module process function (IIR) twice causes noisy signal

[daniel]

The filters are stateful. If you feed different signals through the same filter, there are discontinuities in the fed signal that will become audible as noises.

Jules tried to give a general answer here

#1 most common programming mistake that we see on the forum

This post is here so that we have something we can link to when this same issue comes up, because the JUCE team is getting very bored of spending time replying again and again to the same damned problem! This isn’t a JUCE bug - there’s nothing we can do in our codebase that would stop people making it, which makes it painful for us to watch beginner after beginner come along and fall headfirst into the same trap! It’s also understandable that people will post a new topic when they hit it, beca…

TL;DR: you need a filter per signal you feed. If you want to filter the signal twice with the same filter, you still need two instances that you call one after the other in always the same order.