OS X: Filter system audio-out to remove high-pitched noise?

I am watching a long video on YouTube and the audio is terrible, mostly because there is a lot of high-frequency noise. I'm about to get a headache.

Is there any software that can capture my system audio out (like Airfoil can) and run it through filters, or even a simple multi-band equalizer?

Solution 1:

• Set Soundflower as the default output device (in order to capture output from applications that don't support custom output devices)
• In the application that's used for the effects, set the input device to Soundflower and the output device to a normal output

The default output device can be changed from the Sound preference pane or Audio MIDI Setup:

The application that's used for the effects needs to support:

• Custom input and output devices (Fission doesn't?)
• Playthru with effects (Audacity doesn't?)

I added instructions for Garage Band and AU Lab, but any DAW or standalone version of an effect plugin would probably do as well.

Garage Band only allows you to make a new project by choosing a template. Anyway choose for example the Voice template:

Change the input and output devices in the preferences:

Focus an audio track, and enable playing back the input:

Add effects in the edit tab:

AU Lab was installed with Xcode before 4.3, but it can now be downloaded from https://developer.apple.com/downloads/.

Solution 2:

Soundflower is free and can capture your audio and send it through another application such as iTunes, which has an equalizer.