enabling an audio filter from a C# script

I have an AudioSource “aSource” defined with an AIFF file loaded in the Inspector. I’m trying to activate a filter (I have Unity Pro) as follows:

public AudioSource aSource;
aSource.AudioChorusFilter.enabled = true;

This gives me this error:

Type `UnityEngine.AudioSource' does not contain a definition for `AudioChorusFilter' and no extension method `AudioChorusFilter' of type `UnityEngine.AudioSource' could be found (are you missing a using directive or an assembly reference?)

Can someone point me in the right direction please?

I got it working using GetComponent. If anyone has a similar issue in future, this is what I did:

AudioChorusFilter chorus = aSource.GetComponent(typeof(AudioChorusFilter)) as AudioChorusFilter;
chorus.enabled = true;