But my question really is when should I call the police. I completely agree with you, but trying to determine when it's yelling and when it is actually domestic violence is hard to do. I have a good friend who used to work for me, who had the cops called on him once by a nosey neighbor. He and his wife were arguing, possibly too loudly (I have been guilty of this, and no I am not proud of that, but it happened) anyway he got so mad he threw something. I was not at her, or even in her direction, but when the cops came they took him in. When he didn't show up for work the next morning and won't answer the phone, I called his wife. She explained the whole thing and told me he was in jail, even though she insisted he did nothing to harm her. This is just standard procedure, and unfortunately it can lead to a criminal record in some cases. With that in mind I don't really like calling the cops for such things, but should it happen again, I guess I will. I'll let the police sort it out.
If you hear it regularly then it's probably more than just a little spat. Besides its the police departments job to investigate and try to end it before it gets worse and someone gets hurt/killed.