This post is inspired by Keshi who is wondering what to do. Let me recount one awful night when I was a teen and my mother and us kiddies were staying at our grandparents, my mother's parents, in South Oakleigh.
There was a Hungarian born couple next door and they had two young sons. He was a good bit older than her and very charming and urbane. She was very attractive. They were arguing very loudly. My mother and my grandparents were pacing around the house, occasionally peeking out the windows. I think it was around ten at night. They were all upset by the ruckas. Then came the sound of breaking china and glass. Objects were clearly being thrown.
My mother wanted to call the police. My grandparents discouraged her. It is not our business, don't get involved. We don't want to be bad neighbours with them. My mother did not insist and so the police weren't called. Better that they were.
I can remember with such clarity the words said in almost a stage whisper that carried so well, 'You are so beautiful Helena, I cannot stand it', then a most terrible scream. He had thrown a pot of hot oil over her.
The police arrived soon after. I am not sure if my mother or someone else called them. The victim did have some scarring from the hot oil. They separated and she remained in the house. I do recall that he supported the family financially and used to take the boys out sometimes.
I suppose you don't call the police over raised voices, but there comes a time when it sounds more serious, so let the police know and they can prioritise and decide. They are presumably the experts. Any doubts, call them. There are things you never want on your conscience.