So I was having a conversation with an 'old timer' last night and he was talking about when he had to rewire his father's house, or at least some of it. They had the plaster walls cloth wiring etc. As he was opening the walls he was complaining about all the sand that was in the walls. I guess it used to be used as an insulator? Anyhow, I was mentioning my soon to be home theater build and he said that he thought that sand in the walls was a good way to block noise transmitting to other rooms. He said he found about 2ft of sand in the walls of the house and it was doing a good job of blocking audio from transmitting through the walls. Has anyone ever heard of this or tried putting anything like this into practice?