I’ve experimented a little with this in the past but the simple expedient of dropping something soft down the bell of my tuba (in my case, a small silk rug!) seems to work reasonably well at dropping the level down for practise at home.
As far as I can measure on my phone, the sound comes out at about 65dB – louder than when I’m quietly playing my electroacoustic bass but about the same as having a conversation with somebody where you aren’t particularly trying to keep a lid on the volume. I reckon that isn’t unreasonable for shortish sessions during the daytime, particularly when I think nobody is in next door.
I understand it probably has a tendency to make the instrument a bit sharp but, for solo practise, it doesn’t matter too much as I’m not afflicted with perfect pitch. It also creates a lot more blowing resistance. In theory, that isn’t a bad thing but might function in the same way as practising for a sporting event with extra weights on. I will have to keep an ear on it though – if I find that I start overblowing in situations where I’m not using the “ghetto mute”, I’ll need to reconsider.
It will do for now though.