Noise Complaints (My Tenants Complaining)

Other

I let a flat in converted Georgian house. The current tenants have been resident for four years without a problem. The flat below them changed hands and new owners moved in 2 months ago. They have installed a modern hifi/av system which creates a noise problem, often past midnight. When they spoke to me originally I asked them to speak to the owners directly. Which they did do. The problem has persisted. I then emailed the owner to ask them to be more reasonable. The response I had blamed me as landlord for not insulating the floor to sound. They tell me they had this done but as far as I'm aware none of the other flats above have even considered this. The email also suggested that the noise problem comes from my tenants footsteps and they are complaining to me.

I would like to make a step to convince the owners to be more reasonable with music/tv noise, does anyone have advice on how I could do this?