I sleep badly and I found that anything I play on my phone doesn't hand enough "depth" - whatever you'd call it - to really cover external noises. I bought a (m

I sleep badly and I found that anything I play on my phone doesn't hand enough "depth" - whatever you'd call it - to really cover external noises. I bought a (mechanical) white noise machine, which is better, but I found that runnjng the HVAC fan was much better still because (I assume) the ducts create a much more deep and filling sound that properly overwhelms other stuff you'd hear. It's not about the volume, but the frequency response somehow.