High-bypass turbofans, or just turbofans in general.

In the early days of jet airliners, you had high-velocity exhaust providing all of the thrust. With turbofans, you have much more thrust coming from low-velocity bypass. This is quieter in itself, but also 'shrouds' the high-velocity exhaust.

As bypass ratios get higher, engines generally get more efficient and quieter. The challenge in doing this is mostly engineering the large fans.

Neil degrades Tyson talked about this on the Joe Rogan show. Jet engines became more efficient and were mandated to be quieter. I'm not sure about altitude because I live near a major airport. Always have lived near airports. The general noise is flat out less.