I wondered this for a long time, do having a good sound isolation headphone protects our ears? Cause technically, it'll prevent us from adjusting the volume knob to a higher level right?
So I'm guessing the general rule of thumb is
1. set your volume level where you can hear your music + slightly ambience sound
2. set a volume limit (for iPod users) by adjusting the volume till you can't hear any ambience sound or people can hear what you're listening to
3. prevent listening to your audio device continuously over a long period of time
Correct me if I'm wrong, I enjoy listening to music details but I don't want to spoil my ears at the same time
This would be wildly variable depending on the headphones you are using, and thus not a reliable way set volume.
For example, the Sony Z1000 don't leak sound, at *any* listening volume. the AKG K701 on the other hand allows anyone to hear what you are listening to even at very moderate volume, the Audeze LCD-2 isolate so little that I can hear the AC at moderate volumes.
Without measuring gear, the best way is to find a volume setting that's comfortable for several hours and consciously not go over it. I personally refrain from listening in noisy environments unless I have IEMs or isolating closed headphones.