| I don't know if it truly protects them from damage overall. That would take science. See "complicated" below :) But the basic answer to your question is, no, it doesn't double the sound energy. To the extent the headphones are successful at creating an inverse sound to cancel incoming sound, that will reduce the sound energy reaching the inner ears. The sound and its "mirror" don't add up to increase energy, rather, the energy reaching the ears is reduced where waves cancel. (The same thing happens in, say, optical interference patterns, and coastal waves. All the energy which seems to mysteriously disappear where waves cancel is, in fact, accounted for by increasing at the places the same waves interfere contructively. In the case of headphones, it's going to be rather more subtly accounted for either elsewhere in the spatial pattern around and on the speaker, and/or by the speaker acting as a net energy absorber.) Active noise-cancellers also have a passive component. The big squishy cups on mine (Bose QC-35). These of course reduce incoming sound energy too. But it's more complicated than that. Ears have a sort of physical volume control of their own (using tiny muscles), which reacts to the sound level. As far as I'm aware, damage is more likely deeper in the ear, past that stage. So anything which alters the ear's coarse volume response might produce a more complicated damage response than simply measuring incoming levels would suggest. I apply that theory: I sometimes listen to music on the headphones, in high volume environments. This seems to mask out the environmental noise better, and my theory is this isn't just psychoacoustic masking, it's causing my ears to block out more sound physically, helping to protect them physically. When doing this I listen to music which has a consistent sound level, rather than, say, a podcast or something subtle like classical. I've read that noise-cancellers produce a hiss which annoys some people. I can't say I've noticed much volume to the hiss, and I used to assume it was residual signal from the signal processing. But it may be that's intended to interact with the ear muscles too. |