We found during our review of classes and mock conferences that there were both. There is trauma from sudden sound—for instance, somebody dropping a microphone or putting down their headset. Also, there is long-term degradation of hearing, which is both permanent and non-permanent.
The non-permanent type was experienced by our trainers, by me. I had to walk outside and hear the leaves rustle to confirm that my hearing was coming back after two 90-minute sessions.
The students, after taking two 20-minute turns during classes, have reported, on occasion of a questionnaire we had before and after every session, transient hearing loss as well. That was from two times 20 minutes, which is much shorter than a conference interpreter typically works during a working day.
As well, there is a well-founded suspicion of long-term hearing damage or degradation because of the higher listening volumes from other fields, not from conference interpreting but from other studies with similar sound pressure levels.