FOKKER79
I have tested noise levels in the A320 flight deck on a good number of some of the oldest msns out there in an effort to justify installing ANR headsets as basic equipment in our fleet. Airbus conducted their own tests on some of the same fins as well. We don't have any of the ducting or 2 speed fan mods installed that might lower the noise level.
I don't know about ipads but with calibrated testing equipment outboard and inboard of the crews ears the noise levels only rose above 85 dBA momentarily and only on less than handful of flights when an short aural anunciated ( I forget which one ) Average noise levels were less than 76 dBA and the highest noise levels were actually experienced on the ground (structural/equipment vibrations fed back through contact with the ground?). This is well below OHS standards. Nonetheless, as you said, the noise levels are fatiguing and can be distracting.
We're looking at installing Airman 850s. They're not absolutely perfect in all conditions* but they are pretty effective and we have demonstrated that they meet or exceed certification requirements.
*with hot mic on, the INT volume needs to be high which leads to some background noise in the earpiece, That leads to hot mic probably being selected off during cruise when it's quiet. If ATC does then call, you won't be able to hear the other crew member's reply. The simple fix is to just uncover one ear during cruise.