My guess is that the seal is pretty good. It certainly cuts the perceived volume of the mower considerably even with no music playing (ie in between songs). Music I'm listening to also doesn't disturb the outside world (ie my wife) when I'm doing other activities such as putting a few miles on the stationary bike while she watches TV.
Obviously it's not best practice to give up a sense when mowing the lawn, but I'm not sure how much worse that is than if I were to use ear plugs as hearing protection while I mowed either.
You raise some good points.
The issue of hearing damage is an intensity time curve, so spl and time are both important factors.
Now using a lawn mower, the device is right in front of you, so the chance of coming to grief is small.
I use hearing protection when using things like angle grinders, strimmers, leaf blowers and chain saws.
The big problem is the tractors, as the implements are behind me. When I first got them, I used hearing protection, but had some significant problems not being able the hear problems.
The old JD Model A makes quite a racket. However, I don't use it day in and day out, mainly in the winter to move snow. Fortunately a heavy blanket of snow in the trees really muffles the sound.
The problem now is that people these days seem to need sound where ever they go and their ears never get a break. The other problem is that this ghastly modern pop music has about zero dynamic range, so in reality it is always loud. A lot of this has to do with how music is produced these days with programs like Abelton Live, which really in essence have no dynamic range at all within a single voice.
I have been sitting thought auditions of student work this last year. And really what is on offer and what can actually be achieved with these highly touted expensive systems is appalling and pathetic. None of it worth the time of day.
The worst of it is that I think these types of productions have a very high chance of inducing permanent hearing damage, even if peak level is not excessive. I keep tabs on the spl. and it is in a very narrow range indeed. This means the average listening level is high compared to music that is worth serious consideration as suitable program.
All this is born out by the medical literature showing an explosive increase in noise induced hearing damage at earlier ages then seen previously.
This whole picture of the modern music industry I find highly discouraging.
