Bring earplugs! Then you have the choice. It can be really hard on the ears, standing there during F3, F3, Porsche Cup, F1 etc. Even if the sound might not be painful, it's still bad for your ears. But it's not the sound level alone, it's also how long you are exposed. I was at Spa in 2013, the last year of the V8's. They were painfully loud. What surprised me the most were the Porsches. I was expecting them too be rather dull, like on tv. But boy was I surprised when they passed up the Kemmel straight
I guess perception of sound level is individual, to a degree. I've abused my hearing so much, I prefer to protect what's left as best as I can. I use earplugs at rock concerts too. Makes the sound better, actually. I don't keep them in 100% of the time, though.