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The Russian incident wasn't just any Class IV. It was a 40W+ KTP/YAG 'Laserscope' style setup running in Q-switched mode. See the problem? (no pun intended)
It's a rule that you never, never, under any circumstance, use a Q-switched or pulsed (such as CuBr) laser for crowd scanning, due to the high peak powers involved.
They were going to sky-scan with it originally, but adverse weather (strong wind and rain) meant they couldn't run the projector out in the rain.
Instead, the event organisers wheeled the unit on-stage and aimed it at the audience instead. Whoops, bad idea.
Audience scanning with projectors in excess of 10W CW is acceptable, in the hands of a trained and competent technician.
It isn't the power that does damage -- it's a combination of the MPE (maximum permissible exposure) and the laser type.
I'm not going to do a full write-up of MPE here(every laser 'enthusiast' or 'hobbyist' should understand this concept), but it's the maximum exposure your eyes can take before your retina goes up in smoke. It's a combination of scan speed (exposure time) and power density (how much light you get hit by).
Example: scanned fast enough, a 1W 445nm laser can be used to audience scan safely.
And then there's the laser type, and as I've said, with a Q-switched laser, the peak (instantaneous) power is often magnitudes higher than the average power. In that case, even scan duration won't save you.
The bottom line doesn't change, though: Class IV is safe. Bad practices are not.
The firework would have well been in excess of 170dB SPL (given distance to your ears) , and well above the threshold of pain. If it were an actual explosion it could well have been even louder, depending on how close you were holding it.
To put things into perspective, that's at least twice as loud than a .22 handgun being fired at 1m from your body. The pressure differential between the two is a factor of 10.
We've all seen (or, at the very least, heard of) people using small arms (including larger cartridges such as the 7.62mm NATO rounds) without hearing protection. They don't go deaf instantly, even though the instantaneous sound pressure level is well above 130dBA, which is the threshold of pain (and possible damage).
As for a nightclub or pub band gig - the average 500W powered speaker that you'd find in smaller clubs can output up to 135dB SPL (on average) RMS, with 138dB peaks.
That's at a distance of 1m from the speaker. Move away further, and loudness drops off rapidly.
Take a bigger speaker -- for instance, the EAW LA460. The high-frequency section can do 130dB SPL long-term (RMS) at one meter.
That's not a lot of sound pressure, really. By the time you throw in some humans (and being bags of blood and water, they soak up sound like nothing else), you're looking at an average SPL of 100-110dBA. Which, in the grand scale of things, really isn't much.
Mind you, though, speakers can move enough air to cause serious damage (and by that, I mean burst eardrums). Dual 21" cone drivers can move a lot of air.
If you wanted to talk something a little bigger, say, a metal concert, then you're dealing with systems that can easily do 150dB SPL at close (<5m) range. But then again, these are massive flown line arrays with up to 20 boxes at a time.
120dB SPL RMS at the mix position (usually halfway down the venue) is not unheard of at larger rock/metal concerts.
There's a good reason why you're advised to bring a set of earplugs to a heavy metal concert. Most times you'll need 'em.
Digression: I don't know why everyone still thinks louder is better -- it's not. Beyond 100dB SPL your ears begin selectively shutting down (and your ears' frequency response goes out the window) and everything sounds like arse once you're missing a large chunk of information in the 2kHz range.
Anyway -- most times in a club, you'll be just fine. The normal rule-of-thumb is if you can feel pain, then it's too loud.
Another bit of perspective: I've had 126dB of speaker in my face before. Was putting up a measurement mic when a colleague accidentally hit the unmute button for the pink noise channel. Whoops. It hurts, but it's nothing permanent. 15 minutes later and everything feels normal again.
Comparisons between light and sound are invalid because of the difference in the way our eyes and ears handle bright (or loud) things.
The threshold that is considered safe for sound pressure levels can be safely exceeded in certain circumstances for small amounts of time, with little chance of permanent damage. Of course, as was bobhaha's case, instant deafness is possible (and that's how flashbangs work), but in most cases, it only lasts for seconds to hours, depending on the intensity of the exposure.
The same cannot be said for the retina. Any damage beyond the threshold of pain is usually permanent.
Of course, long-term exposure to loud noises is not a good idea nonetheless. After all, 85dB SPL is the long-term damage threshold.
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