It is very close to an expensive unit Brian. You would be surprised how close.
It varies on very loud bikes that are way over 110 dB. But in the legal levels it is dead on.
The way I see it, once you see a reading over 99, you will fail and the iPhone meter is a bit off.
The app on my phone (iPhone 4 w Otterbox Defender) is off by a lot (~10 dB high at 40dBA, completely useless over 100 dBA as it basically stops increasing with sound level). I haven't tested it at levels in between, but if it's off at the bottom and the top, I don't trust the middle. I know other people have had better luck, so I don't know if it is my phone or the case causing problems.
If people really care, I can bring out a calibrated meter and see what we get, that's what we did with Brian P's bikes. Based on those tests, the tests conducted by the vintage bike guys and the tests the cops did in Mississauga (or was it Oakville?), as Brian has stated, this isn't a battle worth fighting. Unless you are actively trying to be an ***, you will pass and they aren't even likely to bother enforcing this law.
As for that Harley guy saying revving his bike saved him, why didn't he just use his horn like everybody else? Why does his bike need to be ear-bleeding long 100% of the time for the 0.01% of the time he needs it loud for safety? Such a stupid argument.
I agree that this should be for everyone, not just bikes, but as long as they don't mess with the SAE standard, it is an entirely reasonable test that everyone should easily pass, it is basically an almost useless law to make people feel better about doing something.
City busses get lots of complaints in some areas. They are loud, discharge sound out the top (towards condo windows), run a high throttle a lot of the time and run much of the night. As with most other governmental agencies, they is no political will to touch this problem. If someone brings forward a law that creates money, politicians are all over it, if someone wants to create a law that will cost huge taxpayer dollars to fix, it is DOA.