I read the reviews on that app and the permissions - it wants wifi state and network state so I assume advertising?
There seems to be varying degrees of accuracy, i.e. 50db in a quiet room with no noise and a maximum of around 80-90 or so no matter how loud.
"A Google User - November 30, 2012 - Version 1.4.7
Bulls***
App, my system pumps 120 db of sound that I know for a fact (EPA and police measured) 5 metres outside my house , yet free version measures up to 80. Uninstalled."
Another app - "Noise Meter" has no advertising or permissions needed but has the same high rating with no noise and max of around 80-90.
So I didn't bother.
On the other hand the critical levels of over 85 db if measured incorrectly could damage your hearing.
From what I read somewhere - at higher levels - that 3db increase cuts the time in half that is safe.
i.e a rock concert or jackhammer at 100db is dangerous and has a maximum safe time. 3db more and that time is cut in half.