Got sound problems in your office space ? This article investigates how good smartphone apps are at measuring sound levels. It's a reasonably old article (2013) and the conclusions are that they are normally within 1-2 decibels of what high level measurement equipment can achieve.
Our assumption is that the 7 years that have passed will have resulted in greater accuracy - so our conclusion is that you can get pretty accurate measurements from an app.
So next time the sales guy next door is screaming on the phone - get your app out and confront him with his noise generation :-)