Old timer here, new account. Sorry to resurrect an old topic but I was looking for some sound apps for my work (entertainment/sporting events) and I found some relevant information that I think would be very helpful to the forum. There are several articles and studies out there, the most comprehensive is done by scientists at the National Institute of Occupational Safety and Health. Evaluation of smartphone sound measurement applicationsa)
They looked at hundreds of apps, both iOS and Android. Found out that iOS apps SoundMeter by Faber Acoustics and SPLnFFT by Fabien Lefebvre are the most accurate and within 2 decibels of their type 1 sound level meter. Other apps that did well, Noise Hunter by inter.net2day and NoiSee by Noise Lab. Android apps didn't do so well.
There are a couple of other studies where SoundMeter and SPLnFFT show up, one done by Safety Awakenings and one by grad students at a couple of different universities, though none of those studies are peer-reviewed and not as thorough.
Finally, if anyone interested in an app for audio or professional work, get an app that allow you to calibrate to a known source and invest a bit of money in an external mic, there are some really good ones out there.