The National Institute for Occupational Safety and Health (NIOSH) developed the smartphone decibel meter several years ago and has updated it and researched about the smartphone sound level meter apps.
NIOSH Sound Level Meter app is easy to use, we can use built-in microphone of smartphone(of course, we can preferably use external, calibrated, microphone) and check the workplace or environmental noise level as easy as we use the smartphone camera. This app also contains some useful information on workplace noise and noise-induced hearing loss prevention. This app cannot replace the professional instrument, but it can raise worker's awareness, easily detect potential noise hazards, and promote better prevention effort.
I have added several useful research and information on this app.
Download this app on the App Store (This app is exclusive to iPhone)
A brief summary of the research on smartphone sound level meter apps "So How Accurate Are These Smartphone Sound Measurement Apps?"
Celestina, M., Hrovat, J., & Kardous, C. A. (2018). Smartphone-based sound level measurement apps: Evaluation of compliance with international sound level meter standards. Applied Acoustics, 139, 119-128.