Over the past few years more and more apps have been developed for safety and industrial hygiene. Of those, there are many sound level meter apps that are both useful and inexpensive. They can be used as a screening tool to pinpoint noisy machinery or a noisy area. Like most professional sound level meters they measure noise levels in decibels (dB). Unfortunately, they cannot be reliably calibrated and should not be used to document noise levels.
I have used a Studio SPL Meter by Studio Six Digital which cost at the time $10. I have taken measurements and compared them to a professional Type I sound level meter. Results were within 1 dB. While I don’t endorse this device, I tried some other sound level meter apps that were not as reliable. Whatever one you get make sure you follow up your measurements with a calibrated sound pressure level meter.