The National Institute for Occupational Safety and Health (NIOSH) has released a Sound Level Meter (NIOSH SLM) App. Currently, it is only available for iOS devices.
The app provides a readout of the sound level using the built-in microphone. It provides an instantaneous sound level (dB) reading as well as Time Weighted Average (TWA) and dose. Information can be saved and stored. The app also has some noise and hearing loss prevention information.
It would be a good hands-on demonstration for HOSTA Task Sheet 3.2 Noise Hazards and Hearing Protection.