Sound Pressure Level Meter. This app uses the microphone to detect sound and convert it into an SPL value. FOR INDICATION ONLY. Results depend on your device and its hardware. The range between the noise floor and saturation may be only 20 dB on the worst devices, but could exceed 100 dB on the best devices.
Features of this SPL meter include:

Analogue Dial with Max and Min indicators.
Weighting – A, C or None . (A weighting filters the high and low frequencies according to how the ear perceives sound loudness). Results are in dB, dBA or dBC depending on weighting.
Averaging of SPL, Clear and Pause buttons.
Octaves and third octaves - Frequency spectrum of the sound.
Graph – Shows the time depedence of the sound.
Autoscale or manual (pinch & pan) Y-Axis.
Relative button – if looking for differences, tapping REL will remove the current averaged value from the reading.
Calibrate option – If you have a calibrated SPL meter or known loudness source, you can use this option to calibrate the meter. (However its still for Indication Only).

More technical details can be found on the website.