Sound Level Meter is in the 4th set of the Smart Tools collection (noise, decibel meter).
SPL(sound pressure level) meter app uses your built-in microphone to measure noise volume in decibels(db), and shows a reference. We had calibrated many android devices with the actual sound level meter with dB(A).
REMEMBER!! The built-in microphones were aligned to human voice (300-3400Hz, 40-60dB). Therefore the maximum values are LIMITED by the hardware limitation, and very loud sound(100 db) cannot be recognized. Moto Droid (max.100), Galaxy S3 (81dB), Galaxy Note (91dB), Galaxy s2 (98dB)...
See the 4-6th pictures. I had calibrated many android devices with an actual sound level meter. You can trust the result in routine-noise levels (40-70dB). But, please use it as an auxiliary tool.

* Pro version added Features :
- Sound Meter and Vibrometer are integrated
- Statistic menu with saving log data
- CSV file exporting
- Level notification
- Line-chart duration
- More models are calibrated
- No ads
* Do you want more tools?
Get [Sound Meter Pro] and [Smart Tools] package.
For more information, see the manual, Youtube video and the blog. Thank you.
** Not compatible with: Xperia(X8, X10 mini), lg Optimus One, P350), GT-I9001, ZTE, etc