SPL(sound pressure level) meter app uses your built-in microphone to measure noise volume. We had calibrated many android devices with the actual sound level meter with dB(A).
REMEMBER!! The smart phone 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), HTC Desire(82dB)...
Vibrometer app uses the phone sensors to measure vibration or the earthquake, and it shows a reference as a seism detector.
The measured values are the Modified Mercalli Intensity scale(MMI), and just for reference.