SPL (sound pressure level) meter app uses your microphone to measure noise volume in decibels(db), and shows a reference. We calibrated a lot of android devices using an actual sound meter with dB(A). REMEMBER!! Most smart phone microphones were aligned to human voice (300-3400Hz, 40-60dB).
Therefore maximum value is LIMITED by manufacturers, and very loud sound(100+ dB) cannot be recognized. Moto G3 (max.80), Galaxy S6 (85dB), Nexus 5 (82dB), HTC Desire(82dB).
You can trust the result in routine-noise levels (40-70dB). Please use it as an auxiliary tool.

Vibrometer uses the phone sensors to measure vibration or an earthquake, and it shows a reference as a seism detector.

The measured values are related to Modified Mercalli Intensity scale (MMI). If it is inaccurate, you can calibrate it so that maximum value is about 10-11.
Please use the results only for reference because android devices have a wide variety of performance and sensitivity.