Best Sound Meter (sound pressure level) is an app which can measure the actual sound level meter with dB.
It uses your built-in microphone and shows a reference.
You have to know that 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.
Remember that it's only a device for fun and please use it as an auxiliary tool.