Sound Meter

Simple design. Easy to use.
Measured value, maximum value, indicates the minimum value.
Smartphones microphone to measure the sound pressure level.
Smart phone microphone is optimized for voice bandwidth (40 ~ 60dB).
Other than voice bandwidth can have measurement errors.
Depending on the capabilities of the smartphone can be a measurement error.

If you want a detailed measurements over a wide bandwidth, it is better to buy a professional Sound Meter.