Just to urge some caution with Apps for measuring noise. I spent my career working in acoustics (although not in the automotive industry), and as part of that, ran some tests on various apps to see if they were any good. The bottom line was: As Jonathan says, they are better for relative measurements rather than absolute. They are more consistent on Iphones than other devices because the microphones have broadly the same sensitivity as they come from the same supplier. There is a big difference between dB and dB(A) – not all measure the dB(A) level (the level expressed in dB(A) will always be lower). Also, some tests require averaging over time (to measure the LAeq) which is an added complexity some apps cant deal with. They are very sensitive to wind noise so not ideal for measuring out of doors The microphones in phones are directional so how you point the phone is important. The phone case can also make a difference. A professional sound level meter will have an omni-directional microphone so is much less affected by direction (important if you are doing a drive-by test). Its very difficult to calibrate a phone – protocols for noise measurements almost always require the noise meter to be calibrated before and after use with a standard sound source that fits over the microphone. The test we ran in a semi-controlled environment gave a range of +/- 20dB(A) from a mix of different phones and software packages. That’s a big variation.