When you're dealing with a noise issue, the frequency of that noise is just as important as the decibel level. A decibel is the loudness of the sound, and measuring decibels can alert you when a sound is more than just a nuisance, but dangerous to your health.
However, decibel levels on their own don't tell the whole story. The frequency of the sound is also very important. The frequency is a measure of how many sound waves per second are produced by a sound, and determines its pitch. A low-frequency sound might be really annoying, but it might not be that loud. The lower frequencies are typically below 150 Hz, and often sound like a deep bass sound that's not necessarily loud, but the vibrations from that sound can travel through the entire frame of a building and disrupt sleep or cause stress.
Knowing both the decibel level and the frequency of an unwanted sound can help you determine the right soundproofing solution. For example, a low-frequency noise won't be stopped by fabric or acoustic foam or fiberglass batts. It often requires mass and decoupling, combined with absorption.
Luckily, there's a free app in Apple's App Store that can measure both the decibel level and the frequency of a sound at the same time. It's called "Decibel Meter," and the developer's name is Ashraf Thoppukadavil.
Here's a Soundproofist video that demos the app in action.