If you have a smart phone, download an app such as "smart meter" to record decibel levels. Do a screen grab of levels if necessary for proof. There are many apps that do this. Once you have decibel readings, you can look online for examples of what it compares to.

For example, a jet has a decibel level of 140. Sound levels above 85 are harmful.
You will at least be well informed if you have to approach their landlord.

You could also record their noise, and then play it back to the landlord at the equivilant decibel level so they hear exactly what you are putting up with. You would however need an app that shows the decibel level as the recording is in playback mode.

And that would be the decibel level you are hearing, it would be even louder in their home.