How loud a noise can MacBook Air speakers produce?
Output statistics such are usually reported as Watts. This sounds like what you are looking for. This is, unfortunately, a totally worthless data point, and the reasons why are beyond the scope of an answer on this forum.
Your best bet for measuring output, such as decibels/watt/meter, is an SPL (sound pressure level) meter. This would give you the best data set of output at different volumes and distances with different types of audio. You can actually get one for iOS right here that can measure the output of your speakers.