Hmmm. I use several different types of sound testing at work for our adjusters. I do have a portable unit, but getting it out of the plant for personal use might prove difficult.
One thing I might add to this, is video to prove that you do not have a leak anywhere. For this, an ultrasonic tester can be used. Ultrasonic measurement can detect air flow where the human ear cannot. I use a portable ultrasonic test weekly to test machinery for air leaks.
I’ll see what I can do on the Db test. These are quite common little units. I’m sure there are others on this board that work for automotive suppliers or similar which require sound quality evaluation?