In cabin, we hear noises in commercial aircrafts. Wondering if it is in general over or under 100 hz. There should be a band width. Thanks!
In a jet it's going to be pretty much all above 100hz. Most of the noise at cruise is broad spectrum "white noise" from the air flow and the engine core exhaust. The fan however, running at, typically 4000 rpm, with 60 blades say, will be producing a 4000 Hz noise, and this along with the exhaust white noise will be the dominant sound prior to cruise.
In a propeller driven aircraft however, you will have noise below 100 hz from the prop. A 2 blade propeller spinning at 2200 RPM is imparting a 73Hz pressure wave pulse on the windshield and this is one of the dominant forms of noise in that case. Next is the exhaust pulse from a 4 cyl engine, 2 per revolution, also giving 73 Hz at 2200 RPM. Then random frequency mechanical noise from the engine and white noise from the airflow.
So in a Cessna 172 at take off, running at 2700 rpm, you're going to have mostly a 90 Hz sound from the prop and the engine, then some random noise from other mechanical noise from the engine plus airstream white noise.
This is a problem for pilots who use foam earplugs only when flying. Foam plugs only attenuate sound above maybe 5000 Hz and don't help all that much with the prop and engine noise. ANR headsets are a huge help here because ANR systems work best on low frequencies although not so good on high frequencies.
So when I'm flying I use both an ANR headset and ear plugs.
After I tried out a set of Isolate Earplugs (and promptly lost them, although they do work), I've also developed my own low frequency earplugs by wrapping about 3" of half inch lead tape (used for ballasting golf clubs) around the shaft of a 3M Push-In earplug (the kind with the little yellow bulb on a blue plastic shaft) and covering the tape wrap with heat shrink tubing. I put them in with a bit of lanolin on the foam bit to lubricate and improve the seal. They do a fantastic job of attenuating low frequencies below 200 Hz, and make the ANR headset nearly redundant.