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!

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    $\begingroup$ Your question would be a lot better with some context. Why are you asking? The noise is going to be broad spectrum and beyond the range of human hearing in the low frequencies but probably attenuated at high frequencies by the acoustics of the cabin. If you can find a video recording from inside a cabin you can run it through an audio spectrum analyser program to answer your question. $\endgroup$
    – Transistor
    Jun 21 '19 at 21:08
  • $\begingroup$ @Transistor I don't think you can use the playback of a video recording to find the answer: The recording device can only record certain frequencies. $\endgroup$ Jun 21 '19 at 22:04
  • $\begingroup$ Almost everything you hear is way, way over 100 hz. Are you asking if there are frequencies as low as 100hz in the cabin? $\endgroup$
    – Ralph J
    Jun 21 '19 at 23:40

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.

  • $\begingroup$ Thanks! This is an amazing answer! $\endgroup$ Jun 22 '19 at 17:04
  • $\begingroup$ Very interesting answer indeed! $\endgroup$
    – Quentin H
    Jun 22 '19 at 18:45
  • $\begingroup$ @John K Just did a study of the performance of ANC headsets for non-professionals. It appears that one Bose headset was ok for hz range of over 100 hz, and acceptable for hz range of under 100 hz. One Sony headset was good for hz of over 100 hz but none for hz under 100 hz. If you were offered the two option what would you choose? You mentioned most jets have all over 100 hz so Sony seemed to be the clear winner? $\endgroup$ Jun 25 '19 at 18:57
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    $\begingroup$ Do you mean just audio headsets to wear as a passenger? $\endgroup$
    – John K
    Jun 25 '19 at 20:02
  • $\begingroup$ Hi John, for now yes I do. I haven't got a change to obtain the license yet. $\endgroup$ Jun 25 '19 at 20:37

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