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I live in India, and near an airbase so there are a moderate amount of helicopters flying overhead. My friend claims that he can distinguish between a civilian helicopter or a military one based on the engine noise. He says that military helicopters have a higher pitched whine in their noise that is absent from civilian ones. I have heard it myself but I have no idea if it's perception after he told me or reality. Is there any truth to this? Keeping in mind that most military choppers are Russian/Soviet designs in India while a lot of civilian choppers are Bells

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    $\begingroup$ I think the real answer is different engines sound different regardless whether they are military or civilian. And since most of the sound emanates from the inlet and exhaust, the same engine on a different helicopter can sound vastly different. The shape, length, structure, and orientation of the ducts will have a marked effect on the sound of a helicopter. $\endgroup$ – Gerry Dec 2 '18 at 20:23
  • $\begingroup$ This question is kind of vague, I'll explain why: your question asks about military turboshafts in general, but on civilian models that are converted to military types generally keep the engine. Granted, armoring, small modifications regarding battlefield/military use will be made, but they are the same in the big scheme. On the flip side, these modifications can indeed change the sound if they influence sound output sources, like exhaust shielding for lower IR signature, etc. If you could be more specific, like troop transport, recce, etc helo engine noise then you could get a better answer. $\endgroup$ – Jihyun May 8 at 0:55
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I think your friend is playing a joke on you. I have worked with helicopters since the late 1970's and there is no way to distinguish military from civilian simply by engine noise. You may be able to distinguish between models of helicopters, and in so doing identify a model used by the military versus a civilian one, but that's about it.

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    $\begingroup$ Most of the noise probably comes from the rotors instead of the engine, anyway. $\endgroup$ – Izzy Jan 23 at 15:11

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