Ground vibrations caused by chopper above
 

Hi, a chopper just flew overhead (I'm in a large city). The ground shook a bit and I felt vibrations on the floor under my feet as it flew over. It happened last week as well.

Can any of you rotorheads explain how/why this happens? I can't find anything in google because it's all mobbed with those "strange reports" of sounds that the internet loves!

Here's an unrelated bit of eye candy for ya in return, it's a concept craft (3D visualization)

http://www.igorstshirts.com/blog/con...ganarid_01.jpg

Simply resonance from soundwaves: "when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion."

wow .. the pilot sits in the open cockpit ?? good I guess, for ejection .. ;)

Could be problematic in any Auto rotation, to ground, also as rockets seared by the pilot it would quite possibly toast his arm(s)..:ooh:

Are you sure that isn't a prop from a forthcoming Transformer movie?

Matthew

sometimes doing seismic exploration (oil and gas) the recording crews have to stop shooting/recording due to "noise" from the aircraft. this is fairly common as even small machines (astars and 407's) can make enough "noise" to interfere with their operations; they can even watch you flying across their spread with their geophones. not sure what a medium would look like; probably just like snow on a television.

fp

@Hawkeye Thanks for the explanation. That makes sense, I have wooden flooring in the house. It's the equivalent of a speaker cone vibrating I assume.

What I find interesting is that the volume perceived at the point of contact with my eardrum, is not proportional to the amount of vibration caused. It's almost as if the sound decays faster than the resonance vibration.

@ecurelix & others: It's a silly craft really, looks shiny though.

No it's just that the frequency that affects your floorboards sets up a resonance which helps it persist due to lack of damping whereas the frequencies perceived by your eardrums are just naturally damped.

The gap between your floorboards and whatever is below them will act as a resonant cavity in much the same way as the body of a musical instrument like a guitar or violin.

The floor, the walls, the windows, and the ceiling are affected by soundwaves you can't hear, because they're below the threshold of human hearing, but can still be felt. The distance a soundwave travels, and its intensity at any distance, are inversely proportional to the frequency. High frequencies are quickly attenuated, but the lower the frequency, the less attenuation with distance, thus the further it travels and the greater the intensity. There is a lot of high-frequency noise coming from the helicopter, but you don't hear it at a distance, all you hear are the lower frequencies. And frequencies at the top of the human hearing spectrum won't affect buildings anyway, because they're so high, even if they're generated very near the building. A speaker sounding a high frequency barely moves, but a subwoofer sounding very low bass notes moves a lot.

Living close to the home of the RAF's 'Wokkas' (Chinooks), the first sign that one is on its way over, is when our lounge door starts resonating. This can be a considerable time before the Wokka can be actually heard.

Mechta Minor's first word for anything that flew was 'Bop Bop', which was obvious to us, but less so to others, especiallly when the aircraft wasn't a Chinook, or even a helicopter sometimes.

Some Chinooks make a very strange slapping noise which sounds like they are about to fall apart. Those in the know reckon it is just the blade tape coming adrift and is nothing to worry about.

If you think of a Chinook as a 9000hp loudspeaker at 675Hz (225rpm rotor) then you can appreciate why the ground shakes.