I bet it is much lower than that. One can almost count the cycles per second ! But it is definitley a few octaves higher than 0.01 Hz and I am also convinced that it is only perceived because it is quite loud.
The funny thing is, that usually the culprit (i.e. the one who opened the window) can't hear it because he/she is in a position of high velocity and low pressure.
Regards
Charles
Having measured this effect several times, I can assure you that the peak is not as low as you suggest. It is mostly broad band noise with a few significant resonances. And of course this all depends on the volume of the vehicle and the area of the window opening.
You may be hearing the buffeting of the noise as is common with wind noise. It is not stable. But what you hear is the noise, not the buffeting - it is a modulated noise.
You may be hearing the buffeting of the noise as is common with wind noise. It is not stable. But what you hear is the noise, not the buffeting - it is a modulated noise.
I perceive it more as a "whomp whomp whomp whomp" . . . slow enough to be almost countable. And that's more consistent with what I'd expect from the volume (a 50 ft^3 ported subwoofer will have a resonance around 15Hz, for example), something down in the 3-5 Hz range for my Scion, lower for my Aerostar . . .
I consider a change in atmospheric pressure to be a sound. I can sense changes with my ears. I consider changes that I sense with my ears as hearing.
Such pressure changes that I can sense with my ears include
airplanes going up and down,
elevators,
cars driving on hills,
and so on.
They are not meaningful as sounds.
But I can sense such pressure changes with my ears.
But an open window is a very small mass - both mass and compliance make the resonance. I think what you are hearing is the modulation of the broadband noise.
That is a very broad definition that I think you would find little support for. I certainly do not consider atmospheric changes to be sound even if I could sense them. I suspect that your ability to do that would be rather limited.
I once had a discussion with Prof. Zwicker on the pressure sensation that one gets in a car that has high noise. He said that there was was no actual static pressure change, but it was the nonlinearity in the ear at these high amplitudes that gave a signal offset that is sensed as a static pressure. We have to be very careful about interpreting our sensations to actual physical parameters.
I don't think that we are very sensitive to static pressure. In an airplane, once your ears neutralize (which usually happens very quickly unless you have a cold) there is no sense of pressure change by the ears even though that is in effect a huge change in atmospheric pressure. What you are sensing is the static pressure differential between your inner ear and the ear canal. Once this equalizes then you can no longer sense this pressure.
"Modulation" by what? It is periodic, vlf, and clearly a resonance in something.
Oh, and by the way, the air in the mouth-hole of a flute has "very low mass" as well . . . that doesn't stop blowing across it from exciting a pronounced resonance in the tube (that is not just "modulation of . . . broadband noise") . . .
Dewardh - I'm just telling you what I know from years of noise control work at Ford. Wind noise is not stationary and your flute example does not apply since the "wind" is quite steady.
I'm not talking about "wind noise" . . . I'm talking about a very distinct cavity resonance that is excited by "wind" passing across the (open) window of a moving car . . . much like the cavity resonance excited in a flute, or by blowing across a Coke bottle . . .
Have you ever been in a car in a controlled wind situation - I have. It was at Lockheeds wind tunnel in a Town Car. The "wind" was extremely stable, but yet the car was buffeting all over the place. At first I thought this odd but then it was obvious that the turbulence around the car was not stable. This mean that the excitation force - the wind - is highly unsteady. In an unsteady force any resonance that occurs will also be unsteady. This will occur at a very low frequency - but the interior resonance is much higher. The unsteady wind will "modulate" the resonance.
Otherwise assuming there should be a steady state, is the buffeting somehow related to non-linear compression?
An example I notice when bike riding, buffeting can be relatively smooth and hard to hear. A crosswind might for example set up what seems to be a combined resonance (low but higher f, and not necessarily clean). Higher up, wind noise can be introduced which can be significantly modulated by the buffeting state.
Hey Earl (and others). Do you remember how cars from 40 years ago and earlier didn't buffet? For years I drive a 1960's Buick and an old Dodge they never had that horrible oscillation. Why?
Oh yeah, and no way it's 0.01Hz. 300x that high, maybe. A horrible sound or sensation. Yuck.
- Bad aerodynamics. The wind didn't cling to the body.
- Vent windows! How I loved vent windows. Drive down the road at 70 mph with no turbulence inside the car. Just a cool breeze.
Oh yeah, and no way it's 0.01Hz. 300x that high, maybe. A horrible sound or sensation. Yuck.
Yes, vent windows (we call them quarter windows), my first car had them and they are the best, but they were also handy if I lost my keys.
Modern cars are well sealed. A few years ago I broke down and had to sleep in the car, which is almost impossible without putting the windows down a little.
Interesting point about the stream clinging to the body
Modern cars are well sealed. A few years ago I broke down and had to sleep in the car, which is almost impossible without putting the windows down a little.
Interesting point about the stream clinging to the body
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Concur
A correct observation. I have experienced the same phenomenon. The fundamental is huge and below the frequency response of most microphones. WHG
A correct observation. I have experienced the same phenomenon. The fundamental is huge and below the frequency response of most microphones. WHG
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In this part of the world most bikies wouldn't even let their brother and buddies ride their bikes - but Aussie blokes let even their pets do it ! 😉
Regards
Charles
Sorry, but couldn't resist.
Yes, that is a correct application of the term.
PS. for those who think that the resonance of a car is very low consider that the lowest mode in my home theater is 30 Hz. How can a car be lower than a room?
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