15 ms·

How far behind a plane is its noise?

108 points by otras 4y ago

roeles 4y ago
I happen to write control and processing software for wind tunnel experiments. I am currently working on an acoustic setup. I believe things like wind speed I influence the delay too, as it affects the distance the sound has to travel between the source and you.
DavidPeiffer 4y ago
Neat writeup! From the title, I had my fingers crossed that they integrated ADS-B flight tracking data to show a map of where the sound of airplanes in the air is currently observable. If anyone wants to go down that rabbit hole really far, I'm imagining general profiles of the sound each airplane makes, considering altitude, different sound propagation by frequency depending on distance the sound travels, and geography. Might as well throw air properties in too, to minimize the overall error. The user could provide their location and see the estimated arrival time and frequency of the sound from the airplanes in the sky.
- kqr 4y ago
  Oh it could get messy quickly. One of the playgrounds I like to hang out with my son at is right under the most common approach to a small local airport. It's also walled off by seven-story apartment buildings on two sides. The noise from approaching planes is dampened by the building between me and them, but bounce off the opposite building. So it always sounds like they're coming from the opposite direction, until they clear that angle, when they sound correct again, until obscured by the other building...
- bernulli 4y ago
  Air properties are actually super relevant, the speed of sound distribution (slower with increasing altitude) can have lensing effects for sonic booms.
  - idealmedtech 4y ago
    Would time of day (air temperature) also have effects on it?
    - savoytruffle 4y ago
      yes, air pressure and thus propagation speed varies with temperature. Colder air is denser, and thus faster to sound. But this is probably not a huge difference for sound as compared to airfoil performance.
bernulli 4y ago
Nice, but very wrong. This describes the case of a plane suddenly appearing in mid-air and starting to make noise, something, planes rarely do (maybe in the Bermuda triangle). It's like thunder after lightning, or seeing a ball fly before hearing it being kicked when you're far away. The aircraft, however, is flying for a long time, certainly it was flying and making noise much earlier than when it is passing the observer. As long as it flies subsonically, i.e. sound outpaces the aircraft - which is the case for every single commercial plane - the sound may be able to reach you much much earlier than the plane: As an example, take an aircraft flying with 100 m/s directly towards you. With every second flying, the sound will gain another 200 m distance relative to the aircraft (speed of sound ~300 m/s). If you're 100km away, the aircraft will reach you after 1000s, the sound has reached you after 333s, i.e. ahead of the aircraft. If you're 200km away, the aircraft will reach you after 2000s and the sound has reached you after 667s. So, how come it sounds like the sound of the plane is behind the plane? It's got to do with sound attenuation in the atmosphere and your hearing threshold. So, it's not at all like in the article. Somewhat minor nitpicks: - The aircraft is drawn to essentially fly with Mach 1, i.e. at the speed of sound, as the position of the plane relative to the wave does not linearly increase with time. Essentially all airplanes you see are flying subsonically (unless you're in the military). - "If the plane was moving very slowly, it wouldn’t outpace its sound by much." That's completely wrong. "very slow" aircraft are much slower than their sound, and all commercial aircraft still are slower than their sound, all of them are outpaced by their sound rather than the other way around. [Edit: typos & math]
- Deritio 4y ago
  Isn't that only correct if the plain is heading towards you? The article states that the sound of the plane is not were you hear it. At least this is true
  - bernulli 4y ago
    Use the construction the author is using, i.e. the emanating sound waves, but you'll have to start them where it all starts, i.e. at take-off, and not simply appearing right next to you. Then, correct the drawing by having the plane move slower than the sound waves. The first sound you should be hearing as observer will be the take-off (if you could hear it) at the airport, and the aircraft will be wherever it is afterwards.
mav88 4y ago
I was in the boondocks last week and had a striking example of a jet climbing up to its ceiling around 2 or 3 miles away. I tried to figure out the answer to this in my head and didn't get anywhere - was hoping a kind geek would come up with something.
temptemptemp111 4y ago
knolan 4y ago
It’s quite common in research to plot the aircraft noise footprint. It looks like this: http://2.bp.blogspot.com/_3cvpfoj7mb8/TJpQu-_IqfI/AAAAAAAAC7w/hoJ0ZkoVoDo/s1600/noisecomparison3.jpg http://2.bp.blogspot.com/_3cvpfoj7mb8/TJpQu-_IqfI/AAAAAAAAC7... A significant part of the noise signature is from the jet of hot turbulent air shearing against the surrounding cool air. This happens behind the aircraft. The Doppler effect plays a role too but for jet aircraft the noise is generated behind the aircraft and much is projected backwards. A lot of work has gone into geometries to reduce this shear noise.
kazinator 4y ago
> How far behind a plane is its noise? Somewhere between the last $2200 business class seat and the first $900 economy seat.

108 points by otras 4y ago

I happen to write control and processing software for wind tunnel experiments. I am currently working on an acoustic setup. I believe things like wind speed I influence the delay too, as it affects the distance the sound has to travel between the source and you.

DavidPeiffer 4y ago

Neat writeup! From the title, I had my fingers crossed that they integrated ADS-B flight tracking data to show a map of where the sound of airplanes in the air is currently observable. If anyone wants to go down that rabbit hole really far, I'm imagining general profiles of the sound each airplane makes, considering altitude, different sound propagation by frequency depending on distance the sound travels, and geography. Might as well throw air properties in too, to minimize the overall error. The user could provide their location and see the estimated arrival time and frequency of the sound from the airplanes in the sky.

kqr 4y ago
Oh it could get messy quickly. One of the playgrounds I like to hang out with my son at is right under the most common approach to a small local airport. It's also walled off by seven-story apartment buildings on two sides. The noise from approaching planes is dampened by the building between me and them, but bounce off the opposite building. So it always sounds like they're coming from the opposite direction, until they clear that angle, when they sound correct again, until obscured by the other building...
bernulli 4y ago
Air properties are actually super relevant, the speed of sound distribution (slower with increasing altitude) can have lensing effects for sonic booms.
- idealmedtech 4y ago
  Would time of day (air temperature) also have effects on it?
  - savoytruffle 4y ago
    yes, air pressure and thus propagation speed varies with temperature. Colder air is denser, and thus faster to sound. But this is probably not a huge difference for sound as compared to airfoil performance.

bernulli 4y ago

Nice, but very wrong. This describes the case of a plane suddenly appearing in mid-air and starting to make noise, something, planes rarely do (maybe in the Bermuda triangle). It's like thunder after lightning, or seeing a ball fly before hearing it being kicked when you're far away. The aircraft, however, is flying for a long time, certainly it was flying and making noise much earlier than when it is passing the observer. As long as it flies subsonically, i.e. sound outpaces the aircraft - which is the case for every single commercial plane - the sound may be able to reach you much much earlier than the plane: As an example, take an aircraft flying with 100 m/s directly towards you. With every second flying, the sound will gain another 200 m distance relative to the aircraft (speed of sound ~300 m/s). If you're 100km away, the aircraft will reach you after 1000s, the sound has reached you after 333s, i.e. ahead of the aircraft. If you're 200km away, the aircraft will reach you after 2000s and the sound has reached you after 667s. So, how come it sounds like the sound of the plane is behind the plane? It's got to do with sound attenuation in the atmosphere and your hearing threshold. So, it's not at all like in the article. Somewhat minor nitpicks: - The aircraft is drawn to essentially fly with Mach 1, i.e. at the speed of sound, as the position of the plane relative to the wave does not linearly increase with time. Essentially all airplanes you see are flying subsonically (unless you're in the military). - "If the plane was moving very slowly, it wouldn’t outpace its sound by much." That's completely wrong. "very slow" aircraft are much slower than their sound, and all commercial aircraft still are slower than their sound, all of them are outpaced by their sound rather than the other way around. [Edit: typos & math]

Deritio 4y ago
Isn't that only correct if the plain is heading towards you? The article states that the sound of the plane is not were you hear it. At least this is true
- bernulli 4y ago
  Use the construction the author is using, i.e. the emanating sound waves, but you'll have to start them where it all starts, i.e. at take-off, and not simply appearing right next to you. Then, correct the drawing by having the plane move slower than the sound waves. The first sound you should be hearing as observer will be the take-off (if you could hear it) at the airport, and the aircraft will be wherever it is afterwards.

mav88 4y ago

I was in the boondocks last week and had a striking example of a jet climbing up to its ceiling around 2 or 3 miles away. I tried to figure out the answer to this in my head and didn't get anywhere - was hoping a kind geek would come up with something.

temptemptemp111 4y ago

knolan 4y ago

It’s quite common in research to plot the aircraft noise footprint. It looks like this: http://2.bp.blogspot.com/_3cvpfoj7mb8/TJpQu-_IqfI/AAAAAAAAC7w/hoJ0ZkoVoDo/s1600/noisecomparison3.jpg http://2.bp.blogspot.com/_3cvpfoj7mb8/TJpQu-_IqfI/AAAAAAAAC7... A significant part of the noise signature is from the jet of hot turbulent air shearing against the surrounding cool air. This happens behind the aircraft. The Doppler effect plays a role too but for jet aircraft the noise is generated behind the aircraft and much is projected backwards. A lot of work has gone into geometries to reduce this shear noise.

kazinator 4y ago

> How far behind a plane is its noise? Somewhere between the last $2200 business class seat and the first $900 economy seat.