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Great to see so much innovation in aerospace. Boom has said they're going to reduce noise, but they've also said they'll only fly supersonic over water with buf

Great to see so much innovation in aerospace. Boom has said they're going to reduce noise, but they've also said they'll only fly supersonic over water with buffers between supersonic zones and populated areas. So the 'boom', as it were, is still a concern. I'm super interested to see how quiet their planes are at subsonic. If you ever saw the Concorde flying subsonic, it was unbelievably loud. Nothing to do with being supersonic - their engines were just obnoxiously loud. Came into Cape Town a long time ago and made the whole town rumble on final. In this blog post: https://blog.boomsupersonic.com/booms-principles-of-sustainability-2f4a7b638f48 https://blog.boomsupersonic.com/booms-principles-of-sustaina... ..Boom says: "Today’s subsonic commercial aircraft are 80% more fuel efficient than those of the 1960s, and noise footprints have shrunk up to 90% in the last 50 years. This technological progress has fueled Boom’s efforts to design a supersonic airliner that makes economic sense for airlines and their customers. " However, the innovation that enabled this is high bypass turbofan engines. Turns out if you move more air slower, it's way quieter and more fuel efficient because physics. Boom can't take advantage of this - at least directly, because they have to go supersonic. A high bypass turbofan engine is huge, by it's very nature. At supersonic speeds this presents a lot of drag. That's why I'm super curious how they plan to be quiet and fuel efficient while also being supersonic.

aeternum 5y ago
How about some powerful noise cancelling speakers around the turbofan?
- coopsmoss 5y ago
  Turbofan noise is nothing compared to sonic boom noise.
- ehnto 5y ago
  I wonder what the result would be of using pulsejets offset in timing by 50% of eachother.
  - AnimalMuppet 5y ago
    Lots of weird mechanical resonance, and not noise cancellation.
- BurningFrog 5y ago
  I think noise cancelling is only practical at the listening location.
  - aeternum 5y ago
    Cancel at the source tech definitely exists, but it generally works best for repetitive predictable noise (same with noise cancelling in general) Here's an example: https://www.silentium.com/technology-2/quiet-at-the-source/ https://www.silentium.com/technology-2/quiet-at-the-source/
    - Toenex 5y ago
      Yep. Many years ago I was a junior at an engineering consultancy that specialised in noise & vibration. I spent many a happy hour out in the sun replacing melted loudspeakers that were being used to feed 'anti noise' into the exhaust ports of a pair of RB211 jet engines. The engines were ground mounted vertically to introduce pressure into the gas network should an emergency loss arise. This was rural Cheshire (UK) and some of the local properties were very old and expensive and needed protection from the vibrations these things produced. In reality they were never used but they were tested twice a year, for a total of about 2 minutes. After that the speakers (yep lots of hi-fi cones) were mush. They did their job though. And I got to travel up to Cheshire, see my then girlfriend who lived in Manchester and spend a week up on a roof in the sunshine replacing them.
Multicomp 5y ago
NASA is working with Lockheed on the X-59 QueSST to tackle the noise concerns. Construction is ongoing but over half complete according to wp . first flight planned for 2022. 2025 or so is when the icao expects to establish a new sonic boom standard. If things go well it could be much less of a blunt instrument than 'no overland flights ever' like what was required for the birds in the days of Concorde and Boeing's SST competitor, the 2707 or Lockheed L-2000
themeiguoren 5y ago
Worth noting that human sound perception is logarithmic, not linear. A 90% reduction in sound is -20dB, which is significant. But in human perception terms, that’s only about a fifth of the range of the typical soundscape which ranges from the 20dB of a quiet room to the 120dB of an ambulance siren.
- voldacar 5y ago
  10db actually. 20db would be a linear scaling of 100x.
  - thrdbndndn 5y ago
    It depends on the the physical property you're measuring. For power/intensity ratio, 10dB is 10x; for amplitude ratio, 20dB is 10x. Loudness is typically measured by "sound pressure level" (dB SPL), which uses the latter (amplitude ratio). Also, it's worth pointing out that while sound perception (like almost all the perceptions people have, Stevens's power law [1]) is logarithmic, its base isn't exactly 10 or 100. [2] claims that every 20dB (10x) in SPL, the perceived loudness is 4x. [1] https://en.wikipedia.org/wiki/Stevens%27s_power_law https://en.wikipedia.org/wiki/Stevens%27s_power_law [2] http://www.sengpielaudio.com/calculator-levelchange.htm http://www.sengpielaudio.com/calculator-levelchange.htm
onlyrealcuzzo 5y ago
Why is there suddenly a lot more drag once you break the sound barrier? I would imagine drag increases linearly or exponentially with speed - not as a step function once you cross the sound barrier. Or are you saying because it's going to be flying twice as fast?
- genericone 5y ago
  I understand this is probably an honest question, so I'll just point to this wiki article: https://en.wikipedia.org/wiki/Drag-divergence_Mach_number#:~:text=The%20drag-divergence%20Mach%20number,times%20its%20low-speed%20value https://en.wikipedia.org/wiki/Drag-divergence_Mach_number#:~.... [Increasing Mach Number] can cause the drag coefficient to rise to more than ten times its low-speed value. I only know this because of a number of fluid-dynamics courses that are only required for Mechanical and Aeronautical engineering majors. Barely anyone else is expected to know this information. Mach numbers represent fluid-flow discontinuities. If there is fluid flow in a varying inner-diameter tube and there is a Mach number change from <1 -> >1 at any point in the tube, as long as the Mach discontinuity is there, fluid flow characteristics before and after the discontinuity are decoupled from each other, they no longer influence each other if the Mach discontinuity is present.
- carabiner 5y ago
  It actually IS a step function. A shock wave by definition is an instantaneous change in fluid properties. At the molecular level, the change in properties is observed as occurring within a mean free path length (average distance a gas molecule travels before colliding). Imaging that shows the jump: https://phys.org/news/2015-08-schlieren-images-reveal-supersonic.html https://phys.org/news/2015-08-schlieren-images-reveal-supers...
carabiner 5y ago
It's not just a high bypass ratio that has helped. It's computational acoustics (we can predict the sound something makes based on its geometry and movement in a medium), nacelle design, materials. In supersonic flight, the most pressing issue is suppressing the sonic boom. There was a lot of work on this in the late 00's, with even Cessna rumored to be working on a quiet supersonic business jet. Various attempts - a bulging, ogival nose will increase the local density raising the local mach number leading to a weaker shock, and other thing. It was a lot of fine tuning and deep insight into transonic phenomena.
- wiz21c 5y ago
  I can assure you that the planes the take off from the airport located 7 km, plain line of sight makes measurable noise, with infra basses I guess. Not unbearable but clearly a nuisance. So maybe there's progress, but it'd be better with just less planes...
fy20 5y ago
Concorde was actually quieter (subsonic) than a lot of other aircraft in use when it was first released. Over time however as other commercial aircraft became quieter, Concorde wasn't developed any further, so by the end of its life it was very noticeable. The sonic boom really isn't an issue as long as you can fly that part of the route over the sea. Concorde often took longer routes to avoid flying over land, and it was still quicker than a more direct subsonic flight. Or you just fly supersonic over countries you don't care about - flights from London to Bahrain would fly supersonic from the Adriatic Sea crossing the Middle East over land to Bahrain. The Bahrain route is interesting, as today it is 5.5 hours and Concorde was 4 hours, so not that much difference. I'd rather spend an extra hour and a half on a much more comfortable and bigger aircraft. They also tried routes to Singapore (stopping in Bahrain to refuel), the total flight time was 9 hours, compared to non-stop flights today which are 13 hours. I wonder if developments in reducing the time subsonic aircraft take to reach and descend from cruising altitude could narrow that gap even further.
bertil 5y ago
> However, the innovation that enabled this is high bypass turbofan engines. Turns out if you move more air slower, it's way quieter and more fuel efficient because physics. Boom can't take advantage of this - at least directly, because they have to go supersonic. A high bypass turbofan engine is huge, by it's very nature. At supersonic speeds this presents a lot of drag. That's why I'm super curious how they plan to be quiet and fuel efficient while also being supersonic. I’m curious if either shielding the engines, or having a scramjet (like a turbofan but without the blades) could have a similar effect.

aeternum 5y ago

How about some powerful noise cancelling speakers around the turbofan?

coopsmoss 5y ago
Turbofan noise is nothing compared to sonic boom noise.
ehnto 5y ago
I wonder what the result would be of using pulsejets offset in timing by 50% of eachother.
- AnimalMuppet 5y ago
  Lots of weird mechanical resonance, and not noise cancellation.
BurningFrog 5y ago
I think noise cancelling is only practical at the listening location.
- aeternum 5y ago
  Cancel at the source tech definitely exists, but it generally works best for repetitive predictable noise (same with noise cancelling in general) Here's an example: https://www.silentium.com/technology-2/quiet-at-the-source/ https://www.silentium.com/technology-2/quiet-at-the-source/
  - Toenex 5y ago
    Yep. Many years ago I was a junior at an engineering consultancy that specialised in noise & vibration. I spent many a happy hour out in the sun replacing melted loudspeakers that were being used to feed 'anti noise' into the exhaust ports of a pair of RB211 jet engines. The engines were ground mounted vertically to introduce pressure into the gas network should an emergency loss arise. This was rural Cheshire (UK) and some of the local properties were very old and expensive and needed protection from the vibrations these things produced. In reality they were never used but they were tested twice a year, for a total of about 2 minutes. After that the speakers (yep lots of hi-fi cones) were mush. They did their job though. And I got to travel up to Cheshire, see my then girlfriend who lived in Manchester and spend a week up on a roof in the sunshine replacing them.

Multicomp 5y ago

NASA is working with Lockheed on the X-59 QueSST to tackle the noise concerns. Construction is ongoing but over half complete according to wp . first flight planned for 2022. 2025 or so is when the icao expects to establish a new sonic boom standard. If things go well it could be much less of a blunt instrument than 'no overland flights ever' like what was required for the birds in the days of Concorde and Boeing's SST competitor, the 2707 or Lockheed L-2000

themeiguoren 5y ago

Worth noting that human sound perception is logarithmic, not linear. A 90% reduction in sound is -20dB, which is significant. But in human perception terms, that’s only about a fifth of the range of the typical soundscape which ranges from the 20dB of a quiet room to the 120dB of an ambulance siren.

voldacar 5y ago
10db actually. 20db would be a linear scaling of 100x.
- thrdbndndn 5y ago
  It depends on the the physical property you're measuring. For power/intensity ratio, 10dB is 10x; for amplitude ratio, 20dB is 10x. Loudness is typically measured by "sound pressure level" (dB SPL), which uses the latter (amplitude ratio). Also, it's worth pointing out that while sound perception (like almost all the perceptions people have, Stevens's power law [1]) is logarithmic, its base isn't exactly 10 or 100. [2] claims that every 20dB (10x) in SPL, the perceived loudness is 4x. [1] https://en.wikipedia.org/wiki/Stevens%27s_power_law https://en.wikipedia.org/wiki/Stevens%27s_power_law [2] http://www.sengpielaudio.com/calculator-levelchange.htm http://www.sengpielaudio.com/calculator-levelchange.htm

onlyrealcuzzo 5y ago

Why is there suddenly a lot more drag once you break the sound barrier? I would imagine drag increases linearly or exponentially with speed - not as a step function once you cross the sound barrier. Or are you saying because it's going to be flying twice as fast?

genericone 5y ago
I understand this is probably an honest question, so I'll just point to this wiki article: https://en.wikipedia.org/wiki/Drag-divergence_Mach_number#:~:text=The%20drag-divergence%20Mach%20number,times%20its%20low-speed%20value https://en.wikipedia.org/wiki/Drag-divergence_Mach_number#:~.... [Increasing Mach Number] can cause the drag coefficient to rise to more than ten times its low-speed value. I only know this because of a number of fluid-dynamics courses that are only required for Mechanical and Aeronautical engineering majors. Barely anyone else is expected to know this information. Mach numbers represent fluid-flow discontinuities. If there is fluid flow in a varying inner-diameter tube and there is a Mach number change from <1 -> >1 at any point in the tube, as long as the Mach discontinuity is there, fluid flow characteristics before and after the discontinuity are decoupled from each other, they no longer influence each other if the Mach discontinuity is present.
carabiner 5y ago
It actually IS a step function. A shock wave by definition is an instantaneous change in fluid properties. At the molecular level, the change in properties is observed as occurring within a mean free path length (average distance a gas molecule travels before colliding). Imaging that shows the jump: https://phys.org/news/2015-08-schlieren-images-reveal-supersonic.html https://phys.org/news/2015-08-schlieren-images-reveal-supers...

carabiner 5y ago

It's not just a high bypass ratio that has helped. It's computational acoustics (we can predict the sound something makes based on its geometry and movement in a medium), nacelle design, materials. In supersonic flight, the most pressing issue is suppressing the sonic boom. There was a lot of work on this in the late 00's, with even Cessna rumored to be working on a quiet supersonic business jet. Various attempts - a bulging, ogival nose will increase the local density raising the local mach number leading to a weaker shock, and other thing. It was a lot of fine tuning and deep insight into transonic phenomena.

wiz21c 5y ago
I can assure you that the planes the take off from the airport located 7 km, plain line of sight makes measurable noise, with infra basses I guess. Not unbearable but clearly a nuisance. So maybe there's progress, but it'd be better with just less planes...

fy20 5y ago

Concorde was actually quieter (subsonic) than a lot of other aircraft in use when it was first released. Over time however as other commercial aircraft became quieter, Concorde wasn't developed any further, so by the end of its life it was very noticeable. The sonic boom really isn't an issue as long as you can fly that part of the route over the sea. Concorde often took longer routes to avoid flying over land, and it was still quicker than a more direct subsonic flight. Or you just fly supersonic over countries you don't care about - flights from London to Bahrain would fly supersonic from the Adriatic Sea crossing the Middle East over land to Bahrain. The Bahrain route is interesting, as today it is 5.5 hours and Concorde was 4 hours, so not that much difference. I'd rather spend an extra hour and a half on a much more comfortable and bigger aircraft. They also tried routes to Singapore (stopping in Bahrain to refuel), the total flight time was 9 hours, compared to non-stop flights today which are 13 hours. I wonder if developments in reducing the time subsonic aircraft take to reach and descend from cruising altitude could narrow that gap even further.

bertil 5y ago

> However, the innovation that enabled this is high bypass turbofan engines. Turns out if you move more air slower, it's way quieter and more fuel efficient because physics. Boom can't take advantage of this - at least directly, because they have to go supersonic. A high bypass turbofan engine is huge, by it's very nature. At supersonic speeds this presents a lot of drag. That's why I'm super curious how they plan to be quiet and fuel efficient while also being supersonic. I’m curious if either shielding the engines, or having a scramjet (like a turbofan but without the blades) could have a similar effect.