Was watching a program on the BBC on the growth of airports in the 70s with the introduction of the jumbo jets, and the protests of people in the surrounding areas over the noise.
Seems to me, that if we’d just have silent jets, the whole problem would go away, yet as far as I can tell, jets are still as noisy today as they were in the 70s.
So why can’t we build “silent” jets?
If you think about what a jet engine does, it’s hard to imagine how you could build one that was silent. Great strides have been made in the past 30 years reducing the noise that jets make, but there is only so much they can do. GE claims to make much more powerful, yet quieter engines than they did in the past (http://www.geae.com/aboutgeae/presscenter/genx/genx_20070322.html). My guess is that you could never make them quiet enough to satisfy those who live near a busy airport.
Also consider that a lot of the noise is not due to the jet engine itself.
A large body like a plane moving quickly is going to displace a lot of air, quickly.
This will always be very noisy.
pdts
Imagine the size of the muffler.
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FWIW, we were at Boeing Field a few years back visiting their Museum of Flight. It sits right next to the runway. The current museum was under construction so we were closer to the runaway. Anyway, we were having a bite to eat in the cafeteria and watched a 747 land right there, right next to us. I’ve never been that close to an active flightline and this was a super opportunity.
An hour or two later we were outside, on the fence right next to the taxiway (between us and the runaway). We noticed that same 747 was approaching us on the taxiway. As it passed us by to swing around onto the runway for takeoff we noticed two things: (1) the aircraft was painted pure white with no visible markings, and (2) we could carry on a normal conversation because the jet noise was practically non-existent. After turning and sitting on the end of the runway for probably five minutes, the pilot finally received permission to take off. It took off with all four engines throttled up, and we continued to carry on a normal conversation without yelling. In other words, even during takeoff the 747 jet noise was practically non-existent, and we were right there as close as one could be to the taxiway and runway.
I went to the Dayton airshow about 15 years ago and one of the things they had was a quiet jet. It was a 727 with special engines and it made almost no noise. I remember it very well because when we saw it in the program we thought it was pretty lame- we came to see the Thunderbirds, not some boring 727. But you know what, when that thing took off and flew around and landed with barely a whisper of noise, we were blown away. In fact, I don’t remember the Thunderbird act from that show, or anything else they had there, but I do remember that quiet jet. It was amazing.
As posted above, just think about what a jet engine does and how it works. Just look at how much work those relatively small machines must do, having to accelerate and maintain a huge mass at hundreds of miles per hour. Good luck keeping that quiet.
As someone who’s lived around an airport all their life, I can tell you they are considerably quieter then the were even 15 years ago. It wasn’t that long ago that a plane taking off would rattle the windows and force you to put your phone conversation on hold until the plane had passed. Nowadays, the only time that really happens is if you are under a flight path.
Also keep in mind that many of the complaints about living near an airport are ridiculous. The property values tend to be lower in near an airport because there is an airport there. There are relatively few people in the U.S. that bought a house only to have a major airport pop up beside them. Most of them have been there for quite a while and airports require a large amount of free land. It is like complaining about train tracks next to the house you bought or even an interstate highway. It is even difficult for new runways to be constructed because people try to block it but they have to be built somewhere due to increased demand. It is simple NIMBY philosophy like nuclear power plants or industrial facilities. They have to go somewhere.
Plane manufacturers have made major strides in reducing jet engine noise since the 70’s so that part isn’t right although there are many older planes still in service. The FAA also has stringent noise abatement rules in place. Supersonic flight isn’t allowed over most of the U.S. mainland for example so it isn’t as if the regulators don’t listen. Research in engineering in physics can probably bring down the noise level incrementally over time even more but jet engines can’t ever be silent or close to it because they move a tremendous amount or air and cause it to vibrate which is what sound is in the first place. There is no way around that.
Here, at any rate, it’s no defence to point out that the complainant ‘came to the nuisance’ by moving in next door to it.
Compared to the old 707, newer engines are much quieter. They used to build HS 146 (now BAe 146) shorthaul airliners here and from some angles you couldn’t hear the noise at all.
Progress is being made. I live relatively close to the San Diego airport. Without looking up, I know immediately when an MD-80 is taking off. They are considerably louder than the newer 737s.
How long have you lived there? My dad used to work for a regional airline that bought a bunch of those and it was his job to go there, check out the plane, and fly it back.
The late PSA used to fly the BAe 146, which was sometimes called the ‘Whisperjet’ because it was so quiet.
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You may think you’re kidding, but they put these things on older jet engines to quiet them down a bit.
Looks like they raided the local Honda Civics club. 
Lived there for many years, factory’s long closed now. They made Saving Private Ryan and Band of Brothers there before pulling most of it down.
If we ever have SSTs again - which I think is probably only a matter of time - they may be much quieter, thanks in part to research like this: Quiet Spike - Wikipedia
Interesting fact pulled out of my head without checking, so it could be bullshit: The engines of the 146 are actually the same engines that power chinooks, as bizzare a statement as that sounds at first.
The cores essentially are, with some tweaking. The 146 engines have fans on them instead of shaft output, though.
Short version: Engines are much quieter now than they used to be, and are going to be much quieter than that in the near future. The basic sources of noise are shear between the discharge velocity and the ambient air, and blade/stator passing frequencies. The former are reduced with higher bypass ratios (larger and slower fans pumping larger and slower airflows) and with some exhaust nozzle tweaking. The latter are being reduced by wider spacing, but with better computer modeling permitting loss of efficiency to be avoided. The next dominant airliner engine design is likely to involve gearing down the fan to a lower speed, so that both it and the turbine that drives it can operate at their optimal speeds while also reducing both blade tip noise and passing frequencies (lower tones are less annoying). An A320-type aircraft with a Pratt & Whitney geared turbofan will be nearly inaudible outside the airport fence, per current predictions.
Quick trick: Fan blade passing frequencies can be reduced only to turbulence, with the fundamental tone eliminated, just by having more than twice as many stators as blades. The Canadair RJ is much quieter than the A-10, for instance, despite having essentially the same engine, because of this small change. The BaE146 is exceptionally quiet because it has more than 4X the number of stators as fan blades, thereby eliminating both the fundamental tone and the first harmonic. It costs some efficiency, though.
The airplane manufacturers are also constantly working on improving “smoothness” of airflow, which improves efficiency as well as noise. In fact, for a landing aircraft with the engines at idle, the landing gear and flaps/spoilers *each *typically produce more noise than the engines.
One of the things NASA discovered (rediscovered) in the 60’s was the noise reduction when cold air from a turbo prop air hits the hot turbine exhaust. Metropolitan-Vickers discovered this in 1942 with their work on axial flow turbines.
The high bypass engines today are much quieter than the earlier low bypass engines by a wide margin. This trend will continue.
