Aeroplane Noise — Variations Between Jet and Turboprop Engines

[Dr_Paul]

I've noticed over the years that although when at or near an airport jet aeroplanes are considerably more noisy than turboprop ones, there's quite a difference between them elsewhere. For example, I live about four miles to the south of Heathrow's approach, roughly when they're at about 1500 feet, and can't hear jet planes on the approach but can hear, if faintly, turboprop planes such as the DHC-8. At further distance and greater height, the difference is more marked: a jet plane at, say, 20,000 feet needs to be pretty much vertically above me to be heard, yet I can hear a turboprop plane at that height at some ten miles away; I can distinctly hear the nightly Saab 340 on its way over West London to Dublin, and RAF A400s are very audible at that distance and height. I once heard a C-133 Cargomaster cruising at about 30,000 feet some 40 miles away, but they were exceptionally noisy planes. Can anyone explain this phenomenon?

 

[edwin_m]

At a guess, the turboprop sound includes a component at the frequency given by the revs per second of the propeller multiplied by the number of blades. The equivalent sound from a jet is the rotational speed multiplied by a much larger number of fan or turbine blades, so will be at a higher frequency that carries less well.

 

[bangor-toad]

Hi,
Many modern jet engines are "high bypass" ones. That means some of the air compressed by the turbine doesn't go through the actual jet but bypasses it around the outside but within the overall engine.

This effectively 'wraps' the burning jet exhaust in a ring of compressed air. This suppresses the jet noise. Most noise is directed backwards rather than expanding outwards evenly.

Turbo props don't have this ability. The noise of the props rotating spreads out far more evenly.
That's why you can hear prop planes more from the side.

Cheers,
Mr Toad

 

[thejuggler]

Re noise on landing. Jet engines benefit from autothrottles which maintain approach speed, very little power is needed so they are in effect gliding on final approach. Turbo props land at much slower speeds and need forward thrust to prevent stalling, rather than gliding. Forward speed is managed by maintaining the engine revs and flying the plane along the glodeslope, adjusting manually the throttle and prop condition to maintain approach speed, its a two person job. There was a tragic accident last year in Nepal which showed just how quickly things can go out of shape in a turboprop during approach. The prop condition was set incorrectly and the blades were providing no forward thrust, despite full power being applied. This caused a stall and the plane crashed.

Re general noise. Modern high bypass jet engines have been designed to be as quiet as possible. They use the cold air moving around the outside of the internal parts of the engine to significantly reduce noise. Compare the diameter of a jet engine on a 737-200 against one on a 737 300, 800 or max. The diameter increase is required to accommodate a larger fan so more cold air can bypass the engine and reduce the noise level.

I live under the high level routes to US and can hear jets at 37,000 feet. A330s and 777 are particularly noisy. The only turboprops I hear are occasional very old Russian AN12 transporters and military aircraft that are both flying at lower altitudes.

 

[randyrippley]

Hi,
Many modern jet engines are "high bypass" ones. That means some of the air compressed by the turbine doesn't go through the actual jet but bypasses it around the outside but within the overall engine.

This effectively 'wraps' the burning jet exhaust in a ring of compressed air. This suppresses the jet noise. Most noise is directed backwards rather than expanding outwards evenly.

Turbo props don't have this ability. The noise of the props rotating spreads out far more evenly.
That's why you can hear prop planes more from the side.

Cheers,
Mr Toad

Which brings in an important point: modern "jets" are turbofans........if you were to go back to the early days of turbojets with the likes of the 707, Comet or especially the VC10 you'd be looking at completely different comparisons. The VC10 especially was an auditory overload

 

[Ted633]

Which brings in an important point: modern "jets" are turbofans........if you were to go back to the early days of turbojets with the likes of the 707, Comet or especially the VC10 you'd be looking at completely different comparisons. The VC10 especially was an auditory overload

The VC-10 actually had turbofans, of the low bypass variety, not turbojets. Only a handful of jetliners were ever powered by pure turbojets: 707, comet, concorde, DC-8 are the ones I can think of off the top of my head.
The VC-10 was incredibly loud though, found that out watching the Dunsfold one do an engine run a few years back.

 

[Ken X]

The BAC 1-11 was quite good at converting fuel to noise.

One notable example I heard about, shut down an engine leaving Gatwick in the early hours and decided to return to base via a low level circuit over Crawley. With the remaining engine doing the work of two, all the residents were very aware of it's presence.

 

[Peter Mugridge]

The Trident made an impressive amount of noise.

Seriously, I'd really love to hear an old style straight jet airliner once again; I enjoyed feeling the deep bass vibration in my stomach for a minute or so after they passed overhead, and still being able to hear a gently decreasing rumble from the sky up to five minutes afterwards.

 

[route101]

Props are quite noticeable even at cruise. The nosiest ones are the Hercules and Antonov's.

 

[Ken X]

I suspect the main reason is the dramatic reduction in jet noise over the years. I was brought up under the Heathrow flight path in the sixties and the noise was impressive, although you soon became immune to it and ceased to notice the aircraft.
Still miss Concorde
Recently I was parked alongside the runway at Gatwick near to the departure end working on the windsock and watching an A350 taking off. The lack of noise was really noticeable. Propeller aircraft have not had the same reduction in noise for various reasons and now stick out more.

 

[66701GBRF]

Forward speed is managed by maintaining the engine revs and flying the plane along the glodeslope, adjusting manually the throttle and prop condition to maintain approach speed, its a two person job.

Being a prop plane doesn’t automatically mean it’s a two person job.

 

[Meerkat]

The A400s and C-130s are REALLY noisy. They go over here at cruise height and you can hear them for ages.
Also the single engine Turboprop planes on approach to the local airport are notably louder than twin jets whistling in.

The most annoying however are the little Cessnas etc as they are noisy and SOOO slow. And don’t get me started on the stupid little petrol/diesel helicopters…….

 

[Strathclyder]

The A400s and C-130s are REALLY noisy. They go over here at cruise height and you can hear them for ages.

I saw two A400s (I think) take off from Glasgow about a month ago. No idea what they were doing here, thought they were C-17s until I heard that distinctive droning. Haven't seen a C-17 in the metal yet, but I suspect it won't be quite as loud as a Atlas.

 

[TravelDream]

Re noise on landing. Jet engines benefit from autothrottles which maintain approach speed, very little power is needed so they are in effect gliding on final approach. Turbo props land at much slower speeds and need forward thrust to prevent stalling, rather than gliding. Forward speed is managed by maintaining the engine revs and flying the plane along the glodeslope, adjusting manually the throttle and prop condition to maintain approach speed, its a two person job. There was a tragic accident last year in Nepal which showed just how quickly things can go out of shape in a turboprop during approach. The prop condition was set incorrectly and the blades were providing no forward thrust, despite full power being applied. This caused a stall and the plane crashed.

Re general noise. Modern high bypass jet engines have been designed to be as quiet as possible. They use the cold air moving around the outside of the internal parts of the engine to significantly reduce noise. Compare the diameter of a jet engine on a 737-200 against one on a 737 300, 800 or max. The diameter increase is required to accommodate a larger fan so more cold air can bypass the engine and reduce the noise level.

I live under the high level routes to US and can hear jets at 37,000 feet. A330s and 777 are particularly noisy. The only turboprops I hear are occasional very old Russian AN12 transporters and military aircraft that are both flying at lower altitudes.

I think you have a bit of a misunderstanding on a few things.

Planes don't come down with very little power. The landing gear and flaps cause significant drag. The 737, for example, will be at about 55-60% power which is very similar to cruise power.
I am not sure what difference an autothrottle (a/t) makes to noise levels. The pilot and a/t should manipulate the throttle in a very similar manner. Though you're right prop aircraft like the Dash 8 and ATR 42/72 don't have an autothrottle.
And no matter what, the person flying manipulates the throttle (or is in charge of the a/t) on landing. It is not a two person job. In that Nepal crash, the pilot flying called for flaps whatever. Instead of selecting the flaps lever, the training captain sat next to her accidentally selected the propellor pitch lever (they are next to each other) and feathered the propellers. The prop pitch lever should only have been manipulated by the pilot flying.

On the bypass ratio, I'd say you are broadly right. Though the compressed air isn't 'cold' as such as compressed air increases in temperature quite a lot, but is obviously colder than the air being combusted. Compare the 737-800 and 737 Max. You'll notice the Max engine is much larger. The bypass ratio increases from 5.5 to 1 to 9 to 1 in the Max. This not only makes the engine much quieter, but also a lot more efficient at this compressed bypass air also provides a lot of thrust.

I find prop planes much louder at altitude. The Aer Lingus Regional ATRs frequently pass overhead and are much more noticeable than the 737s/A320s on the same routes.
A big part of the noise is the prop. It might be traveling at 2800 RPM which means it is very near the speed of sound and it's operating in free air.

 

[route101]

Anyone notice the newer A320s series make a distinctive noise when passing over? Say for example around 6000 ft coming into an airport you hear the aircraft approaching then there is a whoosh type of noise and the aircraft noise disappears for a few seconds, then it comes back and passes over head.

 

[Watershed]

Anyone notice the newer A320s series make a distinctive noise when passing over? Say for example around 6000 ft coming into an airport you hear the aircraft approaching then there is a whoosh type of noise and the aircraft noise disappears for a few seconds, then it comes back and passes over head.

Yes, there is a video about it here:

The noise is only present on some A320s as most of the "better" airlines have paid for a vortex generator modification that removes the whoosh sound.

 

[Lost property]

The A400s and C-130s are REALLY noisy. They go over here at cruise height and you can hear them for ages.
Also the single engine Turboprop planes on approach to the local airport are notably louder than twin jets whistling in.

The most annoying however are the little Cessnas etc as they are noisy and SOOO slow. And don’t get me started on the stupid little petrol/diesel helicopters…….

Yes indeed, those Lycoming's are noted for their noise imprint...and, sadly, (for you) a cruising speed of around 100kts would be "slow". even slower with a headwind. How do you cope with vintage pistons !

But, I would like to get you started on the "stupid little petrol / diesel helicopters" as you describe them. Types and fuels please.

As for Hi Bypass ratio fans, 75 % (approx) thrust is produced by the fan. "Cold stream" reverse thrust proved beyond the understanding of a "manager" on the former railway I was on. To say he was thick would be an understatement you understand. "Hot stream " and reverser buckets, he could, just, understand. Mind you, when he "thought" reverse thrust meant the engine went into reverse, there was little hope of educating him.

 

[Meerkat]

How do you cope with vintage pistons !

Piston engines are fine at a decent speed - the lovely noise of Merlins! The Cessnas landing locally are a bit annoying because they are lower and slower .
The A400/C130 engines produce a deep booming drone that is really intrusive

But, I would like to get you started on the "stupid little petrol / diesel helicopters" as you describe them. Types and fuels please.

I don't know the details, but its the silly little Robinson things (possibly other suppliers are available). They aren't that loud, but VERY slow, and when they are being used for training they just don't go away!

 

[Lost property]

Piston engines are fine at a decent speed - the lovely noise of Merlins! The Cessnas landing locally are a bit annoying because they are lower and slower .
The A400/C130 engines produce a deep booming drone that is really intrusive

I don't know the details, but its the silly little Robinson things (possibly other suppliers are available). They aren't that loud, but VERY slow, and when they are being used for training they just don't go away!

Oh dear.

Was the airfield closed due to bad weather when you viewed the property, or did the estate agent "forget" to mention the proximity. Always makes me laugh when people move into their "dream home" and then, suddenly notice, say an arterial route at the end of the garden, or an incinerator plant, or "country smells" or an airfield etc. etc

The Robinson Lycoming engine runs on 100LL fuel. There may be a very good reason, let's call it.. training and safety, why, when training, they tend to stay in the vicinity of an airfield.

But look on the bright side. If you are lucky enough to hear / see a Herc, it won't, unfortunately, be an RAF one and if you think they are bad outside, try sitting in one for "xx2 hrs.

 

[Meerkat]

Oh dear.

Was the airfield closed due to bad weather when you viewed the property, or did the estate agent "forget" to mention the proximity. Always makes me laugh when people move into their "dream home" and then, suddenly notice, say an arterial route at the end of the garden, or an incinerator plant, or "country smells" or an airfield etc. etc

The Robinson Lycoming engine runs on 100LL fuel. There may be a very good reason, let's call it.. training and safety, why, when training, they tend to stay in the vicinity of an airfield.

But look on the bright side. If you are lucky enough to hear / see a Herc, it won't, unfortunately, be an RAF one and if you think they are bad outside, try sitting in one for "xx2 hrs.

The house where the Cessnas were mildly annoying was 3 miles from the airfield. Not exactly next door.
But the point was that they were actually more annoying than the business jets coasting over, which are barely noticeable (despite some being 737s etc)
When I was a kid it was under the flightpath for RAE Farnborough which was great, including the Snoopy Hercules.
I’m not sure why it being training means being a nuisance is ok.
Still regular Hercules going over - Canadian, USMC, Italian, Jordanian, Saudi…..all sorts!

Of course the daddy of noise was Concorde - over 10 miles from Heathrow and conversation had to stop when that was departing overhead! Though Chinooks in formation made the windows rattle.

 

[Spamcan81]

Props are quite noticeable even at cruise. The nosiest ones are the Hercules and Antonov's.

The A400 is rather noisy too.

 

[Tetchytyke]

Always makes me laugh when people move into their "dream home" and then, suddenly notice, say an arterial route at the end of the garden, or an incinerator plant, or "country smells" or an airfield etc. etc

Or they move the flight paths, which is what happened to me in Newcastle. It wasn’t under the flight path for Newcastle Airport right up until it was.

 

[TravelDream]

Or they move the flight paths, which is what happened to me in Newcastle. It wasn’t under the flight path for Newcastle Airport right up until it was.

If you live near an airport, it's a risk.
What isn't a flight path today, might be one tomorrow.
What's a quiet regional airport, might become a big base for Ryanair or easyjet.

 

[Dr_Paul]

Always makes me laugh when people move into their "dream home" and then, suddenly notice, say an arterial route at the end of the garden, or an incinerator plant, or "country smells" or an airfield etc, etc.

I've lived near Heathrow for most of my life, and it makes me laugh when I hear folk complain about aeroplane noise. I remember the 1960s, and some jet planes were very loud. The BAC-111 was extremely loud for its size. As for 'country smells', my grandparents bought a cottage near Lowestoft, downwind of the fat and bone factory when the wind was in the east; the most revolting smell I've ever smelt, rotten meat being boiled for cooking fat (very good quality, by the way), tallow and bone meal. The estate agent must have checked the wind direction before taking them for a viewing; they wouldn't have bought it had they had a whiff of the factory's fumes.

Back to the original topic of jet versus turboprop noise -- thanks for all the information, very interesting.