High flying aircraft loudly audible today
 

I have noticed than sometimes high flying airliners, which are normally inaudible from the ground, can be heard quite loudly as they pass overhead. An AA 777 Heathrow to NY has just passed overhead at 36,000' (according to Fight Radar), con-trail clearly visible in the cloudless sky, and with a distinct roar.

Normally such flight may be visible, more often not due cloud, but very rarely can they be heard clearly from the ground. I have noticed this before when out walking in the Peak District on cold clear days, when the noise can be quite intrusive with busy skies above evidenced by con trails..

Weather today is a NW air stream on the chart (surface wind nearer W) with a low over Norway, slight ridge in the Atlantic, low surface air temperatures due to clear skies. No visible inversion.

Anyone any idea why this should happen on rare days, whereas most of the time it doesn't?

Doesn't the presence of con trails indicate moist air rather than dry air? On these 'noisy days' the con trails are quite visible if not quite as long or long lived as on other days.

It would seem that today there is enough moisture to allow con trail formation, but not enough to sustain them for more than several seconds.

Quite intrusive noise from FL360 .... really!

I suspect your definition of intrusion varies from that of the vast majority!

Well intrusive seems quite a reasonable adjective to use. I suppose it depends on what you are used to, where you are and what you are doing.
Is it me or does aircraft noise become more objectionable as one grows older? I've become a grumpy old man of late, roundly cursing the Yanks and their after-burning F15s as they rocket around the skies about here, so bloody anti-social! They are without a doubt intrusive.
Certainly the Emirates 380 afternoon service from Manchester to Dubai often makes a good old noise just before reaching top of climb over the North Sea here in North Norfolk, enough to make you look up. We see up to twenty helicopter transits a day; after twenty years one ceases to notice them.
Para-motors - don't get me started!

I felt it was a bit of a mix, soundwise, today. I was a mile or so to the East of Heathrow and, while the sky was very clear, I heard some of the overhead flights but not others. At times like that, I think how difficult life must have been in WW2 for the Observer Corps. You hear the aircraft and look up; nothing there! Because it has already passed!

Top Bunk, I suspect you are not familiar with the peace and quiet of the White Peak, miles from any villages or roads. It's total. To have this peace, quite unusually, drowned out by the frequent roar of usually inaudible high flying airliners IS obtrusive on anyone's radar, not least because of the rarity of its happening. But that is NOT the point of this thread.

It's not aircraft noise per se I am talking about, it's aircraft noise one doesn't usually hear, and I'd love to have conformed by someone who might know what is the physics that cause it to be heard only very occasionally.

The point of my question, which you could have a go at answering if you like, was not the obtrusiveness of the noise, but why it is such a very occasional occurrence.

I noticed the clearer sound some years ago on a clear wintery day and remarked on it here. I believe fitter2 gave me a good answer though I regret I can't recall what it was!

I think that's being a bit unfair on the ROC - if they had failed to spot an aircraft until it was close enough to hear it, they wouldn't have been doing their job. :O

Sound waves can be refracted through the atmosphere in different ways depending on the temperature and wind structure. Sometimes the refraction can lead to a dispersal of the sound, other times it can be brought to what you might term a 'focus'. I have more knowledge of this refraction as applying to sources of sound on the ground and how it can sometimes result in some areas receiving a louder sound and some a weaker sound, but no doubt the same physics apply to sound sources in the air.
Bear in mind that for an aircraft at an altitude around 35,000feet, the sound that you are hearing at the surface was created 20 to 30 seconds ago, not at the current position of the aircraft.

I am not being unfair on the ROC. I am merely saying what a difficult job they had!

Bear in mind that most of what the ROC were sighting and plotting in the early 1940s were generally lower and slower than your average airliner, and often in formations.

However, when the V-1s started their one way trips from northern France crews on the posts in the relevant areas became very adept at sighting them early and quickly pushing the info up the line.

Used to hear the sound of our RAF jets coming from the fireplace in our house at five miles final.
The sound of freedom!
Perhaps I should have said coming from the fireplace at the speed of heat ;)

Two penn'orth - depends if they are climbing or cruising. Or upper-level winds, I'd guess.

Having grown up under the old "Amber One" near Castle Don., now living under "some airway or other, roughly OTR-NEW", going "home" makes me realise how different the sounds are.

In North Yorkshire, west-bound transatlantic stuff is generally in the cruise. The only things that make a distinctive racket are Qatari and Etihad climbing out of Edinburgh for The Gulf. Occasionally Emirates out of Glasgow, too.

Back in the East Midlands, the noise of upper airways traffic climbing is quite distinctive and noticeable.

Even further back in the 1970s, the -100 series 747s were very noisy, particularly Pan Am West Coast services, low-pressure fan drone being very clear.

I've noticed that A330's at cruise alts to be quite noticeable also, for some reason...(not complaining... (prefer the old 732's 1-11's etc :-))

Same here in Cheshire...I was thinking the high pressure and the dense cold air all the way to the ground.

I was distracted while gardening a while ago in inner London by sound from an A330 doing Frankfurt to Charlotte at altitude. I suspect that if they have just got a clearance to a higher level and throttle up that is what makes it occasionally more noticeable.

Long ago lived in The Wirral, about the midpoint of London to Belfast, and Vanguards on that run were always audible from the ground.

With there unmistakable drone audible from miles away making its way up Red 3.

I think modern airliners are prob quieter that the Vangaurd.

Every high flying jet I saw overhead today was unusually loud....not all climbing I would think....got to be to do with atmospherics I'm sure.

As I mentioned on another thread, it was great to hear a pair of Tynes attached to a Transall overflying Croydon just before Christmas!

Where I live -near Camberley there are overflights in all directions but seldom here much jet noise from anything high up=-turboprops are another story and can sound very loud, lots of FlyBe in and out of Southampton , Mil to and from Brize and the random tramping Antonovs . Are turboprops louder because of
a) the low frequency of the prop noise
b) Just because they cruise in FL 20s not FL30s

Anyone enlighten me please
PB

I noticed that yesterday in West Yorkshire while removing external Chritmas lights, that the high level A/C heading either out of Europe for the Atlantic and the large ex LHR 747 heading up country for American Wext Coast and SW were noticably louder. Conditions were cold at 1c but no cloud at all with straight vapour trails visable behind each A/C for about 20miles. The A/C sound also seemed to lag further behind where the A/C was by more than normal in my observations.


Regards
Mr Mac