OT but kind of an audio question.

[Jeff Leites]

I don't know why it's taken me so long to wonder about this. I live about 4 miles from Burbank airport, so depending on the take off and landing patterns, I can often hear the planes. Here's the question, why isn't the roar of the jet engines at my ground position a constant build up, and fade out? The sound is full of peaks and dips as it approaches, and then fades away. What is causing that?

[philboking]

I suppose it depends on things like throttle position, angle of departure/arrival relative to your location, aircraft speed causing doppler effects, and what sort of buildings the sound bounces off on its way to you. Add all this stuff up and you get an equation only Sheldon Cooper could solve...

[ggm1960]

Reminds me of my childhood a bit. I grew up not too far away on Lloyd Ave in North Hollywood, went to Saticoy grade school. I remember sonic booms being a fairly common occurrence back then in the 60's and sometimes we were not allowed to go outside at recess due to smog alerts there in the valley.

[Phil O'Keefe]

Reminds me of my childhood a bit. I grew up not too far away on Lloyd Ave in North Hollywood' date=' went to Saticoy grade school. I remember sonic booms being a fairly common occurrence back then in the 60's and sometimes we were not allowed to go outside at recess due to smog alerts there in the valley.[/quote']

 

I remember all of that as well. I grew up at the base of the mountains in the San Gabriel valley. I'm glad we don't have that kind of smog level here anymore. As far as sonic booms, I haven't heard a sonic boom in the greater LA area since I was a kid - only out in the desert. You haven't lived until you've had an F/A-18 come flying in on the deck at supersonic speed and startle the heck out of you while you are rock climbing out in Joshua Tree...

 

 

[Ernest Buckley]

I live around 4 miles from JFK. Back in the day of the Concorde it was a living hell twice day. When that rocket took off, house and car alarms went off, windows rattled, and you had to cover your ears. It was insane now that I think about it.

 

I got so used to it, so that I could tell if it was going to be tolerable or excruciatingly loud depending on the weather. I don't know the science behind it but it seemed that on cold winter days, the sound was not as bad. As the weather would warm up, the Concorde got louder.

 

And the humidity seemed to factor in and of course, the wind. Glad those days are over.

[MikeRivers]

I have more helicopters flying over my house every month. I'm near Washington DC and it's how the bigwigs get around, particularly between the airports and the Pentagon. They rattle the windows when they're on the right path. There's a small hospital just a block away from my house that has a contract with the county to care for mental patients that aren't quite ready for release. Every now and then one will walk away, sometimes in his pajamas, and then they start flying search patterns over and over and over and over.

 

 

[Mandolin Picker]

Use to live south of DC in Waldorf. Before 9-11 we always had aircraft flying around. General aviation, commercial, military, fix wing and rotary wing. After 9-11 it got real quiet as the no fly zone was expanded. Even the College Park Airport (oldest operating airport in the world) was affected - nearly caused them to shut down permanently. When I moved from there about 1 1/2 years ago, the air traffic had picked up - mostly military - but was never back to the same level. Hardly ever saw any general aviation craft. As a bit of an aviation/space buff, I always felt a bit sad about that. That we had lost something special.

[Jeff Leites]

When I was a kid we moved to an area north of Philadelphia located between two military bases, the Willow Grove Naval Air Station, and the Johnsville Naval Air Development Center. As kids we liked seeing the military planes flying around close up.

 

Buy getting back to my original question, I found this YouTube video that demonstrates the variety of sounds and sound levels I was referring to when a jet passes by overhead. It's a lot more complicated than the Doppler pitch shift.

[MikeRivers]

I'm guessing that the whistle the drops in pitch is Doppler shift of the engine turbine moving away from you. I don't know why we don't hear it rise in pitch as the airplane approaches, perhaps because it's masked by the exhaust. Since the exhaust is toward the rear it doesn't reach its loudest point until the airplane has just passed overhead, and then drops in level from there as the distance increases. The modulation of the exhaust noise is probably due to phase cancellation from reflections from the ground, or maybe even clouds (which would tend to reflect the high frequency component of the noise.

 

No, I didn't read this anywhere, just some wild guesses based a little on theory.

[nat whilk II]

We lived for about 4 years just a couple of blocks from the Austin airport (back in the days when the airport was located mid-town.) Really loud jet engine noise was an everyday thing - so was the fine black, gritty soot that settled over everything. My guess is that it was from the tires...

 

There is a lot going on - the movement of the plane to/from your point of reference, the angle of the jets, and a whole, whole lot of air movement. In the video Jeff posted, as the plane gets further away, the dynamic range of the fade in/fade out of the sound increases (more air mass between the source and the hearer, more masking??)

 

I'm guessing the wind currents are the biggest factor in the modulation - we live about a mile from a pretty heavily travelled state highway, and depending on the wind direction, we can hear every single vehicle and brappy motorcycle engine, or hardly anything at all from the highway.

 

I would think reflections would also cause some cancellation and buildup, too.

 

I'm still amazed at just how incredibly loud airplanes and helicopters are....do they really make any attempts to develop technology to tamp down the racket??

 

nat

 

[arneal]

Pacoima guy from the early '50s here. Oh yes, I remember those days and conditions well. The worst were the days when, if you played a little too hard on borderline days, your chest would hurt when you breathed. Millions of dirty internal combustion engines and millions of backyard incinerators that burned everything - garbage, plastic, wood, vinyl, fiberglass, etc. Since unleaded gas and better fuel efficiency requirements, at least the visibility improved noticeably, but I'm sure it's still not the best thing for breathing.