I once went to an outdoor music festival west of Washington DC. I am not sure how high the planes from Dulles were when they went overhead, but we saw them without noticing the sound. Then a Concorde flew over - they stopped playing the music until it had passed.
ETA: the snap of a bullwhip is a shock wave–right?
I lived near a SAC base as a kid. So I remember B-36’s taking off. (Actually, most of them were RB-36’s.) They impressed me.
Here’s a takeoff from Strategic Air Command, a fairly lame film despite Jimmy Stewart & some fine aerial photography.
(Scroll down this page to see a B-17, B-29 & B-36 sharing the tarmac.)
When I was first joining the Army my recruiter had to bring me to Fort Dix for a test. Until recently Fort Dix and McGuire Air Force Base were separate installations but only separated by a fence. The main runway is near the main street of Fort Dix. The loudest aircraft I ever heard was that day when a pair of Air Guard F-4 Phantoms were practicing take off and landings. I’ve been to many air shows and nothing compared to that. It was completely impossible for any verbal conversation even while shouting. Of course that’s cheating because there were 2 aircraft flying in tandem.
Some confusion here.
- Any given blob of air will generate a sonic boom when a solid object hits that blob at or above Mach 1. The air blob has no “warning” the solid object is approaching. So the blob is shoved violently out of the way molecule by molecule as the solid object arrives.
Thunder is similar in that air is violently pushed aside and we sense that violent pressure pulse as a loud noise. The big difference is that thunder is a momentary phenomenon & sonic booms are continuous.
If you imagine an ocean-going ship driving across the ocean for days it will leave a V-shaped wake behind it which expands behind the ship, extends for hundreds of miles, and eventually washes up (much dissipated) on the ocean’s shores. From space we can see pictures of ships dragging wakes hundreds of miles long & wide. But any single dinghy out in the ocean will experience that multihundred-mile wake as a single (or maybe double) wave going past the dingy for just a few seconds.
A sonic boom is effectively the same thing, just with a very sharp leading edge to the wavefront. The entire time a, say, Concorde is going above Mach 1 across the ocean it’s dragging this V-shaped wake behind it. Actually in 3D it’s a cone, not a V. Wherever the edge of the cone touches the ground, anyone there hears the sharp “bang” of the boom as it passes.
The objection to overland SST flight was the recognition that soon hundreds of supersonic airliners would be dragging the continuous explosion sounds across the land from coast to coast.
IOW, the sonic boom is a continuous phenomenon. It sounds like a momentary thing to a stationary observer because the continuous wake/sound is being dragged past your position at several hundred miles an hour.
2/2a) The energy delivered to a particular spot on the ground depends on the size & speed of the generating aircraft. Bigger & faster is louder up to a point. But the human perception of loudness and its startle value depends on more than just the total physical energy of the pulse. Our perception depends greatly on the *shape *of the pressure pulse(s).
Sonic booms are created by various points on the aircraft, principally the nose and the leading edges of the wings and tail surfaces. There is active research now on the idea of shaping an aircraft so the booms generated by the various spots on the aircraft mutually interfere and effectively muffle one another. See NASA's Sonic Boom Research Takes "Shape" - NASA for an older effort and NASA Tests Supersonic Aircraft Without Boom – UAS VISION for the latest efforts.
Absent special shaping there’s a tendency for the propagating boom to settle into two mutually reinforcing pressure pulses. The shockwave becomes more coherent and is experienced at the ground as a “boom-BOOM”. Which is commonly called an “N-shaped” pressure pulse and seems to be doubly startling to humans.
3) Supersonic props produce their racket because each blade is dragging it’s own corkscrew-shaped sonic boom around with it. And as the aircraft advances through the air the blade corkscrews are wrapped around each other and drug forward through space. Finally the Bear has counter-rotating props, which affords a whole 'nother pile of noise caused by the aft blades slicing through the chaotic turbulence and Mach shockwaves created by the leading blades.
Finally, the Doppler effect get its licks in as well. if you were standing alongside a stationary Bear or Thunderscreech. on the ground you’d experience each blade spinning towards and then away from you 5-10 times per second at Mach 1+ speeds. The noise frequencies would be alternately greatly raised and lowered. Which spreads the racket out in frequency space & produces the “wall of noise” sensation.
The noise of pure turbojets is predominantly the result of turbulent flow. The gasses coming out the back are traveling at Mach or higher speeds. And are still expanding thermally. When this blast hits the more or less stationary ambient air, lots of shaking, rattling, and rolling occurs very quickly. Which you hear as a crackling distant-thunder-but-louder sound.
Low noise turbofans make much less noise by the expedient of moving a lot of air slowly instead of moving a small amount very, very fast. So the resulting collision between the exhaust flow and the ambient flow is more gentle.
From the noise POV an afterburner is essentially a turbojet on steroids. Compared to a turbojet the resulting exhaust is even faster and hotter and more energetic. Which makes the resulting crash engagement with the ambient flow even noisier. See Shock diamond - Wikipedia
True, but irrelevant. The boom is not formed by the transition of the aircraft or prop (or other part) from below to above Mach 1. It’s caused by any part of the aircraft moving at > Mach 1 and colliding with the more or less stationary ambient air.
As a final tidbit to this essay I’ll point out that for an aircraft at an overall speed of below but near Mach 1, it’s still possible for airflow to be accelerated above Mach 1 in some spots around the aircraft. This is the so-called “transonic” speed range. The booms generated there are weak to inaudible simply because there’s not as much volume of shocked air being created.
I had a moment of inspiration, and…aha! Preliminary Measured and Predicted XB-70 Engine Noise.
Looks like…136 db, roughly, when taking off under full afterburner, but with a “perceived noise level” of 145 db, on takeoff.
I think the comment is right on - the Harrier is F-in loud because it can get up close and stay there. I saw one during the airshow in Toronto decades ago, that came into the (old) stadium and went up and down a bit; also saw one doing that up-and-down while rotating at the Oshkosh fly-in. It’s loud, really close, and it’s staying there, not receding at a few hundred mph.
I have seen many Harriers at airshows – they are nowhere near as loud as a top fuel dragster, which have been measured at 150 to 180 db. However the Harrier continues for a long time if hovering nearby, which contributes to the impression. But unlike a top fuel car, the ground is not shaking under your feet and your eyeballs are not vibrating.
There is not a direct correlation between thrust and horsepower since they are different measurements. But in some regimes it takes roughly 2 lbs thrust to equal 1 hp, e.g, Tu-95 shaft horsepower vs B-52 thrust at same altitude and speed. If that is correct a Harrier is producing about the same power as the 10,000 hp of a top fuel car.
The Harrier exhaust stream is very high speed, which is inefficient relative to accelerating a larger volume of air to a lower speed – like a helicopter or V-22. Some of that wasted energy from the high speed exhaust is manifested as noise which is why the Harrier is so loud. That is also why newer airliners with high-bypass turbofan engines are much quieter than a Harrier, despite having vastly more thrust. The bypass fan is accelerating a larger volume of air to a slower velocity, which is more efficient and produces less wasted noise energy.
No one’s mentioned SR-71’s … I thought they were pretty loud … maybe that’s just frequencies humans can’t hear.
I would describe Top Fuel dragsters as feeling like you were being carpet bombed for 3-4ish seconds. Fun times.