Sonic boom

[2] Sonic booms due to large supersonic aircraft can be particularly loud and startling, tend to awaken people, and may cause minor damage to some structures.

For today's supersonic aircraft in normal operating conditions, the peak overpressure varies from less than 50 to 500 Pa (1 to 10 psf (pound per square foot)) for an N-wave boom.

The strongest sonic boom ever recorded was 7,000 Pa (144 psf) and it did not cause injury to the researchers who were exposed to it.

[5] In recent tests, the maximum boom measured during more realistic flight conditions was 1,010 Pa (21 psf).

Ground motion resulting from the sonic boom is rare and is well below structural damage thresholds accepted by the U.S. Bureau of Mines and other agencies.

[6] The power, or volume, of the shock wave, depends on the quantity of air that is being accelerated, and thus the size and shape of the aircraft.

These secondary shockwaves are caused by the air being forced to turn around these convex points, which generates a shock wave in supersonic flow.

On most aircraft designs the characteristic distance is about 40,000 feet (12,000 m), meaning that below this altitude the sonic boom will be "softer".

In the late 1950s when supersonic transport (SST) designs were being actively pursued, it was thought that although the boom would be very large, the problems could be avoided by flying higher.

This assumption was proven false when the North American XB-70 Valkyrie first flew, and it was found that the boom was a problem even at 70,000 feet (21,000 m).

Richard Seebass and his colleague Albert George at Cornell University studied the problem extensively and eventually defined a "figure of merit" (FM) to characterize the sonic boom levels of different aircraft.

Small airplane designs like business jets are favored and tend to produce minimal to no audible booms.

[7] Building on the earlier research of L. B. Jones,[9] Seebass, and George identified conditions in which sonic boom shockwaves could be eliminated.

[7] This remained untested for decades, until DARPA started the Quiet Supersonic Platform project and funded the Shaped Sonic Boom Demonstration (SSBD) aircraft to test it.

The SSBD was tested over two years culminating in 21 flights and was an extensive study on sonic boom characteristics.

After measuring the 1,300 recordings, some taken inside the shock wave by a chase plane, the SSBD demonstrated a reduction in boom by about one-third.

As a follow-on to SSBD, in 2006 a NASA-Gulfstream Aerospace team tested the Quiet Spike on NASA Dryden's F-15B aircraft 836.

Valuable data was gathered from the experiment, but 15,000 complaints were generated and ultimately entangled the government in a class-action lawsuit, which it lost on appeal in 1969.

Windows would rattle and in some cases, the "torching" (masonry mortar underneath roof slates) would be dislodged with the vibration.

Research by acoustics experts under this program began looking more closely at the composition of sonic booms, including the frequency content.

Several characteristics of the traditional sonic boom "N" wave can influence how loud and irritating it can be perceived by listeners on the ground.

Even strong N-waves such as those generated by Concorde or military aircraft can be far less objectionable if the rise time of the over-pressure is sufficiently long.

The intensity and width of a sonic boom path depend on the physical characteristics of the aircraft and how it is operated.

Ground width of the boom exposure area is approximately 1 statute mile (1.6 km) for each 1,000 feet (300 m) of altitude (the width is about five times the altitude); that is, an aircraft flying supersonic at 30,000 feet (9,100 m) will create a lateral boom spread of about 30 miles (48 km).

Temperature variations, humidity, atmospheric pollution, and winds can all affect how a sonic boom is perceived on the ground.

Hard surfaces such as concrete, pavement, and large buildings can cause reflections that may amplify the sound of a sonic boom.

Conversely, grassy fields and profuse foliage can help attenuate the strength of the overpressure of a sonic boom.

However, work is underway to create metrics that will help in understanding how humans respond to the noise generated by sonic booms.

The end of the whip, known as the "cracker", moves faster than the speed of sound, thus creating a sonic boom.

Goriely and McMillen showed that the physical explanation is complex, involving the way that a loop travels down a tapered filament under tension.