In 1953, U.S. Air Force pilot, Chuck Yeager broke a new Mach 2 record. For his efforts he was called to the White House by President Eisenhower, who presented him with the Harman International Trophy. Yeager eventually reached a speed of 1,650 miles per hour and an altitude of over 90,000 feet, feats that helped earn him the Distinguished Service Medal.
Ever since Chuck Yeager blew through the sound barrier in his X-1 in 1947, residents living near testing ranges have been bombarded by sonic booms. The thunderous booms rattle windows and scare pets, and are among the reasons the supersonic Concorde never flew across the continental U.S.
As Audiologists we are totally familiar with routine noise sources: airplanes, guns, loud manufacturing machinery, race cars, motorcycles, rock concerts, etc. But we may fail to consider other less familiar sources of noise that can also be very loud, such as sonic booms. While most of us are rather well-versed in acoustics and the physics of sound, it is unlikely that we have given much thought to sonic booms. A sonic boom is the thunder-like noise a person on the ground hears when an aircraft or other type of aerospace vehicle flies overhead faster than the speed of sound or supersonic.
What Causes Sonic Booms?
Air reacts like a fluid to supersonic objects. As objects travel through the air, the air molecules are pushed aside with great force and this forms a shock wave much the way a boat creates a bow wave. The bigger and heavier the aircraft, the more air it displaces. The shock wave forms a cone of pressurized air molecules that move outward and rearward in all directions and extend to the ground. As the cone spreads across the landscape along the flight path, the molecules create a continuous sonic boom along the full width of the cone’s base. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom. The change in air pressure associated with a sonic boom is really not very great–only a few pounds per square foot, or about the same pressure change experienced while riding an elevator down two or three floors.
It is not so much how much pressure but the rate of the change sudden onset of the pressure change, that makes the sonic boom audible. All aircraft generate two cones: one at the nose and one at the tail. They are usually of similar strength and the time interval between the two as they reach the ground is primarily dependent on the size of the aircraft and its altitude. Unless the vehicle is rather large, such as the space shuttle, most people on the ground cannot distinguish between the two, and they are usually heard as a single sonic boom. Now, if the plane is traveling at the supersonic speeds (greater than that of sound), it is going faster than its own sound. As a result, a pressure (sound is variation in pressure) wave is produced in the shape of the cone whose vertex is at the nose of the plane, and whose base is behind the plane. The angle opening of the cone depends on the actual speed the plane is traveling at. All of the sound pressure is contained in this cone.
So, imagine now this plane in a level flight. Before the plane passes you, you can see it, but you can not hear anything. The pressure cone is trailing behind the plane. Once your ears intersect the edge of this cone, your will hear a very loud sound – the sonic boom. Therefore you hear the sonic boom when your ears intersect this cone, and not when the plane breaks the sound barrier, as is commonly misunderstood by most of the population.
Effects of Sonic Booms
The sonic booms can sometimes be quite loud. For a commercial supersonic transport plane (SST), it can be as loud as 136 decibels, or 120 Pa (in units of pressure). While studies of sonic booms date well back into the 1960s, Rylander (1974) was among the early few who studied the effects of sonic booms on humans and animals and found the appearance of a startle response, the disturbance of ongoing activities, and the disturbance of sleep and rest.
NASA has conducted research both in the field and in laboratories that simulates these sounds and their effects. To summarize the effects that have been found, sonic booms are measured in pounds per square foot of overpressure. This is the amount of the increase over the normal atmospheric pressure that surrounds us (2,116 psf/14.7 psi). At one pound overpressure, no damage to structures would be expected. Overpressures of 1 to 2 pounds are produced by supersonic aircraft flying at normal operating altitudes. Some public reaction could be expected between 1.5 and 2 pounds. Rare, minor damage to buildings and glass etc., may occur with 2 to 5 lb overpressure.
As overpressure increases, the likelihood of structural damage and stronger public reaction for disruption, noise levels, and sleep disturbance also increase. Tests, however, have shown that structures in good condition have been undamaged by overpressures of up to 11 lb. Sonic booms produced by aircraft flying supersonic at altitudes of less than 100 feet, creating between 20 and 144 lb. overpressure, have been experienced by humans without injury. Damage to eardrums can be expected when overpressures reach 720 lb. Overpressures of 2160 lb. would have to be generated to produce lung damage. Typical overpressure of aircraft types that create sonic booms are:
- SR-71: 0.9 lb, speed of Mach 3, 80,000 feet
- Concorde SST: 1.94 lb, speed of Mach 2, 52,000 feet
- F-104: 0.8 lb, speed of Mach 1.93, 48,000 feet
- Space Shuttle: 1.25 lb, speed of Mach 1.5, 60,000 feet, landing approach
But these rather benign physiological issues are only part of the story. Cunningham (2004) reports that NASA researchers have made findings much the same as Rylander’s in that booms are disruptive, annoying, and cause loss of sleep. They have also found that people’s attitudes toward the source of the sonic booms play a big role in how willing they are to put up with it. For example, people are less likely to accept a sonic boom if they are concerned about its impact on wildlife, if they have negative attitudes toward the military, or if they live in a rural area where the noise would be more pronounced.
Until low-sonic-boom aircraft move beyond the development stage, making it possible for tests to be conducted in the real world, public tolerance of the boom will remain an open question. Currently sonic booms are prohibited over the United States.
So…..Is the sonic boom another source of noise exposure? Probably not, as it is a rare impulse noise that we do not hear over the US, but could be of interest in other countries where these “booms” are more frequent.
Armstrong Fact Sheets: Sonic Booms. National Aeronautical and Space Administration. Retrieved May 13, 2015: http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html#.VVNSs_lViko
Cunningham, A., Sonic booms and human ears: How much can the public take? Popular Science. Retrieved May 13, 2015: http://www.popsci.com/military-aviation-space/article/2004-07/sonic-booms-and-human-ears?image=0
Rylander, R. (1974). The Sonic Boom -Effects on humans. Sozial-Praventivmedizin. 19 (3),pp.217-. Retrieved May 13, 2015: http://link.springer.com/article/10.1007%2FBF01999428#page-1
Hewitt, P., Pearson Education/Addisson-Wesley, 2006: Retrieved May 13, 2015: http://imgarcade.com/1/shock-wave-physics/
University of New South Wales, Sydney Australia, Sonic Booms, http:/you.be/-d9A2oq1n38
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