Sonic boom: Sound like thunder

Are you familiar with the word 'sonic boom'? You probably are not. This concept is highly scientific and combines the latest technology. Through this week's Science page, we explain this concept to you.

A sonic boom is a sudden noise similar to thunder. This is caused by an object moving faster than sound, about 750 miles per hour at sea level. An aircraft travelling through the atmosphere continuously produces air pressure waves similar to the water waves caused by a ship's bow. When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves which travel forward from the generation or 'release' point.

As an aircraft flies at supersonic speeds, it is continually generating shock waves, dropping sonic booms along its flight path, similar to someone dropping objects from a moving vehicle. If you see this from the aircraft, the boom appears to be swept backwards as it travels away from the aircraft. If the plane makes a sharp turn or pulls up, the boom will hit the ground in front of the aircraft.

The sound we hear as the 'sonic boom' is the sudden onset and release of pressure after the build up by the shock wave. The pressure change caused by a sonic boom is small, but it occurs in a very short period. It's similar to the pressure change we feel in an elevator, as it comes down two or three floors.

There are two types of booms - 'N' waves and 'U' waves. They are named according to their shape. The strongest sonic boom ever recorded was 144 pounds per square foot, but did not cause any injury to the researchers who had been exposed to it. This boom had been produced by an F-4 aircraft, flying just above the speed of sound, 100 feet above sea level. Sometimes, sonic booms might shatter glass, but not cause much harm.

The energy range of sonic booms is concentrated in the 0.1 - 100 Hz frequency range, that is considerably below that of gunfire and most industrial noises.

The duration of the sonic boom is really short; sometimes, even less than a second. Depending on the aircraft's altitude (height above sea level), sonic booms reach the ground two to 60 seconds later.

However, not all booms are heard at ground level. The speed of sound at any height depends on the air temperature; when the temperature increases, the speed of sound also increases. Sonic booms happen not only with aircraft that fly at supersonic speed(such as the Concorde)but also with space. The intensity and width of a sonic boom's path depends on the physical characteristics of the aircraft and how it is operated.

Most fighter aircraft of the US Airforce are capable of super sonic speeds. Sounds amazing, isn't it? Though it is difficult to observe this in Sri Lanka, it could happen.