You look up into the sky and see an aircraft flying towards you at Mach speed.
Everything is silent as the plane approaches. You expected to be able to hear the roar of
its engines as it got closer, but this is not the case. You look through a pair of binoculars at the
object and can see the flames from the jets coming out of the back, yet there is still no sound. When
you glance down at the trees below the plane, it seems as if some unseen force is bending them.
The supersonic jet passes over you. There is a sudden increase of pressure and a deafening boom!
You just experienced a sonic boom from the aircraft that flew over you.
Mach 1 means that an object is traveling at the speed of sound.
Anything over Mach 1 means an object is going faster than the speed of sound and will create a
sonic boom. The loudness and intensity of this phenomenon is determined by several factors.
But what is a sonic boom, and what actually causes it? Or perhaps a better question is:
what are the dangers of standing in the path of a sonic boom?
On October 14, 1947, U.S. AirForce pilot Captain Chuck Yeager was the first person to travel faster
than the speed of sound. For years the military had been trying to break the sound barrier,
but every aircraft that approached the speed of sound experienced intense turbulence. The
aircraft would shake so violently that some were torn apart, costing the lives of several pilots.
The problem was that as an aircraft approached the speed of sound, there would be a build-up
of pressure pushing on the plane from all sides. The vessel would then begin shaking violently,
which would continue until the pilot slowed down. This led many to assume that the sound
barrier was something that humans were just not meant to get past.
But Chuck Yeager proved them wrong. On a sunny October day, Chuck Yegar was strapped into his
bright orange Bell X-1 aircraft named “Glamorous Glennis” after his wife. The X-1 was loaded onto
a B-29 and flown to an altitude of 25,000 feet above Dry Lake, California. The X-1 was released
through the bomb bay doors and rocketed to 40,000 feet at over 662 miles per hour. This meant Yeager
was traveling faster than the speed of sound at that altitude. It was a combination of Yeager’s
piloting skills and modifications made to the X-1 that allowed it to be more aerodynamic and stable
during the turbulence that allowed humans to achieve their first supersonic flight.
But what does this actually mean?
A good way to think about the concept of sonic booms is to imagine that air is a fluid. When
a boat travels through water, it creates ripples that radiate outward from the front of the boat.
The same thing happens with a plane in the air. As an aircraft travels through the air, the sound
waves it generates radiate out in a similar way to ripples in water from a moving boat.
However, when an aircraft goes supersonic or exceeds the speed of sound, it means that the
plane is moving faster than the soundwaves it is creating. This is why an aircraft that is
approaching you at Mach speeds appears to be silent until it passes over you. This
phenomena results in a sudden change of pressure between the front and tail of the aircraft,
which results in a shockwave or what we call a sonic boom. But a sonic boom is
not just generated at a single point. It is actually a cone shape called the Mach cone.
Anyone within the area of the Mach cone will experience the sonic boom of an aircraft going
faster than the speed of sound. So, the plane does not need to fly directly above you. The
sonic boom is not the noise of the jet engines or the object itself; it is actually the sharp
release of pressure that occurs between the built-up shockwave in front of the aircraft
that collapses into the space the aircraft previously occupied as it moves through the air.
An easier way to think of it is that due to the natural forces of physics, the incredibly high
pressure that builds up in front of the plane wants to fill in the space that the aircraft
occupies. A simplified example of this can be seen when you wave your hand back and forth.
You can feel the air “move” over your hand. This is because as your hand moves,
the air molecules around it flow over your hand to fill the empty space that your hand once occupied.
This all has to do with changes in pressure; the space where your hand was has a lower
concentration of air molecules than the surrounding area. This causes the molecules
to flow towards the area of low concentration in order to equalize the pressure. If you could
move your hand at 761 miles per hour, you would create a mini sonic boom as your hand
moved through the air. The same thing happens when an aircraft reaches supersonic speeds.
The build-up of pressure at the front of the plane instantly fills the space behind the tail
of the plane where the aircraft was a moment before. This is what causes the sonic boom.
The size and distance of the sonic boom created by the Mach cone is influenced by several factors.
Things like the weight, size, and shape of the aircraft all impact the sonic boom's intensity.
Also, the speed, altitude, and flight path the aircraft takes will affect the Mach cone. A large,
heavy aircraft will displace a lot more air than a thin, small aircraft.
This means that the larger aircraft’s sonic boom will be much stronger and louder.
The more air displaced by an object going supersonic speeds,
the bigger and stronger the sonic boom will be when the pressure change occurs.
Altitude also plays an important factor in how intense a sonic boom is. The higher a supersonic
aircraft is flying, the wider the area of the Mach cone will be. This is because the shock
wave needs to travel a further distance to reach the surface of the Earth than a low-flying object.
However, altitude also influences the speed at which sound moves. At sea level, the speed of
sound is around 761 miles per hour. This means any object that travels faster than that speed
will break the sound barrier. The record for fastest land speed is held by Andy Green, who
drove a Thrust SSC 763 miles per hour, breaking the sound barrier by just over 2 miles per hour.
When the Concorde was still in operation, it flew passengers across the Atlantic at about
55,000 feet above sea level. At this altitude the sound barrier is approximately 660 miles per hour,
almost 100 miles per hour slower than at sea level. Both created sonic booms when they
passed the sound barrier, but the Thrust SCC needed to travel much faster than the Concord
to achieve this phenomenon due to its lower elevation.
The area within a Mach cone where the sonic boom can be heard is called the boom carpet.
Researchers can calculate the exact dimensions of the boom carpet to know how much area will
be affected by the sonic boom of an aircraft flying overhead. The width of the boom carpet
is about one mile for every 1,000 feet of altitude. This means that if a supersonic
jet is flying at 60,000 feet, its boom carpet will be about 60 miles in width.
That being said, not all points within the boom carpet will experience the same intensity of
shock waves. The spot directly beneath the aircraft will have the highest intensity
and experience the most powerful sonic boom. The further away someone is from
the flight line of an aircraft, the less intense the sonic boom will be.
Any object traveling faster than the speed of sound actually generates
two sonic booms. This is because there are two changes in pressure
as an object travels quickly through the air. The first is at the front of the plane,
where a huge amount of pressure builds up around the nose. The second is at the tail of
the aircraft, where the pressure suddenly returns to normal. The two sonic booms are normally about
the same strength. The time between the two is dependent only on how large the aircraft is.
The longer and bulkier a plane is, the more time there will be between the two sonic booms.
Conversely, if an aircraft is small and thin, the time between the two sonic booms will be
so short that it may seem like there is just a single shockwave. However,
this is just an illusion as every aircraft going over Mach 1 creates two sonic booms.
A good example of this phenomenon occurred when NASA used to utilize space shuttles for their
missions. During reentry, the shuttles would be traveling much faster than the speed of sound.
Since these crafts were so large, the time between the two sonic booms was very noticeable.
Interestingly, speed doesn’t have as much of an effect on the intensity of a sonic boom as
you might think. The faster an aircraft goes over Mach 1 does not necessarily correlate to a bigger
sonic boom. At speeds slightly faster than Mach 1, like Mach 1.3, there is a significant increase
in the power of a sonic boom. However, at speeds faster than Mach 1.3, the change in the intensity
of a sonic boom is negligible. On the other hand, the maneuvers an aircraft makes while
traveling at supersonic speeds can have a huge impact on its shockwave. Maneuvers like an “S”
turn can significantly increase the intensity of a sonic boom due to a greater buildup of pressure.
Currently, supersonic aircraft are restricted from flying over populated areas due to the
effects sonic booms can have on people, animals, and structures. During the time that the Concorde
was being used, people who lived along the flight paths would complain about
the disruptions the sonic booms had on their daily lives. The sonic booms from these jets
passing overhead would startle people and were even connected to increased levels of anxiety.
In unique circumstances, the sonic booms from the Concords could even cause hearing loss.
Therefore, aircraft that fly above Mach 1 take routes that are away from populated areas
or decrease their speeds when above land inhabited by people.
It is important to note that sonic booms are measured in pounds per square foot
because they are the result of a change in pressure and not the loudness of a sound.
A Concorde SST flying Mach 2 at 52,000 feet generates a sonic boom of 1.94 psf.
This is enough to cause mild discomfort for someone standing directly below the aircraft
and could even result in minor damage to structures, such as the cracking of glass.
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