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Sound Barrier & speed of sound

Updated on June 4, 2009

    Sound Barrier & the Speed of Sound 

                             An insight on the past, the present, and the future


            Ever since the Wright brothers made the first flight on December 17, 1903, man has made it his job or vision to fly faster and faster than the speed of sound. The first question that pops up to our minds is: “What does the term speed of sound refer to? How does breaking the sound barrier occur and did man really achieve speeds greater than the speed of sound, if so by how much? Well, when trying to study this matter we are faced with diverse questions concerning this subject, yet our main goal behind this simplified research is to answer these questions in the most effective yet simple manners to avoid confusion and misinterpretations.


1-       Definition: What is the speed of sound:

                 As for a start, the speed of sound is one of the most important properties of atmosphere. It does not know a constant value since it changes or varies according to atmospheric temperatures. For the ideal atmospheric conditions, the speed of sound is defined as the velocity at which sound waves travel through substances such as air. This value as stated previously depends on the altitude since sound travels in slightly different values at different altitudes, since altitudes affect the temperature. For simplicity, the speed of sound at sea level is 761 miles per hour whereas the speed is 660 miles per hour at 20,000 feet. Can you notice the difference? Thus, the speed is not a single value. To be precise the speed of sound (a) can be directly measured by applying the given relation taking in consideration the temperature of the air and that as a function of height.

Such that:

1.        A= speed of sound in m/sec

2.      Specific heat ratio= 1.4

3.      R= a gas constant (287 J/kg/K , where K refers to degrees Kelvin)

And once the speed is known it is very easy for you to calculate the Mach number of your given vehicle. Nowadays, some sights have provided their viewers with atmospheric calculators which are capable of computing such calculations effortlessly.

2-     Mach number:

     This number is the value of the velocity of a certain vehicle with respect to the speed of sound at a given altitude and specific conditions. For example a missile traveling at mach 3 travels at a speed three times the speed of sound during those fixed conditions. The Mach number is named after Czech/Austrian physicist and philosopher Ernst Mach who is one of the 20th century philosophers. The Mach number named after this philosopher (M) is simply the ratio of the vehicle’s velocity also symbolized by (V) over that speed of sound at that altitude. So the relation obtained is as the following:

M= V/a


3- Sound Barrier:

         It was believed in the past by many people that a physical barrier existed which prevented people flying aircraft from obtaining speeds higher than the speed of sound. Well, this is now out of use and proven wrong after the “sound barrier” has been broken many times since the World War II. Breaking the sound barrier stands for the phenomena in which aircrafts or vehicles jump or accelerate form subsonic to super sonic speeds. A vehicle is considered subsonic when it travels slower than the speed of sound or Mach 1 (< 1), whereas it is considered supersonic when it travels in a speed (M>1). According what has been presented in our book, when a plane flies at speed M=1 sound waves accumulate in the front of the plane forming what we call shock waves or sound barrier. Yet, the plane is capable of catching these waves thank to its forward push. Unlike the above example, a plane flying at supersonic speed has waves that accumulate on its sides having different crests. Thus, these waves superpose forming shock waves with high amplitude and enormous sounds. This explains the boom sound we usually hear when Israeli planes fly above our Lebanese territory.




Achievements of the Past:

           Ever since the Wright brothers, man has made it his job to discover the unlimited world of aircraft and aerodynamics. For the last two decades, the humans have achieved numerous successful experiments and exceeding the speed of sound by many times. Thus, not only flying planes at speeds equal to Mach 1 but also, five and above, therefore reaching hypersonic speeds.


           The first real flight to fly at a velocity greater than that of sound occurred on the 14th of October on the year 1947. The plane was named Bell X-1 and was piloted by Captain Charles Yeager, the American test pilot who was the first to travel at the age of 24 in a plane at a speed higher than Mach 1 flying at an altitude 45,000 feet. Then Bell X-1 eventually reached a speed of Mach 1.46. Later, Yeager was honored in the White House in the presence of President Truman.


           After Yeager’s success, the next flight to achieve a speed twice as that of sound occurred on November 20, 1953, i.e. six years after the first official mach one flight. The Pilot was Scott Crossfield with his aircraft Douglas D-558-2 Skyrocket. The aircraft first climbed to a 32, 000 feet by a US bomber. Then he climbed with his aircraft to a 72, 000 feet to attempt breaking the record set by Yeager and those who followed. Scott achieved a Mach 2.005 (1291 m/hr).


           This happened on the 7th of March 1961 with Captain Robert White and his North American X-15. The plane was released from a special Boeing B-52 and it actually achieved on this flight a peed of 2,905 m/hr (Mach 4.43). Similarly, Captain Robert was the first to achieve on the 23th of June 1961 a speed of Mach 5.27 (3603 m/hr).


Achievements of the Present& the Future:


           The present records scored by many prolific pilots have turned breaking the sound barrier as a piece of cake. On the 27th of March 2004, the aircraft X-43A which was unpiloted due to some hazards and fears was able to break all past records by achieving an approximate speed of Mach 7. The experiment took place 100,000 feet above California by the help of a modified Boeing B-52 bomber. Yet, this record was beaten by the unpiloted X-43A that reached a speed equal to ten times the speed of sound!

1-Faster than the Speed of Sound by TEN TIMES!!


           As we have seen in the past two decades that have gone by, many pilots and scientists have achieved great speeds with their aircraft that beat the speed of sound seven or eight times. Yet, technology is still moving on, forcing humans to make aircraft that beat this record as well. So what occurs to your mind when you hear the Phrase: “Faster than the speed of sound by ten times?” Scramjets are hypersonic jets that can fly at speeds exceeding those experimented on before. (A Mach 5 would be like a piece of cake for it). The most recent X-43 flight occurred in 2004 and reached a speed very close to TEN Mach (9.8 or 7546 miles/hr).

           Flying at a great speed like that is a danger or a hazard to the pilot, so test flights such as this that took place on the year 2004 were unpiloted as well. Yet, how did the scientists achieve that? As a matter of fact, engineers at Ohio State University designed a computer that controls the plane during its flight, thus calculating at high speeds all the factors that might affect it during its flight. The computer has flawlessly performed several maneuvers with very slight error. Fiorentini, a doctoral student at the university said concerning the importance of the controller: “It guides the jet along its trajectory and keeps it stable by the help of sensors that measure factors (Altitude, Velocity and Acceleration).



Though this technology is still under development, one can’t stop himself from wonder if it were accomplished and perfected how it would really affect the human race. Scramjets could deliver missiles to mobile targets; they could also carry people halfway around the world in less than an hour. Yet how will this technology be applied?

calculating the speed of sound taking in consideration the temperature
calculating the speed of sound taking in consideration the temperature


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    • Uriel profile image

      Uriel 8 years ago from Lebanon

      i sure will do:D thanks

    • meteoboy profile image

      meteoboy 8 years ago from GREECE

      Excellent job Uriel.Very interesting and unique hub. I like very much to hear the aircrafts breaking the sound barrier.You can use some videos from Youtube in this hub like ..... to enhance this. Thank you for sharing.