Velocity in “Speed of Sound”

“All that noise, and all that sound
All those places I have found
And birds go flying at the speed of sound
To show you how it all began
Birds came flying from the underground
If you could see it then you’d understand”
– Coldplay, “Speed of Sound”

Can you imagine birds flying as fast as sound?

Sound results from collisions between particles. For example, one molecule hits another molecule, and then that molecule hits another molecule, and so on. The speed or velocity of sound depends on the material that it travels through. In general, sound travels faster in solids than liquids and faster in liquids than gases. Solids are denser than liquids; liquids are denser than gases. This means that the particles in solids are much closer together and can quickly bump into one another, allowing sound to propagate through more quickly. On the other end, gas particles tend to be farther apart, meaning that the particles don’t collide as quickly and sound takes longer to propagate.



Typical dry air is made of about 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.03% carbon dioxide, and trace amounts of other gases. At 20 degrees Celsius (68 degrees Fahrenheit), sound travels through dry air at a speed of 343 meters per second (769 miles per hour). At 0 degrees Celsius (32 degrees Fahrenheit), sound travels through dry air at a speed of 331 meters per second (740 miles per hour).

For comparison to other materials, sound travels through helium at 972 meters per second (2174 miles per hour) sea water at 1450 meters per second (3244 miles per hour); water at 1493 meters per second (3340 miles per hour); gold at 3240 meters per second (7248 miles per hour); iron at 5130 meters per second (11,475 miles per hour); and diamond at a whopping 12,000 meters per second (26,843 miles per hour).

In general, if we say something is traveling at supersonic speed, or faster than the speed of sound, we mean the speed that sound travels in air. After all, that is where much of human travel occurs. Exceeding the speed of sound is also known as breaking the sound barrier. The first recognized moment for breaking the sound barrier occurred on October 14, 1947, when US Air Force test pilot Chuck Yeager brought the experimental Bell X-1 aircraft to just over the speed of sound.

As an aircraft approaches the speed of sound, it begins to “catch up” to the sound waves it’s emitting. These sound waves build up in front of the aircraft until the aircraft surpasses the speed of sound. The aircraft then begins to “out run” the sound waves it generates. Air molecules that try to propagate the sound are pushed outward and back of the aircraft, forming a pressurized cone of air molecules (with the tip of the cone being the aircraft). The pressure of the air eventually returns to normal, but the changing pressure creates a shockwave or explosive sound known as a sonic boom.



For comparison, the fastest bird clocked has been the peregrine falcon, which can travel at 240 miles per hour (107 meters per second) while diving. It’s pretty fast, but not quite the speed of sound.