The Speed of Sound

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Podcast Transcript

Sound travels fast. Very fast. However, it is not so fast that we can’t recognize that it has a finite speed. 

The speed of sound was estimated and later measured centuries ago, but it was only recently that it has been possible actually travel faster than the speed of sound.

It turns out that while it is possible to travel faster than the speed of sound, there are a whole host of problems that go along with it.

Learn more about the speed of sound on this episode of Everything Everywhere Daily.

Before I get into the speed of sound, I should at least briefly explain what sound is and how it works. 

Sound is nothing more than a wave that travels through a material medium. We mostly encounter sound as it travels through air, but it can also travel through solids and liquids as well. 

A sound wave is not so much the movement of atoms and molecules as it is energy going through those particles. 

One particle is pushed slightly, which pushes the particle next to it, which pushes the particle next to it, and so on. 

What is moving through the medium is kinetic energy.

The maximum speed that these waves can go through something is the speed of sound. 

Unlike light, there is no one speed of sound. The speed of sound will differ depending on the substance that the sound wave is moving through. 

Generally speaking, sound moves faster through solids than liquids and faster through liquids than gases. The two major factors that determine how fast sound travels through a substance are density and temperature. 

For the rest of this episode, however, I’m going to be dealing with the speed of sound in the atmosphere at standard pressure and temperatures because that is how we usually experience sound. 

The speed of light and the speed of sound are both very fast with respect to speeds humans can comprehend; however, there is an enormous difference between the two. 

If you have heard thunder seconds after you saw a lightning bolt, then you’ve experienced firsthand the difference in speed between the two. 

If you have ever been somewhere where you could hear an echo of your voice, you’ve also experienced the speed of sound. 

One of the first attempts to measure the speed of sound was made in 1635 by the French philosopher Pierre Gassendi. 

Assuming that the speed of light was instantaneous, he measured the time between seeing the flash of light from a gun and hearing the sound.

He did this over various distances on days with no wind.

The value he obtained was 478.4 meters per second or 1,569.6 feet per second, which was too high, but he did figure out that the speed of sound was independent of frequency.

Isaac Newton took a stab at trying to estimate the speed of sound just based on calculations.

His first estimate was off by about 15% because he didn’t precisely know how heat affected sound. 

However, by the mid-18th century, the measurement of the speed of sound was shockingly accurate, given the instruments at the time. They had measured it at 332 meters per second.

Over time, this measurement was refined. By 1942, it had been measured at 331.45 meters per second. 

The currently accepted measurement of the speed of sound at 0 °C  or 32 °F is 331 meters per second or 1,086 feet per second. Or, to put it yet another way, sound travels 1,192 kilometers per hour or 740 miles per hour.

Once the speed of sound was established, it raised the question of what happens if you go faster than sound itself.

This was a theoretical question up until the 20th century. Airplanes kept flying faster and faster, but they were never able to go faster than the speed of sound. Some people actually thought that there was a sound barrier and that it was impossible to fly faster than the speed of sound. 

Some pilots of propeller-driven aircraft believed that they broke the sound barrier in powered dives, but it turned out that there were problems with instrumentation. 

An entire system of measurement for supersonic speeds was developed by the Austrian physicist Ernst Mach. He developed the system that bears his name, which measures speeds relative to the speed of sound. 

Under this system, the speed of sound is March 1. Twice the speed of sound is March 2, etc. 

The first person to actually break the sound barrier was Colonel Chuck Yeager, who flew faster than the speed of sound on October 14, 1947.  He did it in the specially designed Bell X-1 rocket aircraft. The quest to break the speed of sound I will leave for another episode, but suffice it to say it was a huge achievement, even if it wasn’t made public until nine months after it happened. 

One of the things that ground observers noticed when Yeager broke the sound barrier was a very loud noise. It was the world’s first sonic boom.

Well… wasn’t technically the world’s first sonic boom. There have been lots of them, but it was the first that could be identified as a sonic boom. 

I should explain exactly what a sonic boom is, because there is a great deal of confusion about the matter. 

A sonic boom is a shockwave created when an object, such as an aircraft or a bullet, travels through the air at a speed faster than the speed of sound. 

It occurs because the object displaces air faster than the air molecules can move out of the way, causing a build-up of pressure that is released as a sonic boom.

When I say Chuck Yeager didn’t create the first sonic boom, it is because small sonic booms occur whenever high-caliber bullets are fired or artillery shells are launched. Most people have no clue that those are sonic booms because they occur almost simultaneously with the explosion of gunpowder, which also causes a loud noise. 

Perhaps the simplest sonic boom you might be familiar with is the cracking of a whip. A whip makes a distinctive cracking sound because the tip of the whip very briefly goes faster than the speed of sound, resulting in a small sonic boom. 

In the case of an aircraft creating a sonic boom, the aircraft is much larger, which creates a much larger boom, and because of its altitude, it can be heard over a much greater distance. 

There is one big misconception people have about sonic booms. They assume that it is something that only occurs a single time when a plane passes Mach 1. 

This is incorrect.

A plane is continually making a sonic boom so long it is flying above the speed of sound. 

A sonic boom is actually analogous to a wake created by a fast-moving boat. A boat will create a two-dimensional wake behind it, but a plane will create a three-dimensional wake behind it in the shape of a cone. 

When the cone reaches the ground, it will do so in the shape of an arc. That arc will follow the plane as it flies. That is known as the sonic boom carpet. 

Another thing that many people don’t realize is that all sonic booms are actually double sonic booms. There is a boom when you hear the shockwave and another when the pressure returns to normal. However, most airplanes will produce sonic booms so close together that they sound like one.

When the Space Shuttle re-entered the atmosphere, it produced a very noticeable double sonic boom. This was due to the large size of the orbiter and the shockwave it created. 

Sonic booms cannot be heard by people inside the aircraft that creates them. Because they are literally traveling faster than sound, the pressure wave can’t catch up to them. 

The issue of sonic booms has actually been the sticking point for supersonic travel. Many people have wondered why commercial air travel hasn’t gotten faster. The truth is that we have the technology to fly passengers faster than the speed of sound.

The Concorde flew at supersonic speeds across the Atlantic for years. From 1976 until 2003, the Concorde flew passengers on a regular schedule.

It set the record for transatlantic commercial travel on February 7, 1996, when it went from New York JFK to Heathrow in just 2 hours, 52 minutes, 59 seconds. It was aided by a 175-mile-per-hour or 282-kilometer-per-hour tailwind.

However, the Concorde was limited to flying over the ocean because of the problem with sonic booms. 

If a plane the size of the Concorde flew from New York to Los Angeles entirely over land, it would lay down a sonic boom carpet across the entire country. 

The higher the altitude a plane flies, the wider the sonic boom carpet will be. A plane flying at an altitude of 50,000 feet or 15240 meters will create a sonic boom carpet 50 miles or 80.4 kilometers wide. 

Sonic booms are extremely annoying and can cause damage. NASA and the Federal Aviation Administration ran tests in 1964 over Oklahoma City. For six months, they created eight sonic booms per day over Oklahoma City. 

The end result was 15,000 registered complaints and a class action lawsuit. 

Many countries have banned or put restrictions on sonic booms for this reason. 

It has also resulted in one of the most active areas of aeronautical research right now. 

The intensity of a sonic boom can be mitigated by the shape of an aircraft and how it creates the shockwave cone that follows it. One method was to try to shape the cone so it would expend most energy horizontally instead of vertically. 

In 2018, NASA commissioned Lockheed Martin to build the X-59 Quesst, which hopefully will demonstrate that a sonic boom can be kept as quiet as a closing car door. 

I also should note the theoretical design called a Busemann biplane. The Busemann biplane has two wings that would, in theory, create no sonic boom at all. However, the design would also not provide any lift, which is important for an airplane wing. 

Another design that is being considered is just making planes smaller. There are current proposals on the board for supersonic private jets, which would make less noise simply due to their size.

The current record for supersonic air travel was set back in 1976 by the SR-71 Blackbird, on which I’ve done a previous episode. 

The pursuit of breaking the sound barrier wasn’t just something that occurred in the air. There were also attempts for decades to break the sound barrier on land. 

The land sound barrier was broken on October 15, 1997, by British RAF fight pilot Andy Green, who drove the jet-powered ThrustSSC past Mach 1 in the Black Rock desert of Nevada. 

To date, it is the only time any land vehicle has broken the sound barrier. 

The speed of sound is no longer a seemingly impossible barrier to overcome. It is broken on a daily basis by military aircraft, and it was broken on a regular basis for years by the Concorde. 

Today, the sound barrier is something to be tamed and controlled. Perhaps a future generation of aircraft that can dampen sonic booms will allow everyone the ability to travel faster than the speed of sound.

The Executive Producer of Everything Everywhere Daily is Charles Daniel.

The associate producers are Peter Bennett and Cameron Kieffer.

Today’s review comes from listener Brenda over on Castbox. They write:

Not sure if Gary sees comments here or not, but 5 stars! After changing jobs a year ago, I went from doing my bookkeeping and answering a phone to just entering bookkeeping data. I started out listening to just music, then tried out Audio Books and Podcasts. Several sounded interesting, but the narrator/podcaster’s voice was not pleasant to my ears. Gary has a smooth voice that is easy to listen to. In less than 3 months, I have listened to every one of the podcasts. Great job Gary!

Thanks, Breanda! As you can see, I do notice that comments that are left over on Castbox. I’m glad to keep you company during your workday. You can both be productive as well learn something in the process….and of course, welcome to the completionist club.

Remember, if you leave a review or send me a boostagram, you too can have it read on the show.