By: Mark Saunders
AudigyGroup Freelancer

Traveling at the Speed of Sound

Sound is a form of energy made when air molecules move. This movement is called sound waves, which are a series of compression waves created by the vibration of some object (e.g., banging a drum, speaking through the cone in a radio loudspeaker, clapping hands, etc,). The waves are very versatile and can pass through just about any medium, including all forms of matter (gas, solids, and liquids). Sound waves share characteristics of frequency, wavelength and amplitude.

How Often, How Far, How Loud

Frequency, measured in cycles per second (or Hertz), is the rate at which sound waves vibrate; the higher the frequency, the higher the pitch. The distance between one sound wave compression and the next is called the wavelength. Faster sounds have shorter wavelengths and higher frequencies. Amplitude measures the amount of energy in a sound wave; the more energy, the louder the sound.

The Sound Barrier

So just how fast is sound? The speed of sound is measured as the velocity of sound through a medium.  For example, the speed of sound in air is 768.095 mph at a temperature of 20°C or 68°F. At sea level, this means “sound” moves faster than 750 miles per hour.

In general, the denser the medium, the faster sound travels. Sound travels faster through steel than through wood, and it travels four times faster through water than through air. That’s why inhaling from a helium balloon gives your voice that temporary, goofy Donald Duck sound. Your voice is much higher than normal because sound travels faster through helium than through air.

Now Arriving

The eardrum (tympanic membrane) gathers the sound and passes it to the ossicles, which send it to the cochlea—home of your hearing organs. In sum, the ear receives and sends sounds to the auditory cortex at the back of the brain for processing. Message sent, received, and processed. Additionally, the brain computes the different arrival times of sound (measured in one millionth of a second) into each ear to estimate the location of its source.

The Science of Hearing – Sidebars

Sidebar 1: Earth Sounds Are Easy

As part of the Apollo 15 space crew that landed on the Moon in July 30, 1971, NASA astronaut David Scott performed an experiment by dropping a hammer and a feather at the same time to see which would land first. Because the Moon has the thinnest of atmospheres, a near-vacuum estimated to be five-trillionth as dense as Earth’s, the two items landed at the exact same time, accelerated only by the force of gravity. And since sound needs to travel through something—air, water, gas—to get from point A to point B, astronaut Scott’s dropped objects made no sound.  Or put another way, the Moon, for all intents and purposes, is silent. Earth, on the other hand, is noisy. Indoors we hear alarm clocks, vacuum cleaners, dishwashers, washing machines, television sets, conversations, and so on. Outside it’s chirping birds and barking dogs, lawnmowers and chain saws, even planes, trains, and automobiles.

Sidebar 2: The Big Boom Theory

A sonic boom is the sound of an object moving faster than the speed of sound. At sea level, this means the sound is moving faster than 750 miles per hour. As a jet flies through the air, it creates a wave of pressure in front; if the jet travels less than the speed of sound, air particles have enough time to part in front of it, much like waves part in front of a ship. However, when the jet flies faster than the speed of sound, also referred to as Mach 1, the pressure waves are compressed and overlap, resulting in what’s known as a sonic boom. The loudest sonic boom every recorded was 144 pounds of overpressure – created by an F-4 fighter jet flying just over Mach 1 at an altitude of 100 feet. There were no injuries, which is amazing since just 5 pounds of overpressure can cause minor damage to buildings. Recently, an experimental jet set a record for hypersonic flight, traveling at Mach 6 – six times the speed of sound!

Sidebar 3: Faster than a Speeding Bullet—Sometimes

Is sound faster than a speeding bullet? The answer depends on the type of bullet and the weapon discharging it. Most pistols and revolvers are slower than the speed of sound. On the other hand, most military rifles are faster, which explains why sometimes you’ll see the shot land before hearing its impact.