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Two Sound Phenomena Explained: Sonic Boom and Acoustic Shadow

Settle down, class, and take your seats. Bobby, what did I say about chewing gum?

Alright. Now that we’re all settled in, let’s talk about sound. Today we’re going to address concepts relating to sound traveling through, getting lost in, and accumulating within airspace.

Much of how we perceive our surrounding environment is influenced by our ability to hear sound in all directions. But how the human brain is able to piece together information about a sound, and how the sound itself is constantly being shaped by the medium it needs to travel through to arrive at our ears (i.e., air), are both subject to a number of natural processes and environmental factors.

How do we hear specific sonic events when they happen, and how do we hear them differently based on certain variables such as distance, altitude, and physical obstructions between the sonic event and our ears?

Let’s learn about two phenomena relating to auditory perception!

Sonic Boom

You may have heard of the term sonic boom before, whether through your interest in aviation, physics, or Sonic the Hedgehog’s animated TV series.

This phenomenon can sound like an incredibly loud explosion, depending on the object producing it. When an object travels through air, it displaces air molecules, creating waves of pressure in concentric circles around it (like the ripples that occur when you throw a pebble into water).

In other words, sound waves radiates in all directions spherically around an object. Since the waves move at the same rate consistently, they don’t overlap.

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But when the moving object increases to a speed greater than the speed of sound, moving through air faster than the pressure waves themselves, it forces the waves to compress and slam into each other, creating a shock wave trailing behind the object. The shock wave creates a buildup of pressure, and it is that sharp, sudden change in pressure between the cone of the shock wave and the air surrounding it that creates an audible boom.

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+ Learn more on Soundfly: Capture and process found sounds in your electronic compositions in our new free course with sampling innovator Andrew Huang, Any Sound Will Do.

The speed of sound varies quite a bit depending on altitude (thus, air pressure) and temperature. At sea level, around 68° F, the speed of sound moving through air is about 761 miles per hour. At an altitude of 20,000 feet, where the air is thinner and the temperature might be between -10 and -30° F, the speed of sound is much slower, around 660 mph.

When an object, likely an aircraft, is traveling at the speed of sound, it’s called “Mach 1” (named after the Austrian physicist Ernst Mach). Mach measures airspeed in relation to the speed of sound, so Mach 1 is one times the speed of sound, Mach 2 is two times, etc.

Back to the sonic boom. If an aircraft travels at Mach 1 or faster consistently, it generates a sonic boom that’s constant. Anyone situated beneath or along the flight path of the airplane will only hear the sonic boom once, however, since that area of pressure change follows the aircraft and only passes over the listener once. This path is known as the “boom carpet.” (There are technically two sets of audible “booms” produced — one at the nose and one at the tail, similar to the wake at the bow and stern of a boat — but it takes something the size of a spacecraft before your brain can distinguish between the two sounds.)

Effects on Hearing

The most powerful sonic boom ever recorded had no measurable effect on the researchers nearby, even though it’s theoretically proven that the force of sonic energy from a sonic boom can shatter glass.

Because the impact of the boom lessens the further it travels, it’s assumed that the closer a supersonically traveling object is, the stronger the effect. But this isn’t the only factor. Other factors include temperature, atmospheric conditions, wind, the surface of the ground, other sound-absorbing objects, and finally, the shape and angular design of the aircraft’s fuselage and wings.

When the short-lived commercial aircraft, the Concorde flew over the ocean during transatlantic flights, it reached up to Mach 2. Residents in the UK underneath the Concorde’s flight path complained about the sound, citing rattling windows and roof tiles being displaced by the vibration, but there are no reports of hearing loss.

And just to answer the question that’s probably bubbling up in your mind right now: No, neither the pilot nor the passengers aboard an aircraft can hear the sonic boom. As the aircraft travels faster than sound itself, the shock wave moves in a cone shape behind the object.

Here’s a video that features a recording of the sonic boom produced by the Concorde.

Micro-Sonic Booms

The cracking sound produced by a supersonic bullet traveling through the air, as well as the crack of a bullwhip, are tiny sonic booms. The end of a bullwhip is designed to pick up speed as it unravels, looping through the air, eventually surpassing the speed of sound.

+ Read more on Flypaper: “14 of the Most Influential Latin American Composers of the 20th Century”

Acoustic Shadow

If the sonic boom is all about a sound event that you hear, the acoustic shadow is in some ways the opposite. An acoustic shadow is a sound event you cannot hear.

Briefly, an acoustic shadow is an area where sound waves cannot propagate due to physical obstructions or disruptions which send the waves off course. What does that mean exactly? Let’s look at a couple of examples.

Sound and the American Civil War

The most famous story of acoustic shadow’s effect on human history occurs not once, but pretty frequently throughout the Civil War, causing many historians to assert that this phenomenon had a significant impact on its outcome.

Communication back in the 1860s was pretty primitive. Most battle decisions between regiments were made either via couriers on horseback, riding back and forth from the front lines to commanders miles away, via telegraph for less time-sensitive messages, or simply by listening to the sounds of one’s surroundings and making adjustments in real time.

However, the contours of the land (whether an area is heavily wooded or barren, features absorptive water surfaces like rivers and swamps, includes dew and snow cover, or consists of hills and valleys) and the weather characteristics of one particular day versus another (a strong wind blowing in one direction, cloud/fog cover, distracting weather sounds like rain and thunder, and temperature) both contribute to the way sound waves travel through air and how they’re restricted.

In multiple instances throughout the Civil War, residents many miles away from a battle in one direction reported to have heard the cannons clearly, while regiments awaiting those sounds upwind or over a wooded ridge heard nothing at all.

The Battle of Gettysburg, which occurred in early July 1863, resulted in more deaths than any other battle and is heralded as the major turning point of the Civil War.

Due to the rocky natural formations of Cemetery Ridge and Culp Hill, where General Robert E. Lee’s two Confederate regiments were positioned, a unidirectional wind, and the unexpected temperature change on the second morning of the battle, one general never heard the other’s attack on the Union army.

As Confederate General Ewell was ordered to wait for the sound of General Longstreet’s artillery barrage before sending troops from the other side, he was delayed and Union General Meade was able to thwart the attack.

+ Read more on Flypaper: “Sounds of Battle: Compositions That Defined the Wars of the Early 20th Century”

Sound waves generally travel outward from a source in all directions, but moving through molecules by gas (air), liquid (water), or solid (wood, like the body of a guitar, for example) has an enormous impact on how far, fast, and intact a sound ultimately travels.

Sound travels faster in warmer air because the molecules are more “excited,” vibrating against each other quickly. This is contrary to the common notion that sound travels farther in cold air due to the relative “stillness” of the molecules and the way refraction amplifies a signal. (Though other factors, including increased humidity in warm air, can play a part.)

Why We Need Two Ears

Human auditory perception takes into account a range of sensory indicators to help us locate where sounds are coming from and define the parameters of that sound for survival. In other words, we hear sound spatially.

Interestingly, the fact that our head contains one ear — one auditory receptor — on either side, is both a hindrance to the auditory perception of our environment and the thing that helps our brain understand it.

If a sound event occurs to our right side, our right ear is mostly responsible for capturing the information contained within the sound. Our left ear, in turn, receives the sonic information on a slight delay (due to distance) and much less of it (due to the obstruction of the head reflecting away vital frequencies).

So the left ear’s responsibility is to map the differences in sonic information received, essentially telling the brain what it “doesn’t know” in order to synthesize a complete analysis of the sound event.

This is called interaural time difference and interaural level difference. Essentially, what occurs is a spatialized acoustic shadow on one side of the head.

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Interaural time differences between the right and left ear. (Image by Christopher J. Plack)

Waveforms containing higher frequencies are closer together when moving through air than those containing low frequencies, as high frequencies oscillate more rapidly. Because of this, sounds containing higher frequencies are more susceptible to being blocked from one ear to the other, depending on which way you’re facing when a sound is made.

As you can see in the image below, lower frequencies have the ability to travel around the head to arrive at (in this case) the left ear without much signal loss in the first diagram.

But in the second diagram, higher frequencies aren’t wide enough to travel around the head, so they bounce off the right side, creating a shadow around the left ear. This is why we need two ears!

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(Image courtesy of Gureckislab.org)

+ Read more on Flypaper: In music, size matters. Check out the largest piano in the world, an 11-foot tall bass, a 50-foot-long flute, and an organ made out of stalactites!

And lastly, before we depart, there’s something very strange that both of these two acoustic topics have in common: “Shadow” is the name of another Sonic the Hedgehog character. In fact, he’s none other than Sonic’s arch rival…

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Who knew this SEGA game had so many sound-science implications?

*This piece was guest edited by Nicholas R. Nelson. 

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Jeremy Young

Jeremy is a music business guru and loves giving advice to young, emerging bands on how to make their tours more effective. He also plays guitar, publishes audiobooks, runs a record label, and is an artist working in sound media. He has performed and released material throughout Europe, Asia, the US, UK and Canada, mostly with his trio Sontag Shogun.

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