20 hertz to 20 kilohertz: the range of frequencies humans are able to hear. Sound is a natural constant, as essential and expected as the light we see and the air we breathe. From good music to blaring alarms, our daily lives are full of these vital vibrations in familiar forms.
However, there is more to sound than meets the eye (or ear). There are volumes of wonder to be heard in the exploration of acoustics, full of brilliance - and danger.
10. The Visual Microphone
After striking an object, sound is generally absorbed and dissipated through it without any noticeable effect. However, the tiniest of vibrations reverberate across the affected surface at barely tenths of a micrometer - still enough to be captured by a high-speed camera, at lengths less than a hundredth of a pixel. Adobe, Microsoft, and MIT researchers have collaborated to develop a brilliant algorithm that is able to extract information from these microscopic motions and use it to recover the original sound of a silent video. Everyday items from boxes of tissues to glasses of water become visual microphones through the impressive tech. Though significantly blurred, the notes from “Mary Had a Little Lamb” in both music and singing are easily heard from a recording of a bag of chips or the leaves of a potted plant.
The technology is nascent, but there is obviously huge potential in accurately retrieving sound for enhanced surveillance, bypassing the limited range of normal microphones and going beyond soundproof walls (provided they are clear, of course). The scientists who created the visual microphone have less controversial intentions, though, believing its best purpose is to eventually be able to identify structural properties of materials without making contact.
9. DolphinAttack
DolphinAttack is a recently published technology that is able to command speech recognition systems via ultrasound, aka frequencies above 20 kHz, above human hearing (but well within dolphin echolocation). The undetected voice control is confirmed to affect Siri, Cortana, Alexa, and even vocal navigation systems in Audi cars, creating a major security issue. The extent of the invasion is naturally the extent of the functions available, from dialing a particular number to visiting malicious websites and downloading malware.
A DolphinAttack hacker could easily reduce screen brightness, mute volume, or set a device offline through airplane mode to assist in the assault, but fortunately, the distance limit is currently only 1.7 meters (5.6 ft). Surprisingly, the method is fairly low-tech and cheap, involving only an amplifier to increase signal, an ultrasonic transducer for the frequency conversion, and a battery hooked to a speaker of some sort, which could simply be that of one’s own smartphone. As improvements are made on the setup, though, DolphinAttack is something all voice assistants should be prepared for.
8. Infrasound Horror
There is little evidence that common ultrasound affects people, usually only found in unverified reports of dizziness and discomfort. On the other side of the sound spectrum, infrasound also can’t be heard, but at the right frequencies, it evokes much more disturbing psychological and physical sensations. The entire body is affected by these deep vibrations, causing hyperventilation, hallucinations, panic, and depression. At 19 hertz, infrasound resonates with the human eyeball to greatly distort peripheral vision. A tiger’s roar has been shown to emit infrasound at the same frequency, an incredibly useful tool to biologically blind and frighten prey and opponents.
Since infrasound is often found in nature and prevalent in machinery, it may be the explanation for allegedly haunted sites. “The Ghost in the Machine” is the most popular true story of the false paranormality of infrasound. Vic Tandy was a medical engineer who had been hired to work at a laboratory producing life support equipment. Though at first skeptical of the hauntings his colleagues told him about, he, too, began to experience a slight depression and the occasional cold shiver. One night, while Tandy was the only person remaining in the lab, he became aware of a grey, silent figure emerging to his left. Fearing the worst, Tandy turned, only for the apparition to disappear. The following day, he found the source of the ghost to simply be a hidden fan, vibrating at about 19 hertz.
7. Sonic Weaponry
Photo credit: US Navy/Photographer’s Mate 3rd Class Tucker M. Yates
The earliest idea of a sonic weapon was researched in secret by Hitler’s chief architect and minister of armaments and war production, Albert Speer, but it never saw light or sound due to the termination of the Nazi regime. The acoustic cannon would have been able to produce a deafeningly focused, insanely amplified sound beam that could vibrate a person’s body so vigorously that anyone standing within 90 meters (300 ft) would horrifically die if exposed for more than 30 seconds. Since the 1950s, sonic weaponry has undergone serious investigation and development by various military and research organizations for counterterrorism and crowd control uses. Depending on the decibel level, sound waves are capable of causing mild nausea and vomiting, incapacitating pain and vertigo, complete disorientation, or explosion of the target’s eardrums and organs.
As previously mentioned, the horrific effects of infrasound (commonly blasted below patrolling drones) are easily able to scatter riotous crowds or demoralize enemy soldiers, while ultrasound of intense frequency and power is able to burn bodies to death through extreme overheating and fracture entire skeletons through bone resonance. Fortunately, most technology is made for non-lethal, non-permanent damage and debilitation, much like the Taser. The US Army is currently developing a long-range, nonlethal acoustic weapon to repel even the most determined suicide bombers and to rouse Al-Qaeda terrorists from their resonating cavernous hideouts with their hands over their ears.
6. Herring Flatulence
During the Cold War, a Soviet nuclear submarine had intentionally navigated difficult waters to reach Sweden’s most restricted naval areas before it ran aground. The Swedes were not fooled by the Russian government’s claim of a simple navigation accident. Later, in the 1980s, the Swedish Royal Navy detected hostile-sounding underwater noises during the night (high-pitched squeaks and pops) and classified them as Russian submarines. The questionable Soviets denied all accusations while the noises continued to be recorded.
In 1994, Swedish prime minister Carl Bildt wrote an aggravated letter to Boris Yeltsin, complaining about the blatant breach of diplomacy. The sounds were still going although the Soviet Union had collapsed, compelling the Swedes to search for another explanation. As depth charges detonated, only schools of dead herring rose to the surface. The source of their political conflict was Swedish fish.
Herring have the strange behavior of gulping air and anally releasing it, producing sounds scientifically named Fast Repetitive Ticks (FRTs). Evidence concludes that the FRTs are used for communication, especially during nightly gatherings, when they can’t clearly see each other. The massive schools that gather in the Baltic Sea are able to produce a huge amount of these FRTs, enough to make an uncomfortable nation frenzied. Unfortunately, Bildt never had the opportunity to apologize and explain that he had mistaken flatulent herring social parties for a terrorizing Russian nuclear threat.
5. Nightingale Floors
Photo credit: Blondinrikard Froberg/CC BY 2.0
If light travels so fast, how come it’s never caught a ninja? Say no more, because sound already has! The nightingale floors were specially designed as a security measure for important Japanese residences such as Nijo Castle, squeaking loudly to alert guards while also beautifully imitating the chirping of nightingales for daily usage. Dry wooden boards naturally creak under pressure, so Japan’s best carpenters, woodworkers, and craftsmen designed floors that greatly magnified the effect. The nightingale floors made sure that assassins would never have a chance to pass unheard through the corridors of the shogun’s Kyoto residence as well as those of other important leaders and national treasures. The clamps of the floors are rubbed against the flooring nails with every step, screeching and scaring off ninjas.
If the shogun visited the castle, bodyguards would wait in hidden doors until an intruding ninja walked through the floors. The timber technique is so precise that the position of the person could be determined through the volume of the noisy chirping heard. The silent but deadly would suffer a loud death. Nowadays, tourists enjoy employing various means of locomotion to see if they are able to sleuth through the floors. None have been successful.
4. Superior Canal Dehiscence Syndrome
In 1929, biologist Pietro Tullio made pin-sized surgical openings in the semicircular canals of pigeons to research sound-induced disbalance and dizziness. Needless to say, the inner ear is awfully delicate, and after the procedure, the pigeons became heavily disoriented when exposed to loud sound. Even the most minuscule defect causes a condition of absolute horror when it comes to the ear: superior canal dehiscence syndrome. The ordinarily protected canal becomes vulnerable to all kinds of sound, causing crippling pain and sensation. Symptoms include all the worst hearing conditions: tinnitus, hearing loss, oversensitivity, Tullio phenomenon (sound-induced vertigo), and the most terrifying: autophony.
Autophony is the disturbingly loud hearing of a person’s own body, whether involuntary or voluntary. Sufferers are able to hear their heart beating and have reported the mere motion of their eyes moving in their sockets as sounding like sandpaper on wood. Daily life becomes dreadful, with simple tasks such as turning one’s neck to see or walking somewhere experienced as unbearably agonizing. Fortunately, the disorder affects less than one percent of the population, and the defunct superior canal may be plugged during an extensive procedure to retain perfect auditory function.
3. Shepard Scale
Notes are not lone frequencies. Often, several sound waves are subconsciously combined to create a single note. This knowledge allows for the creation of an impressive auditory illusion. A Shepard tone is composed of three sine waves, each separated by an octave. Played in a series, a Shepard scale forms, seemingly forever ascending or descending in pitch, depending on the direction of the scale, in an aural corkscrew effect. The sine waves all rise (or fall) an octave, at which point another sine wave, starting from the position of the previous lowest pitch, is subtly introduced. Since two frequencies are always in the same direction, our brains skip the loop to always hear a continuous, infinite sweep, cherry-picking for continuity.
The impressive auditory illusion is ideal for introducing tension in sound effects and music, famously used in both The Dark Knight ’s intensely roaring Batpod and in Hans Zimmer’s score of the recent war thriller Dunkirk, being a favorite cinematic technique of Christopher Nolan.
2. Sound Barrier
Photo credit: US Navy/Ensign John Gay
During World War II, propeller aircraft experienced dangerously decreased performance as they approached the speed of sound, losing control against overpowering shock waves and often nose-diving into crashes. The term “sound barrier” was coined to define the abrupt increase in drag and presence of unexpected aerodynamic effects suffered by aircraft reaching supersonic speed, rendering the idea of surpassing it extraordinarily difficult and prompting research into the jet engine. Above 1,234 kilometers per hour (767 mph), the air ahead of an aircraft is suddenly pushed away, creating a shock wave of pressure difference that greatly shifts the plane’s center of pressure. After the swift incorporation of the turbojet during the 1950s, aircraft ordinarily entered the supersonic.
The distinction of being a supersonic object is not reserved to vehicles, though. Bullets regularly broke the sound barrier before planes were ever invented, and in modern times, assassins and special forces actually use subsonic bullets instead (in conjunction with silencers) to provide stealthier kills. Even a child can experience a sonic boom by popping an inflated balloon and listening to the torn latex pieces contract at supersonic speed. Finally, it should be noted that the crack of a whip, commonly thought to occur only at the tip, is actually from the wave motion of the traveling loop, gaining speed until breaking the sound barrier and releasing sonic booms for the rest of the distance.
1. Black Hole Decibel
One of the loudest animals in the world is the pistol shrimp, which hunts by shooting jets of water so fast that a bubble is formed against the weight of the ocean, collapsing in microseconds in an epic implosion, producing a sonic shock wave of 210 decibels, louder than a gunshot, and reaching a temperature of 5,000 degrees Kelvin, near that of the surface of the Sun. The loudest recorded event on Earth, heard from 4,800 kilometers (3,000 mi) away, was the 1883 eruption of Krakatoa, which destroyed 70 percent of the volcanic island, caused permanent hearing loss within a 160-kilometer (100 mi) radius, and reverberated through the atmosphere seven times.
However, the final phenomenon of this list is potentially the most colossal, impactful of them all. In 2003, NASA discovered the first, tremendous sound waves of a supermassive black hole, rippling through the gas contained in the Perseus galaxy cluster. The pitch of the note was the deepest detected from anything ever heard in the universe, a cosmic B-flat 57 octaves below middle-C, over one quadrillion times beneath the audible.
That was a sound created from a black hole, but there is also a sound that can create one. If an 1,100-decibel sound could be generated, the intense waves would create enough gravity to rip space-time into a quantum singularity. Instead of colossal amounts of mass gathering together, an even more titanic amount of sound energy would collect to forge a black hole through the cosmic compression of the matter in an area.
None of this information is truly frightening, though, since it is all impossible. There is no medium pressurized enough for an 1,100-decibel sound to originate in and no technology even close to providing the necessary amount of energy. But gathering the extremely dense fluids of an alien atmosphere and having mastered the energy of an entire galaxy, the most advanced civilization could produce this ultimate weapon. Maybe.
Top image: Sound waves of a super-massive black hole (right) residing in the Perseus cluster (left). Credit: NASA/CXC/IoA/A.Fabian et al.
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