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7 sounds in nature that humans rarely hear

BY   November 11, 2015 at 5:28 PM EST
You won't believe the pipes on this one. Koala (Phascolarctos cinereus) sitting in eucalyptus tree. Photo by John Warden/via Getty Images

You won’t believe the pipes on this one. Koala (Phascolarctos cinereus) sitting in eucalyptus tree. Photo by John Warden/via Getty Images

Shhh….be quiet. Do you hear that? It’s all around you. Natural sounds occupy one-fifth of our human senses, and it’s easy to miss the subtler tones. But scientists around the globe have put microphones in clever places to document the lesser known twangs and beats. Here are seven examples. (You may want to use headphones).

The sound of a choking ocean

Red tide bloom of toxic dinoflagellates, like this one in southeast Alaska, can choke the sound from underwater life. Photo by Jeff Foott/via Getty Images

Red tide bloom of toxic dinoflagellates, like this one in southeast Alaska, can choke the sounds from underwater life. Photo by Jeff Foott/via Getty Images

A red tide is usually seen, not heard.

Red tide consists of colorful hazardous algal blooms (HABs) that spawn in coastal seas where excessive organic nutrients from agricultural runoff or human sewage feed the reproduction of microscopic dinoflagellates, a type of phytoplankton. Some of these microorganisms produce neurotoxins that fatally paralyze marine life. At high concentrations, an algal bloom can also decompose and sap the water of oxygen. The result is a dead zone, void of fish and other marine animals.

Such was the case in 2005, when the worst regional red tide in 30 years struck Tampa Bay, Florida. But these toxic algae did more than just paint the water, according to a study published this September in Royal Society Open Science.

This red tide also silenced the water.

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As part of a long-term survey on dolphins, marine biologist Shannon Gowans and ecologist Peter Simard placed audio recorders 15 feet underwater in Tampa Bay. These recorders — called hydrophones — were in place during the red tide summer of 2005, but also during non-red tide summers of 2006, 2011 and 2012. And the team was surprised by the absence of sound near Bunces Pass — a channel connecting the bay to the Gulf of Mexico — among the 2005 recordings.

“Bunces Pass is normally healthy. It’s bordered by very healthy mangroves and sea grasses. There are usually lots of fish, dolphins and marine life,” said Gowans, who works at Eckerd College in St Petersburg, Florida.

The investigation, which was led by Eckerd undergraduate Kate Indeck, examined the calls of three marine species that were silenced by the algae. And the high-frequency claps of snapping shrimp, normally plentiful in the area, were muted, because the shrimp had died off. This marked the first evidence that this species is influenced by algal blooms, according to the authors.

Bunces Pass without red tide in 2006:

Bunces Pass with red tide in 2005:

“There were literally millions of fish dying in various parts of the bay, but no one had looked at the effects on snapping shrimp,” Gowans said. The low-pitched choruses of spotted seatrout were gone too, along with the calls of silver perch.

How to crack the ideal coffee roast

If a coffee bean roasts in the woods, does it make a sound? Photo by Mongkol Nitirojsakul / EyeEm

If a coffee bean roasts in the woods, does it make a sound? Photo by Mongkol Nitirojsakul / EyeEm

If you want to test the acumen of your cafe’s barista, ask them about cracking. Raw coffee beans must be roasted before they can be brewed. The degree of roasting — from light to dark — ultimately dictates the taste and aroma of your morning caffeine boost. Roasters use many signs to monitor the transformation process, such time, color, aroma and temperature. Another option is cracking, a sound emitted when heat splits open a coffee bean. Though roasters have likely relied on this audio cue for decades, in 2014, mechanical engineer Preston Wilson charted the first sound profile for cracking in a drum-based coffee roaster.

As he describes, the first crack occurs when beans reach 392 degrees Fahrenheit, releasing steam and gases. This initial wave of cracking beans lasts two minutes, and is 15 percent louder than the second wave of cracking, he found in his analysis. The cracks sound more like popcorn popping or wet Rice Krispies® cereal than splitting wood. The first cracks also have a lower pitch, “by a factor of nearly 19.”

The second wave comes at 446 degrees Fahrenheit due to additional gas release and internal breaking of the bean. The second crack happens at a faster pace than the first crack, Wilson found. “The first crack has a peak rate of about 100 cracks per minute, while second crack has a peak rate of over 500 cracks per minute,” he wrote.

The rate of individual cracking events for both first crack and second crack as a function of time during the roast. Cracks were tallied within time intervals of 15 seconds and 5  seconds for first and second crack choruses, respectively. Photo by J. Acoust. Soc. Am. 135, EL265 (2014)

The rate of individual cracking events for both first crack and second crack as a function of time during the roast. Cracks were tallied within time intervals of 15 seconds and 5  seconds for first and second crack choruses, respectively. Photo by J. Acoust. Soc. Am. 135, EL265 (2014)

By creating this profile, Wilson laid the foundation for building an auditory-based software app for measuring the perfect roast.

Did that koala just burp or hiccup?

Koala bellows sound like a person coughing, hiccuping and burping at the same time. But research by zoologist Benjamin Charlton has shown these boisterous noises say a lot about a koala’s personality. Much like human voices, male koalas sound highly individualistic, and in 2011, Charlton reported that computer analysis of 287 bellows from 20 males could accurately identify an individual 87 percent of the time.

He published a female-centric sequel last month, which showed that lady calls are also individualistic and change as they age. The fundamental frequency of female bellows sped up or became higher pitched with age. But other female noises — squeaks, wails, squawks and screams — did not change with age. Overall, male bellows lasted longer than female ones. Female voices had faster frequencies, meaning higher pitches.

No word yet on the prevalence of vocal fry.

The burps and moans of sand

Sliding down on the seat of your pants and creating a large sand avalanche on the  656-foot-high Eureka Dune in Death Valley National Park, Calif.  Photo by Nathalie Vriend

Sliding down on the seat of your pants and creating a large sand avalanche on the 656-foot-high Eureka Dune in Death Valley National Park, Calif. Photo by Nathalie Vriend

Next time you’re at the beach, traipsing along a sand dune, keep in mind that each step might be creating a little chorus of Gregorian chants.

Physicists from the California Institute of Technology and the University of Cambridge recorded these groans by visiting huge sand mounds in Death Valley National Park and the Mojave Desert. Looming more than 30 feet tall, these dunes experience avalanches that can last for minutes at a time. Geomechanicist Nathalie Vriend and her colleagues decided to record these avalanches using geophones, devices that measure the velocity particles being pushed by sound waves in the earth or underwater.

“The waves travelling through the dune move individual grains of sand, which exert a force on the geophone that we use for measurements,” Vriend told AIP News. To start avalanches, she and her colleagues would slide down dunes on their butts or run a hand through the loose earth.

The avalanches produced a mixture of harmonies, but in their analysis, the group paid special attention to booming and burping noises.


Avalanching sand from dune faces in the Mojave Desert can trigger loud, rumbling “booming” or short bursts of “burping” sounds that resemble a tuned musical instrument. Courtesy of Nathalie M. Vriend/California Institute of Technology/University of Cambridge

Though related, the two sound effects originated from different physical properties of sand. The burps started first, within the first three seconds, and were caused by Rayleigh waves. Rayleigh waves travel slowly on top of a solid surface and are often the last seismic vibrations felt during earthquakes (They also cause the most damage). In this scenario, the Rayleigh waves were nonlinear, meaning the early waves didn’t match the late waves in terms of amplitude and speed. This has to do with the composition of individual grains of sand. The burps also exhibited dispersion, or the tendency to break down from a complex noise into basic pitches and frequencies. (Fun fact: You can make burps at home by shaking sand from a desert dune inside of a jar.)

Climbing with heavy field equipment to the top of the 656-foot-high Eureka Dune in Death Valley National Park, Calif. Photo by Nathalie Vriend

Climbing with heavy field equipment to the top of the 656-foot-high Eureka Dune in Death Valley National Park, Calif. Photo by Nathalie Vriend

Booms came later relative to burps, 15 to 17 seconds after the avalanche starting point. Booms were primarily composed of fast-moving P-waves that traveled internally through the sand dune. During an earthquake, P-waves are the first to reach and be recorded by a seismograph. In contrast to the wave responsible for burps, these P-waves were linear and non-dispersive, showing a more regular and harmonious pattern. Slower S-waves were also detected, but faintly.

“A blow of a hammer on a plate triggered a natural resonance — around the booming frequency — inside the dune, which is something we’ve never seen described in literature,” said Vriend, who now works at the University of Cambridge.

These dual characteristics — burps and booms — suggest that the sound produced by large dune avalanches lands somewhere in between that of a kid kicking sand at the beach and the tectonic shifts that cause major disasters. In an earlier study, Vriend and her colleagues found that smaller dunes only emit burps, while earthquakes produce a spectrum of P-, S-, Rayleigh and other waves. Large dune booms are largely confined to arid settings, and likely aren’t responsible for the mysterious booms sometimes felt along the Carolinas.

Earthquakes across the decades

Speaking of earthquakes, did you know that most seismic waves are rarely heard by humans? P-waves can create a rumble if they move rocks along the Earth’s surface, but most seismic waves register at frequencies below 20 Hertz, which can’t be perceived by human ears, according to the U.S. Geological Survey. Most sound heard during an earthquake comes from stuff falling or moving buildings.

Structural and earthquake engineer Karl Steinbrugge archived some of the earliest recordings of earthquake-related noise in a 1974 publication in the journal, Bulletin of the Seismological Society of America. The backstories for these recordings echo their serendipitous nature. The earthquake audio was captured during church sermons, pay phone calls, court depositions and studio recording sessions.

Since seismic waves permeating through the Earth’s crust are often inaudible, scientists have been known to take a seismograph recording and increase its speed until it falls into an audible range. Take this example from the 1992 7.3 M Landers earthquake:

Other scientific sensors can unexpectedly capture earthquakes too. Such was the case when a hydrophone captured this underwater recording of the 2011 Tōhoku earthquake:

[You may want to turn down your earbuds]

Singing Spiders

If you need a Ringo Starr for your jam band, call the “purring” wolf spider, Gladicosa gulosa. Many species of male wolf spiders are known for their mating calls, which feature leg vibrations against a piece of woodland debris, like a dead leaf. But G. gulosa wolf spiders play a slightly different tune, according to study presented in May by researchers at the University of Cincinnati.

The spiders don’t only beat against a “drum” but they also produce an accompanying vibration, like a backup vocal, that floats through the air.

However, the lead author of the research, biologist Alexander L. Sweger, isn’t sure if these secondary vibrations’ are intentional.

“They’re quiet — nothing on the order of crickets,” Sweger told Live Science. “We think this airborne sound is primarily a byproduct. As far as we can tell, they may not deliberately be producing a sound.”


Jumping spiders, like Habronattuscoecatus species, make vibrating love calls too. h/t The Nerdist

A whale song for climate change

For a sad song from a whale, talk to oceanographer Kathleen Stafford from the University of Washington. Stafford’s team has collected acoustic recordings of whale songs to track their migratory habits at the boundary of the Pacific and Arctic oceans. Over a five-year span, they’ve secured hydrophones on moorings strategically placed along this oceanic border to catch the calls of seals, walruses as well as fin, humpback, bowhead and killer whales.

“This passive acoustic monitoring technique allows us to detect the presence of vocalizing marine mammals continuously — 24 hours per day — in all weather conditions, over periods of weeks to months, over distances of 20 to 30 kilometers, and is a proven sampling method in the waters offshore Alaska,” Stafford said in a statement.

Whale songs may reveal Arctic territorial battles between bowhead whales (Balaena mysticetus; pictured) and summertime freeloaders.  Photo by Michael Nolan/via Getty Images

Whale songs may reveal Arctic territorial battles between bowhead whales (Balaena mysticetus; pictured) and summertime freeloaders. Photo by Michael Nolan/via Getty Images

They’ve documented a climate-based shift in whale habits. Male humpback whales sing throughout the autumn in the Chukchi Sea, located north of the Bering Strait. This behavior is typically only observed at their tropical breeding grounds and comes in parallel with the humpbacks extending their summer stays in Arctic into the fall. Fin whales, another whale that summers in the Arctic, acted in kind and stayed longer than normal. Killer whale calls were recorded irregularly.

“Summer whales have always occurred north of Bering Strait, although not in great numbers, and not in September, October, or November, when we hear them now,” Stafford said.

Winter-loving bowhead whales arrived in the fall, after the departure of the summer whales, but as Arctic ice melts due to global warming, Stafford projects more overlap between the seasonal species. As a result, the bowhead could face more competition for food and territory.

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