This exoplanet - as astronomers call planets around a star other than the Sun - is the smallest ever found up to now [1] and it completes a full orbit in 13 days. It is 14 times closer to its star than the Earth is from the Sun. However, given that its host star, the red dwarf Gliese 581 [2], is smaller and colder than the Sun - and thus less luminous - the planet nevertheless lies in the habitable zone, the region around a star where water could be liquid! The planet's name is Gliese 581 c.
"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explains Stéphane Udry, from the Geneva Observatory (Switzerland) and lead-author of the paper reporting the result. "Moreover, its radius should be only 1.5 times the Earth's radius, and models predict that the planet should be either rocky - like our Earth - or fully covered with oceans," he adds.
"Liquid water is critical to life as we know it," avows Xavier Delfosse, a member of the team from Grenoble University (France). "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."
The host star, Gliese 581, is among the 100 closest stars to us, located only 20.5 light-years away in the constellation Libra ("the Scales"). It has a mass of only one third the mass of the Sun. Such red dwarfs are intrinsically at least 50 times fainter than the Sun and are the most common stars in our Galaxy: among the 100 closest stars to the Sun, 80 belong to this class.
"Red dwarfs are ideal targets for the search for low-mass planets where water could be liquid. Because such dwarfs emit less light, the habitable zone is much closer to them than it is around the Sun," emphasizes Xavier Bonfils, a co-worker from Lisbon University. Planets lying in this zone are then more easily detected with the radial-velocity method [3], the most successful in detecting exoplanets.
Two years ago, the same team of astronomers already found a planet around Gliese 581 (see ESO 30/05). With a mass of 15 Earth-masses, i.e. similar to that of Neptune, it orbits its host star in 5.4 days. At the time, the astronomers had already seen hints of another planet. They therefore obtained a new set of measurements and found the new super-Earth, but also clear indications for another one, an 8 Earth-mass planet completing an orbit in 84 days. The planetary system surrounding Gliese 581 contains thus no fewer than 3 planets of 15 Earth masses or less, and as such is a quite remarkable system.
The discovery was made thanks to HARPS (High Accuracy Radial Velocity for Planetary Searcher), perhaps the most precise spectrograph in the world. Located on the ESO 3.6-m telescope at La Silla, Chile, HARPS is able to measure velocities with a precision better than one metre per second (or 3.6 km/h)! HARPS is one of the most successful instruments for detecting exoplanets and holds already several recent records, including the discovery of another 'Trio of Neptunes' (ESO 18/06, see also ESO 22/04).
The detected velocity variations are between 2 and 3 metres per second, corresponding to about 9 km/h! That's the speed of a person walking briskly. Such tiny signals could not have been distinguished from 'simple noise' by most of today's available spectrographs.
"HARPS is a unique planet hunting machine," says Michel Mayor, from Geneva Observatory, and HARPS Principal Investigator. "Given the incredible precision of HARPS, we have focused our effort on low-mass planets. And we can say without doubt that HARPS has been very successful: out of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS!"
HARPS is also very efficient in finding planetary systems, where tiny signals have to be uncovered. The two systems known to have three low mass planets - HD 69830 and Gl 581 - were discovered by HARPS.
"And we are confident that, given the results obtained so far, finding a planet with the mass of the Earth around a red dwarf is within reach," affirms Mayor.
More Information
This research is reported in a paper submitted as a Letter to the Editor of Astronomy and Astrophysics ("The HARPS search for southern extra-solar planets : XI. An habitable super-Earth (5 MEarth) in a 3-planet system", by S. Udry et al.) The paper is available from http://obswww.unige.ch/~udry/udry_preprint.pdf.
The team is composed of Stéphane Udry, Michel Mayor, Christophe Lovis, Francesco Pepe, and Didier Queloz (Geneva Observatory, Switzerland), Xavier Bonfils (Lisbonne Observatory, Portugal), Xavier Delfosse, Thierry Forveille, and C.Perrier (LAOG, Grenoble, France), François Bouchy (Institut d'Astrophysique de Paris, France), and Jean-Luc Bertaux (Service d'Aéronomie du CNRS, France)
Notes
[1]: Using the radial velocity method, astronomers can only obtain a minimum mass (as it is multiplied by the sine of the inclination of the orbital plane to the line of sight, which is unknown). From a statistical point of view, this is however often close to the real mass of the system. Two other systems have a mass close to this. The icy planet around OGLE-2005-BLG-390L, discovered by microlensing with a network of telescopes including one at La Silla (ESO 03/06), has a (real) mass of 5.5 Earth masses. It, however, orbits much farther from its small host star than the present one and is hence much colder. The other is one of the planets surrounding the star Gliese 876. It has a minimum mass of 5.89 Earth masses (and a probable real mass of 7.53 Earth masses) and completes an orbit in less than 2 days, making it too hot for liquid water to be present.
[2]: Gl 581, or Gliese 581, is the 581th entry in the Gliese Catalogue, which lists all known stars within 25 parsecs (81.5 light years) of the Sun. It was originally compiled by Gliese and published in 1969, and later updated by Gliese and Jahreiss in 1991.
[3]: This fundamental observational method is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. The evaluation of the measured velocity variations allows deducing the planet's orbit, in particular the period and the distance from the star, as well as a minimum mass.
Contact
Stéphane Udry, Michel Mayor
Observatory of Geneva University, Switzerland
Phone: +41 22 379 22 00
Email: Stephane.Udry (at) obs.unige.ch, Michel.Mayor (at) obs.unige.ch
Xavier Delfosse, Thierry Forveille
LAOG, France
Phone: +33 476 51 42 06
Email: Xavier.Delfosse (at) obs.ujf-grenoble.fr, Thierry.Forveille (at) obs.ujf-grenoble.fr
Xavier Bonfils
Lisbonne Observatory, Portugal
Phone: +351 21 361 67 43
Email: xavier.bonfils (at) oal.ul.pt
The Dwarf Carried Other Worlds Too!
Astronomers have discovered the most Earth-like planet outside our Solar System to date, an exoplanet with a radius only 50% larger than the Earth and capable of having liquid water. Using the ESO 3.6-m telescope, a team of Swiss, French and Portuguese scientists discovered a super-Earth about 5 times the mass of the Earth that orbits a red dwarf, already known to harbour a Neptune-mass planet. The astronomers have also strong evidence for the presence of a third planet with a mass about 8 Earth masses.
Friday, April 27, 2007
Wednesday, April 25, 2007
Potentially habitable planet found
WASHINGTON - For the first time astronomers have discovered a planet outside our solar system that is potentially habitable, with Earth-like temperatures, a find researchers described Tuesday as a big step in the search for "life in the universe."
The planet is just the right size, might have water in liquid form, and in galactic terms is relatively nearby at 120 trillion miles away. But the star it closely orbits, known as a "red dwarf," is much smaller, dimmer and cooler than our sun.
There's still a lot that is unknown about the new planet, which could be deemed inhospitable to life once more is known about it. And it's worth noting that scientists' requirements for habitability count Mars in that category: a size relatively similar to Earth's with temperatures that would permit liquid water. However, this is the first outside our solar system that meets those standards.
"It's a significant step on the way to finding possible life in the universe," said University of Geneva astronomer Michel Mayor, one of 11 European scientists on the team that found the planet. "It's a nice discovery. We still have a lot of questions."
The results of the discovery have not been published but have been submitted to the journal Astronomy and Astrophysics.
Alan Boss, who works at the Carnegie Institution of Washington where a U.S. team of astronomers competed in the hunt for an Earth-like planet, called it "a major milestone in this business."
The planet was discovered by the European Southern Observatory's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths. Those wobbles can reveal the existence of other worlds.
What they revealed is a planet circling the red dwarf star, Gliese 581. Red dwarfs are low-energy, tiny stars that give off dim red light and last longer than stars like our sun. Until a few years ago, astronomers didn't consider these stars as possible hosts of planets that might sustain life.
The discovery of the new planet, named 581 c, is sure to fuel studies of planets circling similar dim stars. About 80 percent of the stars near Earth are red dwarfs.
The new planet is about five times heavier than Earth. Its discoverers aren't certain if it is rocky like Earth or if its a frozen ice ball with liquid water on the surface. If it is rocky like Earth, which is what the prevailing theory proposes, it has a diameter about 1 1/2 times bigger than our planet. If it is an iceball, as Mayor suggests, it would be even bigger.
Based on theory, 581 c should have an atmosphere, but what's in that atmosphere is still a mystery and if it's too thick that could make the planet's surface temperature too hot, Mayor said.
However, the research team believes the average temperature to be somewhere between 32 and 104 degrees and that set off celebrations among astronomers.
Until now, all 220 planets astronomers have found outside our solar system have had the "Goldilocks problem." They've been too hot, too cold or just plain too big and gaseous, like uninhabitable Jupiter.
The new planet seems just right — or at least that's what scientists think.
"This could be very important," said NASA astrobiology expert Chris McKay, who was not part of the discovery team. "It doesn't mean there is life, but it means it's an Earth-like planet in terms of potential habitability."
Eventually astronomers will rack up discoveries of dozens, maybe even hundreds of planets considered habitable, the astronomers said. But this one — simply called "c" by its discoverers when they talk among themselves — will go down in cosmic history as No. 1.
Besides having the right temperature, the new planet is probably full of liquid water, hypothesizes Stephane Udry, the discovery team's lead author and another Geneva astronomer. But that is based on theory about how planets form, not on any evidence, he said.
"Liquid water is critical to life as we know it," co-author Xavier Delfosse of Grenoble University in France, said in a statement. "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."
Other astronomers cautioned it's too early to tell whether there is water.
"You need more work to say it's got water or it doesn't have water," said retired NASA astronomer Steve Maran, press officer for the American Astronomical Society. "You wouldn't send a crew there assuming that when you get there, they'll have enough water to get back."
The new planet's star system is a mere 20.5 light years away, making Gliese 581 one of the 100 closest stars to Earth. It's so dim, you can't see it without a telescope, but it's somewhere in the constellation Libra, which is low in the southeastern sky during the midevening in the Northern Hemisphere.
"I expect there will be planets like Earth, but whether they have life is another question," said renowned astrophysicist Stephen Hawking in an interview with The Associated Press in Orlando. "We haven't been visited by little green men yet."
Before you book your extrastellar flight to 581 c, a few caveats about how alien that world probably is: Anyone sitting on the planet would get heavier quickly, and birthdays would add up fast since it orbits its star every 13 days.
Gravity is 1.6 times as strong as Earth's so a 150-pound person would feel like 240 pounds.
But oh, the view. The planet is 14 times closer to the star it orbits. Udry figures the red dwarf star would hang in the sky at a size 20 times larger than our moon. And it's likely, but still not known, that the planet doesn't rotate, so one side would always be sunlit and the other dark.
Distance is another problem. "We don't know how to get to those places in a human lifetime," Maran said.
Two teams of astronomers, one in Europe and one in the United States, have been racing to be the first to find a planet like 581 c outside the solar system.
The European team looked at 100 different stars using a tool called HARPS (High Accuracy Radial Velocity for Planetary Searcher) to find this one planet, said Xavier Bonfils of the Lisbon Observatory, one of the co-discoverers.
Much of the effort to find Earth-like planets has focused on stars like our sun with the challenge being to find a planet the right distance from the star it orbits. About 90 percent of the time, the European telescope focused its search more on sun-like stars, Udry said.
A few weeks before the European discovery earlier this month, a scientific paper in the journal Astrobiology theorized a few days that red dwarf stars were good candidates.
"Now we have the possibility to find many more," Bonfils said.
By SETH BORENSTEIN, AP Science Writer
The planet is just the right size, might have water in liquid form, and in galactic terms is relatively nearby at 120 trillion miles away. But the star it closely orbits, known as a "red dwarf," is much smaller, dimmer and cooler than our sun.
There's still a lot that is unknown about the new planet, which could be deemed inhospitable to life once more is known about it. And it's worth noting that scientists' requirements for habitability count Mars in that category: a size relatively similar to Earth's with temperatures that would permit liquid water. However, this is the first outside our solar system that meets those standards.
"It's a significant step on the way to finding possible life in the universe," said University of Geneva astronomer Michel Mayor, one of 11 European scientists on the team that found the planet. "It's a nice discovery. We still have a lot of questions."
The results of the discovery have not been published but have been submitted to the journal Astronomy and Astrophysics.
Alan Boss, who works at the Carnegie Institution of Washington where a U.S. team of astronomers competed in the hunt for an Earth-like planet, called it "a major milestone in this business."
The planet was discovered by the European Southern Observatory's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths. Those wobbles can reveal the existence of other worlds.
What they revealed is a planet circling the red dwarf star, Gliese 581. Red dwarfs are low-energy, tiny stars that give off dim red light and last longer than stars like our sun. Until a few years ago, astronomers didn't consider these stars as possible hosts of planets that might sustain life.
The discovery of the new planet, named 581 c, is sure to fuel studies of planets circling similar dim stars. About 80 percent of the stars near Earth are red dwarfs.
The new planet is about five times heavier than Earth. Its discoverers aren't certain if it is rocky like Earth or if its a frozen ice ball with liquid water on the surface. If it is rocky like Earth, which is what the prevailing theory proposes, it has a diameter about 1 1/2 times bigger than our planet. If it is an iceball, as Mayor suggests, it would be even bigger.
Based on theory, 581 c should have an atmosphere, but what's in that atmosphere is still a mystery and if it's too thick that could make the planet's surface temperature too hot, Mayor said.
However, the research team believes the average temperature to be somewhere between 32 and 104 degrees and that set off celebrations among astronomers.
Until now, all 220 planets astronomers have found outside our solar system have had the "Goldilocks problem." They've been too hot, too cold or just plain too big and gaseous, like uninhabitable Jupiter.
The new planet seems just right — or at least that's what scientists think.
"This could be very important," said NASA astrobiology expert Chris McKay, who was not part of the discovery team. "It doesn't mean there is life, but it means it's an Earth-like planet in terms of potential habitability."
Eventually astronomers will rack up discoveries of dozens, maybe even hundreds of planets considered habitable, the astronomers said. But this one — simply called "c" by its discoverers when they talk among themselves — will go down in cosmic history as No. 1.
Besides having the right temperature, the new planet is probably full of liquid water, hypothesizes Stephane Udry, the discovery team's lead author and another Geneva astronomer. But that is based on theory about how planets form, not on any evidence, he said.
"Liquid water is critical to life as we know it," co-author Xavier Delfosse of Grenoble University in France, said in a statement. "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life. On the treasure map of the Universe, one would be tempted to mark this planet with an X."
Other astronomers cautioned it's too early to tell whether there is water.
"You need more work to say it's got water or it doesn't have water," said retired NASA astronomer Steve Maran, press officer for the American Astronomical Society. "You wouldn't send a crew there assuming that when you get there, they'll have enough water to get back."
The new planet's star system is a mere 20.5 light years away, making Gliese 581 one of the 100 closest stars to Earth. It's so dim, you can't see it without a telescope, but it's somewhere in the constellation Libra, which is low in the southeastern sky during the midevening in the Northern Hemisphere.
"I expect there will be planets like Earth, but whether they have life is another question," said renowned astrophysicist Stephen Hawking in an interview with The Associated Press in Orlando. "We haven't been visited by little green men yet."
Before you book your extrastellar flight to 581 c, a few caveats about how alien that world probably is: Anyone sitting on the planet would get heavier quickly, and birthdays would add up fast since it orbits its star every 13 days.
Gravity is 1.6 times as strong as Earth's so a 150-pound person would feel like 240 pounds.
But oh, the view. The planet is 14 times closer to the star it orbits. Udry figures the red dwarf star would hang in the sky at a size 20 times larger than our moon. And it's likely, but still not known, that the planet doesn't rotate, so one side would always be sunlit and the other dark.
Distance is another problem. "We don't know how to get to those places in a human lifetime," Maran said.
Two teams of astronomers, one in Europe and one in the United States, have been racing to be the first to find a planet like 581 c outside the solar system.
The European team looked at 100 different stars using a tool called HARPS (High Accuracy Radial Velocity for Planetary Searcher) to find this one planet, said Xavier Bonfils of the Lisbon Observatory, one of the co-discoverers.
Much of the effort to find Earth-like planets has focused on stars like our sun with the challenge being to find a planet the right distance from the star it orbits. About 90 percent of the time, the European telescope focused its search more on sun-like stars, Udry said.
A few weeks before the European discovery earlier this month, a scientific paper in the journal Astrobiology theorized a few days that red dwarf stars were good candidates.
"Now we have the possibility to find many more," Bonfils said.
By SETH BORENSTEIN, AP Science Writer
Friday, April 20, 2007
Sun's Atmosphere Sings
Astronomers have recorded heavenly music bellowed out by the Sun's atmosphere.
Snagging orchestra seats for this solar symphony would be fruitless, however, as the frequency of the sound waves is below the human hearing threshold. While humans can make out sounds between 20 and 20,000 hertz, the solar sound waves are on the order of milli-hertz--a thousandth of a hertz.
The study, presented this week at the Royal Astronomical Society's National Astronomy Meeting in Lancashire, England, reveals that the looping magnetic fields along the Sun's outer regions, called the corona, carry magnetic sound waves in a similar manner to musical instruments such as guitars or pipe organs.
Making music
Robertus von Fay-Siebenburgen of the Solar Physics and Space Plasma Research Center at the University of Sheffield and his colleagues combined information gleaned from sun-orbiting satellites with theoretical models of solar processes, such as coronal mass ejections.
They found that explosive events at the Sun's surface appear to trigger acoustic waves that bounce back and forth between both ends of the loops, a phenomenon known as a standing wave.
"These magnetic loops are analogous to a simple guitar string," von Fay-Siebenburgen explained. "If you pluck a guitar string, you will hear the music."
In the cosmic equivalent of a guitar pick, so-called microflares at the base of loops could be plucking the magnetic loops and setting the sound waves in motion, the researchers speculate. While solar flares are the largest explosions in the solar system, microflares are a million times smaller but much more frequent; both phenomena are now thought to funnel heat into the Sun's outer atmosphere.
The acoustic waves can be extremely energetic, reaching heights of tens of miles, and can travel at rapid speeds of 45,000 to 90,000 miles per hour. "These [explosions] release energy equivalent to millions of hydrogen bombs," von Fay-Siebenburgen said.
"These energies are plucking these magnetic strings or standing pipes, which set up standing waves--exactly the same waves you see on a guitar string," von Fay-Siebenburgen told SPACE.com. The "sound booms" decay to silence in less than an hour, dissipating in the hot solar corona.
Solar physics
The musical finding could help explain why the Sun's corona is so hot.
While the Sun's surface is a steamy 10,000 degrees Fahrenheit (5,538 degrees Celsius), plasma gas in the corona soars to more than 100 times hotter.
"How can the atmosphere above the surface of the Sun be hotter if nuclear fusion happens inside the Sun?" von Fay-Siebenburgen said. If astronomers can get a clearer picture of what's going on inside these magnetic loops in the Sun's atmosphere, they have a better chance of finding the answer.
Another recent study using images from Hinode's telescope revealed twisted magnetic fields along the Sun's surface, which store huge amounts of energy. The magnetic fields can snap like a rubber band; when they do, they might release energy that could heat up the corona or power solar eruptions and coronal mass ejections, the researchers say.
Video: Sun Storms
Images: Solar Flares
Twisted Solution to Sun's Mystery Heat
Original Story: Sun's Atmosphere Sings
Visit SPACE.com and explore our huge collection of Space Pictures, Space Videos, Space Image of the Day, Hot Topics, Top 10s, Multimedia, Trivia, Voting and Amazing Images. Follow the latest developments in the search for life in our universe in our SETI: Search for Life section. Join the community, sign up for our free daily email newsletter, listen to our Podcasts, check out our RSS feeds and other Reader Favorites today!
Jeanna BrynerStaff WriterSPACE.com
Snagging orchestra seats for this solar symphony would be fruitless, however, as the frequency of the sound waves is below the human hearing threshold. While humans can make out sounds between 20 and 20,000 hertz, the solar sound waves are on the order of milli-hertz--a thousandth of a hertz.
The study, presented this week at the Royal Astronomical Society's National Astronomy Meeting in Lancashire, England, reveals that the looping magnetic fields along the Sun's outer regions, called the corona, carry magnetic sound waves in a similar manner to musical instruments such as guitars or pipe organs.
Making music
Robertus von Fay-Siebenburgen of the Solar Physics and Space Plasma Research Center at the University of Sheffield and his colleagues combined information gleaned from sun-orbiting satellites with theoretical models of solar processes, such as coronal mass ejections.
They found that explosive events at the Sun's surface appear to trigger acoustic waves that bounce back and forth between both ends of the loops, a phenomenon known as a standing wave.
"These magnetic loops are analogous to a simple guitar string," von Fay-Siebenburgen explained. "If you pluck a guitar string, you will hear the music."
In the cosmic equivalent of a guitar pick, so-called microflares at the base of loops could be plucking the magnetic loops and setting the sound waves in motion, the researchers speculate. While solar flares are the largest explosions in the solar system, microflares are a million times smaller but much more frequent; both phenomena are now thought to funnel heat into the Sun's outer atmosphere.
The acoustic waves can be extremely energetic, reaching heights of tens of miles, and can travel at rapid speeds of 45,000 to 90,000 miles per hour. "These [explosions] release energy equivalent to millions of hydrogen bombs," von Fay-Siebenburgen said.
"These energies are plucking these magnetic strings or standing pipes, which set up standing waves--exactly the same waves you see on a guitar string," von Fay-Siebenburgen told SPACE.com. The "sound booms" decay to silence in less than an hour, dissipating in the hot solar corona.
Solar physics
The musical finding could help explain why the Sun's corona is so hot.
While the Sun's surface is a steamy 10,000 degrees Fahrenheit (5,538 degrees Celsius), plasma gas in the corona soars to more than 100 times hotter.
"How can the atmosphere above the surface of the Sun be hotter if nuclear fusion happens inside the Sun?" von Fay-Siebenburgen said. If astronomers can get a clearer picture of what's going on inside these magnetic loops in the Sun's atmosphere, they have a better chance of finding the answer.
Another recent study using images from Hinode's telescope revealed twisted magnetic fields along the Sun's surface, which store huge amounts of energy. The magnetic fields can snap like a rubber band; when they do, they might release energy that could heat up the corona or power solar eruptions and coronal mass ejections, the researchers say.
Video: Sun Storms
Images: Solar Flares
Twisted Solution to Sun's Mystery Heat
Original Story: Sun's Atmosphere Sings
Visit SPACE.com and explore our huge collection of Space Pictures, Space Videos, Space Image of the Day, Hot Topics, Top 10s, Multimedia, Trivia, Voting and Amazing Images. Follow the latest developments in the search for life in our universe in our SETI: Search for Life section. Join the community, sign up for our free daily email newsletter, listen to our Podcasts, check out our RSS feeds and other Reader Favorites today!
Jeanna BrynerStaff WriterSPACE.com
Friday, April 06, 2007
Mystery of Greek Amphitheater's Amazing Sound Finally Solved
Cut the chatter! The ancient mystery surrounding the great acoustics of the theater at Epidaurus in Greece has been solved.
The theater, dating to the 4th century B.C. and arranged in 55 semi-circular rows, remains the great masterwork of Polykleitos the Younger. Audiences of up to an estimated 14,000 have long been able to hear actors and musicians—unamplified—from even the back row of the architectural masterpiece.
How this sonic quality was achieved has been the source of academic and amateur speculation, with some theories suggesting that prevailing winds carried sounds or masks amplified voices.
It's in the seats
Now, researchers at the Georgia Institute of Technology have discovered that the limestone material of the seats provide a filtering effect, suppressing low frequencies of voices, thus minimizing background crowd noise. Further, the rows of limestone seats reflect high-frequencies back towards the audience, enhancing the effect.
Researcher Nico Declercq, a mechanical engineer, initially suspected that the slope of the theater had something to do with the effect.
“When I first tackled this problem, I thought that the effect of the splendid acoustics was due to surface waves climbing the theater with almost no damping,” Declercq said. “While the voices of the performers were being carried, I didn’t anticipate that the low frequencies of speech were also filtered out to some extent.”
However, experiments with ultrasonic waves and numerical models indicated that frequencies up to 500 hertz (cycles per second) were lowered, and frequencies higher than 500 hertz went undiminished, he said.
Acoustic traps
The corrugations on the surface of the seats act as natural acoustic traps. Though this effect would seem to also remove the low frequencies from the actors' voices, listeners actually fill in the missing portion of the audio spectrum through a phenomenon known as virtual pitch. The human brain reconstructs the missing frequencies, producing the virtual pitch phenomenon, as in listening to someone speaking on a telephone with no low end.
The findings are detailed in the April issue of the Journal of the Acoustics Society of America.
Amazingly, the Greek builders of the theater did not themselves understand the principles that led to the exceptional audibility of sound from the stage.
Attempts to recreate the Epidaurus design never quite matched the original. Later seating arrangements featured other materials, such as wood for the benches, an approach which may have ultimately derailed the design duplication effort.
Music Tickles Strong Memories
Singing Sand Dunes: The Mystery of Desert Music
Top 10 Ancient Capitals
Original Story: Mystery of Greek Amphitheater's Amazing Sound Finally Solved
Tom ChaoLiveScience Staff WriterLiveScience.com
The theater, dating to the 4th century B.C. and arranged in 55 semi-circular rows, remains the great masterwork of Polykleitos the Younger. Audiences of up to an estimated 14,000 have long been able to hear actors and musicians—unamplified—from even the back row of the architectural masterpiece.
How this sonic quality was achieved has been the source of academic and amateur speculation, with some theories suggesting that prevailing winds carried sounds or masks amplified voices.
It's in the seats
Now, researchers at the Georgia Institute of Technology have discovered that the limestone material of the seats provide a filtering effect, suppressing low frequencies of voices, thus minimizing background crowd noise. Further, the rows of limestone seats reflect high-frequencies back towards the audience, enhancing the effect.
Researcher Nico Declercq, a mechanical engineer, initially suspected that the slope of the theater had something to do with the effect.
“When I first tackled this problem, I thought that the effect of the splendid acoustics was due to surface waves climbing the theater with almost no damping,” Declercq said. “While the voices of the performers were being carried, I didn’t anticipate that the low frequencies of speech were also filtered out to some extent.”
However, experiments with ultrasonic waves and numerical models indicated that frequencies up to 500 hertz (cycles per second) were lowered, and frequencies higher than 500 hertz went undiminished, he said.
Acoustic traps
The corrugations on the surface of the seats act as natural acoustic traps. Though this effect would seem to also remove the low frequencies from the actors' voices, listeners actually fill in the missing portion of the audio spectrum through a phenomenon known as virtual pitch. The human brain reconstructs the missing frequencies, producing the virtual pitch phenomenon, as in listening to someone speaking on a telephone with no low end.
The findings are detailed in the April issue of the Journal of the Acoustics Society of America.
Amazingly, the Greek builders of the theater did not themselves understand the principles that led to the exceptional audibility of sound from the stage.
Attempts to recreate the Epidaurus design never quite matched the original. Later seating arrangements featured other materials, such as wood for the benches, an approach which may have ultimately derailed the design duplication effort.
Music Tickles Strong Memories
Singing Sand Dunes: The Mystery of Desert Music
Top 10 Ancient Capitals
Original Story: Mystery of Greek Amphitheater's Amazing Sound Finally Solved
Tom ChaoLiveScience Staff WriterLiveScience.com
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