Universe Today

October 20, 2010December 24, 2015 by Jason Rhian

NASA’s Ames Director Announces “100 Year Starship”

NASA's Ames Center Director, Simon "Pete" Worden has announced that development of next-generation propulsion technologies are underway. Image Credit: NASA

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The Director of NASA’s Ames Center, Pete Worden has announced an initiative to move space flight to the next level. This plan, dubbed the “Hundred Year Starship,” has received $100,000 from NASA and $ 1 million from the Defense Advanced Research Projects Agency (DARPA). He made his announcement on Oct. 16. Worden is also hoping to include wealthy investors in the project. NASA has yet to provide any official details on the project.

Worden also has expressed his belief that the space agency was now directed toward settling other planets. However, given the fact that the agency has been redirected toward supporting commercial space firms, how this will be achieved has yet to be detailed. Details that have been given have been vague and in some cases contradictory.

The Ames Director went on to expound how these efforts will seek to emulate the fictional starships seen on the television show Star Trek. He stated that the public could expect to see the first prototype of a new propulsion system within the next few years. Given that NASA’s FY 2011 Budget has had to be revised and has yet to go through Appropriations, this time estimate may be overly-optimistic.

One of the ideas being proposed is a microwave thermal propulsion system. This form of propulsion would eliminate the massive amount of fuel required to send crafts into orbit. The power would be “beamed” to the space craft. Either a laser or microwave emitter would heat the propellant, thus sending the vehicle aloft. This technology has been around for some time, but has yet to be actually applied in a real-world vehicle.

The project is run by Dr. Kevin L.G. Parkin who described it in his PhD thesis and invented the equipment used. Along with him are David Murakami and Creon Levit. One of the previous workers on the program went on to found his own company in the hopes of commercializing the technology used.

For Worden, the first locations that man should visit utilizing this revolutionary technology would not be the moon or even Mars. Rather he suggests that we should visit the red planet’s moons, Phobos and Deimos. Worden believes that astronauts can be sent to Mars by 2030 for around $10 billion – but only one way. The strategy appears to resemble the ‘Faster-Better-Cheaper’ craze promoted by then-NASA Administrator Dan Goldin during the 1990s.

DARPA is a branch of the U.S. Department of Defense whose purview is the development of new technology to be used by the U.S. military. Some previous efforts that the agency has undertaken include the first hypertext system, as well as other computer-related developments that are used everyday. DARPA has worked on space-related projects before, working on light-weight satellites (LIGHTSAT), the X-37 space plane, the FALCON Hypersonic Cruise Vehicle (HCV) and a number of other programs.

Source: Kurzweil

October 20, 2010December 24, 2015 by Matt Williams

First Law of Thermodynamics

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Ever wonder how heat really works? Well, not too long ago, scientists, looking to make their steam engines more efficient, sought to do just that. Their efforts to understand the interrelationship between energy conversion, heat and mechanical work (and subsequently the larger variables of temperature, volume and pressure) came to be known as thermodynamics, taken from the Greek word “thermo” (meaning “heat”) and “dynamis” (meaning force). Like most fields of scientific study, thermodynamics is governed by a series of laws that were realized thanks to ongoing observations and experiments. The first law of thermodynamics, arguably the most important, is an expression of the principle of conservation of energy.

Consistent with this principle, the first law expresses that energy can be transformed (i.e. changed from one form to another), but cannot be created or destroyed. It is usually formulated by stating that the change in the internal energy (ie. the total energy) contained within a system is equal to the amount of heat supplied to that system, minus the amount of work performed by the system on its surroundings. Work and heat are due to processes which add or subtract energy, while internal energy is a particular form of energy associated with the system – a property of the system, whereas work done and heat supplied are not. A significant result of this distinction is that a given internal energy change can be achieved by many combinations of heat and work.

This law was first expressed by Rudolf Clausius in 1850 when he said: “There is a state function E, called ‘energy’, whose differential equals the work exchanged with the surroundings during an adiabatic process.” However, it was Germain Hess (via Hess’s Law), and later by Julius Robert von Mayer who first formulated the law, however informally. It can be expressed through the simple equation E2 – E1 = Q – W, whereas E represents the change in internal energy, Q represents the heat transfer, and W, the work done. Another common expression of this law, found in science textbooks, is ?U=Q+W, where ? represents change and U, heat.

An important concept in thermodynamics is the “thermodynamic system”, a precisely defined region of the universe under study. Everything in the universe except the system is known as the surroundings, and is separated from the system by a boundary which may be notional or real, but which by convention delimits a finite volume. Exchanges of work, heat, or matter between the system and the surroundings take place across this boundary. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments (such as steam engines, for which the study was first developed).

We have written many articles about the First Law of Thermodynamics for Universe Today. Here’s an article about entropy, and here’s an article about Hooke’s Law.

If you’d like more info on the First Law of Thermodynamics, check out NASA’s Glenn Research Center, and here’s a link to Hyperphysics.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/First_law_of_thermodynamics
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/firlaw.html
http://en.wikipedia.org/wiki/Internal_energy
http://www.grc.nasa.gov/WWW/K-12/airplane/thermo1.html
http://en.wikipedia.org/wiki/Thermodynamics
http://en.wikipedia.org/wiki/Laws_of_thermodynamics

October 20, 2010December 11, 2016 by Matt Williams

What are Earthquake Fault Lines?

False-color composite image of the Port-au-Prince, Haiti region, taken Jan. 27, 2010 by NASA’s UAVSAR airborne radar. The city is denoted by the yellow arrow; the black arrow points to the fault responsible for the Jan. 12 earthquake. Image credit: NASA

Every so often, in different regions of the world, the Earth feels the need to release energy in the form of seismic waves. These waves cause a great deal of hazards as the energy is transferred through the tectonic plates and into the Earth’s crust. For those living in an area directly above where two tectonic plates meet, the experience can be quite harrowing!

This area is known as a fault, or a fracture or discontinuity in a volume of rock, across which there is significant displacement. Along the line where the Earth and the fault plane meet, is what is known as a fault line. Understanding where they lie is crucial to our understanding of Earth’s geology, not to mention earthquake preparedness programs.

Definition:

In geology, a fault is a fracture or discontinuity in the planet’s surface, along which movement and displacement takes place. On Earth, they are the result of activity with plate tectonics, the largest of which takes place at the plate boundaries. Energy released by the rapid movement on active faults is what causes most earthquakes in the world today.

The Earth's Tectonic Plates. Credit: msnucleus.org — The Earth’s Tectonic Plates. Credit: msnucleus.org

Since faults do not usually consist of a single, clean fracture, geologists use the term “fault zone” when referring to the area where complex deformation is associated with the fault plane. The two sides of a non-vertical fault are known as the “hanging wall” and “footwall”.

By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining. Basically, when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall hanging above him. This terminology has endured for geological engineers and surveyors.

Mechanisms:

The composition of Earth’s tectonic plates means that they cannot glide past each other easily along fault lines, and instead produce incredible amounts of friction. On occasion, the movement stops, causing stress to build up in rocks until it reaches a threshold. At this point, the accumulated stress is released along the fault line in the form of an earthquake.

When it comes to fault lines and the role they have in earthquakes, three important factors come into play. These are known as the “slip”, “heave” and “throw”. Slip refers to the relative movement of geological features present on either side of the fault plane; in other words, the relative motion of the rock on each side of the fault with respect to the other side.

Transform Plate Boundary — *Tectonic Plate Boundaries. Credit:*

Heave refers to the measurement of the horizontal/vertical separation, while throw is used to measure the horizontal separation. Slip is the most important characteristic, in that it helps geologists to classify faults.

Types of Faults:

There are three categories or fault types. The first is what is known as a “dip-slip fault”, where the relative movement (or slip) is almost vertical. A perfect example of this is the San Andreas fault, which was responsible for the massive 1906 San Francisco Earthquake.

Second, there are “strike-slip faults”, in which case the slip is approximately horizontal. These are generally found in mid-ocean ridges, such as the Mid-Atlantic Ridge – a 16,000 km long submerged mountain chain occupying the center of the Atlantic Ocean.

Lastly, there are oblique-slip faults which are a combination of the previous two, where both vertical and horizontal slips occur. Nearly all faults will have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant.

*Map of the Earth showing fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com*

Impacts of Fault Lines:

For people living in active fault zones, earthquakes are a regular hazard and can play havoc with infrastructure, and can lead to injuries and death. As such, structural engineers must ensure that safeguards are taken when building along fault zones, and factor in the level of fault activity in the region.

This is especially true when building crucial infrastructure, such as pipelines, power plants, damns, hospitals and schools. In coastal regions, engineers must also address whether tectonic activity can lead to tsunami hazards.

For example, in California, new construction is prohibited on or near faults that have been active since the Holocene epoch (the last 11,700 years) or even the Pleistocene epoch (in the past 2.6 million years). Similar safeguards play a role in new construction projects in locations along the Pacific Rim of fire, where many urban centers exist (particularly in Japan).

Various techniques are used to gauge when the last time fault activity took place, such as studying soil and mineral samples, organic and radiocarbon dating.

We have written many articles about the earthquake for Universe Today. Here’s What Causes Earthquakes?, What is an Earthquake?, Plate Boundaries, Famous Earthquakes, and What is the Pacific Ring of Fire?

If you’d like more info on earthquakes, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources:

October 19, 2010November 7, 2016 by Jon Voisey

Stolen: Magellanic Clouds – Return to Andromeda

The Magellanic Clouds are an oddity. Their relative velocity is suspiciously close to the escape velocity of the Milky Way system making it somewhat difficult for them to have been formed as part of the system. Additionally, their direction of motion is nearly perpendicular to the disk of the galaxy and systems, especially ones as large as the Magellanic Clouds, should show more orientation to the plane if they formed along side. Their gas content is also notably different than other satellite galaxies of our galaxy. The combination of these features suggests to some, that the Magellanic Clouds aren’t native to the Milky Way and were instead intercepted.

But where did they come from? Although the suggestion is not entirely new, a recent paper, accepted to the Astrophysical Journal Letters, suggests they may have been captured after a past merger in the Andromeda Galaxy (M31).

To analyze this proposition, the researchers, Yang (from the Chinese Academy of Sciences) and Hammers (of the University of Paris, Diderot), conducted simulations backtracking the positions of the Magellanic Clouds. While this may sound straightforward, the process is anything but. Since galaxies are extended objects, their three dimensional shapes and mass profiles must be worked out extremely well to truly account for the path of motion. Additionally, the Andromeda galaxy is certainly moving and would have been in a different position that it is observed today. But exactly where was it when the Magellanic Clouds would have been expelled? This is an important question, but not easy to answer given that observing the proper motions of objects so far away is difficult.

But wait. There’s more! As always, there’s a significant amount of the mass that can’t be seen at all! The presence and distribution of dark matter would greatly have affected the trajectory of the expelled galaxies. Fortunately, our own galaxy seems to be in a fairly quiescent phase and other studies have suggested that dark matter halos would be mostly spherical unless perturbed. Furthermore, distant galaxy clusters such as the Virgo supercluster as well as the “Great Attractor” would have also played into the trajectories.

These uncertainties take what would be a fairly simple problem and turn it into a case in which the researchers were instead forced to explore the parameter space with a range of reasonable inputs to see which values worked. In doing so, the pair of astronomers concluded “it could be the case, within a reasonable range of parameters for both the Milky Way and M31.” If so, the clouds spent 4 – 8 billion years flying across intergalactic space before being caught by our own galaxy.

But could there be further evidence to support this? The authors note that if Andromeda underwent a merger event of such scale would likely have induced vast amounts of star formation. As such, we should expect to see an increase in numbers of stars with this age. The authors do not make any statements as to whether or not this is the case. Regardless, the hypothesis is interesting and reminds us how dynamic our universe can be.

October 19, 2010April 26, 2016 by Nancy Atkinson

Underground Acquifers Fed Long-Lived Oceans, Lakes on Ancient Mars

Artist's impression of water under the Martian surface. If underground aquifers really do exist, the implications for human exploration and eventual colonization of the red planet would be far-reaching. (Illustration: ESA)

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Images from the spacecraft orbiting Mars seem to indicate the Red Planet may once have had oceans and lakes, and researchers are still trying to figure out how these bodies of water could have developed. A new explanation is that underground aquifers fed water to the surface, forming the floors of ancient continental-scale basins on Mars. The groundwater emerged through extensive and widespread fractures, leading to the formation of river systems, large-scale regional erosion, sedimentary deposition and water ponding in widespread and long-lasting bodies of water in Mars northern plains.

J. Alexis Palmero Rodriguez, research scientist at the Planetary Science Institute PSI, has been studying the Martian northern lowlands region, finding extensive sedimentary deposits that resemble the abyssal plains of Earth’s ocean floors. It is also like the floors of other basins on Mars where oceans are thought to have developed.

The origin of these deposits and the formation of Martian lakes and seas has been a controversial subject over the years. One theory is that there was a sudden release of large volumes of water and sediment from zones of apparent crustal collapse known as “chaotic terrains.” However, these zones of collapse are on the whole rare on Mars, while the plains deposits are widespread and common within large basin settings, Rodriguez said.

From evidence in the planet’s northern plains (south of Gemini Scopuli in Planum Boreum), Rodriguez’ new model does not require sudden massive groundwater discharges. Instead, it advocates for groundwater discharges being widespread, long-lived and common in the northern plains of Mars.

Large gully on Mars, seen by the Mars Reconnaissance Orbiter's HiRISE camera. Credit: NASA/JPL/University of Arizona

“With the loss over time of water from the subsurface aquifer, areas of the northern plains ultimately collapsed, creating the rough hilly surfaces we see today. Some plateaus may have avoided this fate and preserved sedimentary plains containing an immense record of hydrologic activity,” Rodriguez said. “The geologic record in the collapsed hilly regions would have been jumbled and largely lost.

“This model implies that groundwater discharges within basin settings on Mars may have been frequent and led to formation of mud pools, lakes and oceans. In addition, our model indicates this could have happened at any point in the planet’s history,” he said. “There could have been many oceans on Mars over time.”

If life existed in Martian underground systems, life forms could have been brought up to the surface via the discharges of these deep-seated fluids. Organisms and their fossils may therefore be preserved within some of these sedimentary strata, Rodriguez said.

His paper was published in the journal Icarus.

Source: Planetary Science Institute

October 19, 2010December 24, 2015 by Nancy Atkinson

365 Days of Astronomy Podcast to Continue in 2011

If you’ve been considering contributing a podcast to the 365 Days of Astronomy but just haven’t gotten around to it yet, there’s good news: the project will be continuing for another year — its third — in 2011. As far as we can tell, 365 Days of Astronomy is the most popular and successful user-generated podcast ever, as each podcast is listened to thousands of times. If you’re looking to share your experiences, thoughts, feelings, discoveries, or anything about space and astronomy, this is your big chance to find your voice and an audience to listen.

Since it is now the Year of the Solar System, it seemed like a good reason to keep this Energizer-Bunny project from the International Year of Astronomy going for another year. As the Project Manager, I hope you’ll join in, or at least check it out and start listening daily — if you aren’t already. Here’s the official press release:

The award-winning 365 Days of Astronomy Podcast is proud to announce the project will continue for yet another year – its third year — and is now accepting sign-ups for participants for another 365 podcasts in 2011. 2011 encompasses the Year of the Solar System, which marks an unprecedented flurry of robotic exploration of space, and is the perfect opportunity for more of the public to become involved in creating podcasts to share astronomy with the world.

365 Days of Astronomy is a legacy project of the International Year of Astronomy (IYA), and in 2009 was a major project of the IYA. For two years now, the project has published one podcast for every day of the year. The episodes are written, recorded and produced by people all around the world. “This podcast gives a voice to everyone in astronomy – professionals, amateurs, and those who just enjoy the amazing discoveries and images of our Universe,” said Dr. Pamela Gay, chair for the IYA’s New Media Group.

The 365 Days of Astronomy podcast is now looking for individuals, schools, companies and clubs to submit 5 – 10 minutes of audio for our daily podcast.

The 365 Days of Astronomy has gained a wide audience, and each podcast is heard by 5,000 – 10,000 listeners. The project was awarded a Parsec Award in 2009 for “The Best Info-tainment” podcast in 2009, and was nominated for the “Best Fact Behind the Fiction” award in 2010.

Participants can sign up to do just 1 episode or up to 12 episodes (one per month, subject to editorial discretion). People from every continent except Antarctica have submitted podcasts the past two years, and the 365 Days of Astronomy team encourages a more diverse population from even more countries to sign up for a particular day (or days) of 2011. A calendar of astronomical events is available on the project’s website to provide ideas but the podcasts can be about virtually any astronomical topic. “We are seeking a wide range of contributions, from simple concepts or how-tos to more in-depth discussions of complex concepts,” said Dr. Gay. “Over the past two years, we received a wide range of contributions, from simple at-home first-time podcasts to highly polished and professional recordings. We expect the same for 2011 and are looking to sign up a variety of participants, from amateur astronomers, classroom teachers and students to scientists, science bloggers and big media companies.”

The project is also asking individuals and organizations for financial support.

The podcast team also invites people and organizations to sponsor the podcast by donating $30 to support 1 day of the podcast, with your dedication appearing at the start of the show. For just $360, it is possible to sponsor 1 episode per month. Alternatively, you can also have a dedication message at the end of the show for a week, for a donation at the $100 level. These donations will help pay for editing, and posting of the podcasts.

For more information visit:

365 Days of Astronomy: http://365DaysOfAstronomy.org.

Astrosphere New Media: http://www.astrosphere.org/

Year of the Solar System: http://solarsystem.nasa.gov/yss/index.cfm

October 19, 2010December 24, 2015 by Jon Voisey

The Habitability of Gliese 581d

The Gliese 581 system has been making headlines recently for the most newly announced planet that may lie in the habitable zone. Hopes were somewhat dashed when we were reminded that the certainty level of its discovery was only 3 sigma (95%, whereas most astronomical discoveries are at or above the 99% confidence level before major announcements), but the Gliese 581 system may yet have more surprises. When the second planet, Gliese 581d, was first discovered, it was placed outside of the expected habitable zone. But in 2009, reanalysis of the data refined the orbital parameters and moved the planet in, just to the edge of the habitable zone. Several authors have suggested that, with sufficient greenhouse gasses, this may push Gliese 581d into the habitable zone. A new paper to be published in an upcoming issue of Astronomy & Astrophysics simulates a wide range of conditions to explore just what characteristics would be required.

The team, led by Robin Wordsworth at the University of Paris, varied properties of the planet including surface gravity, albedo, and the composition of potential atmospheres. Additionally, the simulations were also run for a planet in a similar orbit around the sun (Gliese 581 is an M dwarf) to understand how the different distribution of energy could effect the atmosphere. The team discovered that, for atmospheres comprised primarily of CO₂, the redder stars would warm the planet more than a solar type star due to the CO₂ not being able to scatter the redder light as well, thus allowing more to reach the ground.

One of the potential roadblocks to warming the team considered was the formation of clouds. The team first considered CO₂ clouds which would be likely towards the outer edges of the habitable zone and form on Mars. Since clouds tend to be reflective, they would counteract warming effects from incoming starlight and cool the planet. Again, due to the nature of the star, the redder light would mitigate this somewhat allowing more to penetrate a potential cloud deck.

Should some H₂O be present its effects are mixed. While clouds and ice are both very reflective, which would decrease the amount of energy captured by a planet, water also absorbs well in the infrared region. As such, clouds of water vapor can trap heat radiating from the surface back into space, trapping it and resulting in an overall increase. The problem is getting clouds to form in the first place.

The inclusion of nitrogen gas (common in the atmospheres of planets in the solar system) had little effect on the simulations. The primary reason was the lack of absorption of redder light. In general, the inclusion only slightly changed the specific heat of the atmosphere and a broadening of the absorption lines of other gasses, allowing for a very minor ability to trap more heat. Given the team was looking for conservative estimates, they ultimately discounted nitrogen from their final considerations.

With the combination of all these considerations, the team found that even given the most unfavorable conditions of most variables, should the atmospheric pressure be sufficiently high, this would allow for the presence of liquid water on the surface of the planet, a key requirement for what scientists maintain is critical for abiogenesis. The favorable merging of characteristics other than pressure were also able to produce liquid water with pressures as low as 5 bars. The team also notes that other greenhouse gasses, such as methane, were excluded due to their rarity, but should the exist, the ability for liquid water would be improved further.

Ultimately, the simulation was only done as a one dimensional model which essentially considered a thin column of the atmosphere on the day side of the planet. The team suggests that, for a better understanding, three dimensional models would need to be created. In the future, they plan to use just such modeling which would allow for a better understanding of what was happening elsewhere on the planet. For example, should temperatures fall too quickly on the night side, this could lead to the condensation of the gasses necessary and put the atmosphere in an unstable state. Additionally, as we discover more transiting exoplanets and determine their atmospheric properties from transmission spectra, astronomers will better be able to constrain what typical atmospheres really look like.

October 19, 2010December 24, 2015 by Nancy Atkinson

Hubble Spins the Wheel on Star Birth

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Galaxies are like snowflakes, with no two looking exactly the same. The latest image released from the Hubble Space Telescope shows a striking face-on spiral galaxy named NGC 3982, which is a swirl of activity and star birth along with its winding arms. The arms are lined with pink star-forming regions of glowing hydrogen, newborn blue star clusters, and obscuring dust lanes that provide the raw material for future generations of stars. The bright nucleus is home to an older population of stars, which grow ever more densely packed toward the center.

NGC 3982 is located about 68 million light-years away in the constellation Ursa Major. The galaxy spans about 30,000 light-years, one-third of the size of our Milky Way galaxy. This color image is composed of exposures taken by three different instruments, taken over a substantial portion of the space telescope’s life, from March 2000 and August 2009: The Wide Field Planetary Camera 2 (WFPC2), the Advanced Camera for Surveys (ACS), and the Wide Field Camera 3 (WFC3). The observations were taken between The rich color range comes from the fact that the galaxy was photographed in visible and near-infrared light. Also used was a filter that isolates hydrogen emission that emanates from bright star-forming regions dotting the spiral arms.

Source: HubbleSite

October 18, 2010October 18, 2010 by Nancy Atkinson

First Rickroll in Space

Those pranksters from Zug have now gone to the edge of space, sending their own DIY satellite up to 89,000 feet above Earth, and doing a little Rickrolling along the way. They claim they have now pulled the famous prank on the entire planet. Hmmm, hopefully this wasn’t the source of the radio signals that caused ESA’s Soil Moisture and Ocean Salinity (SMOS) probe to be “blinded from interference.” Surely strains of “Never Gonna Give You Up” could never do that….

The video above is a quick look at their balloon satellite launch and their results; here’s the whole story on Zug.

October 17, 2010April 26, 2016 by Matt Williams

Destructive Interference

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Sound travels in waves, which function much the same as ocean waves do. One wave cycle is a complete wave, consisting of both the up half (crest) and down half (trough). Waves also have a certain amplitude which is the measure of how strong the wave is; the higher the amplitude, the higher the crests and deeper the troughs. Waves don’t usually reflect when they strike other waves. Instead, they combine. If the amplitudes of two waves have the same sign (either both positive or both negative), they will add together to form a wave with a larger amplitude. This is called constructive interference. If the two amplitudes have opposite signs, they will subtract to form a combined wave with lower amplitude. This is what is called Destructive Interference, which is a subfield of the larger study in physics known as wave propagation.

An interesting example of this is the loudspeaker. When music is played on the loudspeaker, sound waves emanate from the front and back of the speaker. Since they are out of phase, they diffract into the entire region around the speaker. The two waves interfere destructively and cancel each other, particularly at very low frequencies. But when the speaker is held up behind baffle, which in this case consists of a wooden sheet with a circular hole cut in it, the sounds can no longer diffract and mix while they are out of phase, and as a consequence the intensity increases enormously. This is why loudspeakers are often mounted in boxes, so that the sound from the back cannot interfere with the sound from the front.

Scientists and engineers use destructive interference for a number of applications to levels reduce of ambient sound and noise. One example of this is the modern electronic automobile muffler. This device senses the sound propagating down the exhaust pipe and creates a matching sound with opposite phase. These two sounds interfere destructively, muffling the noise of the engine. Another example is in industrial noise control. This involves sensing the ambient sound in a workplace, electronically reproducing a sound with the opposite phase, and then introducing that sound into the environment so that it interferes destructively with the ambient sound to reduce the overall sound level.

For a hands-on demonstration of how destructive interference works, click on this link.

We have written many articles about destructive interference for Universe Today. Here’s an article about constructive waves, and here’s an article about the Casimir Effect.

If you’d like more info on destructive interference, check out Running Interference, and here’s a link to NASA Science page about Interference.

We’ve also recorded an entire episode of Astronomy Cast all about the Wave Particle Duality. Listen here, Episode 83: Wave Particle Duality.

Sources:
http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29
http://en.wikipedia.org/wiki/Loudspeaker_enclosure
http://en.wikipedia.org/wiki/Sound_baffle
http://www.windows2universe.org/earth/Atmosphere/tornado/beat.html
http://library.thinkquest.org/19537/Physics5.html
http://zonalandeducation.com/mstm/physics/waves/interference/destructiveInterference/InterferenceExplanation3.html