Engineering behind "Headphones"

Headphones

If you've ever been to a rock concert and heard music thumping out of giant loudspeakers, you'll know sound can pack a powerful punch. Sometimes, however, we want to enjoy music more quietly and intimately or in places where others don't want to hear what we're listening to. Trains and planes are noisy enough—just imagine the cacophony there would be if everyone sat with massive stereo systems in front of them! For times like this, headphones let us retreat quietly into our own imaginary worlds. Let's take a closer look at what's inside them and how they work!

Headphones: miniature loudspeakers fixed to your ears

Headphones (which are often called "cans" by DJs and people who work in radio broadcasting) work in exactly the same way as speakers, so you might want to consult our article on loudspeakers if you're not sure how they use magnetism to turn electrical energy into sound.

 
Photo: A pair of earbud phones from an MP3 player. The metal gauze on the front is "acoustically transparent": it lets soundout without letting (too much) dirt and dust in. The backs of some earbuds are completely sealed to stop sound from escaping (so they're similar to closed-back headphones), though other earbuds do have small vent holes in them (making them equivalent to larger, open-backed headphones).
The biggest difference between loudspeakers and headphones is, of course, size. A loudspeaker needs to set all the air moving in a room so you can hear the sound it's making, but the speaker in a headphone only has to move the volume of air inside your ear canal. That's why it can be so much smaller and more discreet.
Large headphones are essentially just two loudspeakers mounted on a strap that clamps firmly over your head. Earbuds work the same way but, as you would expect, everything inside them (the magnet, the coil of wire, and the diaphragm cone that makes sound) is shrunk down to a much smaller size.
Speakers tend to be built into "enclosures" (as engineers call them—the rest of us call them "boxes") to amplify their sounds and keep them safe from damage. If you look closely, you'll see speaker enclosures usually have openings at the front or the back so air can move more freely in and out of them to generate decent sound. The same is true of headphones and earbuds, which come in two main types. As their name suggests, closed-back headphones are sealed at the back so (theoretically) no sound escapes (or leaks in from outside) while open-back headphones are open to the air at the back as well as the front. Many people find that open-back headphones sound better but much of the noise will leak into the room around you and annoy other people, while "ambient" noise from the room can easily penetrate open-back headphones and annoy you too. If that's a problem, you need closed-back headphones ,which make it easy to cut yourself off completely.

How earbuds work

Taking broken things apart is a great way to find out how they work. If you're a young person, ask an adult first to make sure what you want to dismantle is really safe.

1. Here's my broken earbud and I've popped the back off it. You can see how the wires run up through the main case to the coil inside. We need two wires to make a circuit: one carries the current into the coil from the stereo; the other carries it back again.

2. Next, I've tipped the earbud over and popped the front cover off. The front cover is a plastic disc with holes in it to let the sound enter your ear. Just behind it there's a very small cone. It's hardly cone-shaped, though: it's a flattish, transparent disc made of very thin and flexible plastic, and it's quite crinkly and crackly when you move it. You can just see the tiny metal coil (colored red) attached to it.

3. In summary, then, these are all the bits that make up your earbuds:

• Back case: holds everything together. The wires run up through a hole at the bottom.
• Front case: This is the part that faces into your ear. Sometimes it's covered with a little fabric pouch to keep it clean.
• Seal: This rubbery circle clips the front case to the back case, holding the two together.
• Wires: Carry signals from the stereo to the speaker.
• Magnet: The permanent magnet at the back of the speaker. This is the heaviest part of an earbud and makes up the vast majority of its weight.
• Coil: This becomes an electromagnet when electricity flows through it.
• Transparent plastic cone: This makes the sound when it moves.

How bigger headphones work

As you might expect there's nothing radically different inside bigger headphones: they're just a scaled-up version of what you find inside earbuds.
As luck would have it, the arrival of my new Sennheiser headphones was followed quite quickly by the final collapse of my old pair. I could have repaired them, my friends, but in the interests of your curiosity, they've agreed to leave their insides to medical science! Let's see what we find when we open them up:

1. This is one of the two earpieces from my old pair of open-backed headphones with the light foam cover and the cable removed. Note that "open-backed" is a bit of an understatement for what you see here: the headphones are almost completely open to the air and built around a kind of plastic spoke design. The diaphragm that makes the sound is in the center. The open spokes radiating outwards are there to make the headphone the right size to cover your ear, without making it too heavy or uncomfortable.

2. Now I've popped out the central part that contains the loudspeaker:

3. Break off the protective plastic "spokes" and you can see the transparent plastic cone/diaphragm behind. You can also just see some small holes (beige dots) behind it that let sound out of the back and allow the diaphragm to move back and forth more freely.

4. Now the painful part. Break open the diaphragm cone (it's much thicker plastic than the ones in earbuds) and you can see the coil (red band) fastened on to it and the permanent magnet (silver and gold) behind. The coil sits in the slot like a band running loosely around the outside of the magnet:

5. Here's another shot of the diaphragm cone and the coil:

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Engineering behind "Swing Bowling"

Physics of swing bowling

The essence of swing bowling is to get the cricket ball to deviate sideways as it moves through the air towards or away from the batsman. To do this, the bowler makes use of six factors:

• The raised seam of the cricket ball
• The angle of the seam to the direction of travel
• The wear and tear on the ball
• The polishing liquid used on the ball
• The speed of the delivery
• The bowler's action

The asymmetry of the ball is encouraged by the constant polishing of one side of the ball by members of the fielding team, while allowing the opposite side to deteriorate through wear and tear. With time, this produces a marked difference in the aerodynamic properties of the two sides. 

Both turbulent and laminar airflow contribute to swing. Air in laminar flow separates from the surface of the ball earlier than air in turbulent flow, so that the separation point moves toward the front of the ball on the laminar side. On the turbulent flow side it remains towards the back, inducing a greater lift force on the turbulent airflow side of the ball. The calculated net lift force is not enough to account for the amount of swing observed. Additional force is provided by the pressure-gradient force. 

To induce the pressure-gradient force the bowler must create regions of high and low static pressure on opposing sides of the ball. The ball is then "sucked" from the region of high static pressure towards the region of low static pressure. The Magnus effect uses the same force but by manipulating spin across the direction of motion. A layer of fluid, in this case air, will have a greater velocity when moving over another layer of fluid than it would have had if it had been moving over a solid, in this case the surface of the ball. The greater the velocity of the fluid, the lower its static pressure.

Animation of an inswinger bowl from a right arm over the wicket bowler to a right arm batsman.

When the ball is new the seam is used to create a layer of turbulent air on one side of the ball, by angling it to one side and spinning the ball along the seam. This changes the separation points of the air with the ball; this turbulent air creates a greater coverage of air, providing lift. The next layer of air will have a greater velocity over the side with the turbulent air due to the greater air coverage and as there is a difference in air velocity, the static pressure of both sides of the ball are different and the ball is both 'lifted' and 'sucked' towards the turbulent airflow side of the ball.

When the ball is older and there is an asymmetry in roughness the seam no longer causes the pressure difference, and can actually reduce the swing of the ball. Air turbulence is no longer used to create separation point differences and therefore the lift and pressure differences. On the rough side of the ball there are scratches and pits in the ball's surface. These irregularities act in the same manner as the dimples of a golf ball: they trap the air, creating a layer of trapped air next to the rough side of the ball, which moves with the surface of the ball. The smooth side does not trap a layer of air. The next layer of air outward from the ball will have a greater velocity over the rough side, due to its contact with a layer of trapped air, rather than solid ball. This lowers the static pressure relative to the shiny side, which swings the ball. If the scratches and tears completely cover the rough side of the ball, the separation point on the rough side will move to the back of the ball, further than that of the turbulent air, thereby creating more lift and faster air flow. This is why a new ball will swing more than an old ball. If the seam is used to create the turbulent air on the rough side, the tears will not fill as quickly as they would with laminar flow, dampening the lift and pressure differences.

Reverse swing occurs in exactly the same manner as conventional swing, despite popular misconception. Over time the rough side becomes too rough and the tears become too deep – this is why golf ball dimples are never below a certain depth, and so "conventional" swing weakens over time; the separation point moves toward the front of the ball on the rough side. When polishing the shiny side of the ball, numerous liquids are used, such as sweat, saliva, sunscreen, hair gel (which bowlers may apply to their hair before a game) and other illegal substances like Vaseline (applied to the clothing where the ball is polished). These liquids penetrate the porous surface of the leather ball. Over time the liquid expands and stretches the surface of the ball (which increases the surface area meaning more lift) and creates raised bumps on the polished side, due to the non-uniform nature of the expansion. The valleys between the bumps hold the air in the same manner as the tears on the rough side. This creates a layer of air over the shiny side, moving the separation point towards the back of the ball on the shiny side. The greater air coverage is now on the shiny side, giving rise to more lift and faster secondary airflow on that side. There is therefore lower static pressure on the shiny side, causing the ball to swing towards it, not away from it as in conventional swing.

The rough side tears hold the air more easily than the shiny side valleys, so to maintain the air within the valleys the initial air layer must have a very high velocity, which is why reverse swing is primarily, but not necessarily, achieved by fast bowlers. Due to the less static nature of the initial air layer it takes longer for the swing to occur, which is why it occurs later in the delivery. This is why conventional and reverse swing can occur in the same delivery.

Cold and humid weather are said to enhance swing. Colder air is denser and so may affect the differential forces the ball experiences in flight. When looking at humidity, changes between 0% and 40% humidity appear to have little to no effect on the ball's swing; yet, when approaching 100% humidity "condensation shock" has been observed enhancing the swing of the ball. 

Conventional swing

Jimmy Anderson, a swing bowler for the England cricket team

Typically, a swing bowler aligns the seam and the sides of the ball to reinforce the swing effect. This can be done in two ways:

• Outswinger: An outswinger to a right-handed batsman can be bowled by aligning the seam slightly to the left towards the slips and placing the roughened side of the ball on the left. To extract consistent swing, a bowler can also rotate his wrist toward the slips while keeping his arm straight. To a right-handed batsman, this results in the ball moving away to the off side while in flight, usually outwards from his body.
• Inswinger: An inswinger to a right-handed batsman can be bowled by aligning the seam slightly to the right and placing the roughened side of the ball on the right. To extract consistent swing, a bowler can also rotate or "open up" his wrist towards leg slip. To a right-handed batsman, this results in the ball moving in to the leg side while in flight, usually inwards towards his body.

The curvature of swing deliveries can make them difficult for a batsman to hit with his bat. Typically, bowlers more commonly bowl outswingers, as they tend to move away from the batsman, meaning he has to "chase" the ball to hit it. Hitting away from the batsman's body is dangerous, as it leaves a gap between the bat and body through which the ball may travel to hit the wicket. Also, if the batsman misjudges the amount of swing, he can hit the ball with an edge of the bat. An inside edge can ricochet on to the wicket, resulting in him being out bowled, while an outside edge can fly to the wicket keeper or slip fielders for a catch.

An inswinger presents relatively fewer dangers to the batsman, but on some particular days can be devastating due to risk of being bowled or LBW. Shane Bond was considered a great exponent of inswing bowling.

An inswinger combined with a yorker can be especially difficult for the batsman to defend against, especially if used as a surprise delivery after a sequence of outswingers.

There has been a distinct lack of left-arm swing bowlers in the game.Some of the most famous left-arm bowlers were Pakistan's Wasim Akram, India's Zaheer Khan, Australia's Alan Davidson and Sri Lanka's Chaminda Vaas.

Reverse swing

"Reverse swing" redirects here. For reverse swing in doors, see door.

Normal swing occurs mostly when the ball is fairly new. As it wears more, the aerodynamics of the asymmetry changes and it is more difficult to extract a large amount of swing. When the ball becomes very old—around 40 or more overs old—it begins to swing towards the shine. This is known as reverse swing—meaning a natural outswinger will become an inswinger and vice versa.In essence, both sides have turbulent flow, but here the seam causes the airflow to separate earlier on one side. The result is always a swing to the side with the later separation, so the swing is away from the seam.

Reverse swing tends to be stronger than normal swing, and to occur late in the ball's trajectory. This gives it a very different character from normal swing, and because batsmen experience it less often, they generally find it much more difficult to defend against. It is also possible for a ball to swing normally in its early flight, and then to alter its swing as it approaches the batsman. This can be done in two ways one for the ball to reverse its direction of swing, giving it an 'S' trajectory: the other is for it to adopt a more pronounced swing in the same direction in which the swing is already curving; either alteration can be devastating for the batsman. In the first instance, he is already committed to playing the swing one way, which will be the wrong way to address swing which is suddenly coming from the opposite direction: in the second instance, his stance will be one which is appropriate for the degree, or extent, of the expected swing, and which could suddenly leave him vulnerable to LBW, being caught behind, or bowled. Two consecutive deliveries from Wasim Akram, one of each type, were considered to be the turning point of the 1992 World Cup Final^.

Pioneers and notable practitioners of reverse swing have mostly been Pakistani fast bowlers. In the early days of reverse swing, Pakistani bowlers were suspected of ball tamperingto achieve the conditions of the ball that allow reverse swing. According to Shaharyar Khan, reverse swing was invented by Salim Mir, who played for the Punjab Cricket Club in Lahore and taught it to his team-mate Sarfraz Nawaz.Sarfraz Nawaz introduced reverse swing into international cricket during the late 1970s, and passed their knowledge on to their team-mate Imran Khan,who in turn taught the duo of Wasim Akram and Waqar Younis. The English pair of Andrew Flintoff and Simon Jones, having been taught by Troy Cooley and the Indian bowlers like Zaheer Khan and Ajit Agarkar, are also well known for the ability to reverse swing.Bowlers tend to disguise the direction of reverse swing by running up starting with the opposite hand before switching hands and covering the ball for as long as possible before release. Neil Wagner utilizes this to show the ball is reversing, but disguises the direction of swing.

Playing swing bowling

Playing swing bowling is considered to be a hallmark of a batsman's skill.

Firstly, a batsman needs good eye reflexes which are considered to be a key skill when facing swing bowling. Secondly a batsman often needs to anticipate beforehand what the ball will do and adjust accordingly to play swing bowling. This can be done by observing the bowler's grip and action (which may have a marked difference depending on which type of swinger is to be delivered), by observing the field set, which may depend on the types of deliveries expected (as a rule outswingers will have more slips assigned) or by means of prior knowledge of the bowler; many can bowl or are proficient in only one type of swing. Traditional methods include the batsmen playing the ball as late as possible, and not playing away from the body. Other effective measures for combating swing bowling include standing well outside the crease, thus giving the ball less time to swing; and guessing the direction of swing based on the seam position observed in the ball's flight.

Since reverse swing occurs at faster speeds, later in the trajectory of the ball and with no real obvious change in action and grip (Waqar Younis from Pakistan for example had the same action and grip for nearly all his deliveries, batsmen with a quick eye and reflexes will do well. In his autobiography Wasim Akram mentions four batsmen— Adam Gilchrist, Brian Lara, Aravinda De Silva and Martin Crowe —who had such reflexes and who were exceedingly difficult to bowl to.

Controversy regarding reverse swing has never left modern cricket, as the Pakistani team was accused of ball tampering by the Australian umpire Darrell Hair during the fourth test against England in 2006 when the ball began to reverse swing after the 50th over.His co-umpire Billy Doctrove supported him. A hearing subsequently found that there was insufficient evidence to convict anyone of ball tampering.

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Engineering behind "Rain"

The Cycle of Water

Scientists explain the origin of rain via what is referred to as, “The Cycle of Water”. Typically the water cycle can be broken down into five main events including evaporation, condensation, precipitation, infiltration, and surface runoff. Below is a brief description of each event in the order in which they occur.

1. Evaporation. Water has the ability to transform into a gas, liquid, or solid. Evaporation occurs when water goes from a liquid state to a vaporous state. In term of the cycle of rain, this occurs as a result of the heat from the brazing sun, beaming down on large bodies of water such as oceans. The heat from the sun causes small amounts of ocean water to turn into vapor; the vapor then begin to rise into the air. Evaporation can occur at any temperature, however it tends to occur at a faster pace with warmer temperatures.

2. Condensation. As the water vapor from the ocean rises into the air, it begins to cool down again. As a result, the water vapor then begins to return to its liquid state which causes the formation of tiny water droplets. These trillions of water droplets all come together to form the huge clouds that we see in the sky.

3. Precipitation. If the clouds, formed by the condensed water droplets, become heavy enough, they then begin to fall out of the sky. The result of this is either rain or snow depending on how cold temperatures are.

4. Surface Runoff. Surface runoff is the process in which water travels across the ground. The water can be infiltrated (as defined below), accumulated in wells and holes within the ground, flow back into large bodies of water, or evaporated back into the air.

5. Infiltration. Infiltration occurs after the rain and refers to water soaking into the ground as a mean to provide moisture for the soil.

Although a rainy day can sometimes put a damper on outdoor plans, it is important to remember that rain is both necessary and beneficial. Not only does it put moisture back into the ground which allows farmers to grow and harvest the food that we eat, but it also cools the air down so that those hot summer days are a little less hot.

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Engineering behind Walking and Running

Physics behind Walking and Running

Problem

Walking is energy efficient. In a walking human, one leg swings forward while the other leg’s foot stays planted on the ground. When walking at natural speed (defined below), the swinging leg uses muscle force to move forward and immediately relaxes, allowing the force of gravity to move it to the ground. Simultaneously, the planted leg moves forward with largely passive rotation at the hip. The plant leg only needs to stay straight and the swinging leg’s knee only slightly bends to allow it to pass underneath the body.

Figure 1: Leg positioning. Plant leg in blue, swinging leg in red.

The swinging leg can be modeled as a physical pendulum: a thin uniform rod of mass $m$m rotating about a point a distance $r$r from its center of mass. Swinging freely under gravitational acceleration, $g$g, such a physical pendulum with moment of inertia $I$I will swing with a period $T$T:

$T=2π\sqrt{\dfrac{I}{mgr}}$ (Equation 1)

For a uniform, thin rod of length l with a pivoted end, $I=\dfrac{{ml}^2}{3}$I, equals, start fraction, m, l, start superscript, 2, end superscript, divided by, 3, end fraction. The natural walking step length is roughly $\dfrac{l}{5}$start fraction, l, divided by, 5, end fraction. The natural speed of walking, $v$v, is the step length divided by the time required to take the step.

To move faster or slower than the natural speed, the legs do not move at their natural frequencies or with the natural step length. Instead, the muscles produce forces (hence torques) to move the body forward. The maximum force a muscle can produce, $F_{max}$F, start subscript, m, a, x, end subscript, is proportional to its cross sectional area, $A$A, which is proportional to the square of the length: $F_{max}∝A∝l^2$. The maximum torque that the muscle can exert about its pivot point, $L_{max}$L, start subscript, m, a, x, end subscript, is proportional to the product of $F_{max}$F, start subscript, m, a, x, end subscript and its length: $L_{max}∝F_{max}l$. The mass of the leg is proportional to the mass of the muscle, which is proportional to the product of the area and the length. Unlike in the case of walking, the period of a pendulum acted on by maximum torque $L_{max}$L, start subscript, m, a, x, end subscriptis (constant of proportionality not organism-specific)

$T∝\sqrt{\dfrac{I}{L_{max}}}$ (Equation 2)

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