What is sound?

"What is sound and how does it travel?

Sound is a pressure disturbance that moves through a medium in the form of mechanical waves. When a force is exerted on an atom, it moves from its rest or equilibrium position and exerts a force on the adjacent particles. These adjacent particles are moved from their rest position and this continues throughout the medium. This transfer of energy from one particle to the next is how sound travels through a medium. The words "mechanical wave" are used to describe the distribution of energy through a medium by the transfer of energy from one particle to the next.

Waves of sound energy move outward in all directions from the source. Your vocal chords and the strings on a guitar are both sources which vibrate to produce sound waves. Without energy, there would be no sound. Let's take a closer look at sound waves.

What do waves consist of?

Sound or pressure waves are made up of compressions and rarefactions. Compression happens when particles are forced, or pressed, together. Rarefaction is just the opposite, it occurs when particles are given extra space and allowed to expand. Remember that sound is a type of kinetic energy. As the particles are moved from their rest position, they exert a force of the adjacent particles and pass the kinetic energy. Thus sound energy travels outward from the source.

Sound travels through air, water, or a block of steel; thus, all are mediums for sound. Without a medium there are no particles to carry the sound waves. The word "particle" suggests a tiny concentration of matter capable of transmitting energy. A particle could be an atom or molecule. In places like space, where there is no atmosphere, there are too few atomic particles to transfer the sound energy.

Let's look at the example of a stereo speaker. To produce sound, a thin surfaced cone, called a diaphragm, is caused to vibrate using electromagnetic energy. When the diaphragm moves to the right, its energy pushes the air molecules on the right together, opening up space for the molecules on the left to move into. We call the molecules on the right compressed and the molecules on the left rarefied. When the diaphragm moves to the left, the opposite happens. Now, the molecules to the left become compressed and the molecules to the right are rarefied. These alternating compressions and rarefactions produce a wave. One compression and one rarefaction is called a wavelength. Different sounds have different wavelengths.

What do sound waves look like?

We cannot see the energy in sound waves, but a sound wave can be modeled in two ways. One way is to create a graph of the diaphragm's position at different times. Think of a number line. We call the diaphragm's rest position zero. As it travels to the right, it moves to an increasingly positive position along the number line. As is travels to the left, its position becomes more and more negative. The graph of the diaphragm’s position as it vibrates looks like the sine graph, with its highest point when the diaphragm is the farthest right and its lowest point when it is farthest left.

Another graph can be made using the amount of force on the molecules versus time. The force is greatest when the diaphragm is moving through its original position. This is similar to the way we feel the greatest force on a swing as we move through the center, where we started. As the diaphragm moves to the right, there is less and less force. At its rightmost position, it is exerting no force (due to pressure) and begins its trip the opposite way. Similarly, the diaphragm is exerting no force at its leftmost position. For our graph, we say the force is least when the diaphragm moves through its starting position heading the opposite way. When the force is exerting a pulling force, we assign negative values to it. A graph of the force versus time also resembles the sine graph.

More about compression and rarefaction

Compression and rarefaction are terms defining the molecules near the diaphragm. Compression is the point when the most force is being applied to a molecule and rarefaction is the point when the least force is applied. It is important to note that when a molecule to the right of the diaphragm is experiencing compression, a molecule to the diaphragm's left is experiencing rarefaction. For right side molecules, compression occurs when the diaphragm is in its original position, moving towards the right. This is where the molecule experiences the most force. Rarefaction happens when the diaphragm is once again in the center, this time moving towards the left. At this point, the molecule is experiencing the least force. Of course, this is the opposite for molecules to the diaphragm's left.

Different types of waves

As the diaphragm vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that travel in the same direction as the particle movement are called longitudinal waves. Longitudinal sound waves are the easiest to produce and have the highest speed. However, it is possible to produce other types. Waves which move perpendicular to the direction particle movement are called shear waves or transverse waves. Shear waves travel at slower speeds than longitudinal waves, and can only be made in solids. Think of a stretched out slinky, you can create a longitudinal wave by quickly pushing and pulling one end of the slinky. This causes longitudinal waves for form and propagate to the other end. A shear wave can be created by taking one end of the slinky and moving it up and down. This generates a wave that moves up and down as it travels the length of the slinky.

Another type of wave is the surface wave. Surface waves travel at the surface of a material with the particles move in elliptical orbits. They are slightly slower than shear waves and fairly difficult to make. A final type of sound wave is the plate wave. The particles of these waves also move in elliptical orbits but plate waves can only be created in very thin pieces of material.

To read the whole text http://www.ndt-ed.org/EducationResources/HighSchool/Sound/introsound.htm

Need it simple ?

How Do Sound Waves Travel?

This will throw you back to 7th grade science class. Remember when your teacher was talking about molecules and atoms, those building blocks of the universe that were so tiny you can't see them? Well, if you missed that class, this will be a refresher because sound waves travel through air (among other things that are made of molecules). Sound is vibration, or energy, that gets all the molecules around the source of that vibration all excited, so they vibrate too and start bumping into each other and pass along the vibrations. It's kind of a molecular mosh pit. The molecules lose some of their excitement the further away they are from the source of the vibration so the noise gets quieter and quieter until the molecules run out of vibrations and there the sound ends.

9th Grade Science, a Little More Advanced

Sound waves are essentially variations of pressure that exists, in this example, in the air. (Air is made up of molecules). To help visualize the nature of sound waves, think of yourself in a tub of water with a plastic toy boat floating in front of you. If you create waves by pushing your hand along the top of the water, the waves push against the boat and move it from one end of the tub to the other. Now think of the waves and the boat as sound. As the waves move along, so does the boat (which is also the sound). The bigger the wave, the more the boat moves up and down and the farther it can travel because the sound waves, like vibrating molecules, dissipate over distance. Now if you were in a large pool, you'd need much bigger waves to push the boat (sound) all the way to the other end.

Time to Play Dominoes

When a vibration causes the molecules to vibrate, it isn't the first molecule that just gets so excited that it rushes to your ear, bypassing all the other molecules. When it gets excited, and jumps all around, it starts bounding off all the other molecules around it that bound around to the ones next to them and so on. Now set up your dominoes on their side in a line, close enough together so that when the first one falls it will fall into the second and then the second onto the third...you get the idea. Now push over the first one and it will cause the entire line to fall. Now instead of a straight line make a Y pattern using a few of the dominoes. At the top of each Y, start two more branches, and at the top of those branches do the same until you run out of dominoes. Again, knock over the first and watch how they all fall around the Y tree you built. That more accurately shows how sound waves don't move in just a straight line but in a spherical or conical way.

Time to Put the Toys Away

For sound to travel, it needs a medium that contains molecules. It can be water, steel, concrete, anything that has molecules close enough to one another that they can vibrate and transmit the sound. In fact, the closer the molecules are bunched, the faster the sound travels. That's why when you put your ear to a railroad track you can hear it coming long before you hear it's noise in the air. Steel molecules are packed much more tightly than air molecules. That is also why there is no sound in space. The molecules in space are so far apart that when one of them near a vibration starts dancing the noise jig, it has no other molecules to pass the vibrations to. In essence, there is no medium through which sound waves can travel.

On to College

As sound waves travel from a source, they are known as compression waves. Because of the excited state of the molecules, they compress while vibrating. But once the vibration has been passed along the molecules left behind slow their vibration because they have essentially transferred the energy along. The ones left behind experience rarefaction. This is a more technical explanation of what scientists mean by sound wave. It looks like this and can be charted as a two-dimensional graph.

Why We Hear Things

When those vibrating molecules reach the outer ear, they are funneled into the inner ear, where a thin stretched membrane "catches" the vibrations, which are fluctuations in air pressure. The body then translates these fluctuations into electrical signals that are transmitted to the brain, which further deciphers them. Pretty complicated process but those are the basics.