The origin of sound is always some vibrating body. In some cases the vibrations of the source may be very small or very large that it may not be possible to detect them. This type of vibrations is produced by tuning fork, drum, bell, the string of a guitar etc. Human voice originates from the vibrations of the vocal chords and the sound from the musical instruments is due to the vibrations of the air columns. Sound travels in the form of longitudinal wave and it requires a material medium for its propagation.

fter reading this section you will be able to do the following:

• Explain what can happen to the energy of sound waves when the waves interact.
• Compare and contrast constructive interference and destructive interference.
• Explain what a critical angle is.

Questions

1. What is the difference in sound between the overlap area and the single color area? 
2. What is the difference in sound in the white area?

Wave Interference

When two or more sound waves from different sources are present at the same time, they interact with each other to produce a new wave. The new wave is the sum of all the different waves. Wave interaction is calledinterference. If the compressions and the rarefactions of the two waves line up, they strengthen each other and create a wave with a higher intensity. This type of interference is known as constructive.

When the compressions and rarefactions are out of phase, their interaction creates a wave with a dampened or lower intensity. This is destructive interference. When waves are interfering with each other destructively, the sound is louder in some places and softer in others. As a result, we hear pulses or beats in the sound.
Dead spots
Waves can interfere so destructively with one another that they producedead spots, or places where no sound at all can be heard. Dead spots occur when the compressions of one wave line up with the rarefactions from another wave and cancel each other. Engineers who design theaters or auditoriums must take into account sound wave interference. The shape of the building or stage and the materials used to build it are chosen based on interference patterns. They want every member of the audience to hear loud, clear sounds.
Sound Traveling Between Materials
Remember that sound travels faster in some materials than others. Sound waves travel outward in straight lines from their source until something interferes with their path. When sound changes mediums, or enters a different material, it is bent from its original direction. This change in angle of direction is called refraction. Refraction is caused by sound entering the new medium at an angle. Because of the angle, part of the wave enters the new medium first and changes speed. The difference in speeds causes the wave to bend.
Critical Angle
The angle of refraction depends on the angle that the waves has when it enters the new medium. As the angle from the wave to the barrier between the two mediums gets smaller, the angle of refraction also gets closer to the barrier. When the wave’s entering angle reaches a certain point, called thecritical angle, the refraction is parallel to the dividing line between the mediums. The critical angle depends on the two mediums the sound is coming from and going to. The speed of sound is different in every medium. Because of this, even if the sound hits at the same angle, the angle of refraction will vary for different mediums. The greater the difference in speed between the two mediums, the greater the critical angle will be.
If sound hits the new medium with any angle smaller than the critical angle, it will not be able to enter. Instead it will bounce off, or be reflected, from the dividing line. When a wave is reflected, it returns with an angle equal to the one with which it hit. Whenever sound hits a new medium, part of it is reflected back. The rest enters the new medium and is refracted. Imagine sound is traveling through the air and hits the wall of a brick building. Some of the wave is reflected, but much of it enters the brick. The part of the wave going through the brick is now going faster than the part in the air. This is because brick is a solid whose molecules are closer together and can transmit sound more quickly. This difference in speeds caused the wave to bend, or be refracted. Suppose that the wave hits the building with an angle that is smaller than its critical angle. This time, the wave cannot enter the brick and all of it is reflected. If the wave struck the wall with an angle of 15 degrees, it would reflect back with the same angle from the other side. Since there are 180 degrees total, the reflected angle would be 165 degrees, 15 degrees measured from the other direction.

After reading this section you will be able to do the following:

• Discuss the relationship between the speed of sound and speed of light.
• Describe what the sound barrier is.

Questions

1. What conclusion can you draw about the speed of sound relative to the speed of light?

Sound and speed
If you have ever been to a baseball game or sat far away from the stage during a concert, you may have noticed something odd. You saw the batter hit the ball, but did not hear the crack of the impact until a few seconds later. Or, you saw the drummer strike the drum, but it took an extra moment before you heard it. This is because the speed of sound is slower than the speed of light, which we are used to seeing. The same thing is at work during a thunderstorm. Lightning and thunder both happen at the same time. We see the lightning almost instantaneously, but it takes longer to hear the thunder. Based on how much longer it takes to hear thunder tells us how far away the storm is. The longer it takes to hear the thunder, the farther the distance its sound had to travel and the farther away the storm is.
The sound barrier
The speed of sound through warm air at sea level has been measured at 346 meters per second or 0.346 km per second. That is the same as a car traveling about 780 miles per hour! Even most jet airplanes do not travel that fast. When a plane does go faster than speed of sound, it is said to break the sound barrier and a sonic boom is produced. On October 14, 1947, Chuck Yeager did just that. In a small plane called the X-1, he was the first person to fly faster than the speed of sound and the listeners on the ground were the first to hear the loud shock wave of a sonic boom.
Why do we see lightning before the thunder?
The flash of light from lightning travels at about 300,000 kilometers per second or 186,000 miles per second. This is why we see it so much sooner than we hear the thunder. If lightning occurs a kilometer away, the light arrives almost immediately (1/300,000 of a second) but it takes sound nearly 3 seconds to arrive. If you prefer to think in terms of miles, it takes sound nearly 5 seconds to travel 1 mile. Next time you see lightning count the number of seconds before the thunder arrives, then divide this number by 5 to find out how far away the lightning is.