Nondestructive Evaluation Physics : Sound (2024)

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 called interference. 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 produce dead 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. Refraction is also discussed more in the following page.

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 the critical 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.