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# Longitudinal Wave

## Mechanical waves in which energy travels parallel to the wave's oscillation.

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Acoustic Diffuser

### Acoustic Diffuser

Credit: JJ Harrison
Source: http://commons.wikimedia.org/wiki/File:MLS_diffusor.gif

By scattering sound waves, the acoustic diffuser pictured helps remove echoing that might occur in large, vacant rooms.

#### Amazing But True

• An acoustic diffusor is used to diffuse sound energy in an environment. By reflecting incident sound waves in many directions, acoustic diffusors are able to reduce localization of sound and echoes in an environment. Echoes are a result of the same sound wave arriving at a listener at different times.
• The reflective and refractive properties from sound waves are very similar to those of light waves. When sound waves are incident on a smooth flat reflective surface, the angle of reflection is equal to the angle of incidence

θi=θr\begin{align*}\theta_i=\theta_r\end{align*}

• Refraction when dealing with sound waves is a little more complex. The velocity of sound in each medium is dependent on several properties such as the elastic modulus and density. When sound waves enter a medium in which they would have a higher velocity, a portion of the waves speed up compared to the waves that are still outside the medium. This behavior can be modeled by using a modified version of Snell's law when dealing with longitudinal waves:

sinθ1v1=sinθ2v2\begin{align*}\frac{\sin \theta_1}{v_1}=\frac{\sin \theta_2}{v_2}\end{align*}

v1\begin{align*}v_1\end{align*} is the longitudinal wave velocity in medium 1 and v2\begin{align*}v_2\end{align*} is the longitudinal wave velocity in medium 2.

• Watch how ping pong balls can demonstrate sound reflection and diffusion:

#### Show What You Know

Using the information provided above, answer the following questions.

1. If a sound wave is incident on a smooth reflective surface at 0?, what direction would the reflected wave be at?
2. How can you tell that the diffuser in the video causes the energy to be spread in multiple directions?

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