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# Refraction of Mechanical Waves

## Mechanical waves entering a new material change their direction and velocity of travel.

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Acoustic Diffuser

### Acoustic Diffuser

Credit: JJ Harrison
Source: http://en.wikipedia.org/wiki/File:MLS_diffusor.gif

How can you prevent echoes? Read on to learn how acoustic diffusors work.

#### 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 coloration and echoes in an environment. Echoes are a result of the same sound wave arriving at a listener at different times. To be more scientifically accurate, an echo is a sound wave has been reflected off of a discontinuity in a medium, such that there a sufficient delay for the sound to be heard again.
• The reflective and refractive properties from sound waves are very similar to that for light. When sound waves are incident on a smooth flat reflective surface, the angle of reflection is equal to the angle of incidence

\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 enters a medium where it would have a higher velocity, that portion of the wave speeds up compared to the part of the wave that is still outside the medium. This behavior can be modeled by using a modified version of Snell's law when dealing with longitudinal waves

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

where \begin{align*}v_1\end{align*} is the longitudinal wave velocity in medium 1 and \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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