Mechanical waves

Table of Contents

1. Overview
2. Mechanical waves
   1. Types of mechanical waves
   2. Characteristics of mechanical waves
3. Wave behaviour
   1. Reflection
   2. Reflection in 2-D
   3. Refraction
   4. Diffraction
4. Oscillations
   1. Free oscillations
   2. Forced oscillations
   3. Resonance
5. Interference
   1. Principle of superposition
   2. Standing waves
   3. Interference in 2-D
   4. Applications of interference
6. Task: 2004 Sumatran Tsunami

1. Additional resources:
   1. Glossary
   2. Print content

Reflection

Energy in the form of waves will be reflected at the boundary of two media because the oscillations of the particles are not synchronous across the boundary. How much energy is reflected depends on the differences between the density and elasticity of the media.

Acoustic physicists use a term called characteristic acoustic impedance to describe the acoustic character of a medium. If two media have similar impedances there will be little reflected energy; most of the energy will be transmitted. If the difference in impedance is large, a greater proportion of the energy is reflected.

Let's consider the reflection of mechanical waves according to differences in impedance between two media.

Longitudinal waves

Transverse waves

Applications of reflection

Consider the set of protective headphones shown on the right. The difference in the characteristic acoustic impedances of air and the plastic headphones is quite large. As a result a large proportion of the external sound wave energy is reflected, protecting the wearer's hearing.

Ultrasound imaging is a diagnostic medical technique that relies on marked differences in characteristic acoustic impedances. On the right is an ultrasound image of an unborn baby. Just like submarine sonar, reflected sound (echoes) are created due to the impedance mismatch – in this case between the body of the baby and the surrounding embryonic fluid. A gel is smeared onto the skin of the baby's mother to minimise energy reflection due to the large impedance difference between air and skin.

The flute is an open pipe, musical instrument. Sound waves travel down the pipe from the mouthpiece and reach a low impedance boundary at the open end.

There is a difference in impedance because air constrained by the pipe behaves differently to unconstrained air. In the case of the flute compression pulses are reflected back up the tube as rarefaction and vice versa.