Wave Introduction: Types of Waves

  Let's first take a look at waves which move in a material. These waves are called mechanical waves and result from a disturbance in the material. Typically, as the material tries to return to its undisturbed state, the disturbance travels as a wave.

What are the types of Mechanical Waves?

Since mechanical waves need a material to continue to travel there are several types of disturbances that produce these waves. The types of mechanical waves are characterized by how the material moves with respect to the wave's direction of travel. Below are simulations of two kinds of mechanical waves, can you investigate the different properties of these types of waves?

In the simulation below, imagine that you are moving the left end of the string up and down. To do this, move the amplitude slider on the left close to the top of the scale and leave it there. A wave with a height matching the position of the amplitude slider will now move to the right on the string. Move the slider to a new position and see how the amplitude of the wave changes. Look at the part of the string (represented by a red disk) that is moving on the yellow reference line. How is that part of the string moving?

Now imagine in the simulation below that you have struck a tuning fork (press the CLAW play button in upper left corner), which is now vibrating. The as the blades of the fork move left and right, they push and pull on the surrounding air molecules (represented by blue dots). Look at one column of air molecules. In which direction(s) are the air molecules moving?

  We've seen that the organized motion of the medium produces waves. The amount that a piece of the medium has moved from its equilibrium position (where it would normally be if all the material was completely at rest) is called the displacement. This is a very important term and generally applies to any deviation of the medium from equilibrium. In the two cases above, the displacment is actually a distance and direction that a piece of medium has moved away from equilibrium. The displacement also could be the change in other properties of the medium away from equilibrium. It could be a change in temperature, change in pressure, or even a change in orientation. In fact, for a sound wave the displacement of the positions of the air molecules also creates a variation in the pressure of air, which is proportional to the density of air molecules. So we can describe a sound wave either as a periodic displacment of air molecules or as a periodic displacement of air pressure, i.e., a pressure wave.

In the simulations above we have shown a transverse wave, where the motion of the medium is perpendicular to the direction of the wave's travel, and a longitudinal wave, where the motion of the medium is parallel to the direction of wave's travel. In a torsional wave, the medium rotates about the axis along which the wave is travelling. One of the waves in the previous page is a torsional wave. Click the "Prev" button below and see if you can find it.

Mechanical waves are all around us. Can you answer the questions in the pop-up window quiz?