WAVES AND SOUND

To begin our study of waves, we will first need to define some terms:

Wave - A transfer of energy without a transfer of matter through a material (called a medium) that causes the material to vibrate.

Transverse Wave - the particles of the medium vibrate in a direction that is perpendicular to the direction of the wave itself. In the animation below notice how the ends of the wave move relative to the direction that the wave is traveling.

Longitudinal Wave - the particles of the wave vibrate in a direction parallel to (along) the direction that the waves are traveling.

A Pulse is a single disturbance traveling through a medium.

A Periodic Wave is a series of waves caused by a periodic (regular) disturbance. A periodic wave has a vibration or oscillation as its source.

Now we want to learn some basic wave terminology. As you read refer to the following diagram:

WAVE ANATOMY

The Wavelength, , is the horizontal distance between corresponding points on consecutive waves and is measured in meters. Crests are one wavelength apart. Troughs are also one wavelength apart. Points X and Y above are one wavelength apart.

Points that are at the same place on the wave and are going in the same direction at the same time are said to be in phase. Points X and Y are in phase with each other. Crests are in phase with each other. Troughs are in phase with each other.

Points that are moving in opposite directions at the same time are said to be 180 degrees out of phase. Crests are 180 degrees out of phase with troughs. Points Y and Z are 180 degrees out of phase with each other.

Amplitude - The distance a wave rises or falls from a normal rest (equilibrium) position. The greater the amplitude of the wave, the greater the amount of energy carried by the wave. Areas labeled A in the diagram above measure amplitude.

The Frequency of a periodic wave is the number of wavelengths that pass a given point in one second. Frequency is measured in Hertz (Hz). 1 Hz is one wave per second (1/s).

The Velocity of a periodic wave is given by Velocity = (Frequency) X (Wavelength). The speed of a wave does not depend on either amplitude or frequency but only on the properties of the medium.

The Period of a wave is the time required for one complete wave to pass a given point. Period = 1/ Frequency

WAVE BEHAVIORS

Principle of Superposition - at the point where two or more waves meet, the displacement of the medium is the sum of the displacements of the individual waves. Individual waves are not changed by passing through one another. See a simulation (Java).

The effect of two or more waves traveling through a medium is called Interference.

Constructive Interference occurs when two pulses combine to produce a pulse of greater amplitude. See a simulation (Java) (will open in a new window).

Destructive Interference occurs when two pulses combine to produce a pulse with smaller amplitude than either of the original amplitudes. See a simulation (Java) (will open in a new window).

A Standing Wave (black) is produced by the interference of two periodic waves of the same amplitude and wavelength (red and blue) traveling in opposite directions. The points in a standing wave that do not move as waves pass through each other are called nodes. Points where the displacement are largest are called antinodes. See a movie of a standing wave below.

When a wave travels from one medium to another, the wave is both reflected and transmitted.

When a wave is reflected from a more dense medium, it undergoes a phase change of 180 degrees. When a wave is reflected from a less dense medium there is no phase change. See a simulation (Java) (will open in a new window). Simulation shows reflection from a fixed end (more dense medium). To see reflection from a free end (less dense medium) click on "Click here" in the simulation window.