A wave is defined as the transfer of energy from one point to another. There are two large, all encompassing categories of waves: mechanical and non-mechanical.

 

Mechanical waves require a medium for the transfer of energy to occur. For example, water waves are mechanical. Tsunami waves released after an earthquake transfer the energy of the quake to distant shorelines. Sound waves are another type of  mechanical wave. They are compression waves that have a frequency between 20-20000 hertz and travel at an speed of approximately 340 m/sec at room temperature. Different substances carry compression waves at various speeds; metals carry it faster than water which transfers it faster than air. As a mechanical wave travels through a medium, it loses energy to the medium. The molecules in the medium are forced to vibrate back and forth, generating heat. Consequently, the wave can only propagate through a limited distance. When this event happens, we say that the wave has been damped. Damping can be observed by the fact that the wave's amplitude has decreased.

 

Non-mechanical waves do not require a medium for the transfer of energy to occur. The only type of non-mechanical waves are electromagnetic in nature. They can travel through the vacuum of space. Light from distant stars travel hundreds of thousands of millions of years to reach us. Although the electromagnetic radiation spans a large spectrum of wavelengths and frequencies, all electromagnetic radiation travels through a vacuum at 3 x 10⁸ m/sec.

 

Type of radiation	Range of wavelengths
radio	570 down to 2.8 meters
TV	5.6 down to 0.34 meters
microwave	0.1 down to 0.001 meters
infrared radiation	10-3 down to 10-7 meters
visible light red orange yellow green blue indigo violet	700 to 400 nm
ultraviolet	10-7 down to 10-10 meters
x-rays	10-10 down to 10-12 meters
gamma rays	shorter than 10-12 meters

 

Within these two large categories, there are four principle types of waves:

• transverse - waves in which the particles vibrate at right angles to the direction of the wave's velocity (ex: waves along a string). Transverse waves can be polarized since these vibrations can be constrained to move in only one plane.

Acoustics Animations © Dr. Dan Russell, 1999 The above animation was created using a modified version of the   Mathematica ® Notebook "Sound Waves" by Mats Bengtsson.

 

Amplitudes are measured in terms of the distance above or below equilibrium.

Acoustics Animations © Dr. Dan Russell, 1999 The above animation was created using a modified version of the   Mathematica ® Notebook "Sound Waves" by Mats Bengtsson.

 

 

Acoustics Animations © Dr. Dan Russell, 1999

picture courtesy of Carleton University video: Tacoma Narrows Bridge

 

When examining waves, information is usually displayed in two types of graphs, vibration graphs and waveform graphs. The shapes of both types of graphs are the same, the only difference is in the labels for the x-axis. A vibration graph displays the behavior at a SINGLE location along the wave's path as time passes. One vibration can be defined as one complete cycle, or back and forth motion. A waveform graph displays the behavior of a multitude of locations at a SINGLE moment in time.

 

 

Vibration graphs inform the reader of the wave's shape, amplitude, and period. While waveform graphs inform the reader of the wave's shape, amplitude, and wavelength. A wave is either periodic, usually a sinusoidal pattern repeats itself at regular intervals, or it is a pulse, a one-time disturbance. All wave motion is generated by a source that moves or vibrates. Consequently, the frequency of the wave is a property of the source, not of the medium through which the wave subsequently travels. The examples shown below are periodic in nature.

 

The amplitude, A, is the wave's maximum disturbance from it undisturbed equilibrium position and represents the energy being transferred by the wave. Generally, the energy of a mechanical wave is proportional to the square of the wave's amplitude; i.e., if a wave's amplitude triples, its energy content is 9 times greater.

 

On a waveform graph, the wavelength, λ, is the distance between two adjacent in-phase points on a waveform graph.  A crest is a point of  maximum positive amplitude along the wave while a trough is point of maximum negative amplitude.

 

On a vibration graph, the period, T, is the time between two adjacent in-phase points on a vibration graph. The reciprocal of period is frequency, f. It represents the numbers of waves that pass a given location each second along the wave's path.

 

Two points are said to be in-phase if they behave exactly the same; that is, if they are a multiple of a wavelength apart. If two points are not in-phase, then they are out-of-phase. Since a wavelength corresponds to one complete vibration, or one complete revolution, one wavelength is often expressed as 360º.  So in-phase points are separated by n360º. Out-of-phase points can be any number of degrees apart. Although we usually speak of points which are separated by 90º, 180º, or 270º.