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What Is the Direct Measure of a Mechanical Wave?

Mechanical waves represent a periodic disturbance in a medium. They depend upon the interaction between particles and, therefore, cannot propagate through a vacuum. Energy transfers from particle to particle, in the direction of wave propagation. These waves can be directly measured in terms of their wavelength, frequency, period and amplitude.

Types of Mechanical Waves
- There are two types of mechanical waves: longitudinal and transverse. The difference lies in how their particles move. Longitudinal waves displace particles in a direction parallel to their direction of propagation, meaning they are compression waves. Transverse waves displace particles in a direction perpendicular to their direction of propagation. Longitudinal waves move like a slinky, while transverse waves move like a vibrating rope.
Wavelength
- Wavelength is a measure of the distance required to complete one cycle. For a longitudinal wave, this measurement is equal to the distance from one compression to the next, or from one rarefaction to the next. For a transverse wave, this measurement is equal to the distance from one crest to the next, or from one trough to the next.
Frequency
- Frequency is a measure of how many cycles occur over a given amount of time. The most common unit is the Hertz, which is equal to the number of cycles that occur in one second. A cycle is completed when a given particle in the medium returns to its starting position. For a longitudinal wave, a cycle is compression to compression. For a transverse wave, a cycle is crest to crest. However, for both wavelength and frequency, any starting point can be used. This can also be thought of as the number of waves, each defined as one wavelength, per unit of time.
Period
- Period is the amount of time it takes for a wave to complete one cycle. For example, if a wave has a frequency of 20 cycles per minute, its period would be equal to three seconds. Furthermore, by combining the period of a wave with its wavelength, the speed of the wave can be determined. If the above wave had a wavelength of nine feet, its speed would be nine feet per three seconds, or three feet per second, or approximately two miles per hour.
Amplitude
- Amplitude is a measure of a wave's intensity. For a transverse wave, the crest rises above the point of equilibrium or resting position, while the trough drops below it. Amplitude is the distance between the crest or trough and the resting position. Wave height is measured from crest to trough, or twice the amplitude. For a longitudinal wave, there are no crests or troughs. Instead, amplitude represents the maximum change in pressure from rest to compression, or rest to rarefaction.

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