Vibrational Modes

An object will vibrate at a natural frequency or frequencies when strummed, plucked or otherwise disturbed. In this section, we're going to look at vibration more carefully.

The easiest case to consider, as Pythagoras did thousands of years ago, is a string in tension. When you pluck a string which is held in tension with both ends firmly fixed, you will find that, with practice, it can be made to resonate in several different ways.

(picture) todlk
The fundamental vibrational mode of a string

This shows the string in its most normal vibrational mode. The whole string goes up, the whole string comes down. This is the fundamental or base mode. It occurs at the lowest frequency.

(picture) todlk
Second vibrational mode

Here the string is vibrating in two places. Notice that the point in the middle does not move. In effect, it is the same as having a string half as long.

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Third Vibrational Mode

Now we have essentially split the original length into three equal length sections, all vibrating independently.

Circular Membrane Vibrational Modes

This effect becomes especially interesting when we look at objects which are more complex. In this case we'll use a drum, known technically as a circular membrane.

The drum has several modes of vibration.

TODLK Can you remove the gray background?:

It is interesting to note that although this is a loud vibrational mode because the membrane surface is in contact with many air molecules, the vibrational energy is quickly absorbed and the vibration stops. This is the "thump" that you hear when you strike a drum near the middle.

In this mode, a part of the membrane is rising while another part is falling. This mode transmits much less sound energy into the environment and, therefore, takes longer to decay providing an extending sound. See more vibrational modes of drums.

TODLK Can you remove the gray background?:

Superposition of Modes

An object can and usually will vibrate in multiple modes simultaneously. This fits with our understanding of sound, which supports superpositioning of waves.

In the next section we'll look at the standing wave patterns that exist in all resonant vibrational modes. todo - delete after sequencing

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