Ultrasonics – Sound – What is Resonance?

Nearly everyone has heard the term “resonance” or “natural frequency” but everybody has a little different idea of what resonance is all about.  In fact, this is understandable because resonance means different things under different situations and there are many different ways to produce resonance.

Resonance is the condition that arises when, at a specific frequency (or frequencies), the reflection of sound waves within the resonating item provide constructive interference or re-inforcement of vibrations within the item.  A bell, for example, rings with a certain tone because it is resonant at that particular frequecy.  The resonant frequency is also called the “natural frequency.”  It is the frequency at which an item will vibrate naturally.

The following example may help in the understanding of resonance – –

A child’s swing is simply a resonant pendulum.  Try as you might, you can not make the swing’s period (the time for the swing to travel from one extreme position to the other and back) change as long as the length of the supporting ropes remains the same.  No matter if it is a 30 pound child or a 250 pound man sitting in the swing, the period of swing will remain essentially the same.  The swing’s natural or resonant frequency depends almost totally on the length of the ropes!  Lengthening the ropes will increase the period (slower swinging) while shorter ropes will shorten the period (faster swinging).  The period of the swing is also nearly independent of amplitude.  For all practical purposes, the period remains the same if the swing is swinging just a few inches back and forth or if it is swinging all the way from nearly horizontal at one extreme to nearly horizontal at the other extreme.  Of course, if the amplitude is increased, the speed of the swing has to increase in order for it to be able to accomplish the required travel to get from one extreme point to the other in the fixed period of time dictated by the defining rope length.

Resonance of solid items is dependent on shape and size as well as the speed of sound in and the “stiffness” of the material.

Most musical instruments utilize the principles of resonance to produce musical notes but it’s not always the instrument itself that is resonant.  For example, a xylophone has metal bars of varying sizes and shapes which sound a resonant note when struck.  Larger bars resonate at higher frequencies than smaller bars.  The triangle is similar in concept but is designed to resonate at many frequencies all at once which results in its unique sound.  String instruments rely on the resonance of strings of specific lengths, materials, cross-sections and under specific tensions to produce the desired musical notes.  In wind instruments, it is not the instrument itself that is resonant but, rather, the column of air within the instrument.  Wind instruments utilize variable lengths of tubing to change the length of the air column within which is set into resonant vibration by the player’s breath.

Confined liquid volumes also resonate.  The periodic slosh of water in a bucket is an example of resonance.  As a simple experiment, one can fill a shallow pan (cake pan for example) with water.  Start moving the pan back and forth on a smooth surface.  You will soon discover that at one specific rate of oscillating movement the waves within the pan will increase as reflected waves reinforce original waves to the point that the water is no longer confined by the pan and spills over the edge.  This is another example of resonance.  Oscillating the pan at another rate will not produce the same result.

We all utilize or encounter resonance in our everyday lives without realizing it.  In upcomong blogs we will explore how resonance is utilized in the application of sound to a variety of applications.

–  FJF  –