Sound Resonance and Natural Resonant Frequency

Sound Resonance

Resonance comes from the Latin word resonare, meaning to "return to sound." Resonance literally is an induced vibration in an object and means to sound and resound, like a echo.

Vibrating Base
Guitar Strings

Usually we think of resonance in terms of objects such as bells which, when struck, continue to ring or resonate the sound. Although accurate, the field of resonance is actually much more detailed an interesting. It ranges from explaining pleasant music and harmonic relationships to the destruction of the Tacoma Narrows Bridge. (Click to watch a video of the bridge collapsing). The bridge literally vibrated itself to the point of collapse in a 40 mile per hour wind.

Resonance can be demonstrated by observing a guitar string. When it is plucked, it tries to return to its natural, resting position, but because of momentum, it continues to the opposite position, then again tries to return to the resting position, but momentum keeps it going until decay finally causes it to stop.

Similarly, when a bell is struck, it implodes, then tries to return to its resting position, but, again, because of momentum, continues past that position, etc., until decay causes it to stop.

Interestingly, if you fully extend your car antenna, then apply slight pushes back and forth at the base at just the right frequency, you can cause the top of the antenna to whip back and forth, 8" or more.

Natural Resonant Frequency

A sound wave is created as a result of an object vibrating. The vibrating object could be anything from vocal cords to a string of a guitar, a horn or a race car engine. Any object which vibrates will create a sound, although we may not be able to hear it.

An example is the radio antenna of a car. If you pluck it, you will notice that it immediately starts to vibrate at a particular frequency.

Many objects, when hit, struck, plucked or hammered will vibrate. When you drop a penny on a hard floor, it will begin to vibrate. When you pluck a guitar string, it will vibrate. When each of these objects vibrates, they tend to vibrate at a specific frequency, their "natural" frequency. If that frequency is within range of human hearing and loud enough, you will be able to hear the sound vibrations being produced.

All objects have a natural frequency or group of frequencies at which they like to vibrate. Some objects vibrate or ring at a single natural frequency, while others have many "modes" of vibration and can vibrate at any or all of those modes of vibration. A flute or radio antenna tends to vibrate at a single frequency. Other objects, say for example a pencil, will vibrate in many modes, giving it the distinctive sound of a pencil. Some objects, like the string of a guitar, have a number of modes of vibrations that are all mathematically related, as Pythagoras discovered. A guitar string, with its related modes will sound more "musical" than a metal plate, which has unrelated vibration modes thereby making it sound more annoying and unmusical.

At what frequency or frequencies will an object vibrate? That depends on a number of physical factors including the object's mass and elasticity. The frequency of a bell will depend on its thickness, the type of metal, the size and shape of the bell, and other properties.

The natural frequency of a guitar string can easily be changed by pressing the string down on a fret to shorten its length or by changing the tension of the string. A guitarist is, therefore, able to make music by controlling the natural frequencies of the strings and playing a sequence and combination of sounds that is musical.

The same principal applies to the musical instruments where the natural frequency is changed by controlling the qualities of the resonant air chambers. For example, with a trombone, the length of the resonant air chamber is controlled by moving the slide in an out, thereby changing the length of the chamber. This change in length is reflected in a change in pitch of the natural resonance of the instrument.

Conclusion

In conclusion, all objects have one or more natural frequencies at which they will vibrate when stuck or otherwise disturbed. The natural resonant frequency is dependent on the object and various other physical factors.