Sound waves traveling through the air or other mediums sometimes affect the objects that they encounter. Recall that sound is caused by the molecules of a medium vibrating. The molecules vibrate at a specific frequency for each source, called its natural frequency. Steel, brass, and wood all have different natural frequencies.
Occasionally, objects vibrating at their natural frequencies will cause resonance. Resonance is when objects with the same natural frequency as the vibrating source also begin to vibrate. Resonance does not happen very often and only affects object close to the vibrating source. Sometimes, the effects of resonance can be powerful. A singer can make glass vibrate enough to shatter, just by singing a note with the glasss natural frequency!
A string vibrates with a particular fundamental frequency. It is possible, however, to produce pitches with different frequencies from the same string. The four properties of the string that affect its frequency are length, diameter, tension, and density. These properties are described below:
- When the length of a string is changed, it will vibrate with a different frequency. Shorter strings have higher frequency and therefore higher pitch. When a musician presses her finger on a string, she shortens its length. The more fingers she adds to the string, the shorter she makes it, and the higher the pitch will be.
- Diameter is the thickness of the string. Thick strings with large diameters vibrate slower and have lower frequencies than thin ones. A thin string with a 10 millimeter diameter will have a frequency twice as high as one with a larger, 20 millimeter diameter. This means that the thin string will sound one octave above the thicker one.
- A string stretched between two points, such as on a stringed instrument, will have tension. Tension refers to how tightly the string is stretched. Tightening the string gives it a higher frequency while loosening it lowers the frequency. When string players tighten or loosen their strings, they are altering the pitches to make them in tune.
- The density of a string will also affect its frequency. Remember that dense molecules vibrate at slower speeds. The more dense the string is, the slower it will vibrate, and the lower its frequency will be. Instruments often have strings made of different materials. The strings used for low pitches will be made of denser material than the strings used for high pitches.
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