1.5. Basics: what is sound? Frequency? Wavelength?
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This article is from the Active Noise Control FAQ, by Dr. Chris Ruckman
1.5. Basics: what is sound? Frequency? Wavelength?
If you are not familiar with how sound works, the following brief refresher course may help. Don\u2019t be put off by occasional technical jargon; most of the FAQ includes analogies and examples to illustrate ideas in plain language. (The author apologizes to acousticians everywhere for presuming to summarize their craft in a mere three paragraphs!)
Sound is a pressure wave traveling in air or water. A sound wave resembles the surface wave made when you throw a stone into a calm pool of water, except that
* the sound wave consists of tiny fluctuations in the air pressure rather than fluctuations in water height,
* a sound wave can travel in three dimensions rather than two, and
* the wave speed is much faster (340 meters per second in air).
Sound is usually generated by vibration of an object or surface such as a speaker cone, a violin body, or human vocal cords. The vibrating surface "radiates" pressure waves into the adjoining air or water as sound. (Sound can also be generated by turbulent airflow, by bubbles collapsing, or by many other phenomena.)
The frequency (number of wave crests per unit time that pass a fixed location) measures the tone or pitch of a sound. For example, a bass guitar plays lower frequencies than a violin. The wavelength, or distance between wave crests, is related to frequency: lower frequencies have longer wavelengths. In air, all frequencies of sound travel at the same speed. When bending waves travel through a flexible structure, however, low frequencies travel faster than high frequencies.
In this context, noise is simply *unwanted* sound. There is an old trick question: "If a tree falls in the forest and nobody is there to hear it, does it make any noise?" The answer is "no" because sound cannot be *noise* unless somebody hears it and finds it offensive. However, if the question is phrased "Does it make any *sound*," then you have a deep philosophical question to ponder!
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