First of all, the term "frequencies," is a short-hand way of talking about the frequency content of the noise control problem at hand. It is a critically important consideration, because noise control treatments are generally most effective at a particular frequency, or range of frequencies. Some noise regulations even specify sound level limits on a per-frequency basis.
You can think of sound frequencies like musical pitch. Low frequencies would be the low notes, and high frequencies would be the high notes. If a chorus, band, or orchestra all played random notes simultaneously, it would probably sound more like noise than music.
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