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 The Basics of Sound

Acoustic. ​​​​​The Basics of Sound

At the simplest level sound is the movement of air molecules. This movement creates very slight local differences in the air pressure which cause our ear drums to expand and contract - allowing us to perceive the air movement as sound.

Sound travels in waves - the initial movement pushes air molecules together creating a localised area of high pressure and a localised area of low pressure immediately behind. This partial vacuum then “pulls” air behind it creating a moving wave.

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        <i class="icon-chevron-right"> What is frequency?
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        <p> Frequency, measured in Hz, is the wavelength - or how often the wave pattern of the sound repeats. Higher frequencies are perceived as “high pitched” whilst low frequencies are perceived as “low pitched”. It is important to note that human hearing is not equal over the frequency spectrum. Although we can hear across a broad range of frequencies our ears are optimised to hear sound at around 1,000 Hz so noise at these frequencies sound louder to our ears.  </p>
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        <i class="icon-chevron-right"> What is air-borne noise?
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        <p> When a sound wave is carried by air particles the noise is said to be “air-borne” noise.  </p>
<p> Gas &amp; liquid molecules are not strongly bound together and the molecules are free to move independently. Air-borne noise can carry higher frequency noise more easily as a result.  </p>
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        <i class="icon-chevron-right"> What is structure-borne noise?
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        <p>Sound waves can be transmitted through solid materials in the form of vibrations.  </p>
<p>Because molecules in sold materials are strongly bound together it&#39;s harder for them to move far enough to carry higher frequency noises but low frequency noise travels much more easily.  </p>
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        <p>Decibels (dB) are a measure of how powerful a sound pressure wave is (how much air is displaced by the wave). The more powerful a wave the louder we hear it. </p>
<p>Since decibels work on a logarithmic scale, decreasing noise by 3 dB means the noise is only half as loud. Even a small difference in dB ratings of noise can translate to a big difference in “loudness”.  </p>
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        <p>Human hearing is “biased” towards to certain frequencies. We hear noise at frequencies around 1,000 Hz much more loudly than noise at frequencies of 500 Hz or less - even when the noise at 500 Hz is more powerful. </p>
<p>A-weighting is a process that seeks to redress the bias in human hearing by adjusting the dB levels to reflect how loudly we perceive the noise.  </p>
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A simple introduction to the concepts behind acoustic engineering

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