Sound Attenuation Calculator NZ

This calculator works out how a sound level falls as you move away from its source, using the inverse-square law that governs sound spreading in open space. Sound radiating from a small source spreads out over an ever-larger sphere, so its intensity drops with the square of the distance, and its level in decibels falls by a predictable amount. The well-known rule of thumb is that the level drops by about six decibels each time the distance from the source doubles. This matters for assessing noise: working out how loud a machine, concert or road will be at a neighbouring property, planning the placement of speakers, or checking compliance with noise limits. This tool does the calculation. You enter the known sound level at a reference distance, that reference distance, and the new distance you are interested in, and the calculator returns the sound level at the new distance, the drop in decibels, the ratio of the distances, and a reminder of the six-decibel-per-doubling rule. The results update as you type. Use it for noise assessment, acoustics and audio work, or physics study. The level at the new distance is the reference level minus twenty times the base-ten logarithm of the ratio of the new distance to the reference distance. The twenty, rather than ten, appears because sound level in decibels is based on pressure, which falls in direct proportion to distance, and squaring inside the logarithm brings out the factor of two. A key point: this applies to a point source radiating freely in the open, the free-field condition. Indoors, reflections keep the level up, and line sources like a steady stream of traffic fall off more slowly, by about three decibels per doubling, so treat the result as the open-air, point-source case.

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Level at new distance = level - 20 x log10(new distance / reference distance). About -6 dB per doubling of distance. Assumes a point source in free space; indoors and line sources differ.