As everyone who attempts to get someone's attention from a distance intuitively knows, sound level decreases as the distance between the source and the detector increases. For distances less than 50 ft, the rule of thumb states that sound level drops 6 dB for every doubling of the distance between the sound source and the detector.

If your work involves generation of audible signals, the rule of thumb may appear simple, but putting it into practice takes valuable time to ensure that you correctly calculate the conversion. To complicate matters, there's no standard single distance for measuring sound level, and thus conversion of sound levels for different separations or between metric- and nonmetric-measurement units requires rethinking and recalculation.

For example, if an audible signal source measures 90 dBa at a distance of two feet, what's the equivalent sound level at a distance of 10 cm? If you can perform this conversion without putting pencil to paper, you're several steps ahead of your competition. To ease sound-level conversions, you can use an Excel spreadsheet (click here to download). You enter a sound level in decibels acoustic, and the calculation returns sound levels for various commonly used measurement distances.

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