E2S Warning Signals, a manufacturer of “high-performance audible and visual signals for commercial, industrial, marine, onshore and offshore hazardous locations” with offices in the UK and US, has published an educational guide that seeks to ‘de-mystify’ the dB (decibel), one of the most common units in engineering, “yet also one of the most confusing”.
“The main difficulty in comprehending dB measurements,” says E2S, “is that it is a log-linear scale, which, while it is very useful in representing very large differences in a more user-friendly scale, also compresses the scale. Sound is defined as any pressure variation that can be heard by the human ear.
“The threshold of hearing is defined as 0dB [SPL]; the threshold of pain is around 130dB [SPL], a scale that can be readily understood. However, as the power difference between the two levels is 1013 : 1, or, to spell it out in full, 10,000,000,000,000 or ten million million to one, expressing it in these terms suddenly gives a much better idea of the change.”
The guide is all based around the company’s products, of course, but nevertheless it’s an interesting read for the curious or the concerned. For example, the A151 disaster warning sounder is rated at 150dB SPL, the same as a jet engine at 1m. It also discusses the effect of frequency on perceived loudness and includes some rules of thumb for system designers who are specifying the output and location of warning sounders to provide the desired sound levels as part of a warning system.