We've discussed frequency as the oscillation speed of the soundwave, but what about the other dimension, amplitude? A waveform's amplitude, or amount of push and pull, can be measured using sound pressure level, or SPL. SPL is a logarithmic scale measured in decibels, or dB, above a standard reference level. The standard reference level most commonly used for the starting point of 0 dB is 20 micropascals RMS.
Pascals are unit of measurement for pressure or stress. 20 micropascals is usually considered the threshold of human hearing at 1k. The decibel scale is a logarithmic measurement scale. Turning up an audio signal's volume by 1 dB is basically imperceptible. For most of us, it takes at least a 3 dB increase for us to notice it. However, because of its logarithmic scaling, the loudness escalates quickly.
A 10 dB increase represents a perceived doubling of loudness, and an increase of 20 dB is about four times as loud. Sound confusing? Don't worry. All you need to remember is that 0 dB marks the threshold of our ability to sense sound, and as dBs increase, amplitude increases, and so does the perceived loudness of the soundwave. For example, a quiet room has a noise floor about 20 to 30 dB, while a normal conversation at the dinner table sits it at around 40 to 60 dB.
An average vacuum cleaner is about 80 dB, while front row at a rock concert gets up to around 120 dB. Beyond that, our threshold of physical pain starts at about 130 dB.
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