Small values are visible, and large values are practical. This is the real value of using a log scale for signals with low precision and a large dynamic range. Distorted “sounds” that are louder are possible, and can be produced by explosions. Environmental standards that refer to or regulate background noise are usually written to absolute levels as well.Īs a point of reference, at sea level the loudest sound that can be made without distortion peaks at 1 atm pressure, which is 191 dB SPL, and really not a sound you want to hear. There are standards that describe alarms that define the SPL at the sounder itself as an absolute level. Long exposures to levels over 85 dB SPL are thought to cause hearing loss, for instance. Naturally, some absolute sound levels have a natural (and important) uses. It also means that making the music louder by 3 dB has the same perceptual effect (it got louder by about the same amount). Using the logarithmic scale, identifying something as alarming that is 20 dB louder than the ambient level really means 10 times the measured pressure, and is meaningful at all ambient levels. Annoying is hard to characterize, but we can measure loudness as SPL. In a lot of cases it doesn’t really matter how loud an alarm is, as long as it is louder (and more annoying) than the background noise. Since hearing responds to pressure in a generally logarithmic way, using a logarithmic scale for SPL mimics that and makes a number of comparisons of sounds easy. Six and a half orders of magnitude means that the loudest sounds you should be hearing are about three million 5 times the RMS pressure of the quietest sound you can hear. The quietest perceivable sound is taken to be 1 kHz at 20 µPa RMS (0 dB SPL), and the threshold of pain to be 63.2 Pa RMS (130 dB SPL). The scale from 0 dB to 130 dB by definition spans a range of 6.5 orders of magnitude of pressure. That scale spans the whole perceptual range from just perceptibly quiet to painfully loud with numeric values under a hundred or so, and naturally extends to levels too quiet to perceive as well as levels grossly unsafe to perceive.īut that is a log scale, and we haven’t justified why. A sound loud enough that most literature agrees is painful is 130 dB SPL. So the reference point is a sound quieter 4 than a whisper, approximately the sound of a mosquito 3 meters away, which is arbitrarily called 0 db SPL. Finally, it should have a firm mathematical basis on physical measurements. It should also extend to cover sound outside the safe auditory range in a natural way. To be useful, a scale for SPL must describe anything from a whisper 1 to a scream 2 (really, anything heard by “typical” healthy humans 3) with convenient values. Sound Pressure Level (SPL) is almost always expressed on a logarithmic scale and written with the unit dB SPL. Resolution of audio recovered from a PDM data stream.This is part of a series of articles on the general subject of audio signal processing from air to information. In this post, we discuss why we represent SPL on a logarithmic scale, why you should care, and how we implement that efficiently in small CPUs.
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