In the next post, we’ll explore the math behind calculating decibel levels. So, there you have it: a brief explanation of the nature of the decibel and reference values. All other values are expressed as a ratio to the reference. On a decibel scale, the reference value is set to zero. This makes a good choice for a starting point or reference. For example, the reference sound pressure level of 20 micro-Pascals is the smallest perceptible pressure the human ear can hear. While the reference values may seem arbitrary and oddly specific at first glance, these values have clear origins. For example, it’s best to write 63 dB as 63 dB re 20 µPa where re means referencing. MeasurementĪlthough the reference value is assumed to be known often in speech and even in writing, it is good practice to include the decibel reference value, especially in formal reports and documents. The chart below shows some common reference values for various measurements. Without a reference, a decibel is meaningless.įor sound pressure, the reference value is generally 20 micro-Pascals. Every time you come across the word decibel, you can tack on “with reference to _”. Although rarely explicitly mentioned, decibels are always in reference to a chosen value. Furthermore, an increase of about 3 dB is typically perceived by the human ear as a doubling of the sound intensity, while a 10 dB increase is perceived as a doubling of the loudness. what?” In other words, twice has no meaning unless you have a reference. To illustrate, a 20 dB sound is 10 times more intense than a 10 dB sound, and a 30 dB sound is 100 times more intense than a 10 dB sound. However, if you were asked to ring a bell when “twice as much time had passed”, you would be left scratching your head asking, “Twice as much time as. A sound at 20 dB is 10 times more intense than a sound at 10 dB, and would be perceived as twice as loud. Instead, the intensity of a sound grows very fast. This means that loudness is not directly proportional to sound intensity. Five seconds at lunchtime is the same as five second before bedtime. Like the Richter scale for measuring earthquakes, the decibel scale is logarithmic. Five seconds last five seconds regardless of when you start counting. Absolute units can be counted without a reference. Well, the firstly it’s important to understand that the decibel is not an absolute unit like seconds or meters-it’s a relative unit. How could one unit describe so many different things? Shouldn’t different quantities have different units? You also know that it must be less than 30 dB ((1000. For example, if the gain is 850, you know that the decibel gain is just a bit over 29 dB. The result is 9 dB + 20 dB, or 29 dB Note that if the leading digit is not a power of 2, the estimation will not be as precise. With so many usages, it is no surprise that a cloud of confusion lurks around the term. Alternately, the power of 2 literally represents 2 Bels, and thus 20 dB. The word decibel, abbreviated dB, seems to show up everywhere in acoustics: sound pressure levels, sound power levels, intensity levels, and even voltage levels.
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