What is a decibel? 6 simple truths about decibels
1 April , 2022



1. a decibel is one tenth of a bela. "Bel" is named after Alexander Graham Bell. Hence the capital "B" in dB.

2. a decibel (dB) is a unit for measuring sound pressure or signal level by comparison with a reference point on a logarithmic scale. The logarithmic scale makes it easier to compare values over a wide range. For example, instead of saying "A person's hearing threshold is 0.00002 Pa and the level of a normal conversation is about 0.02 Pa", it is much easier to say "A person's hearing threshold is 0 dB SPL and the level of a normal conversation at 0 dB SPL is about 60 dB SPL" .

3. decibels have no intrinsic value. It is not a fixed unit like kilogram, mile, cubic meter or degree Fahrenheit. Decibels are the ratio of one value to another value or a given reference value.

4. To really understand the decibel, you must understand how the logarithmic scale works. With a linear scale, the difference between two values is the difference, while with a logarithmic scale it is expressed as a ratio. For example, a change from 1 to 2 on a linear scale is 1, while on a logarithmic scale the ratio is 1:2. As another example, a change from 3 to 9 on a linear scale is 6, while the ratio on a logarithmic scale is 1:3.

5. there are many types of decibels! The most common in today's audio world are dB SPL, dBu, dBV, and dBFS. Although they have the same prefix (dB) and the same measurement system (comparing two values on a logarithmic scale), they are very different from each other. dB SPL is used to measure the sound pressure level. A sound pressure level of 0 dB refers to 0.00002 Pa (threshold of human hearing) and all other measured values are above this nominal value. As a result, all SPL readings are positive values, e.g. 50 dB SPL or 85 dB SPL. dBFS (dB full scale) is used to measure the levels of digital signals. The reference for this scale is 0 dBFS and almost all other measured values fall below this point. Therefore, all dBFS readings are negative values such as -6 dBFS or -20 dBFS. dBu and dBV are used to measure voltage. The professional standard line level is +4 dBu (1.228 V), while the consumer line level is typically -10 dBV (0.316). Since 0 dBu refers to 0.775 V and dBV refers to 1 V, the reading can be either positive or negative, e.g. -2 dBu, +6 dBu, -12 dBV, or +3 dBV.

6. decibel calculations cannot be done with simple arithmetic or percentages! For example, you cannot say that 80% of 70 dB is 56 dB. This is because the dB scale is logarithmic and percentages are linear! 50% of 70 dBu is 64 dBu because dBu is voltage and halving or doubling is a voltage change of 6 dB. However, if you are measuring power, decreasing or doubling is a 3 dB change. As you can see, the decibel calculation cannot be accomplished by simply adding and subtracting! Suppose you have a speaker with an output sound pressure of 90 dB at 1 meter distance. If you place the same speaker side by side, you won't get 180 dB SPL. Instead, you get a sound pressure level of 93 dB. As another example, if you have three sound sources with output levels of 60 dB SPL, 75 dB SPL, and 80 dB SPL, the total sound pressure level is not 215 dB SPL, but about 81 dB SPL.

We hope these six points will help clear up some of the question marks you may have about decibels.

Source / German: https://arefyevstudio.com/de/2022/04/01/was-ist-ein-dezibel-6-einfache-wahrheiten-ueber-dezibel/