What Is A Decibel And How Can Negative Decibels Exist?

The term ‘decibel’ is used all over the world to measure sound levels in a given setting. A decibel is a unit of measurement used to express the ratio of one value of a physical property to another. Decibels follow a logarithmic scale; you can also say that decibels are an exponential unit. In the medical context, decibels are related to normal hearing.

concert crowd

Sound levels skyrocket during rock concerts. (Image Credit: Pixabay)

In most metropolitan areas, you can’t play music louder than a certain number of decibels after a given time at night. There are certain areas (e.g., hospitals, schools) where they put up signs declaring them to be ‘silent zones’, meaning that the sound levels in and around those areas cannot exceed a certain decibel limit. There are many other places where decibels are used to express the loudness of sound, as they are primarily a unit of volume.

However, it’s quite different from regular units, e.g., meter, kg, Hertz, because the decibel is actually a ratio between two values.

Decibel definition

The decibel is used to measure sound levels. It is represented by dB and is widely used in signals, communication, and electronics. The term decibel can be used to express an absolute value or a change in value (+1 dB or -1 dB). If it’s used in the former case, it signifies the ratio of a value to a reference value.

The decibel is a fairly misunderstood unit of measurement. The thing that makes it so interesting is that the decibel is not a unit in the sense that a kg or meter is. Those two are defined quantities of mass and distance; they never change, regardless of how they are expressed. This is not so in the case of decibels, as a decibel is a relationship between two values of power.


Decibels are designed so that we can conveniently talk about numbers of hugely different magnitude, e.g., 63 and 712,000,000,000,000. Getting the number of zeros right, when you are dealing with such large numbers is quite a nuisance, especially when you need to use such numbers very frequently. Sure, you could scientific notation, but there is still some awkwardness in that approach.

For the sake of convenience and ease, we determine the ratio between the two numbers and convert that into a logarithm. That’s why a decibel expresses the ratio of one value of a physical property to another on a logarithmic scale. When dealing with power quantities, the number of decibels is ten times the logarithm to base 10 of the ratio of two power quantities.

In simple words, decibels tell you that “10 to the power of this number gives you the ratio of these two quantities.” For instance, something that is 10 times as powerful as a reference value is 102 times powerful, so that becomes 20 decibels.

tsunami death map

Number of deaths by country of citizenship, as listed on the Wikipedia article Countries affected by the 2004 Indian Ocean earthquake on 3 December 2008, quantified through a logarithmic scale. (Photo Credit: Kransky/Wikimedia Commons)

Why do decibels follow a logarithmic scale?

As mentioned earlier, a logarithm makes it infinitely more convenient to deal with numbers when discussing values with great differences. This is the primary reason why decibels are logarithmic.

You see, the human ear is capable of hearing a wide range of sounds. In fact, the ratio of the sound pressure that causes permanent damage to the ear to the limit that (undamaged) ears can hear is more than a million! Due to the sheer size of the range, logarithmic units come in very handy in some situations.

How can the value of decibels be negative?

Decibels are not an absolute measure of sound energy; they are actually a comparison with a reference level.

analog volum level sound

Notice the meter has negative decibel values. (Image Credit: Pixabay)

Decibels are similar to the Celsius scale, in the sense that the latter sets 0 as the freezing point of water. What this means is that this scale measures the temperature of things in reference to the freezing point of water. In the same manner, 0 dB is the threshold of human hearing, i.e., the softest sound the human ear can hear (without any artificial help).

In the case of expressing sound levels in the context of human hearing, 0 dB means that you are right at the threshold of human hearing ability. A positive dB value means that the sound is a few times louder than the threshold, while a negative dB value means that you are a few times softer than that threshold.

Additionally, 0 dB also depends on the setting/context of the situation. For instance, when dealing with professional sound equipment, 0 dB usually refers to the loudest level before distortion begins.

All in all, decibels have no objective connection to acoustics, yet we use them because it’s a convenient way to express sound levels in discussions pertaining to human hearing.


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About the Author:

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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