What is a decibel?

A decibel is a tenth of a “Bel”, a unit of level. A Bel is a very large unit, so the prefix “deci” (one tenth) is used.

Decibel uses a logarithmic scale, not a linear scale like volts or watts. It describes the ratio (change) between two values. In audio these values are usually power or voltage. There is no absolute level called a decibel. It is relative to some reference value.

A dB is a dimensionless unit in the same way a percentage is. Percentages are used to express how large or small one quantity is relative to another quantity. Decibels describe the change from one volume/voltage/power state to another.

Where is Decibel used?

In a PA System you will find the decibel almost everywhere you look: on gain and EQ controls, faders, meters, amplifiers, loudspeakers, editing systems etc.. Having an understanding of decibels will therefore make using audio equipment very much easier.

How is Decibel more useful for comparing levels?

The decibel or logarithmic scale is also more useful than linear measures for comparing huge range of levels in sound systems for two important reasons:

1. The greatest sound pressure the ear can accommodate without discomfort is around 1,000,000 (yes, a million) times greater than the smallest sound pressure the ear can detect. However, we can’t distinguish a million different levels (or even a few hundred different levels) over this range. Expressed in decibels, the smallest sound pressure we can hear (our reference point) is 0 dB and sound pressure that is uncomfortably loud is 120 dB. We can just about distinguish 120 different levels over that range. Our hearing doesn’t easily distinguish between differences in level of less than about 3 decibels. To most listeners a difference of 1 decibel is ‘just noticeable’, 3 decibels is ‘clearly noticeable’, and 10 decibels is ‘twice as loud’.
2. We experience sound in a way that corresponds more closely with a logarithmic than with a linear scale. If we listen to ten successive doublings of sound pressure, they sound like ten equal increases. Viewed on a linear scale, the tenth increase would be 512 times bigger than the first, but it doesn’t sound 512 times bigger: it sounds the same. It makes sense, therefore, to use a logarithmic scale when we compare sound levels: the numbers match our experience

How to differentiate Absolute and Relative Decibels?

As a general rule, adding a suffix to a decibel makes the unit absolute. Without suffix it is relative.

Absolute decibels represent comparative change from a known reference.

For example 0dBu is defined as 0.775 volts RMS, 0dBV is defined as one volt RMS, 0dbW is defined as one watt, 0dBSPL is defined as 20 micro-pascals. So, for instance, 22dBu is a change in voltage of 9.75V from the reference level of .775V.

Relative decibels, on the other hand, measure the proportional change between two values. For example, the proportional change from 5V to 8V is 4dB.

 What are the advantages and disadvantages of Decibels?

The advantages of the “logarithmic scale” of the decibel mean that a very large range of ratios can be represented by a convenient number. The level values in decibels can be added instead of multiplying the underlying power values.

Ex-1: If two loudspeakers each individually produce a [sound pressure] level of, say, 90 dB at a certain point, then when both are operating together we should expect the combined sound pressure level to increase to 93 dB, but certainly not to 180 dB !!!”

Ex-2: If a sound source is measured (including the contribution of background noise) and found to be 87 dBA but when the sound source is switched off the background noise alone is measured as 83 dBA. The sound source level may be obtained by ‘subtracting’ the 83 dBA background noise from the combined level of 87 dBA; i.e., 84.8 dBA !!!”

Ex-3: If in order to find a representative value of the sound level in a room a number of measurements are taken at different positions within the room, and an average value is calculated. Compare the logarithmic and arithmetic averages of 70 dB and 90 dB: logarithmic average = 87 dB; arithmetic average = 80 dB !!!”

The disadvantages of the “logarithmic scale” of the decibel mean that quantities in decibels cannot always be just added, subtracted, averaged or multiplied as in linear scales.

How loud is a Decibel?

As a rough guide, the following levels (SPL) approximately correspond with the sounds described:

• 0 dB SPL – Threshold of hearing.
• 10 – 20 dB SPL – A gentle breeze through the trees.
• 20 – 30dB SPL – A soft whisper (at 1 meter).
• 30 – 40dB SPL – A quiet auditorium.
• 40 – 60db SPL – Background music in a cafe, bar or restaurant.
• 60 – 70dB SPL – Typical conversation levels (from the listener’s position).
• 70 – 80dB SPL – The cabin of an aircraft during normal cruise conditions.
• 90 – 100dB SPL – Loud orchestra (playing off, as it would sound in the front row of the audience).
• 110 – 115dB SPL – A loud rock band (front rows of audience).
• 115 – 130dB SPL – Threshold of pain (120 dB SPL), which varies with frequency and individuals..

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