What is a decibel? | Basic Pro Audio Concepts

THE GIST

The decibel is a unit of measurement (equal to one-tenth of a bel) commonly used to measure the strength of audio signals and acoustical volume. Decibels express values on a nonlinear logarithmic scale, always in relation to a fixed reference point. There are many different decibel scales, each with a unique suffix like dBV (voltage) and dB SPL (sound pressure level) denoting their specific purpose.

The Logarithmic Scale

Sound and electromagnetic waves are capable of such huge swings in value that measuring them linearly just isn’t efficient. This is where the decibel comes in handy, representing change on a logarithmic scale which increases sharply and tapers off at higher values (the inverse of an exponential curve).

For example, the level at which sound becomes damaging over time is 500 million times louder than the quietest perceivable sound. On the decibel scale, that difference can be expressed as 85dB. At a factor of 10 trillion, sound becomes outright painful, but this mind-boggling ratio can be written simply as 130dB.

In the audio realm, the logarithmic scale excels at measuring two properties: power (acoustical energy) and amplitude (analog or digital audio signals). However, due to the different physics behind these properties, the decibel scale works a bit differently for each: When measuring power, a change of 3dB represents a factor of two (twice as loud or quiet) and 10dB represents a factor of ten. When describing amplitude/signal strength, the scale is doubled. So, 6dB represents a factor of two and 20dB is tenfold.

Practical Application

Decibels are used in radio, optics, and digital imaging, but most non-scientists will probably encounter the decibel on audio gear. Level meters, pad switches, faders, and knobs are all labelled in decibels, but many people don’t really know what they’re looking at. Why is -20 so common for pad switches? Why is 0 sometimes on the left of a knob, sometimes in the middle, and sometimes on the right?

Take the common fader, for example: 0 is placed near the top with numbers above and below it, with +10 usually at the top and -∞ at the bottom. The 0 represents unity gain (the point at which the signal is not being boosted or cut) simply passing through at full strength.

When you pull that fader down to -6, you’re attenuating the signal to half strength. From there, it continues to decrease logarithmically until it reaches infinite attenuation and the signal is effectively muted. That +10 at the top may not look like much extra gain compared to infinity, but it actually makes the signal five times as strong.

It’s this nonlinear curve that makes for such smooth fade-ins and fade-outs simply by moving knobs and sliders, but it also translates to how we see sound. Digital meters in DAWs, simple LED lights on gear, and even old-school VU meters are all displaying information in decibels—albeit in two different ways: peak level and average level.

The meters in a DAW, for example, are usually showing the real-time peaks of the music, even if they only last a millisecond. The physical needle on a VU meter can’t respond fast enough to measure all those peaks, and instead displays the root mean square (RMS) of the signal—the average level over time.

Types of Decibel Scales

The dB symbol by itself tells us nothing and isn’t capable of measuring anything in absolute terms. Rather, it must always relate to a fixed reference point and can then be used to measure deviations from that value. The decibel has myriad uses in various scientific fields, but below are some of the most common used in audio:

  • dBV: Voltage, relative to 1 Volt. -10dBV = line level for consumer audio equipment.
  • dBu: decibels “unloaded,” roughly equal to .775 Volts. +4dBu = line level for professional audio equipment.
  • dBFS: “Full Scale”, used in digital audio. 0dB = the maximum value the system can express.
  • dB SPL: Sound Pressure Level, or the energy of sound waves. 0dB SPL = the threshold of human hearing.
  • dB (A/B/C): A, B, and C “weightings” are different filters for measuring sound energy with a SPL meter. dB(A) is used to mimic the frequency response of the human ear for measuring loudness.
  • dBVU: Volume Units, as seen on the VU meters of some analog audio equipment. 0dBVU can be calibrated to different operating levels.
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