On Tracks: What Is a Decibel?

“Decibel” is one of the most commonly used terms in the audio lexicon, as well as one of the most commonly misunderstood — and with good reason.

Although the simplest definition is “a unit for measuring the loudness of sound,” that unit isn’t like an “inch” or a “gallon,” with a predetermined value. A decibel is a relative quantity that is expressed as a ratio compared to a reference, or fixed starting point, from which measurements are made.

In other words, you need to know what 0dB is before +5dB or -5dB will have any meaning.

For example, when the cop tells your Motley Crue cover band to turn down because she’s reading 120dB on her sound level meter, “0dB” on the meter is a fixed value that more or less represents the threshold of human hearing.

The difference in level between 2dB and 3dB is not the same as the difference in level between 3dB and 4dB."

The mathematical formulae underlying calculations employing decibels are quite complex and linked to how the human ear perceives changes in sound pressure level (SPL), but the essential point is that decibels are logarithmic ratios and therefore decibel scales are nonlinear; the difference in level between 2dB and 3dB is not the same as the difference in level between 3dB and 4dB.

Technically, a decibel is one-tenth of a “Bel,” which was named in honor of Alexander Graham Bell, and used to quantify signal degradation in Bell Systems telephone lines. Starting from 0dB on the scale, a sound ten times more powerful is 10dB, a sound 100 times more powerful is 20dB, a sound 1,000 times more powerful is 30dB and so on.

One of the easiest ways to get a sense of what this means is to consider the approximate measurements of familiar sounds:

  • Breathing: 10dB
  • Whispering: 20dB
  • Quiet conversation: 40dB
  • Loud conversation: 60dB
  • Vacuum cleaner: 70dB
  • Jackhammer: 100dB
  • Your cover band: 120dB
  • Pain threshold: 130dB
  • Aircraft carrier deck: 140dB

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Of course, all of these measurements have to be qualified by how far away from the sound source the meter or listener is, and that’s just one of many variables.

When speaking in terms of “loudness” the characteristics of human hearing also have to be taken into consideration. For example, we perceive the loudness of different frequencies in different ways.

One way of doing this is by filtering or “weighting” the measurement in various ways (“A weighting,” B weighting,” etc.). The details of these various “scales” are far beyond the scope of this primer, but when you see dB followed by a suffix such as dB(A) and dB(B), that is what is being referred to.

In short, suffixes always indicate the reference that the ratio is being compared to. Other examples include:

  • dBu, compared to a reference of 0.775 volts
  • dBm, compared to a power reference of one milliwatt
  • dBFS, compared to digital full scale

And that last example brings us to a decibel scale nearly everyone has crossed paths with.

If you record using a Digital Audio Workstation (DAW), such as Avid Pro Tools or Apple Logic, you are familiar with the peak level meters that are included on all of your mixer channels that pass audio, be they for audio tracks, auxiliary tracks, instrument tracks, or the master fader.

These meters display audio levels in dBFS, or Digital Full Scale, and 0dBFS is the absolute upper limit of digital audio levels. That’s why all of the values are minus values (-3dB, -6dB, etc.) and there is no such thing as a positive dB in Digital Full Scale. Beyond 0dBFS lies certain doom.

Mathematically inclined readers may delve more deeply into this vast and complex topic with the aid of their favorite search engine.

This new column will address topics of interest to recordists, ranging from remedial tutorials on essential terms and concepts (such as this one) to more advanced examples of studio geekery. Next up in On Tracks: “Why Do Meters Matter?”

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About the Author:

Barry Cleveland
Barry Cleveland

Barry Cleveland is a San Francisco Bay Area-based journalist, author, guitarist and composer. He was an editor at Guitar Player magazine for 12 years and at Mix and Electronic Musician magazines before that. His book, “Joe Meek's Bold Techniques” is a cult classic, and he also contributed to the book “Whole Lotta Led Zeppelin.” He has released five albums and composes music for film and television.

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