What are Decibels?

Learn all about decibels and their unusual scale. Measuring sound isn’t straight forward because of our complicated ears. So what are decibels?


An SPL meter used for measuring decibels

The decibel is the unit that was invented so that we can describe the intensity of a sound. The name honors Alexander Graham Bell and means one tenth of a bel, a unit invented by the famous Scotsman.

The decibel scale is not your normal measurement scale as it has to account for something as complicated as the human ear.

The human ear is so sensitive that it can hear everything from the light rustle of a leaf to the deafening roar of an airplane engine.

If we describe sound in terms of power then the sound of an airplane is around one trillion times more powerful than the quietest sound we can hear.This massive difference causes difficulties, and that is where decibels come in handy.

Decibels are a logarithmic unit. This means that a change in a decibel reading of +10dB actually represents a sound that is ten times louder to us.

The smallest audible sound on the decibel scale is measured at 0dB. A sound ten times more powerful than this would be 10dB. One hundred times more powerful than the quietest sound is 20dB, and one thousand times more powerful is 30dB. An increase of 3dB represents a perceived doubling in sound intensity or acoustic power.

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Let’s take a look at some decibel ratings of sounds to put this into perspective.

  • 0 – The smallest audible sound to someone with normal hearing
  • 10 – Breathing
  • 20 – Whispering at 5 feet
  • 50 – Electric toothbrush
  • 60 – Normal conversation
  • 95 – Electric drill
  • 110 – Car horn
  • 120 – Rock concert
  • 140 – Threshold of pain
  • 150 – Jet engine taking off
  • 160 – Shotgun

These readings are all taken while being close to the sound source. We know that distance is a great way of reducing sound intensity.

It is said that any sound above 85dB has the potential to cause hearing loss. A sound of 85dB or more would be best described by being a sound in which you would have to raise your voice to be heard above it.

Hearing damage depends on the sound intensity and also the length of exposure to the sound. Experiencing a sound of 85dB over the period of 8 hours would cause damage to you hearing. As would being exposed to a sound of 150dB for any length of time, no matter how short.

The chart below shows the recommended allowable exposure times to continuous noise of different levels. Every 3dB over 85dB cuts the exposure time in half.

Decibels are measured using a sound level meter. There are various ways that a decibel reading can be affected. For example, distance will affect sound intensity.

Doubling the distance will lower the decibel rating by 6dB. By increasing the distance by 10 times, the measurement will drop by 20 dB.

Whether the reading is taken indoors (where sound can reverberate) or outdoors (where it can easily dissipate) will affect the outcome too. When reporting decibel levels it is important to specify the conditions under which the reading was taken.

To see how we can reduce the sound intensity in our homes, check out our guide to soundproofing. We show how you can make your home a quieter place today. We pay special attention to how you can improve the sound insulation of your windows.

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How Stuff Works
WikepediaDangerous Decibels
US Department of Labor

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