Understanding the Relationship Between Loudness and Amplifier Power

The Difference Between Decibels and Watts

What makes a loud speaker loud?
What makes a loud speaker loud?.

Decibels (a measure of loudness) and watts (a measure of amplifier power) are common terms used when describing audio equipment. They may be confusing, so here is a simple explanation of what they mean and how they relate.

What Is a Decibel?

A decibel is made up of two words, deci, meaning one-tenth, and bel, which is a unit named after Alexander Graham Bell, the inventor of the telephone.

A bel is a unit of sound and a decibel (dB) is one-tenth of a bel. The human ear is sensitive to a wide range of sound levels from 0 decibels, which is complete silence to the human ear, to 130 decibels, which causes pain. The volume of 140 dB can cause hearing damage if endured for a length of time while experiencing 150 dB can burst your eardrums, immediately damaging your sense of hearing. Sound above this level can be very physically damaging and even lethal.

Some examples of sounds and their decibels:

  • Human breathing heard from close distance is about 10 dB
  • A normal conversation is about 60 dB
  • A vacuum cleaner is often around 80dB
  • A jet engine at close range is about 120 dB (that's why you see the crew on the tarmac of an airport wearing protective earmuffs)
  • An ambulance siren is also about 120 dB up close

The human ear is capable of hearing and recognizing an increase or decrease in the sound level equivalent to about 1 dB. Anything less than +/-1 dB is hard to perceive. An increase of 10 dB is perceived as being approximately twice as loud by most people.

What Is a Watt?

A watt (W) is a unit of energy, like horsepower or joules, named after James Watt, a Scottish engineer, chemist, and inventor.

In audio, a watt is a measure of the energy output of a receiver or amplifier used to power a loudspeaker. Speakers are rated for the number of Watts they can handle. Using an amplifier that produces greater watts than a speaker is rated to handle can blow out, thus damaging, the speaker. (When looking at speakers, you should take into account speaker sensitivity as well.)

The relationship between units of power output and speaker units of volume is not linear; for example, an increase of 10 watts does not translate into a 10 dB increase in volume.

If you compare the maximum volume of a 50-watt amplifier with a 100-watt amplifier, the difference is only 3 dB, barely greater than the ability of the human ear to hear the difference. It would take an amplifier with 10 times more power (500 watts!) to be perceived as being twice as loud—a 10 dB increase.

Keep this in mind when purchasing an amplifier or receiver:

  • Doubling the power output (watts) will result in a 3 dB increase in loudness.
  • A ten times increase to the power output will result in a 10 dB increase, or sound that is twice as loud.