Relationship Between Loudness and Amplifier Power

Decibels (a measure of loudness) and watts (a measure of amplifier power) are common terms used to describe stereos. They can be confusing, so here is a simple explanation of what they mean and how they relate.

What Is a Decibel?

A decibel is actually two words, ‘deci’ (meaning one-tenth) and ‘bel’ (named after Alexander Graham Bell, a scientist and inventor of the telephone). A ‘bel’ is a unit of sound and a decibel is one-tenth of a bel.

The human ear is sensitive to a wide range of sound levels from 0 decibels (complete silence) to 140 decibels (pain). For clarity, a quiet conversation is about 60 dB, a jet engine at close range is about 120 dB. The human ear is capable of hearing and recognizing an increase or decrease in the sound level of about 1 dB. Anything less than +/-1 dB is hard to hear. An increase of +10 dB (easy to hear) is perceived as being approximately twice as loud by most people.

What Is a Watt?

A watt is a unit of energy, like horsepower or joules. In audio, a watt is used to describe the energy output of a receiver or amplifier used to power a loudspeaker. The relationship between power output and speaker loudness or volume is not linear or straight (+10 watts does not equal +10 dB). For example, 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. 2X the power output = +3 dB increase, 10X the power output = +10 dB increase, or twice as loud. So, what is the benefit of more power output?