It's often necessary to estimate how much a sound level changes. Our ears interpret a wide range of sound amplitudes, volume or loudness as change in level and change in loudness. The decibel is a very convenient unit for measuring signal levels in electronic circuits or sound pressure levels in air. However, changes in the loudness of sounds as perceived by our ears do not conform exactly to the corresponding changes in sound pressure level. Loudness is the quality of a sound that is the primary psychological correlation of physical strength (amplitude). Loudness, a subjective feeling, is often confused with objective measures of sound pressure level SPL such as decibels.



Definitions:

Sound level or noise level is a physical quantity measured with measuring instruments.

Loudness is a psycho-physical sensation perceived by the human auditory perception or the human ear/brain mechanism. That is not the same.

We are told by psycho-acousticians that a level 10 dB greater usually means "double the loudness" or "twice as loud".

A decibel is one-tenth of a bel, which is the logarithm of the ratio of any two energy-like quantities or two field-like quantities.



Ratio doubling means:

− a power level of +3 dB, or a sound intensity level of +3 dB

− an electric voltage level of +6 dB, or a sound pressure level of +6 dB

− a loudness level of about +10 dB

−10 dB more SPL means 10 times increase in amplifier (electric) power.



In the newsgroups these often misunderstood statements are explained rather less accurately. Decibels can also mean dBSPL or dBA, while a level change is always in dB. The perceived loudness of the sound depends on several factors: the amplitude, the sound pressure level, the frequency, and the time behaviour of the sound. A typical question on the internet: "Is 3 dB or 6 dB double the loudness?"

The answer is: "It is neither 3 dB, nor 6 dB − it is closer to 10 dB".