The decibel

If you change volume control on the hi-fi set until you can barely tell the difference, the increment is one decibel. If you use volume control to double the actual acoustic wave power coming from the set of speakers, you will hear an increase of 3 dB, or 3 time’s minimum detectable difference.

For the decibels to have meaning there should be a reference level against which everything is measured. Sometimes you will hear decibels used as if they are absolute; for instance, a vacuum cleaner may produce 80 dB of sound, and a jet taking off may generate 100 dB as it passes overhead at the low altitude. These figures are determined with respect to threshold of hearing, which is faintest sound that good ears can hear at the time when there is no background noise.

In hi-fi amplifiers, you will see meters that show the relative sound level sometimes. Such a meter indicates volume units, and accordingly, it is called as volume-unit (VU) meter. These meters are calibrated in decibels with respect to some reference sound level.

While thinking of decibels as they apply to sound, remember that the doubling of actual acoustic energy equals a 3-dB increase in the amplitude. A rise in sound level is indicated by the positive decibel values, and a decrease is indicated by the negative values. A change of 6 dB means that sound power is cut to one quarter of its previous level; the change of   9 dB means an eightfold increase in the sound power. Zero decibels (0 dB) which mean that there is no change in sound level or that 2 different sounds have equal amplitudes.