Our ears detect changes in volume in a non-linear fashion. A decibel is a logarithmic scale of loudness. A difference of 1 decibel is the minimum perceptible change in volume, 3 db is a moderate change, and 10 decibels is doubling of volume. Decibels are designated by the letters: db.
0 db is the threshold of hearing - Other examples inlcude:
Whisper: 15-25 dB
Background noise: about 35 dB
Normal home or office background: 40-60 dB
Normal speaking voice: 65-70 dB
Orchestral climax: 105 dB
Live Rock music: 120 dB+
Pain Threshold: 130 dB
Jet aircraft: 140-180 dB
For one amplifier or receiver to be twice as loud as another, you need 10 times more wattage output. A Receiver with 100 WPC is capable of twice the volume level of a 10 WPC amp. A receiver with 100 WPC needs to be 1,000 WPC to be twice as loud.
