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Correct Answer: Four times louder
The decibel (dB) scale is logarithmic, not linear, increasing by a power of ten with every 10dB, which roughly equates to a doubling in loudness based on average human hearing. Consequently, 30dB is about twice as loud as 20dB, and a computer producing 40dB of noise would be approximately four times louder than one generating 20dB.
A PC emitting only 20dB of noise would be considered a relatively quiet system, while a typical office environment is around 40-50dB, and comfortable hearing levels are said to be below 60dB. A siren 10 meters away and speakers at a rock concert measure around 110dB, heavy weapons and space rockets register at 180-190dB, and at 194dB, sound waves become shock waves.
While on the topic of computer noise, here's a great reference about the different 'sounds' your PC can make: