Instead of increasing linearly, decibels follow a logarithmic scale that raises by powers of ten and every 10dB roughly equates to a doubling in loudness based on the average human hearing. That being the case, 30dB is around twice as loud as 20dB and a computer making 40dB of noise would be about four times louder than one generating 20dB.
A PC producing only 20dB of noise would be considered a pretty quiet system while a typical office is around 40-50dB and comfortable hearing levels are said to be under 60dB. A siren 10 meters away and speakers at a rock concert are around 110dB, heavy weapons and space rockets are 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, courtesy of Quiet PC USA:
- Hum: A "hum" is generally a steady, low-frequency vibration like your refrigerator makes. This is usually generated by the PSU and it's often amplified by the computer case. Sometimes it can be generated by the larger computer fans running at medium RPM.
- Whir: A whir is like a hum but at a higher frequency. Not as high as a squeal or a whine, though. This is most likely caused by case fans, CPU fans, and sometimes GPU fans or power supplies.
- Whining and squealing: A squeal is sort of an abrupt, intermittent whir but at an even higher frequency. A whine is similar to a squeal but steadier. These are often made by smaller-sized fans spinning at higher RPM speeds when starting up and/or running. Sometimes the culprit can be a HDD whining or a graphics card (coil whine) when it's working really hard and begins to vibrate, emitting a high-pitched noise in the process.
- Rattling and buzzing: These types of sounds may or may not be intermittent. They may occur across several frequency ranges simultaneously (and at the same time). Rattling is caused by fans, HDDs, and CD or DVD players, often vibrating the computer case.
- Grinding: A faint intermittent staccato type of sound often blended with a bit of whine. Grinding sounds are usually made by hard disk drives accessing files. SSDs won't emit this kind of sounds, of course.