Silence your PC

Posted at: Friday 7th October 2005 by James Morris

A powerful PC doesn't necessarily have to sound like a jet engine. James Morris investigates how to make a high-end PC more acoustically demure

As a comparison, a very quiet room will have ambient noise levels in the range of 25-40dB (our test facility's average noise level averaged out at 30.75dBA, but it was particularly noisy that day), and for most offices, the figure is over 50dB. Sound levels above 85dB are considered to be harmful, eardrums rupture at between 190dB and 198dB, and death is possible above 200dB.

Totally in-sone

However, the Weber-Fechner law was cooked up in the 19th century, and modern theories of sound disagree with it. Another proposed method of measuring noise emission is the sone measurement. This unit was suggested by S Smith Stevens in 1936, and is heavily based on psychoacoustic theory. Unlike the decibel, the sone takes into account the subjective nature of volume, as different frequencies are considered more annoying than others. One sone is defined as 40 phons, with a phon being a 1KHz tone at 1dB. So a sone is a 1KHz tone at 40dB, and twice as many sones sounds like a 100 per cent increase in subjective loudness. You'll see sone levels quoted on some products instead of dB, such as the Arctic Cooling NV Silencer 5.

Most manufacturers quote dB, though, which can also take into account the nuances of human physiology with a bit of adjustment. According to psychoacoustic theory, specific mid-level frequencies have a greater impact on human hearing than others. Normal sound perception ranges

from 20Hz to 22KHz, although age and long-term use of headphones can considerably reduce sensitivity at the top end of the spectrum. However, the most sensitive area is between 1KHz and 5KHz, with sensitivity tapering off outside of this range. Generally, sound measurements take this into account by applying the so-called 'A-weighting', which uses a frequency contour to adjust decibel sound pressure measurements. Sound levels recorded in this way are usually denoted by quoting readings as dBA or dB(A). Sometimes, when manufacturers just refer to dB, they actually mean A-weighted sound levels.

A-weighting takes into account the fact that noise in the 1-5KHz range seems louder to the human ear than noise with higher or lower frequencies.

As a result, A-weighting implies that very high or low frequencies have less impact on overall noise levels. Unfortunately, many system fans operate at rotational speeds close to the most sensitive frequencies of human hearing - 1,000-5,000rpm, so the sound they emit will be at precisely the worst pitch for noise irritation. However, lower rotational speeds also supposedly produce less overall sound pressure, even if it's still within the primary A-weighting range. This was something we specifically wanted to find out, which brings us to our test scenario.

Testing, testing

As it's the industry norm, the A-weighting measurement system is used by our testing facility, Intertek in Milton Keynes. In other words, the results have been adjusted to conform to standard human sound perception. Intertek's listening room is also built according to International Engineering Consortium (IEC) standards to represent a domestic listening environment, with low background noise. However, there were still some background sounds, so these had to be calibrated prior to testing. During our testing session, the average background noise was measured at 30.75dBA.