Measuring Noise Level
Decibels or Sone?
Decibels and sone as measures of noise level both have advantages and disadvantages. Physical sound pressure is measured in decibel. It's easy to measure, and people are generally familiar with it. However, it doesn't relate to human perception of sound very well. So, even though the sound pressure in decibel might have doubled, that doesn't mean that we actually perceive it to have doubled.
The definition of perceived loudness in sone is based on the sound level. A sine wave audio signal at 1 kHz and 40 dB is equivalent to 40 phons, which, in turn, is defined as one sone. When a person perceives a sound wave as being twice as loud, this is called two sone; half as loud is called 0.5 sone. Although this seems practical, logical, and convenient to use, there are some traps and pitfalls when using sone, which ultimately sway us toward using decibel.
First, sone doesn't scale well below one sone. Even though a sound level increase of 10 phons above one sone is perceived as a doubling of loudness, the situation is more complex below 40 phons, where sound level reductions of less than 10 phons are perceived as halving loudness. Unfortunately, there is no generally-accepted formula for this sound pressure range. As graphics cards at idle typically have sound levels of less than 40 phons, this poses a major problem.
Second, sone is based on a pure sine wave of 1 kHz at 40 dB. Fans, however, generate sounds that are a mix of frequencies, and not a pure sine wave. Different frequencies are perceived to have different loudness at the same sound pressure. What would you rather listen to, a low hum or a high-pitched whine at the same loudness?
In order to emulate, or partially emulate, the sound response of the ear, sound measurements typically use weighted sound power curves, such as the standardized curves specified in EN 61672-1/-2. These curves constitute filters that try to emulate the frequency response of the human ear when conducting a sound level measurement. These curves are, of course, approximations. But, depending on the accuracy of the sound level meter, the resulting data in dB(A) is more relevant than data in sone when we are dealing with sound levels below 40 dB. A dB(A) value has to be accompanied by the distance to the sound source in order to be useful.
Bottom line: We use the so-called A-curve as the filter. In other words, we use dB(A) as the unit for our sound level measurements.
Noise Level Measurements in the Real World
Since our test rig is mobile, we can measure the sound level in a designated chamber that's devoid of sound sources like computers and air conditioning. The chamber features soundproof windows, a thick carpet, and a fold-up three-sided soundproofing compartment with a lid that has an array of soft foam pyramids.
We measure the sound level with a Voltcraft SL 400 sound level meter and log the results with an attached data logger. The instrument is mounted on a tripod, which is placed on a thick felt mat to isolate it from structure-borne sound. The distance to the graphics card is 20” (50 cm), and the microphone is aimed at the fan. If the card has two fans, the microphone is aimed between the two fans. Even though this instrument is certified for measurements down to 30 dB(A), we had it calibrated down to 27 dB(A) with a professional sound level meter; we found only minor discrepancies, all of which may have been random.
Next, we take a look at temperature measurements.
