Measuring And Rating Noise
Sound is commonly measured with a device called a sound level meter, or SLM. We rented a Norsonic SLM116 meter for this story, to make sure our sound measurements were as accurate as possible. Sound is measured in decibels (abbreviated dB), and is based on an equation called A-weighting that compensates for differences between the way the human ear hears sounds and the way that electronics detect them. That's why the measure of sound pressure most commonly used is written dBA, to indicate that the reading has been corrected to match human hearing as much as possible (humans perceive low and high frequencies much less acutely than mid-range ones, whereas good electronics should be equally sensitive throughout the entire audible spectrum, usually from 20 Hz to 20,000 Hz; thus, A weighting reflects non-linear human hearing).
| dBA | Brief description | Remarks |
|---|---|---|
| 0 | Human hearing threshold | Inaudible at any distance |
| <15 | Human body sounds | Normally inaudible(breathing, heartbeat, digestion, etc.) |
| 15-20 | Super, quiet fanless PC | Barely audible |
| 20-25 | Quiet whisper, barely audible PC | Very quiet(ambient sound level in recording studio) |
| 25-30 | Bedroom at night, very quiet PC | Quiet |
| 30-45 | Quiet office or library, typical PC | Somewhat audible |
| 50 | Conversation at 1m, noisy PC | Clearly audible |
| 60 | Restaurant or mall, noisiest PC | Noisy |
| 80 | Curbside on busy street | Very noisy |
| 100 | Machine shop, printing press | Extremely noisy |
| 120 | Boiler room, ship engine room | Nearly intolerable |
| 140 | 30m from jet engine at takeoff | Threshold of pain |
Table 1: Sound levels as humans perceive themSource: "What is Sound? And Noise?" by Mike Chin, www.silentpcreview.com; reproduced by permission.
The background noise level we measured in a tightly-sealed closet set the baseline for our numbers at 31.7 dBA. According to the preceding scale, we were only able to achieve sound levels at the lower end of the typical PC range. As a useful point of comparison, we also measured sound output from a desktop PC that one of our writers assiduously tweaked to reduce noise output. We were able to beat those numbers in all cases but one. This PC produced sound levels between 39.0 dBA and 42.0 dBA. We'll also do our best to characterize the noise that the two builds emitted, such as it was.
