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Measuring And Rating Noise

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9:01 AM - 09/04/2007 by Toby Digby

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 them
Source: "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.

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