With a certified dBA meter we measure how many dBAs originate from the PC. It's slightly subjective as there is always noise in the background, from the streets, from the HD, PSU fan, etc so this is by a mile or two not a precise measurement. You could only achieve objective measurement in a sound test chamber. Take this measurement as an indication, not a precise measurement please.
The human hearing system has different sensitivities at different frequencies. This means that the perception of noise is not at all equal at every frequency. Noise with significant measured levels (in dB) at high or low frequencies will not be as annoying as it would be when its energy is concentrated in the middle frequencies. In other words, the measured noise levels in dB will not reflect the actual human perception of the loudness of the noise. That's why we measure the dBA level. A specific circuit is added to the sound level meter to correct its reading in regard to this concept. This reading is the noise level in dBA. The letter A is added to indicate the correction that was made in the measurement. Frequencies below 1kHz and above 6kHz are attenuated, whereas frequencies between 1kHz and 6kHz are amplified by the A weighting.
TYPICAL SOUND LEVELS
Jet takeoff (200 feet)
Shout (5 feet)
Heavy truck (50 feet)
Normal conversation (3 feet)
Bedroom at night
But let's have a peek at noise levels. We take a dBA gun and point it at the working PC and take a distance of 75 CM.
So explaining sound is always difficult, but up-to 40 dBA is considered to be silent. Once you are in the 41~43 dBA range you can hear the product and after 44 dBA you already have a noisy product. The margins are that tight.
We now put the processor under 100% load again with the processor under full load for a full Prime95 run. Noise pressure is a difficult thing to explain alright.
So if you want silence then up-to 40 dBA is your baseline threshold. Some products that you won't hear at all are the Noctua NH-L12 and the Cooler Master GeminII SF524 low profile coolers.
For the coolers in the 41 dBA range, well you can hear airflow to some extent really. Nothing bad, very normal. That brings all heatpipe coolers within acceptable noise limitations.
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