Archive: A Primer on Noise in Computing

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October 28, 2003 by Mike Chin

A primer on noise and sound was one of the many items on my want list for core articles when Silent PC Review was first launched. There is so much misinformation on the topic that I felt it mandatory to provide some kind of baseline, an introduction to this complex subject. It is, in fact, a subject that seems simple only if you never scratch below the surface. Hopefully, this article serves well enough for its purpose: To provide guidelines for those who seek a quieter computing experience by which they can interpret noise specifications, commentary by others, and what their own ears tell them.

Revised slightly Nov 4, 2003: Section on "Why Such Noise Levels in Typical PCs?," page 3.

Note: A large portion of the content for this article originates from a white paper I wrote for VIA entitled "Noise, Computing and VIA", available at VIA's web site.

* * * * *

WHAT IS SOUND? AND NOISE?

Sound is what we hear. Technically, it is defined as the human perception of airborne pressure waves caused by mechanical vibrations emanating from any source. A pure sound, such as that made by a tuning fork, is a tone. When picked up with a microphone and displayed on an oscilloscope, such a tone looks like a sine wave. Every tone has two components: frequency or pitch, and amplitude or volume. Hertz (Hz) is the measure of sound frequency; the decibel (dB) is the measure of sound pressure level. Most sounds that we hear are not pure tones; they are many tones in complex combinations of frequency, amplitude and timing.

Noise is difficult to define technically - it can be almost any waveform, because fundamentally, noise is any unwanted sound. Unwanted being the operative word, noise can be a dripping faucet when you're trying to sleep in a quiet room, a jackhammer being operated intermittently in the street while you try to read, or the whine of a computer's cooling fans and hard drive as you try to write a paper. It can even be the sound of laughter when you are angry.

An important aspect of noise is that unlike microphones or sound level meters, people share with animals a keen ability to distinguish different kinds of sounds even when these sounds are actually quieter than the overall ambient level. You could call it focus: It is what allows us to pick out a single familiar voice in a noisy crowded restaurant. This phenomenon comes into play with noise perception as well, where an annoying sound is somehow audible even when it should be masked by other sounds. Noise is as much a psychoacoustic phenomenon as it is physical.

Some types of machine sounds can only be described as noise. The high-pitched limited bandwidth sound of small motors, such as an electric drill or a small high speed fan, is almost always perceived as unpleasant. It has high pitch and penetrating volume, worse than the buzzing of bees or wasps, which is about the closest comparison in nature.

RELATIVE SOUND LEVELS

The decibel follows a logarithmic scale, rather than a linear one. This is a complex subject, but for our purpose, it is sufficient to note:

  • 1 dB is generally the smallest difference that can be perceived by human beings
  • A 3 dB difference is clearly audible for just about anyone with normal hearing
  • A 10 dB difference is generally perceived as being twice or half as loud.

This means, for example, that if one source of noise is measured at point of perception at 85 dB, another source that measures 75 dB sounds half as loud. A 95 dB source sounds twice as loud as the 85 dB source, and four times louder than the 75 dB source.

Sounds are additive, but not in a simple linear way.

  • Two 30 dB noise sources make 33 dB
  • Four 30 dB noise sources make 36 dB
  • Eight 30 dB noise sources make 39 dB
  • Sixteen 30 dB noise sources make 42 dB

Each doubling of identical noise sources results in a 3 dB increase in noise.

NOISE REFERENCE TABLE

The following table shows Sound Pressure Levels for common sounds as a frame of reference to PC noise levels.

SPL (dB) TYPICAL ENVIRONMENT AVERAGE DESCRIPTION
140 30 meters from military aircraft at take off Threshold of pain
120 Boiler shop (maximum levels)
Ships engine room (full speed)
Almost intolerable
100 Automatic lathe shop
Platform of underground station (maximum levels)
Printing press room
Extremely noisy
80 Curbside of busy street
Office with tabulating machines
Very noisy
60 Restaurant, Department Store; Noisiest Gamer PC Noisy
50 Conversational speech at 1 meter; Noisy workstation Clearly audible
35 - 45 Quiet office or library; Typical PC Subdued
25 - 30 Bedroom at night, Quiet PC Quiet
20 - 25 Quiet whisper; Very quiet PC
Background in TV and recording studios
Very quiet
15 - 20 Super quiet / fanless PC Barely audible
<15 Sounds of internal organs Normally inaudible
0 'Normal' threshold of hearing Not audible


HUMAN HEARING

The decibel scale gives sound of all frequencies equal weight, while human hearing does not. Our hearing sensitivity varies with frequency: It is most sensitive in the middle range (between 400~4000 Hz), but much less sensitive in the low frequencies, and less sensitive again in the high frequencies.

To compensate for the non-linear frequency response of human hearing, the "A" weighting scale was developed for sound level measurements. Such measurements are expressed as dBA instead of dB and allow sounds of different frequency balances to be fairly compared for relative loudness. For example, a reading of 90 dBA of automotive traffic measured below a bridge (mostly low frequency sound) can be said to have the same loudness as a reading of 90 dBA of massed violins holding a note at 4 octaves above middle C (high frequency sound). Sound level meters (SLM) have the "A" weighting scale built in so that dBA can be read directly off the display.

Note that the "A" weighting scale is an attempt to compensate for non-linearities in average human hearing perception when compared to acoustic measuring devices such as SLMs, frequency spectrum analyzers and other machines. It is an educated approximation based on the research of Munson & Fletcher.



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