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Understanding Noise Levels

loudness of sound

Decibels—the loudness of sound

Decibels (dB) is a logarithmic, not a linear, unit of measurement for the loudness of sound.

Normally a change of 1 dB is the smallest volume change detectable by the human ear. A reduction of 3 dB in noise level is equivalent to about 50% of the sound energy being removed from the system.

Regular exposure to noise at or above 90 dB causes permanent hearing damage.

Decibel Levels of Common Sounds
Example Typical Noise Level
dB(A)
A pin dropping, breathing 10
Recording studio 30
Library 40
Normal conversation 60
Vacuum cleaner 70
Alarm clock 80
Lawn mower 90
Hand drill 100
Chain saw 110
Emergency vehicle siren 115
Jack hammer 120
Jet engine at takeoff 140

Noise "Colour"—the harshness of sound

noise colour

The "colour" or "harshness" of the sound is defined by the frequency of the sound. Frequency is the number of waveforms generated in one second. For example, if you hit the middle C key on a piano, the strings in the piano will vibrate back and forward around 261 times per second. Higher pitched notes have a higher frequency and lower pitched notes have a lower frequency.

The sensitivity of human hearing is frequency dependent. Human ears are most sensitive to the noise around 1,000Hz. Human ears are less sensitive to low frequency noise such as below 60Hz or extremely high frequency such as above 10,000Hz. As a result, noise level is adjusted to reflect the sensitivity of the human hearing and the adjusted noise level is termed dB(A), i.e., "A" weighted noise level.

Harsh noise: noise concentrating at around 1,000Hz and higher. This noise frequency tends to cause discomfort to human ears. A typical example is the high pitch noise of vacuum cleaners.

Tonal noise: when one single frequency noise is about 10dB(A) higher than the all the other frequency noise. Tonal noise is extremely unpleasant especially exposed over long period of time. A typical example is the "humming" noise from a transformer.

Examples of Frequency Levels
Example Frequency
Hz
Lowest note for tuba 16.35
Lowest C on standard 88-key piano 32.70
Lowest note for cello 65.41
Lowest note for viola 130.81
Middle C 261.63
C in middle treble clef 523.25
Approximately the highest note reproducible by the average female human voice 1046.50
Highest note for flute 2093
Highest note on standard 88-key piano 4186
Approximately the tone that a typical CRT television emits while running 16744

Occupational Health and Safety (OHS) Requirements

OHS Requirements for Noise

Occupational Health and Safety Regulations in Victoria, Australia, limit workers' exposure to 85 dB(A) averaged over 8 hours, or to any instantaneous noise in excess of 140 dB(C).