Understanding Noise Levels
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
||Typical Noise Level
|A pin dropping, breathing
|Emergency vehicle siren
|Jet engine at takeoff
Noise "Colour"—the harshness of sound
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
|Lowest note for tuba
|Lowest C on standard 88-key piano
|Lowest note for cello
|Lowest note for viola
|C in middle treble clef
|Approximately the highest note reproducible by the average female human voice
|Highest note for flute
|Highest note on standard 88-key piano
|Approximately the tone that a typical CRT television emits while running
Occupational Health and Safety (OHS) Requirements
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).