Quiet please! Fighting noise pollution
Box 2 | What is noise?
Sounds are created when a vibrating source causes waves of acoustic energy to travel through the air. The sound pressure waves move out from the vibrating source and become weaker the further they travel. The waves may be reflected or scattered by objects so that the sound reaching the ear may be different from the sound originally generated.
Sounds can be characterised by their frequency (or pitch) and intensity (or loudness). The vibrations producing the sound are cyclical and are measured in hertz (Hz), which gives the number of cycles that occur per second. An adult with good hearing can hear frequencies in the range 20 to 15,000 Hz, while children can hear frequencies above 20,000 Hz.
The sound pressure level is measured in decibels (dB). For example, a whisper is in the range 20-30 dB, normal conversation about 60 dB, while someone shouting in your face can easily exceed 80 dB. A sound level meter is used to measure the decibel levels of sound. Usually the sound level meter has a filter that has a frequency response similar to the human ear. These levels are known as dB(A) or A-weighted decibels. Like the ear, this filter reduces the sound levels in the lower frequencies (below 1000 hertz) and in the higher frequencies (above 5000 hertz).
The correspondence between decibel levels and perceived loudness is fairly simple. A difference of 3 dB in noise level is barely noticeable, yet it represents a doubling of the acoustic energy involved. For a noise to sound twice (or half) as loud, a difference of about 10 dB is required. For example, a lawn mower measured at 80 dB will sound about twice as loud as a hair dryer at 70 dB.
When sounds become noise
People react to sounds in different ways. We take most sounds for granted, but in some situations a sound can distract us and break our concentration. When this happens, the sound becomes unwanted noise.
Often it is not the pitch or the loudness that makes a sound an annoying noise. Sometimes it is the repetitive nature of the sound and our inability to control it that makes it annoying. Car alarms and the seemingly endless barking of a dog are good examples.
Boxes
Box 1. Health issues related to noise pollution
Box 3. Planning to reduce airport noise
Box 4. Traffic noise – sources and solutions
Box 5. Soundproofing noisy buildings
Related sites
Noise – basic information (Canadian Centre for Occupational Health and Safety)
Background information on acoustics (State Water Control Board, California, USA)
Page updated February 2007.