Noise Pollution and Control in HK
Two important characteristics of sound or noise
Frequency of Sound
Sound is the quickly varying pressure wave travelling through a medium. When sound travels through air, the atmospheric pressure varies periodically. The number of pressure variations per second is called the frequency of sound, and is measured in Hertz (Hz) - cycles per second. Loudness
A loud noise usually has a larger pressure variation and a weak one has smaller pressure variation. Pressure and pressure variations are expressed in Pascal (Pa) - Newton per square metre (N/m2).
The human audible range
■ Human ear can perceive a very wide range of sound pressure.
| |Sound Pressure expressed in | | |Pa |µPa | |Softest Noise just Heard by a Human Ear (the Threshold of |20 x 10-6 |20 | |Hearing) | | | |Launching of the Space Shuttle |2,000 |2 x 109 |
• To express sound or noise in terms of Pa is quite inconvenient because we have to deal with numbers from as small as 20 to as big as 2,000,000,000.
Response of the human ear to sound - audible range
■ A normal human ear is able to hear sounds with frequencies from 20 Hz to 20,000 Hz. The range of 20 Hz to 20,000 Hz is called the audible frequency range. The sounds we hear comprise of various frequencies. ■ The response of the ear to sound is dependent on the frequency of the sound. The human ear has peak response around 2,500 to 3,000 Hz and has a relatively low response at low frequencies.
Audible range - There is a remarkably wide range of frequencies and sound pressure levels over...
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