Quiet please! Fighting noise pollution
Glossary
dB(A) or A-weighted decibels. Decibels with the sound pressure scale adjusted to conform with the frequency response of the human ear. A sound level meter that measures A-weighted decibels has an electrical circuit that allows the meter to have the same sensitivity to sound at different frequencies as the average human ear. There are also B-weighted and C-weighted scales, but the A-weighted scale is the one most commonly used for measuring loud noise.
deafness. There are two types of deafness: sensori-neural and conductive. In sensori-neural deafness, the defect lies in either the cochlea (the organ that converts vibrations to nerve impulses) or in the transmission of the sound signals to the brain once they have left the cochlea. This form of deafness tends to occur with age, and is accelerated by exposure to loud sounds (eg, at a disco, from a ghetto blaster, from a portable radio used with earphones, from construction projects). Workers on noisy building sites wear ear protectors, as do sporting-rifle shooters and army personnel on a rifle range.
Conductive deafness occurs when something prevents the sound vibrations from reaching the inner ear. This could merely be wax in the ear canal, but it could also occur if infection has caused the ear drum to become perforated so that it does not move normally under the influence of sound pressure. Alternatively, the ossicles (the tiny bones connecting the ear drum to the cochlea) might become stiff so that they lose their 'lever' action. With conductive deafness, the hearing organ is basically normal, and the problem lies in getting sound to the cochlea.
decibel (dB). A logarithmic scale used to denote the intensity, or pressure level, of a sound relative to the threshold of human hearing. A step of 10 dB is a ten-fold increase in intensity or sound energy and actually sounds a little more than twice as loud.
The quietest sound we can hear is 0 dB; a soft whisper has about 100 times more sound energy and so is about 20 dB. A power lawn-mower has a factor of 109 more sound energy and is about 90 dB. A rock band may be as high as 110 dB. Above 120 dB the sound produces discomfort and even pain. The scale is often adjusted to take account of the reduced sensitivity of human hearing to high and low frequencies and is then specified as dB(A). On this adjusted scale (the A-weighted scale), the range of human hearing is about 3 to 140 dB(A).
For more information see What is a decibel? (How Stuff Works, USA); What is a decibel? (University of New South Wales, Australia); and Intensity and the decibel scale (The Physics Classroom, USA).
frequency. A measure of how frequently an electromagnetic wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).
reflection, scattering, absorption. Sound waves can be reflected by hard surfaces, scattered by rough surfaces, or absorbed by soft porous surfaces, in much the same way as light waves. Because the wavelength of sound ranges from centimetres to metres, smoothness must be judged on a similar scale.
When sound meets with a large surface, the sound may be absorbed or reflected depending on the nature of the surface. Hard, glossy surfaces such as glass, bricks and ceramic tiles are efficient reflectors; porous surfaces such as carpets and curtains are good absorbers. These differences are important in the design of living rooms, recording studios and concert halls.
For more information see Behavior of sound waves – reflection, refraction and diffraction (The Physics Classroom, USA).
weighted sound reduction index. A single-number rating of the sound reduction through a wall or other building element. Since the sound reduction may be different at different frequencies, test measurements are subjected to a standard procedure which yields a single number that is about equal to the average sound reduction in the middle of the human hearing range.
Page updated January 2007.