Sounding out the secrets of the sea
Glossary
CT scans. A series of X-ray images of the body. The body is X-rayed from many directions and the results are analysed by a computer. The computer generates images of cross-sections (slices) of the body. CT scans show details of the shape and location of soft tissues, as well as bones and blood. Other names for this technique are computerised tomography, CAT scan and computerised axial tomography. For more information see CAT scans (University of Colorado at Boulder, USA).
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.
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). Since decibels measure relative power level, a reference must be specified, which is normally 20 micropascals, or 10-12 watts per square metre.
For more information see What is a decibel and what is the loudest sound I can listen to before it hurts my ears? (How Stuff Works, USA); What is a decibel? (University of New South Wales, Australia); and Sound properties and their perception – intensity and the decibel scale (The Physics Classroom, USA).
frequency. A measure of how frequently a wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – one oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is one million hertz – one million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).
newton (N). The SI unit of force. One newton gives an acceleration of one metre per second per second to a mass of one kilogram. The weight you feel when you hold an average size apple in your hand is about one newton (mass is about 0.1 kilogram and gravitational acceleration of about 10 metres per second per second). It is named after Sir Isaac Newton.
pascal (Pa). The SI unit of pressure equivalent to one newton acting uniformly over an area of one square metre (newton per metre squared).
radar. The use of reflected radio waves to determine the location of an object and its speed if it is moving. It is an acronym derived from radio detecting and ranging. For more information see How radar works (How Stuff Works, USA).
root mean square. The square root of the average of the squares of a set of numbers.
sound pressure. The pressure deviation from the ambient pressure caused by a sound wave. Sound pressure underwater is measured using a hydrophone. The unit for sound pressure is the pascal (Pa). The reference sound pressure in air is 20 micropascals (root-mean-square). In water, the reference sound pressure is one micropascal (root-mean-square).
sound pressure level (SPL). A logarithmic measure of the root mean square pressure (force/area) of a particular noise relative to a reference noise source. It is measured in decibels.
sound wave. A wave that is transmitted through a solid, liquid, or gas as a result of mechanical vibrations of particles in the medium. The direction of motion of the particles is parallel to the direction of propagation of the wave.
transducer. A device that converts one form of energy into another. For example, a microphone converts sound energy into electricity. A loudspeaker converts electrical energy into loudspeaker sound energy.
Posted August 2007.