Themes > Science > Physics > Acoustics > Physical Properties of Sound

The material in this section gives a brief introduction to some of the terms used in building acoustics and the test methods used to characterize systems and materials. A very basic understanding of some fundamentals of acoustics and terms used in building acoustics, is all that is necessary to understand the material in this and the following chapters. To emphasize the simplicity of the approach, equations are kept to a minimum.

Sound is generated by creating a disturbance of the air, which sets up a series of pressure waves fluctuating above and below the air's normal atmospheric pressure, much as a stone that falls in water generates expanding ripples on the surface. Unlike the water waves, however, these pressure waves propagate in all directions from the source of the sound. Our ears sense these pressure fluctuations, convert them to electrical impulses, and send them to our brain, where they are interpreted as sound.

There are many sources of sound in buildings: voices, human activities, external noises such as traffic, entertainment devices and machinery. They all generate small rapid variations in pressure about the static atmospheric pressure; these propagate through the air as sound waves.

As well as travelling in air, sound can travel as vibrational waves in solids or liquids. The terms airborne and structure-borne sound are used depending on which medium the sound is travelling in at the time. For example, the noise from a radio set may begin as airborne sound, enter the structure of the building and travel for some distance as structure-borne sound, and then be radiated again as airborne sound in another place. (Figure 1) The importance of structure-borne sound will become more apparent when flanking sound transmission is discussed, in Acoustics in Practice.

Figure 1

Air pressure is usually measured in units of Pascals (Pa). Atmospheric pressure is about 100 kPa. Sound pressure is a measure of the fluctuation of the air pressure above and below normal atmospheric pressure as the sound waves propagate past a listener. Generally, the larger the fluctuations, the louder the sound.

The pressure variations in an individual sound wave are much less than the static atmospheric pressure, but the range of sound pressures encountered in acoustics is very large. The threshold of hearing is assumed to correspond to pressure fluctuations of 20 microPascals; some individuals will have more acute hearing than this, some less. The threshold of pain in the ear corresponds to pressure fluctuations of about 200 Pa. This second value is ten million times the first. These unwieldy numbers are converted to more convenient ones using a logarithmic scale, the decibel scale. Sound pressure levels are expressed as a number followed by the symbol dB. Sound level metersconvert electrical signals from a microphone to sound pressure levels in dB. Table 1 gives some representative sound pressure levels encountered in a range of situations.

 Table 1   Typical sound levels dB Jet takeoff, artillery fire, riveting .................. 120 or more Rock band or very loud orchestra ....................... 100­120 Unmuffled truck, police whistle ........................ 80­100 Average radio or TV .................................... 70­90 Human voice at 1 m ..................................... 55­60 Background in private office ........................... 35­40 Quiet home ............................................. 25­35 Threshold of hearing ................................... 20

Decibels are more easily related to the response of the human ear, which also responds logarithmically to sound.The response of our ears, that is, our perception of loudness, does not increase linearly with a linear increase insound pressure. For example, a 10 dB increase in sound pressure level would be perceived as a doubling of the loudness. In practical situations, level changes of about 3 dB are just noticeable.

It is very important to remember that decibels and similar acoustical quantities have properties different from more conventional units. Sound pressure levels, for example, cannot be added together as can kilograms. The combination of two noises with average levels of 60 dB does not give a sound pressure level of 120 dB, but 63 dB.

The addition of a noise with a level of 70 dB to a room with a level of 80 dB will result in no measureable difference in the overall level. This does not mean, however, that a large number of secondary sources can be introduced into an environment without increasing the overall level. If ten 'negligible' 70 dB noise sources are combined with one 80 dB noise source, the resulting level will be 83 dB. Fortunately in building acoustics, there is seldom a need to combine noise levels in this way or to do many complicated calculations with decibels or other logarithmic units. These examples merely emphasize the peculiarities of the decibel scale.

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