10-2 Environmental conditions that can influence measurement

(1) Weather conditions, topography and surface geometry

Noise being propagated outdoors can be significantly influenced by a range of factors such as topography, surface geometry and weather conditions like wind and temperature. For example, sound normally becomes louder in areas downwind of the source and less loud in areas upwind of the source. Also, sound is usually propagated less easily when temperature at high altitudes is relatively lower than the temperature at low altitudes; and propagates more easily when temperature at high altitudes is relatively higher than the temperature at low altitudes. Sound traveling over a ground surface with greater sound absorbing properties, such as paddy fields and grassland, can become dampened more easily and therefore travel less than when traveling over sound reflecting surfaces such as pavement. Therefore, clear records need to be made of the topography and surface geometry as well as the weather conditions around the point of measurement: conditions such as wind direction, wind velocity, temperature and relative humidity.

(2) Influence of wind noise

Wind noise is generated when strong wind hits the microphone of a sound level meter. In that case, if the sound being measured is smaller than the wind noise, the signal to noise ratio is not high enough to make measurement possible. Whenever measuring noise outdoors or near machinery that generates wind, a wind screen must be installed on the microphone. However, measurement should be avoided whenever wind velocity is beyond the wind noise mitigating capacity of the wind screen being used.

(3) Influence of other environmental conditions

Strong electrical and magnetic fields often form around electrical machinery. If sound level meters are placed in such locations, the electrical circuits of the meter and its microphone can be affected, possibly making the reading inaccurate. If microphone cables are extended using extension cords, these cords can be easily influenced by these fields. Vibrations generated by a range of machinery can be transmitted to the sound level meter, possibly influencing its performance. High temperature and high humidity can also affect the performance of sound level meters and other instruments.

As described above, measurements can be influenced by a range of factors, which makes it essential to find out prior to the measurement whether there are such factors and, if there are, how much influence they can have on the measurement. Any factors expected to have significant influence must be countered with shields, vibration damping and other appropriate measures. It is also important to select points of measurement carefully.

10-3 Sound propagation and attenuation with distance

As described in Chapter 1, sound (sound wave) travels through the air at about 340 m/s. Sound loses its intensity (sound pressure level) as it travels farther even when there is no sound shielding or absorbing materials around. In other words, sound is attenuated as it disperses in all directions. This property of sound is called attenuation with distance. Sound is a wave phenomena: as it travels farther from the source, sound spreads over a wider area and hence, the intensity of the sound (sound energy per unit area) weakens. With sound that has a point source, it is propagated in a spherical pattern in free space. It has an area of 4πr² (where r is the distance from the source of the sound) and the intensity of the sound is attenuated in inverse proportion to the square of the distance. This is called the inverse-square law.

In Figure 10-3, when the sound pressure levels at the distance r₁ and the distance r₂ from the point sound source P are L₂ and L₂ respectively, the following equation is true.

For example, doubling the distance reduces the sound pressure level by 6 dB.