Environmental Noise Propagation

Transcription

1 Environmental Noise Propagation How loud is a 1-ton truck? That depends very much on how far away you are, and whether you are in front of a barrier or behind it. Many other factors affect the noise level, and measurement results can vary by tens of decibels for the very same noise source. To explain how this variation comes about, we need to consider how the noise is emitted from the source, how it travels through the air, and how it arrives at the receiver. The most important factors affecting noise propagation are: Type of source (point or line) Distance from source Atmospheric absorption Wind Temperature and temperature gradient Obstacles such as barriers and buildings Ground absorption Reflections Humidity Precipitation To arrive at a representative result for measurement or calculation, these factors must be taken into account. Regulations will often specify conditions for each factor. 16 Environmental Noise Propagation

2 Types of Source Point source If the dimensions of a noise source are small compared with the distance to the listener, it is called a point source, for example, fans and chimney stacks. The sound energy spreads out spherically, so that the sound pressure level is the same for all points at the same distance from the source, and decreases by 6 db per doubling of distance. This holds true until ground and air attenuation noticeably affect the level. For a point source with sound power level, L W (see section on Environmental Noise Parameters and Terminology), located near the ground, the sound pressure level (L p ) at any distance (r, in m) from that source can be calculated from the equation: L p = L W 2log 1 ( r) 8dB Line Source If a noise source is narrow in one direction and long in the other compared to the distance to the listener, it is called a line source. It can be a single source such as a long pipe carrying a turbulent fluid, or it can be composed of many point sources operating simultaneously, such as a stream of vehicles on a busy road. The sound level spreads out cylindrically, so the sound pressure level is the same at all points at the same distance from the line, and decreases by 3 db per doubling of distance. This holds true until ground and air attenuation noticeably affect the level. For a line source with sound power level per metre (L W /m) located near the ground, the sound pressure level (L p ) at any distance (r, in m) from that source can be calculated from the equation: L p = L W 1log 1 ( r) 5dB Environmental Noise Propagation 17

3 Barriers The noise reduction caused by a barrier depends on two factors: 1. The path difference of the sound wave as it travels over the barrier compared with direct transmission to the receiver (a + b c, in the diagram). 2. The frequency content of the noise. The combined effect of these two is shown in the diagram. It shows that low frequencies are difficult to reduce using barriers. a b c 25 Barrier attenuation in db Wavelength =.68 m (5Hz) 5 Wavelength =.34 m (1Hz) Wavelength =.17 m (2Hz) Path difference (metres) 62 Barrier attenuation for a typical screen is shown in the next diagram as a function of barrier height. A barrier is most effective when placed close to the noise source or receiver. 18 Environmental Noise Propagation

4 screen h Barrier Attenuation (db) Frequency (Hz) h = 6m h = 3.5m h = 2.5m 63 Atmospheric Attenuation This is a complex subject and can only be summarised here. The reduction of noise as it passes through air is dependent on many factors including: Distance from source Frequency content of the noise Ambient temperature Relative humidity Ambient pressure The first two factors mentioned above are the most influential and are shown in the diagram below. To summarise, low frequencies are not well attenuated by atmospheric absorption. Attenuation (db) Hz 25Hz 8kHz 4kHz 2kHz 1kHz 5Hz 2 5 1k 2k 5k 1k 2k Distance from Source (metres) 64/1 Environmental Noise Propagation 19

5 Wind and Temperature Wind speed increases with altitude, which will bend the path of sound to focus it on the downwind side and make a shadow on the upwind side of the source. Why Measure Downwind? At short distances, up to 5 m, the wind has minor influence on the measured sound level. For longer distances, the wind effect becomes appreciably greater. Downwind, the level may increase by a few db, depending on wind speed. But measuring upwind or side-wind, the level can drop by over 2 db, depending on wind speed and distance. This is why downwind measurement is preferred the deviation is smaller and the result is also conservative. A-weighted reduction (db(a)) Downwind Sidewind Upwind Distance (metres) 65 2 Environmental Noise Propagation

6 Temperature Temperature gradients create effects similar to those of wind gradients, except that they are uniform in all directions from the source. On a sunny day with no wind, temperature decreases with altitude, giving a shadow effect for sound. On a clear night, temperature may increase with altitude (temperature inversion), focusing sound on the ground surface. Decreasing Temperature Increasing Temperature Environmental Noise Propagation 21

7 Ground Effects Sound reflected by the ground interferes with the directly propagated sound. The effect of the ground is different for acoustically hard (e.g., concrete or water), soft (e.g., grass, trees or vegetation) and mixed surfaces. Ground attenuation is often calculated in frequency bands to take into account the frequency content of the noise source and the type of ground between the source and the receiver. Precipitation can affect ground attenuation. Snow, for example, can give considerable attenuation, and can also cause high, positive temperature gradients. Regulations often advise against measuring under such conditions. Influence of ground surface at 1 m distance between source and receiver. Source and receiver height 2 m 6 Ground Attenuation (db) Octave Centre Frequency (Hz) Hard Ground Mixed Ground Porous Ground Environmental Noise Propagation

8 Noise at the Receiver Reflection When sound waves impact upon a surface, part of their acoustic energy is reflected from it, part is transmitted through it and part is absorbed by it. If absorption and transmission are low, as is generally the case with buildings, most of the sound energy is reflected and the surface is said to be acoustically hard. The sound pressure level near the surface is therefore due to direct radiation from the source and sound arriving from one or more reflections. Typically, the level.5 m from a plain wall is 3 db(a) higher than if there was no wall. Regulations often require the exclusion of the effect of reflection from reported results (free-field conditions). Open and Closed Windows When at home, some people like to keep their windows closed because of climate or tradition. Disturbing noise in the environment is then attenuated by the building, typically offering 2 3 db of protection (façade sound insulation). Windows are often acoustically weak spots, but can be improved with special design. In other countries and climates, people are used to having their windows open and experiencing the full effect of environmental noise. Regulations for environmental noise, therefore, must take into account both the way dwellings are constructed and the way they are used. Environmental Noise Propagation 23