Sound propagation over soft ground without and with crops and potential for surface transport noise attenuation

Abstract

Growing demand on transportation, road, and railway networks has resulted in increased levels of annoyance from road traffic. Optimized use of green surfaces in combination with vegetation may be desirable as a method for reducing the noise impact of road traffic in urban and rural environments. Sound propagation over soft ground and through crops has been studied through outdoor measurements at short and medium ranges and through predictions. At lower frequencies, ground effect is dominant, and there is little or no attenuation due to crops. At higher frequencies above 3-4 kHz, the attenuation in crops is dominant. It was also found that the ground effects and the influence of crops can be treated independently and can be added to obtain the total effect. Sound attenuation by crops is the result of multiple scattering between the stems and leaves, loss of coherence, and viscous and thermal losses due to foliage. The major contribution is associated with viscous and thermal losses. A model for sound attenuation by vegetation is proposed. Insertion losses for a typical road traffic noise source have been calculated that result either by replacing hard ground with different types of acoustically soft ground or by growing crops along the road sides.

Exploiting ground effects for surface transport noise abatement

“…Moreover the ground type descriptions in Table 1 contain the clue that the most effective type of soft ground is uncompacted. Table 4 shows also that soft ground effects can be augmented in an acoustically-beneficial way (for further noise reductions of between 2 and 5 dB) by 1 m high dense crops (or other dense vegetation) [27,44,46]. The planting of crops can have a longer influence on ground effect than the seasonal effect of the vegetation as a result of creating root zones.…”

“…The main surface wave energy content and the first excess attenuation maxima are moved to higher frequencies as the layer depth is increased. Figure 10 shows data and predictions using a slit pore layer impedance 27 considered so far have been acoustically-hard first we look at data and predictions for acoustically-soft surfaces that are roughened. There is considerable laboratory data showing that the effect of adding small scale roughness to a surface, whatever its original impedance, is to modify its apparent impedance [2, 28 -32].…”

“…the attenuation in excess of that due to ground effect divided by the square root of the product of foliage area per unit volume and the scattering parameter (which is the product of wave-number and a characteristic leaf dimension). On the basis of these suggestions, the data in Figure 10, 11 are fitted empirically by [27] EA(dB)…”

“…At higher frequencies above 3-4 kHz the crops effect is dominant. It was also found that the ground and crops effects can be treated independently and can be added to obtain the total effect [27]. Green leaf crops result in more attenuation than dry crops with fallen leaves.…”

“…Green leaf crops result in more attenuation than dry crops with fallen leaves. Although the attenuation due to crops involves viscous and thermal losses, multiple scattering effects and loss of coherence, it is possible to avoid calculating the multiple scattering and loss of coherence effects and yet to obtain reasonable predictions by only adding ground effect to attenuation due to viscous and thermal losses using larger "effective" values for foliage per unit area and mean leaf size [27].…”