Noise barriers are designed to resist the sound waves in the propagation path from source to receiver. In general, the closer the barrier is to the source the more effective it becomes. For simple plane barriers the height and length are the most important factors determining the degree of screening achieved and simple design rules have been developed to determine the reduction in overall noise levels. These are based on the path difference between the direct path from source to receiver through the barrier and the shortest path passing over the top of the barrier. The greater this path difference the greater the screening. The shadow zone of the barrier is the region where the receiver cannot see the source and here the greatest reductions in noise levels are recorded. Some sound will always be diffracted over the top and around the edges of the barrier into the shadow zone so it is not possible to eliminate all noise from the source. However, typical barriers of a few metres high can achieve a worthwhile noise reduction of the order of 10 dB(A). This corresponds to halving the subjective loudness of the sound.
Figure (a)
Figure (b)
For more complex barriers simple methods are not appropriate and numerical methods such as the Boundary Element Method (BEM) have been used to produce accurate solutions.
Many different types of barrier have been installed using a wide variety of materials including wood, steel, aluminium, concrete and acrylic sheeting. Some of these designs have absorptive facings on the traffic side which reduce reflected sound. Barriers over 8 m in height have been used for some applications and novel capped barriers and angled barriers have been tested.
Barriers that may offer improved performance over simple plane barriers can be grouped under the following broad headings.
The above fig (a) shows the Main pathway of the sound propagation from the source to the barrier’s edge for sound walls with or without source-side absorption. Fig (b) shows Absorption material construction.
If smaller vehicles passing by the barrier, the reflection off the vehicle it does not play much of a role. Multiple reflections can only occur if noise barriers are built along both sides of the highway or train tracks.
In the case of large noise emitters, the implementation of source-side absorbent noise barriers can prevent the so-called zigzag effect
- Absorptive barriers—that is, barriers incorporating elements on the traffic face that absorb a significant proportion of incident sound and hence reduce reflected sound which could contribute to overall noise levels in the vicinity.
- Angled barriers—that is, barriers that are tilted away or have contoured surfaces angled to disperse the noise, the aim being to prevent significant sound reflections into the area where screening is required.
ABSORPTIVE BARRIERS
Where a plane vertical barrier is erected on one side of the road then sound reflections to the opposite side take place as illustrated in fig 1(a). In addition, reflections between vehicles and the barrier may lead to loss of screening performance as shown in fig (b). Where plane vertical barriers exist on both sides of the road, as shown in fig(c), they are normally parallel to each other and, in this situation, sound is reflected back and forth between the barriers again leading to a loss in performance. Absorbing panels located on the sides of the barriers facing the traffic can reduce this reflected contribution by absorbing the sound energy from the incident wave.
ANGLED BARRIERS
An alternative to using sound absorptive barriers is to angle the barrier or parts of the barrier away from the road such that the reflected wave from the traffic face of the barrier is deflected upwards, so reducing the contribution to noise at receptor positions relatively close to the ground. The performance of such barriers has been measured at full scale at TRL’s unique Noise Barrier Test Facility (NBTF). The noise source used consisted of an 800 W speaker that can be positioned in front of the test barrier on a specially laid strip of hot rolled asphalt, thereby representing the traffic source on motorways and all-purpose dual carriageway roads. Microphones can be positioned to measure the noise level in the shadow zone of the test barrier at any point on a wide flat grassland area free of reflecting objects. To measure the acoustic performance of the barrier, recorded noise in a broad frequency range is broadcast and noise levels are measured at standard locations behind the barrier. Corrections can be made for variations in speaker output and wind speed and direction. In this way the screening performance of the barriers for a typical traffic noise source can be evaluated.
The above fig shows angled noise barrier.
Source : Various books and research journal
