08 Aug Noise Barriers: How do they work?
Outdoor noise barriers can effectively reduce the transmission of noise from source to receiver. When placed between source and receiver, the barrier diffracts the sound transmitted to the receiver. This reduction is frequency dependent: Noise barriers block high frequencies more effectively than low frequencies.
What determines the effectiveness of a noise barrier?
A noise barrier’s effectiveness is determined by the degree to which it forces sound to bend to reach the receiver. The following sketches show the general principles involved and how changing the barrier height and location effects this critical angle.
As long as the barrier interrupts the straight line path from source to receiver, noise will attenuate as it diffracts around the barrier (shown as dotted lines).
There are a number of ways to increase the effectiveness of a barrier. The first is to construct the barrier close to the source. This is one of the best methods as it benefits all locations past the barrier.
If the barrier can’t be located near the source, the next best location is near the receiver. This is equally effective for that receiver, but the benefit diminishes for receivers at a greater distance. When a noise barrier is close to the receiver, the critical angle also increases.
Generally, the least effective location for a barrier is midway between source and receiver. Yet regardless of location, a barrier’s acoustical benefit improves when the barrier height is increased.
The mass of the barrier is usually not a critical element. The barrier should be constructed so sound that penetrates through the barrier is sufficiently lower than the sound that diffracts over the top. For example, the sound transmission loss of the barrier should be at least 10 decibels lower than the attenuation planned for above the barrier. A solid barrier that supports itself and withstands wind loading will often provide more than adequate sound transmission loss.
Calculating noise barrier attenuation
The actual calculation of barrier attenuation is based on formulas developed by Fresnel, using the following geometrical format:
When the barrier blocks line-of-sight between the source and receiver, the Fresnel Number (N) is found using this formula:
Using the Fresnel Number and the following chart, you can determine the excess attenuation (Ae4 in the chart below) of the barrier.
Remember that other factors can influence a barrier’s effectiveness. Varying air speeds (increasing with height above ground) caused by light winds travelling from source to receiver can refract the sound passing through the air and bend it downward. This action tends to reduce the effectiveness of a barrier. Trees with foliage above the top the barrier can have a similar detrimental effect.
Noise barriers can also cancel the beneficial attenuation that results from ground effect, a phase cancellation effect that can occur when both source and receiver are very near the ground. This loss of ground effect attenuation must be subtracted from the barrier attenuation to determine the net attenuation of the barrier.
The barrier should block all paths from source to receiver. This means that if a barrier is 3 metres high, it should extend 6 metres horizontally past the point where the barrier blocks line-of-sight from source to receiver. This prevents sound from flanking the barrier’s edge. In addition, there can be no gaps or low points in the barrier. An opening, for a roadway for example, would significantly diminish the acoustical performance of the barrier unless it was carefully designed.
Before installing a noise barrier, it’s important to consider the acoustical factors we describe in this article. Yet there are other factors that can’t be overlooked, factors that can affect how the barrier is perceived by residents. During the design stage, ask the following questions:
- Will the barrier create a safety issue by blocking the view of drivers to other cars or to pedestrians?
- Will the barrier block a view that is important to the residents?
- Will the barrier itself look unattractive?
- Will the barrier obstruct a breeze that helps cool residences’ yards?
- Will the barrier adversely effect plants or gardens? Will it cause unacceptable shading?
- Will the barrier require passages for pedestrian and/or vehicle access?
- Will the barrier become a personal security hazard by creating areas where criminals can hide?
- Will the barrier demand ongoing maintenance costs for the municipality?
- Will there be drainage or snow removal issues?