February 2017 Newsletter: Building Walls

In today’s political scene and in the media, there’s been a lot of talk about building walls or not building walls.  With all this talk of walls, we thought now would be a good time to talk about the acoustical performance of walls, or as we acousticians like to call them, sound barriers.  Barriers can be effective solutions for certain types of noise issues, but they have their limitations.  Read on to learn more about how sound barriers (walls) can be a good thing in the acoustical world.

Let’s say there is or there will be a noisy piece of equipment outside of a building.  The equipment could be an air-cooled chiller, cooling tower, or air handling unit, to name a few.  When sound from outdoor equipment is disturbing to nearby residents, the designers must determine ways to reduce the sound transmission, particularly when a noise ordinance violation is a possibility.  The first step should always be to reduce noise at the source, which can be accomplished by selecting quieter equipment and specifying sound attenuating accessories and options from the manufacturer.

When this first step fails to meet the sound control goals, a common practice is to surround the noisy equipment with a wall or barrier.  To understand the acoustical performance characteristics of barriers we first need to acknowledge that sound at lower frequencies travels very differently than sound at higher frequencies.  High frequency sound such as the squeal of a fan belt or failing bearings is very directional sound.  A barrier that blocks the line-of-sight from a receiver to this sound source is likely to provide meaningful attenuation.  As the frequency of sound decreases, it is increasingly able to bend around objects, a phenomenon called diffraction.  When sound diffracts around a barrier, the area where the barrier provides sound attenuation is reduced.  The area of sound attenuation is referred to as the shadow zone as illustrated below.

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Image Credit: BRD Noise Control

In practice, diffraction limits the effectiveness of barriers to attenuate low frequency noise, especially when the distance between source and receiver is large. The primary factors that determine a barrier’s acoustical performance include:

  • The frequencies of interest.
  • The distances between the barrier and the noise source, between the barrier and the receiver, and between the source and receiver.
  • The relative heights of the source and receiver positions.
  • The height, width, thickness, and mass of the barrier.

To complicate matters further, most outdoor equipment requires airflow for cooling or combustion.  The equipment must not incur an unacceptable drop in airflow due to the barrier or its performance, lifespan, and warranty could be in jeopardy.  Barriers can be solid which offer the most benefit; others consist of acoustical louvers to allow for airflow while providing some degree of sound control.


Photo credit: Kinetics Noise Control


Photo credit: Metropolitan Acoustics


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