Noise Mitigation Principles


There are three distinct types of noise mitigation:

1. Reduce the sound level at the source;

2. Interrupt the path of the noise; or

3. Shield the receptor from the noise.


Source Control


Reducing the source sound emissions results in the most desirable effect; lowering sound levels for everyone without those who are impacted having to provide mitigation.  Source control can sometimes be the cheapest form of noise reduction since lower source emissions may preempt the need for alternative mitigation efforts at many other locations.  Noise control engineering can typically reduce source sound emissions, particularly, when it is included into the equipment design or prior to installation.  It is typically more difficult to retrofit noise reductions on existing equipment.  In some cases equipment or operations can not accommodate any or more noise reduction technology and other methods of noise control must be considered.

Path Control


Interrupting the path of sound can reduce sound exposure.  The most common type of path control is a noise barrier.  Noise barriers that block the direct path of the sound reduce the sound exposure to that resulting from refraction by the barrier and the sound that travels over the top of the barrier.  Properly designed noise barriers can offer reductions in sound level of up to 20-25 dB, although a 10 dB reduction may be more likely.  The amount of sound reduction is typically proportional to the size of the barrier, both height and length.  Noise barriers offer the greatest reduction when they are placed close to either the source or the receiver (the receiver is the person, residence, etc.).  The closer the barrier is to the source, the greater the effective height of the barrier.  Barriers that are closer to the receiver provide abatement by creating a “shadow zone”, which typically is only effective for a limited area.  Barriers are least effective when placed in the middle of the source and receiver.  Barriers also lose effectiveness with increasing distance between the source and receiver, particularly when the barrier is neither close to the source or the receiver.  Over large distances (such as those greater than 300 meters) sound can be bent over the barrier by wind or reflected back towards ground by temperature inversions thereby reducing the attenuating effects.  A drawback with noise barriers is they can reflect sound back in other directions.  Often this is not a problem and is commonly mitigated by including sound absorption on the face of the barrier. 


Receptor Shielding


Shielding the receptor is typically utilized as a last resort because the high sound levels are ever present and an impact exists when the shielding is removed.  The most common types of receptor shielding are ear plugs and headphones; which are very effective and inexpensive when utilized.  Home sound proofing is also a type of receptor shielding and is typically the only available solution to noise impacts from airborne aircraft.  Ear plugs and headphones are viable and common solutions to high noise in the work place but are poor solutions to community noise issues.  Home sound proofing can be very effective for reducing sound exposure within the home; however, it does not address the outdoor environment and is typically only considered when other options are not available.  Because the (closed) windows in our homes are typically the part of the structure that allow the greatest amount of noise to enter, the addition of storm windows to the exterior can often substantially reduce the interior noise exposure, particularly with a gap of 2-4 inches between storm and window panes.


Sound Masking


Although not a form of noise reduction, sound masking can often be utilized to reduce noise annoyance.  Sound masking works by “masking” the annoying sound, “the noise”, with a new sound thereby making the noise less or not noticeable.  Sound masking is commonly used to reduce noise annoyance.  Common sound masking devices are air-conditioners, fans, and electronic equipment that produce continuous “white” or “pink” noise to cover up intrusive sounds.  Sound masking is less effective when its levels have to be loud in order to “mask” the noise.