Sound mitigation is as complex as sound perception. The engineering aspects of noise mitigation involve sound source emissions, sound path controls and receptor sound reductions. These aspects are complex but are more predictable by the principles of physics. The psychological aspects of noise mitigation involve changing the perception of sound and are unique to the individual receiver and the collective community. It typically takes a community to protect the rights of the individuals. The community has the power to make a difference. However, community noise requires the community to consider an individual’s right to quiet versus the community’s right to make sound that may be considered noise. Certainly a concept that is beyond engineering; it relies on psychology and sociology, particularly what is fair social conduct.
Physical Noise (Sound) Mitigation Principles
There are three distinct types of physical noise mitigation:
1. Reduce the sound level at the source;
2. Interrupt the path of the sound; or
3. Shield the receptor from the sound.
Reducing the source sound emissions results in the most desirable effect; thus 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. Physical noise control 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 sound reductions on existing equipment. In some cases equipment or operations cannot accommodate any or more sound reduction technology and other methods of noise control must be considered.
The cheapest form of source control is buying the most quiet equipment available. Specifying “quiet equipment” is a way of signaling manufacturers that designing quiet equipment has a real sales value. Quiet equipment will always be more expensive than the cheapest solution, the “lowest bidder”. Manufacturers need to see the reward for the additional expense and effort necessary to produce more quiet equipment. The low bid device (often the noisiest) may seem like a fiscally responsible choice. However, when noise control is the afterthought it will always be more expensive and it is hard to put a value on a negative public perception.
A common physical source control is restricting the time of use for the offending noise source. This is commonly done to limit noise annoyance during nighttime while the majority of the community is sleeping. Strict time limitations are an integral part of OSHA regulations and can also be used for managing community noise.
Interrupting the path of sound can reduce noise exposure. The most common type of path control is the sound source’s structural enclosure, typically a building. Proper design of the structural enclosure will be the most effective means of community noise control if source control is not an option. The walls of a structural enclosure are noise barriers, typically at the closest location to the source, and it includes a roof which can be a very effective noise barrier. The entire system of four walls and a roof offers the greatest means of confining a noise source and reducing community noise emissions.
Exterior sound barriers are typically called “noise barrier” wall systems. Exterior noise barriers lack the roof component which helps prevent sound from passing over the barrier which is a substantial limitation for a wall system. Noise barriers can block the direct path of the sound and thus reduce the sound exposure to that resulting from refraction and the sound that travels over the top of the barrier. Exterior noise barriers are too expensive to be used decoratively and are only present to reduce community noise. Exterior noise barriers have the flexibility to be both solid wall (cement, metal, wood and plastic) or flexible “curtain” walls that are hung on a structural frame. Noise barriers can be a good choice for temporary noise reduction.
Theoretically, sound barriers can offer reductions in sound level of up to 20-25 decibels. However, in real-world circumstances a 10 dBA reduction may be more likely. Sound barriers also do not attenuate all frequencies the same. Higher frequencies are more easily attenuated than lower frequencies. Noise barriers will have a measurable level of noise reduction at each frequency and are often not effective for eliminating tones even though they may attenuate the overall sound level. Thoughtful engineering is required to attenuate low frequencies and “pure tone” issues.
The amount of sound reduction is typically proportional to the size of the barrier, both height and length, and the mass of the wall material. Typically, the heavier the wall material the more effective the sound reduction although the use of composite materials can be very effective at a lessor weight. Composite materials have a layered construction with different material densities and sound transmission properties that provide more attenuation with less weight.
Sound 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 path. 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 sound 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.
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. 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 is also a type of receptor shielding and is typically the only available solution to noise impacts from airborne aircraft. 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 sound 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. Thermopane glass has two glass panes spaced close together which acts as a coupled system and will offer less sound reduction than two panes of glass with at least a 2 inch gap between them.
Psychological Noise Mitigation Principles
Because noise involves the perception of sound, interrupting the perception of sound or changing the perception can offer noise mitigation. Psychological noise mitigation can be effective in changing the emotional perception of noise by increasing the tolerance of the noise source. Psychological noise mitigation typically involves a benefit to offset the noise detriment. Viable offsets include financial incentives, tax abatements or local and community improvements.
Sound masking can often be utilized to reduce noise annoyance. Sound masking is not a form of noise reduction but a method of interrupting the perception of sound. 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 in open office environments and can be very effective. Common residential 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 offending noise.
Noise compensation is a commonly used principle where those affected by noise are somehow compensated for having to endure noisy environments. What constitutes compensation for enduring a high noise exposure is a decision for those that are impacted by the noise. Types of compensation include direct financial payments, tax abatements, property improvements and community improvements. Examples of common noise compensation include (but are not limited to):