 Most of us are exposed to it at one time or another in our daily routines. Some hardly even notice it, accepting it as part of their everyday environment. But to those living in close proximity to busy highways, it can be a major source of annoyance. It can interrupt sleep, interfere with conversation, and cause stress. In recent years, more and more people have moved out of cities and into the suburbs or more rural communities. The result is larger numbers of people commuting greater distances, thus requiring larger and more efficient highways. Relatively little-use roadways become busy thoroughfares that require expansion to meet the demand. And many of these people want easy access, resulting in residential development in close proximity to the busy highways. The noise levels from these highways are often above the recommended levels established by the Federal Highway Administration, as well as state and local municipalities. Noise barriers have been shown to be an effective method of reducing the noise levels in communities adjacent to highways. This video will explain the basics of traffic noise and barrier performance, describe various types of barriers, and discuss concerns related to barrier maintenance and safety. Traffic noise originates primarily from three sources, truck exhaust stacks, tires interacting with the pavement, and vehicle engines. These sources each produce energy that in turn translates into tiny fluctuations in atmospheric pressure as the sources move and vibrate. These pressure fluctuations are measured in units of micro Pascal and range from 20 to 200 million. Because of this wide range, these fluctuations, called the sounds energy, are measured on a logarithmic scale known as the decibel scale. A value of zero decibels is equal to a pressure fluctuation of 20 micro Pascal and corresponds to the threshold of hearing for most humans. A value of 140 decibels is equal to a pressure fluctuation of 20 million micro Pascal, which is the threshold of pain for most humans. Following are sounds encountered in daily life and their approximate decibel values so that you can begin to equate numbers with actual sources. A quiet suburban neighborhood will exhibit an average sound level of about 40 decibels. Normal conversation occurs at around 65 decibels. A gas lawnmower of 31 meters away will create a sound level of approximately 70 decibels. A diesel truck at highway speeds 15 meters away will achieve a maximum level of about 85 decibels. A state-of-the-art jet aircraft at 305 meters altitude will create a maximum level of about 90 decibels. With these common examples in mind, traffic on a busy highway 15 meters away can average as much as 80 decibels during peak travel periods. Sounds typically occur over time, or in other words, they have a particular duration. A graphic time history of neighborhood sound levels over a one-hour period might look like this. The peaks and valleys in the level recording indicate different types of activity. This is the maximum level that occurred during the period. A common noise metric used to describe traffic noise is the hourly equivalent sound level. It can be thought of as the average level for a one-hour time period, although the mathematics are a bit more cumbersome. Hourly LEQ is the most commonly used noise metric for designing highway noise barriers. Typically, the hourly LEQ used for barrier design is for the hour in which traffic is heaviest, but still flowing freely. Not only does sound have loudness and duration, but it also has a frequency or tonality measured in hertz, or cycles per second. Most humans can hear in a range from 20 hertz to 20,000 hertz. As a point of reference, middle C on a piano has a frequency of 261.6 hertz. The notes on a typical 88-key piano range from 28 hertz to 4,186 hertz. Blanche, this is no time for a pause. The spoken voice typically has a range of 70 to 200 hertz for a man, and 140 to 400 hertz for a woman. You find anyone else you think you can work with? Blanche, it's not like hiring a maid. A collaboration is a very sensitive relationship. Following our tones at 250 hertz, 500 hertz, 1,000 hertz, and 2,000 hertz. Notice how the tones, although played at the same level, did not sound equally loud. The last tone at 2,000 hertz should have sounded the loudest. This is because the human ear is not equally sensitive to all frequencies. To account for this, community noise is usually measured using what is known as an A-weighted response network. A-weighting emphasizes sounds between 1,000 hertz and 6,300 hertz. And D emphasizes sounds above and below that range to simulate the response of the human ear. Sound levels measured using the A-weighting network are often denoted by the symbol DBA. Since normal conversation occurs at about 65 decibels, any noise which has a level substantially above this may cause annoyance. Noise barriers reduce the sound which enters a community from a busy highway by either absorbing it, reflecting it back across the highway, or forcing it to take a longer path. This longer path is referred to as the diffracted path. Diffraction, or the bending of sound waves around an obstacle, can occur both at the top of the barrier and around the ends. If designed properly, a noise barrier should be tall enough and long enough so that only a small portion of noise diffracts around the edges. In most instances, any noise which is transmitted through the barrier can be effectively neglected since it will be at such a low level relative to the diffracted noise. Properly designed noise barriers can attain a reduction in noise level of as much as 10 decibels, which is equivalent to a perceived halving in loudness for the homes directly behind the barrier, referred to as the first row of homes. The reduction in noise level due to a barrier is termed insertion loss. A halving in loudness requires a tenfold decrease in sound energy. Such a reduction is readily perceived as substantial by the adjoining community. To illustrate this reduction, here is a busy highway with a sound level of 80 DBA. Here is the same sound source reduced to 70 DBA. To give the viewer a general sense of barrier performance, a 5 DBA reduction can be expected for receivers whose line of sight to the roadway is blocked by the barrier. The general rule of thumb is that each additional 1 meter of barrier height above line of sight blockage will provide about 1.5 DBA of additional reduction. For those residents not directly behind the barrier, an insertion loss of 3 to 5 DBA can typically be provided. An insertion loss of 3 DBA is generally just slightly perceptible to the human ear. Here is the same busy highway, first at 80 DBA and then at 77 DBA. As you notice, it is very difficult to discern a difference. Can the installation of a barrier increase sound levels for those living on the opposite side of the highway? Reflections from a barrier on the opposite side of the roadway can theoretically result in a 3 DBA maximum increase. Practically, some of the sound is diffracted over the barrier and much of the reflected sound energy is dispersed by passing vehicles. Research has shown that typically a 1 or 2 DBA increase can be expected. Such an increase is not readily perceptible to the human ear. What happens when barriers are present on both sides of the highway? Sound reflected between reflective barriers may cause degradations in each barrier's performance anywhere from 2 to as much as 6 DBA. In other words, a single barrier with an insertion loss of 10 DBA may only realize an effective reduction of 4 to 8 DBA if another barrier is placed parallel to it on the opposite side of the highway. Fortunately, the problems caused by both single and parallel barriers can be minimized using one or a combination of these three methods. For parallel barriers, ensure that the distance between the two barriers is at least 10 times their average height. A 10 to 1 width to height ratio will result in an imperceptible degradation in performance. Applying acoustically absorptive material on either one or both barrier facades is another solution. The amount of noise that a barrier absorbs is typically expressed in terms of a noise reduction coefficient, or NRC. The NRC can range from 0 to 1. 0 means the barrier will reflect all the sound that strikes it, and 1 means the barrier will absorb all the sound. A typical NRC for an absorptive barrier ranges from 0.6 to 0.9. And finally, tilt one of the barriers outward away from the road. This solution, however, must consider structures higher than the opposite barrier, because they may be adversely affected by the reflected sound. Barriers can be categorized into two general types, earth berms and walls. Earth berms have the ability to blend more naturally with the surroundings that require large amounts of space to achieve significant noise reduction. Often because of limited right-of-way, earth berms must be transitioned into walls to accommodate highway overpasses. For reasons such as this, and due to the space requirement of berms, wall barriers are far more common. Wall barriers should be designed and constructed with visual aesthetics in mind. Variations in color, texture and form are common ways to achieve this objective. There are two schools of thought on how to make a wall barrier visually acceptable. It can be designed to stand out from its surroundings and become a visual focus. This approach can be costly to build and maintain. And if the barrier is too visually interesting, it may divert the motorist's attention away from the roadway. Or, a barrier can be designed to blend into its surroundings and have little visual impact. These two schools of thought are best exemplified when looking at design considerations associated with barrier ends. If a barrier has an abrupt end, it will stand out and look more unnatural. There are methods to lessen this unpleasant characteristic. A barrier's height can be stepped down gradually. A barrier's last one or two panels can be sloped. Or, a barrier's end panels can be angled inward toward the residence. This technique helps reduce diffraction of noise around the ends. Sometimes barrier ends can be transitioned into an earth berm. And finally, large plants or bushes can be used. In addition to visual quality, the type of material chosen for a wall barrier in a given location is an integral part of the design process. And is dependent on a variety of other factors including cost, weight, durability and maintenance. The needs of the nearby community as well as those of the motorists must also be considered. If a barrier is to blend into its surroundings, it should be constructed of material indigenous to the area. Concrete, metal and masonry block may be used in urban areas, while wood may be more appropriate in suburban or rural areas. Masonry block is a commonly used material for barriers. It is relatively inexpensive to use and maintain, massive enough to allow little sound transmission, and strong enough to survive high winds and minor vehicle impacts. However, this type of wall may become monotonous if visual relief is not provided. Using blocks of various colors and shades can make a wall more aesthetically pleasing. Here, blocks of different colors were used to represent the city's skyline. Precast concrete panels are another popular barrier material. And like masonry blocks, concrete panels are relatively inexpensive to install and maintain, allow for negligible sound transmission and are able to survive winds and minor vehicle impacts. Because the panels are precast, they can be installed in relatively short periods of time, reducing inconvenience for residents and motorists. Pictures and designs can be precast into the concrete for visual appeal. This panel barrier has a stone wall finish on the motorist's side and has an exposed aggregate finish on the resident's side. These concrete panels have been arranged in a zigzag configuration. This design is advantageous because it is structurally sound without the use of a foundation and is visually pleasing to motorists because it provides a variation in form. Wood barriers have the advantage of looking more natural and are thus considered to be more aesthetically pleasing. Because wood barriers are relatively lightweight, they can be used on bridges or other structures where weight is a consideration. However, if treated improperly, wood barriers can be costly to maintain. Moisture and temperature variations can necessitate frequent painting. Warping and cracking are also a maintenance consideration for wood barriers. Metal can also be used as barrier material. It is typically precast, can be installed relatively quickly, and is lightweight for use on bridges and other structures. Unfortunately, however, metal barriers can be difficult to maintain if subjected to vehicle impacts. And once a metal barrier's anti-corrosive finish has been compromised, it may rust and deteriorate further, defeating the aesthetic objective of a barrier. As previously mentioned, earth berms can be more naturally appealing and once landscaped are the easiest to maintain. They are not vulnerable to costly damage by vehicle impact or high winds and can be very economical if fill material and right-of-way space is available. If insufficient right-of-way space is available, a wall can be built on top of a small berm. This is usually more cost-effective than building a wall without a berm. Additionally, the slope of the berm provides an area for extensive landscaping. There are several innovative barrier designs in use throughout the country. Some are still in the experimental stage while others are being used with varying degrees of success. Transparent barriers are being used for maintaining scenic views and for ensuring that business advertisement isn't hampered. It is often necessary, however, for municipalities to underwrite the costly maintenance and cleaning of these barriers to help keep them transparent. Here, a barrier was constructed using a combination of masonry block and transparent panels. However, special care should be taken when using transparent panels so that glare from vehicle headlights does not cause a potential safety problem. Recently, there has been proposals for the use of recycled materials such as tires, either reprocessed as seen here or in discarded form. Plastic is also a potential recycled material per use as noise barriers. Another innovative design uses stacked concrete troughs which are filled with soil and then planted with vegetation. After the vegetation has grown out, the presence of the barrier grows almost unnoticed. These stone-filled wire blocks are extremely cost-effective because the stone was taken from the field created when the highway was under construction and are inexpensive to maintain. Oftentimes, residents resistant to barrier construction suggest that planting a row of trees may solve their noise disturbances. Unfortunately, a limited number of trees and shrubs will only have an out-of-sight, out-of-mind effect. In addition, any benefit deciduous trees may have in summer will be lost in winter. For all types of barriers, it is important that maintenance and safety factors be considered. Graffiti is one such maintenance factor. This is a masonry block barrier that originally used different colored blocks for aesthetic appeal. However, graffiti made it necessary to paint the wall, thus losing the effect. The transparent barrier shown earlier was marred by graffiti scratched into its surface. There are several types of anti-graffiti treatments which help to ease maintenance concerns and municipalities are adopting innovative approaches for both preventing graffiti and encouraging an active community role in the preservation of barriers. It should also be kept in mind that the choice of a barrier surface can naturally aid in the prevention of graffiti. This exposed aggregate finish makes graffiti defacement difficult. This same idea applies to the stone-filled wire blocks shown earlier. Perhaps the most graffiti resistant barrier of all is the earth berm. However, landscaping is of far more concern for earth berms as compared to wall barriers. From a maintenance standpoint, properly planned and maintained landscaping discourages littering as well as graffiti. Plants that are native to the area and require little irrigation will also be more successful than those requiring extensive pruning, mowing and watering. In northern climates, consideration should be given to plants which are resistant to the effects of sand and road salt. And all barriers must be placed far enough from the roadway to allow for cloud snow. Otherwise, snow removal costs will be a factor. Often, the area between the right-of-way fence and a barrier is ignored and becomes overgrown, causing residents to look unfavorably on an otherwise effective noise barrier. Residents usually find an agreement allowing them to use this land in return for its upkeep, quite acceptable. Some states have turned this area into playgrounds or pedestrian and bike paths. Safety is also an important consideration in barrier design. A barrier creating poor drainage can compromise safety for both residents and motorists. This system includes stormwater grates on both sides that carry water under the barrier and roadway. And this barrier has a gravel sub-base which allows some water to pass under it. For barriers of considerable length, some form of emergency access should be incorporated into the design. Here, doors were built into the barrier. A gap with overlapping barrier ends can be built with a negligible reduction in performance. A generally accepted rule of thumb is that the ratio between overlap distance and gap width should be at least 4 to 1 to reduce degradation of barrier performance. Easy accessibility to fire hydrants can mean the difference between life and death. This panel on a concrete barrier has a knockout receptacle to gain access to a fire hydrant in the adjacent community. Similarly, this barrier has a built-in door which is marked by a blue fire hydrant placard to indicate the location of the hydrant. And this barrier has fire hose receptacles built into it. For nighttime safety, lighting standards must be incorporated into the design that did not compromise the barrier's performance. To be most effective, a barrier should either be as close to the residential area it protects, or as close to the road as possible. The latter case can pose another safety issue. For a barrier less than 10 meters from the roadway, some form of vehicle impact protection will be necessary. Here, a guardrail has been placed in front of the barrier. And here, a jersey barrier has been incorporated into the design. A barrier must not block the line of sight between an on-ramp and the highway being traveled. Nor should a barrier block a motorist's view of the intersecting roadway at a stop sign or traffic light. We have seen that the quality of life is enhanced for those living next to busy highways through the use of noise barriers. A 10 DBA reduction, or a halving in the loudness of noise, can be realized by those immediately adjacent to the barrier, and a 3 to 5 DBA reduction by those farther away. Many non-acoustic benefits can be realized from the construction of a noise barrier. Two general types of noise barriers, earth berms and walls, each offer their own unique advantages. Maintenance and safety factors are equally important in ensuring the success of a barrier's design. A fully planned and maintained noise barrier can become a significant part of the environment and a valuable community asset.