Another Brick in the Highway Wall: Noise Barriers & Sound Science

With the constant hum of traffic, engines roaring, and tires screeching against the pavement, silence truly feels golden. How do we keep our neighborhoods and communities peaceful havens that allow us to relax and unwind? The go-to solution has been to erect noise barriers, but science suggests these concrete walls may not be the most effective way to reduce noise. Discover what alternatives we might have for the future and the figurative “walls” blocking innovation. Let’s dive into the details of acoustics, engineering, and how noise affects our health and well-being.

The Problem with Noise

Thanks to the Noise Control Act of 1972, noise is not allowed to bother a community, and highways can generate a lot of noise. These sounds come from different elements: the exhaust and engine of a vehicle, the aerodynamics around a vehicle, and tires against the road. If noise is projected to go over the government threshold of 67 decibels (dB) during the loudest or busiest parts of the day, the goal is then to reduce the noise level by 5 dB for some homes, though 9 dB is more ideal. The goal for noise barriers is to allow surrounding areas enough noise reduction so that people can have conversations without shouting, whether it be in a public park or in someone’s backyard. Noise can also lead to a variety of health issues, such as the following:

1. Circulatory issues such as high blood pressure, heart attack, and stroke

2. Nervous system issues including irritability, mood swings, depression, and anxiety

3. Lack of sleep or restful slumber

4. Immune suppression, difficulty healing, and chronic health issues

According to a 2011 World Health Organization report, environmental noise leads to heart disease, hypertension, and cognitive impairment in children.

What is a Noise Barrier?

With the aforementioned in mind, communities should prioritize a calm and quieter environment. This is where noise barriers come in. What is a noise barrier? Noise barriers are walls constructed alongside highway traffic, between the noise source and areas that require insulation, like neighborhoods and public spaces. They are typically made of metal, block, concrete, or wood, but can also be made of earthberm, insulating wool, or composites. The height of a noise barrier can vary, depending on what’s needed and funding. The idea behind noise barriers is to build walls high and thick so that sound from the highway is reduced for the surrounding areas. But how can we know how to make a noise barrier that will do what it needs to?

A number of studies come into play for determining the height, size, and construction for a noise barrier. Some of these include but are not limited to the mixture of cars and trucks expected on the road, the buildings and vegetation in the area that may block some sound, the configuration and ground quality of the land between the road and the homes, the ways the sound is expected to diffract around the wall, and more. A problem with this way of determining a noise barrier is that it assumes neutral weather conditions, not taking into account winds that may send, or even amplify sound, into surrounding areas. An interesting example of this is morning temperature changes: if warm air heats up cool ground in the morning, sound that would normally be pushed up is refracted downward, amplifying sound for houses some 500 or 1,000 feet from the road. Another issue is called “psycho-coustics,” a phenomenon where if people can see the tops of trucks over the wall they say it’s noisier. By breaking that line of sight, “we achieve a reduction of 5 decibels,” says Mariano Berrios, environmental programs coordinator at FDOT.

How Sound (& Noise) Work

Sound travels in waves. Whenever sound hits the edge of a barrier, it will then travel downward. This is why the higher the barrier, the better efficacy for noise reduction. With highway noise, a complication is refraction, where wind and temperature can alter the shape of the wave, changing how the sound wave travels resulting in a louder or re-directed sound wave. If you’ve ever been into an old cathedral or chapel, or even a large hall, surely you’ve noticed how sound will echo. This is another issue with concrete walls: sound hitting a concrete wall will mostly just bounce against that wall. When a highway is lined with noise barriers on either side, noise is left to bounce in-between the walls. This works in some cases. When we’re at home watching a loud tv, the sound rarely travels outside of the home. If it does, it’s definitely reduced. Depending on the make up of the home, the tv can be muffled, silenced, or even amplified depending on how sound has traveled down different hallways, hit walls, or is absorbed by furniture and rugs.

However, any sound engineer, podcaster, musician, or anyone that has been in a recording studio knows the importance of silence. Recording studios aren’t made with 15ft of concrete walls—they’re instead lined with acoustic panels that absorb sound. This way, when a sound travels throughout the room, it is instantly absorbed into something and dispersed, softened. Enough acoustic panels can muffle any ambient sounds, allowing a mic to pick up only the sounds desired. With this being such common knowledge in the world of acoustics and sound engineering, it feels fairly obvious that more work should go into methods to absorb sound, versus continuing to erect slabs of concrete.

Alternate Methods

Sound absorbing walls are made using materials such as mineral wool between layers of perforated fiberglass, to diffuse and absorb much of the unwanted sound, and can be made to withstand weathering and UV damage. They are also pre-engineered and can even be a more cost-effective method to reduce highway noise, especially since concrete walls cannot be moved or recycled. Since highway walls ran $2 million per mile, an annual $6 billion in 2013, a huge amount of tax payer dollars go to funding noise barriers versus other areas of transportation that also need funding. The University of Pittsburg was even given a grant to study noise absorption methods. Technological improvements to cars have resulted in quieter vehicles, however these are often more focused on internal silence rather than external. This is also true for tire technology and advancements made for quieter tires. The most promising and cost-efficient sound absorption method seems to be altering pavement itself. This is done by adding more porous surfaces to asphalt or by altering the the configuration of grooves in the concrete, to allow tires to travel over the road generating less noise. However, cities that have switched to quieter pavement are still required to put up a sound barrier. Since the pavement needs to be resurfaced every 15 years, states opt for the cheapest noise barrier they can get, which means putting up a permanent concrete wall anyway.

Pros & Cons

So it looks like noise barriers are here to stay—for now at least, so let’s simplify this by discussing the pros and cons. First, the cons: they’re often unattractive, they can re-direct noise into other areas rather than reducing it, sometimes they don’t reduce noise at all, they obstruct the view of drivers, and also can create bird-window collisions as well as prevent wildlife from being able to cross when necessary. However, they have been someone effective otherwise we wouldn’t use them all. Aside from knowing that they do reduce sound for neighboring areas, they can also reduce air pollution levels. The science behind how this works is too much to get into here. Here’s an attempt to word it simply: polluted air goes up versus out when between concrete barriers, and since it continues to spin with non-polluted air, disperses polluted air whenever it crosses the barrier. With highway travel only increasing every year, this is an important benefit seen from noise barriers.

We’ve explored how highway noise barriers represent a cross-section between engineering, environmental factors, and public health. While concrete walls might be the default, hopefully we can shift toward more cost-effective solutions that have been proven to reduce more noise. Regardless, cities continue to grow and traffic will increase with them. Whether in the near or distant future, whenever you travel along a highway, remember that Interstate Signways marked the way.