A soundwall blocks noise from the busy 10 Freeway near Alhambra. (Robert Petersen for LAist)

The tall, beige, unremarkable-looking concrete barriers that stand between freeways and neighborhoods are, in fact, doing something remarkable.

They're called soundwalls, and they're protecting ears from an unending cacophony of traffic that's more than just a nuisance. Freeway noise can lead to hearing impairment, anxiety, sleep disturbance and hypertension.

Faced with a wall, sound can do three things: it can be reflected, absorbed, and/or transmitted (passed through to the other side). Freeway soundwalls are built tall and thick enough so that they reflect soundwaves created by traffic and allow only the soundwaves that travel over and around the wall to reach the other side.

Today, there are thousands of miles of freeway soundwalls all over the country. But the original idea for soundwalls can be traced right back here to Los Angeles — back to the 1940s — when freeway noise started interfering with performances at the Hollywood Bowl.

After the two-mile long Cahuenga Pass Freeway opened in 1940 near the Hollywood Bowl, concertgoers and residents started complaining about traffic noise. So, in 1945, the California Department of Public Works conducted a noise study, which recommended building a 10-foot high wall to block the traffic noise.

But the wall wasn't built and noise complaints grew as the entire Hollywood Freeway was opened several years later, clogging the road with 183,000 vehicles a day — almost double the volume it was designed to carry.

The state highway department conducted additional studies regarding the problem of freeway noise at the Bowl and later recommended building an even taller 30-foot tall wall. These early studies helped lay the foundation for freeway soundwalls.

In 1954, L.A. Mayor Norris Poulsen appointed a committee to study the noise caused by the 101 Freeway. According to L.A. City Council records from that time, the city's general manager recommended developing a sound barrier consisting of high vegetation to screen the Hollywood Bowl.

This aerial shot from 1954 shows Hollywood Bowl and the nearby 101 Freeway, which concertgoers made regular noise complaints about. (Courtesy Los Angeles Public Library archives)

But instead of building the wall, city records show officials decided to crack down on cars without mufflers, hoping they could prevent excessive noise. A new city job was created: Motor Vehicle Muffler Inspector.

The city ordinance creating the job position noted that the noise caused by the 101 Freeway threatened "the peace, health and safety of persons living in areas adjacent to [the] Freeway."

Meanwhile, Hollywood was also complaining about freeway noise.

Movie studios worried that it would interfere with filming. In 1956, the state Department of Public Works conducted a study of freeway noise at the Columbia Pictures Ranch in Burbank — now Warner Bros. Ranch — where dozens of movies and television shows have been filmed, including I Dream of Jeannie and Bewitched.

And the complaints kept coming. As more freeways were built, more people complained about the noise.

Then in 1968, the California Department of Public Works built what is believed to be the state's first official freeway soundwall on Interstate 680 in Milpitas, near San Jose.

This concept art from a 1968 Caltrans report on freeway noise shows an idea for a "noise shield," which became the common soundwalls that you can find along freeways all over Southern California. (Courtesy Caltrans)

Several years later (and decades after they were first proposed), sound barriers started appearing in Los Angeles — and across the country — after the passage of the passage of the Noise Control Act of 1972, which set forth a nationwide program to reduce the effects of freeway noise and provided federal funding to build soundwalls.

Over the years, billions of taxpayer dollars have been spent on freeway soundwalls. And to be built, they all had to meet the same requirement: it had to be noisy enough to merit one.

Sixty-seven decibels was chosen as the threshold "because it was found that that is the level at which you can carry a conversation in the outdoors," said Ben Jong, senior manager of transportation planning for Los Angeles Metro.

Above 67 decibels, people have to raise their voices to be heard, Jong explained. To put that into context, the typical vacuum cleaner measures 70 decibels and a jackhammer measures 102 decibels.

Soundwalls don't eliminate all noise, but they aim to reduce it by a noticeable level.

Whenever the California Department of Transportation recommends a soundwall, they make sure that "the wall will reduce the noise level by a minimum of 5 decibels," said Jin Lee, a senior transportation engineer in charge of Caltrans' Noise and Vibration Unit, "because 5 decibels is what we consider a readily noticeable change." Put another way, a 5 decibel reduction is like a 25% decrease in loudness.

If you want to hear the difference a soundwall makes, here's what it sounds like directly on the side of the 10 Freeway. And here's the difference behind the soundwall.

Soundwalls don't eliminate all noise, but they aim to reduce it by a noticeable level. (Robert Petersen for LAist)

While the vast majority of soundwalls are composed of concrete blocks, they don't have to be. Any material will work, as long as it is dense and thick enough.

The city of Long Beach erected a soundwall in 2013 made of mulch. Dubbed "The Great Wall of Mulch," it consists of two parallel chain link fences (12 feet high and 3 feet apart from each) running for 600 feet, with the space in between the fences filled with wood chips from city street tree trimmings.

"As far as we know it's the only sound barrier of its kind in the United States," said Larry Rich, sustainability coordinator for the city.

The mulch wall was comparatively cheap - costing about $125,000 - and it has the dual benefit of mitigating noise and air pollution. At this point, there are no plans to extend the pilot wall, but it shows that there are other options for soundwalls other than concrete.

About 250 miles of soundwalls have been built throughout L.A. County, according to Caltrans. Another 200 miles of freeway are currently eligible, Jong said. But don't expect them all to be constructed anytime soon.

L.A. Metro inherited a serious backlog of projects when it took over the responsibility for building the majority of freeway soundwalls in the county from Caltrans in the late 1990s. New freeway soundwalls are still being built, but with construction costs of between $10-20 million per mile, according to Jong, Metro simply doesn't have the funding to meet the demand. Agency officials estimate it would cost over $2 billion to build all the soundwalls currently on the eligibility list.

But not everybody loves freeway soundwalls. Some people consider them a view-blocking eyesore. And some argue that the walls don't even work that well.

Caltrans has looked into alternative noise mitigation techniques, such as transparent fiberglass soundwalls or pavement that generates less noise. But according to transportation officials, concrete soundwalls will continue to be the primary tool in the fight against freeway noise as long as they are viewed as the most practical, long-term, and cost-efficient method.

Jin Lee from Caltrans said the state agency is "constantly trying to innovate the methodology as to how to reduce noise as well as come up with new materials and new methods to reduce the noise."

"I can foresee different methods other than soundwalls," he said. "But, meanwhile, for the next ten to twenty years it will probably be mostly soundwalls."