'Smart window' technology opens new possibilities for transportation noise control

Nearly every time a highway or airport expansion is proposed, transportation planners are faced with opposition from residents who fear the increased noise levels in their homes and businesses.  Traffic noise is often mitigated with physical noise barriers, but the large, thick walls often draw opposition as well. 

Now, a new technology developed by University of Minnesota mechanical engineering professor Rajesh Rajamani as part of a research project funded by the National Science Foundation could soon provide a nearly invisible solution for transportation noise cancellation—and give transportation planners another tool for overcoming project opposition.

“For homes close to airports and highways, the primary way noise enters the home is through windows. A window can transmit ten times as much sound energy as a wall can,” says Rajamani. “We realized that of all the possible ways to reduce transportation noise in the home, the best way to do it is by decreasing the amount of noise that is transmitted through the windows.”

To accomplish this goal, researchers set out to create a better method of active noise control for windows.  In the most basic sense, active noise control works by using speakers to generate a sound wave that is a mirror image of the undesirable sound wave. While it may seem counterintuitive, superimposing an “anti-noise” wave of the same amplitude as the undesirable noise wave results in a reduced decibel level of noise in the environment.

In an earlier study, the research team designed extremely thin and nearly transparent speaker panels to fit in the empty space between the two panes of a double-pane window.  In a second study, researchers tackled the problem of how to effectively use the new speaker to cancel out undesirable transportation noise from outside the home while preserving the desirable noise—such as music and conversation—from inside the home. 

Using two small microphones positioned on either side of the window, researchers found they could identify both the undesirable external sound waves and the desirable internal sound waves. Next, they developed a mathematical algorithm that separates out the desirable from the undesirable sound waves based on the direction of travel. This information is then transmitted to the speaker panel within the window; the speakers use this information to produce the proper sound wavelengths to cancel out much of the external noise while preserving the desired sound.

“Using this new method of active noise control with wave separation, we found that we can effectively reduce external noise while better preserving internal sound,” says Rajamani.

In addition to mitigating traffic noise, this new technology offers other surprising benefits. Researchers have found that the “smart window” speakers can actually be used as home audio speakers without losing any of their noise-control benefits. For example, homeowners can listen to their favorite CD through the smart window speakers while also experiencing reduced highway noise inside their home.

Finally, researchers discovered that the small nanotube films they designed to power the smart windows may also be able to help power hybrid electric vehicles in the future.