Engineers design solutions to tackle low-frequency noise

Low frequency noise (500 Hertz and lower), which could stem from various sources such as construction machineries and aircrafts, is a form of environmental noise problem in urban environments. Such noise transmits over long distances and cause disturbance to wide area. It is also known to trigger a number of negative physiological reactions such as changes to blood pressure, vertigo and breathing difficulties even when the noise is not audible.

Most commercially available noise canceling devices and structures today, however, are only effective in reducing high frequency noise, while low frequency noise continues to penetrate through these structures. To more effectively mitigate low frequency noise, a team of engineers from the National University of Singapore (NUS) has therefore designed a set of novel noise attenuating blocks that targets low frequency noise.

Small, customisable blocks to reduce a range of low frequency noise

Each 3-D-printed block can be customised to cancel a specific noise frequency by adjusting the size of the air cavity and the neck opening within the block. The blocks can then be slotted into a grid-like host structure to function as a noise barrier. Using this modular design, the NUS engineers were able to better control the properties of the noise barrier and produce them affordably. The team's experiments also showed that the modular design was capable of cancelling low frequency noise below 500 Hertz by an average of 31 decibel, which is six times more effective than other commonly used noise barriers. Additionally, the modular design was also lighter and thinner compared to certain commonly used noise barriers.

"These noise reduction blocks can be easily incorporated as part of existing wall structures, and it would not be necessary to build a host structure from scratch in order to implement the sound barrier. Such a modular design also means that a variety of blocks targeting different noise frequencies can be used within the same host structure to effectively block a range of low frequency noise," explained Associate Professor Lee Heow Pueh from the Department of Mechanical Engineering at NUS Faculty of Engineering, who led the innovation.

Mobile application for accurate noise data tracking

The team also developed 'Noise Explorer', a mobile application capable of accurately tracking noise data based on a new method established by the team to calibrate the microphones of smartphones. Unlike existing noise monitoring mobile applications, Noise Explorer, when calibrated, can measure sound pressure level and frequency content with an accuracy of less than one decibel for 99.7% of the measurements in the team's experiments.

An immediate application of Noise Explorer is for crowdsourcing of noise data to help authorities identify and mitigate the source of noise more effectively. Other potential uses of the mobile application include tracking of one's exposure to noise over a period of time and coupling with artificial intelligence for condition monitoring of devices and equipment.

Assoc Prof Lee said, "A customisable modular noise barrier, together with a handy and accurate noise meter, will enable authorities and regulators to tailor their noise management strategies effectively to the nature of the noise transmitting in an area. In this way, the people living and working in the area could enjoy greater comfort."

He added that his team is in discussion with a local company to further develop and commercialise the noise attenuating blocks. They are also looking into improving the aesthetics of the blocks such that they can be seamlessly integrated into a building's architectural design.