Fifteen Years of Research on Active Noise Control Systems for Partially Open Windows
Delf Sachau – sachau@hsu-hh.de
Professur für Mechatronik, Helmut-Schmidt-Universität, Hamburg, Hamburg, 22043, Germany
Dr.-Ing Tim Karl
Professur für Mechatronik
Helmut-Schmidt-Universität
Hamburg
Popular version of 3pAA10 – Fifteen Years of Research on Active Noise Control Systems for Partially Open Windows: A Summary of Key Findings
Presented at the 189th ASA Meeting
Read the abstract at https://eppro02.ativ.me/appinfo.php?page=Session&project=ASAASJ25&id=3979397&server=eppro02.ativ.me
–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–
Motivation
In many cities, people want to keep their windows open to allow fresh air into their homes. However, especially in busy urban areas, open windows also let in unwanted noise from traffic, trains, aircraft, and general city activity. Constant exposure to this noise is not just annoying, it can affect sleep, concentration, and even long-term health. To address this problem, researchers at the Helmut Schmidt University in Hamburg have spent the past fifteen years developing systems that can reduce noise coming through partially open windows while still allowing natural ventilation.
Passive Absorbers
The approach combines two methods: passive noise reduction and active noise control (ANC). Passive noise reduction involves using materials that naturally absorb or block sound, such as foam-like acoustic panels or special seals. These materials are very good at reducing high-frequency noise but are less effective for deeper, low-pitched sounds like engines or traffic rumble.
ActiveNoise Control
This is where active noise control comes in. ANC works in a way similar to noise-cancelling headphones. Small loudspeakers placed near the window play “anti-noise sound waves” that are shaped to cancel out incoming noise. When the incoming noise and the anti-noise meet, they interfere with each other and reduce the amount of sound that reaches inside the room. To make this happen, microphones are used to measure the sound, while computer algorithms constantly adjust the sound from the speakers to keep the cancellation effective.
Figure 1: Internoise 2020, J. Hanselka, D. Sachau, Converting an Active Noise Blocker for a Tilted Window from Feedforward Control into a Feedback System
Figure 1: Internoise 2020, J. Hanselka, D. Sachau, Converting an Active Noise Blocker for a Tilted Window from Feedforward Control into a Feedback SystemAlgorithm
worked on improving the computer algorithms that run the ANC system. These algorithms need to react quickly to changing noise, remain stable, and avoid using too much power. Therefor analyses conduction different real-time-controller platforms were evaluated, including DSP and FPGA technology
Figure 2: ISMA 2014, D. Sachau, S. Jukkert, Real-time implementation of the frequency-domain FxLMS algorithm without block delay for adaptive noise blocker
Figure 2: ISMA 2014, D. Sachau, S. Jukkert, Real-time implementation of the frequency-domain FxLMS algorithm without block delay for adaptive noise blockerSimulation
However, using ANC at an open window is much more complicated than inside headphones. The sound field near an open window is irregular and constantly changing because of airflow, reflections, and outdoor conditions. The research team therefore studied how sound moves through small openings of different shapes and sizes. One important discovery is that the depth of the opening relative to the wavelength of the sound plays a enormous role in how much noise gets through. This knowledge helps guide how the ANC system can be designed and placed.
Figure 3: Internoise2020, M. Sandner, D. Sachau, Influence of parameters of small gaps regarding sound transmission and ANC-performance-a numerical simulation
Figure 3: Internoise2020, M. Sandner, D. Sachau, Influence of parameters of small gaps regarding sound transmission and ANC-performance-a numerical simulationPosition Optimization
Another major research effort focused on the best positions for microphones and speakers. Their placement determines how well the noise can be cancelled. The researchers found that placing the speaker near the center of the opening often provides the most even noise reduction throughout the room. Meanwhile, microphone placement is very important for stability, because the microphone input is what guides the control system in real time.
Figure 4: DAGA 2025, T. Karl, D. Sachau, Numerical position optimization approach for sensor and actuator placement in an active noise cancelling system
Figure 4: DAGA 2025, T. Karl, D. Sachau, Numerical position optimization approach for sensor and actuator placement in an active noise cancelling systemConclusion
Overall, the research shows that a combination of passive materials and active noise control is the best approach. Passive elements reduce parts of the noise that are hard to cancel electronically, while ANC handles the deep, low-frequency noise that humans find especially disturbing. Together, these methods make it possible to keep windows open for fresh air -without letting in the city.