A bubble without noise
How does active noise reduction technology in headphones work?
Author:
Title 4
Learning objectives :
This activity allows students to understand the principle of destructive interference used in active noise reduction headphones. It develops the ability to visualize complex wave phenomena.
Concepts covered
Wave interference; Constructive and destructive overlap; Wavelength; Anti-noise; Active noise control
What students will do :
The student explores the phenomenon of sound interference using three smartphones: two simultaneously emitting a pure 680 Hz sound and a third measuring sound intensity. By moving the measuring smartphone between the two sources, the student identifies the areas of constructive (amplified sound) and destructive (attenuated sound) interference and then makes the link with the active noise reduction technology of modern headphones.
What is required :
Three smartphones including at least one with the FizziQ application; A quiet space with few echoes; A tape measure to measure distances; FizziQ experience notebook
Scientific background :
When two sound waves of the same frequency meet, they superimpose according to the principle of interference. This superposition can be: 1) Constructive, when the maxima of the waves coincide, doubling the amplitude and quadrupling the sound intensity; 2) Destructive, when the maximum of one wave coincides with the minimum of the other, ideally leading to complete cancellation of sound. For a pure sound of 680 Hz, the wavelength λ is approximately 50 cm (λ = c/f, where c is the speed of sound, approximately 343 m/s at 20°C). Interference follows a precise spatial pattern: the distance between two consecutive destructive interference zones is λ/2, or approximately 25 cm for 680 Hz. This phenomenon is exploited in active noise reduction technology. Noise-canceling headphones use microphones to pick up ambient sounds, then instantly generate a sound wave of the same amplitude but in opposite phase (shifted by 180°). The superposition of these two waves creates destructive interference which significantly attenuates the noise perceived. This technology is particularly effective for low-frequency, relatively constant sounds (such as the roar of an airplane engine), but less so for high-pitched or sudden sounds due to the system's responsiveness limitations. The proposed experiment presents a static version of the phenomenon: the two sound sources create a fixed interference field in space, with alternating areas of reinforcement and cancellation. In contrast, active reduction headphones must dynamically generate this interference field in real time to adapt to constant variations in the sound environment. The main challenge of this technology lies in the speed and precision with which the system can analyze the incident sound and produce the corresponding anti-noise, with a delay typically less than 1 ms.