Active sound control lab
Interactive explanation
Sound cancellation is possible with a continuous shell of speakers.
A perfectly continuous, zero-delay ring can emit an equal and opposite wave. Outside the shell, the two fields cancel; inside, the original sound remains.
An ideal centered single tone and infinitely dense ring: exact outside cancellation, nonzero interior field.
Discrete speakers can track and cancel sound for a moving pedestrian.
A finite array cannot silence the whole exterior field, but it can steer a local quiet region toward a tracked person and continuously recompute the speaker phases as they move.
The controller follows the human around a slow ellipse. Grab the blue crosshair to take over manually.
The same idea generalizes to multiple people.
Each additional person contributes another tracked quiet region to the same optimization. The array shares its available degrees of freedom across all of them.
All three bubbles enter one least-squares objective. The controller is recomputed as the people move; grab any crosshair to take over manually.
Cancellation also generalizes around known obstacles.
Watch one controller move from a clean open field to a reflective city scene: obstacles break its assumptions, microphone probes reveal the missing paths, and the same array is re-solved against the measured environment.
The naïve controller assumes every sound path is direct.
Real sound must be cancelled frequency by frequency.
A chainsaw is not one clean tone. Its repeating engine cycle contains many harmonics, and its blade noise is broadband. Each narrow frequency band needs its own cancellation amplitude and phase.
Recorded waveform
12.0 s continuous run · analyzed window 4.800–4.900 s