Audio lab · Active noise control

Active noise control (ANC): interactive demo

Active noise control cancels sound by adding a second sound with the same amplitude and the opposite phase, so the two pressure waves sum to nearly zero. In this demo you tune the phase and amplitude of an opposing tone against a 180 Hz hum and watch the residual. At 180° and matched amplitude the modelled point goes silent; at 0° the hum doubles.

SYNTHETIC SIGNALS

These demos are illustrative. Every signal is synthetic, generated and processed in your browser, and every number shown is computed from those signals. They show the principles, not the measured performance of a product.

Press a listen button to play a short sample. Sound starts only when you press a button. No microphone or uploads needed.

Active noise control / ANC

Can sound cancel sound?

Match an opposing tone to a 180 Hz hum. Adjust its phase and amplitude to find the quiet point.

Tune the opposing sound

Phase offset 150°

180° opposes the original tone; 0° reinforces it.

Opposing-tone amplitude 80%

100% matches the amplitude of the original hum.

Two waves. One result.

At one observation point

Amber: original humGrey dashed: opposing toneBlue: combined result

Level change—

Residual amplitude—

Noise frequency180 Hz

Negative dB means attenuation; positive dB means reinforcement. Exact cancellation produces silence in this ideal simulation.

Why the engineering matters

Real ANC depends on microphone and speaker placement, acoustic transfer paths, timing and stability. Cancellation at one point does not imply a quiet room; moving the listener or changing the sound changes the problem.

What this experiment models

Linear superposition of two steady 180 Hz tones at one observation point. Residual amplitude relative to the original is the magnitude of 1 + a·exp(jφ), where a is the amplitude ratio and φ is the phase offset. Playback uses the actual summed waveform with the same gain as the original. The phase and amplitude are set manually: this illustrates the cancellation principle, not a feedback controller, adaptive ANC algorithm or measured headphone performance. Graphs use normalized amplitude. All sounds are synthesized in the browser.

The principle

How it works

Sound waves superpose linearly. The residual amplitude, relative to the original, is the magnitude of 1 + a·e^jφ, where a is the amplitude ratio of the anti-noise and φ its phase offset. Small errors leave a residual, and with matched amplitudes a phase error beyond 60° makes the combination louder than the hum alone.

Real systems do this continuously. A reference or error microphone measures the noise, an adaptive filter such as filtered-x LMS updates the anti-noise, and the controller needs a model of the acoustic path from loudspeaker to error microphone. Cancellation is local: a null at one point does not mean a quiet room, and it is easiest at low frequencies, where wavelengths are long compared with the space to be controlled. The same principles carry over to active noise and vibration control in machines and structures.

This demo sets the phase and amplitude by hand to show the principle; it is not a controller. For adaptive ANC work, see noise reduction and anc.

Audio lab

More demos

01 / Noise filteringLow-pass filter noise reduction – interactive demoLow-pass filtering, input and output SNR 02 / BeamformingMicrophone array beamforming simulatorSteer a delay-and-sum microphone array 04 / Bearing fault vibrationBearing fault vibration analysis – interactive demoEnvelope analysis finds an outer-race defect 05 / Sound event detectionSound event detection – interactive demoHits and false alarms of an energy detector 06 / Speech enhancementSpectral subtraction speech enhancement demoSpectral subtraction and musical noise

All six demos on one page: the audio lab Noise reduction and ANC →

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