Active noise cancellation applies superposition but tops out near 20 dB reduction

Evidence

Active noise cancellation technology is now common in audio headsets. The idea behind this technology is to use external microphones to capture the sound on its way to the ear and, using fast electronics, have the headset speakers apply an opposing pressure field just in time. The principle of superposition does the rest, in theory, canceling the noise. In practice, the cancellation is not perfect, and the sound power can be reduced by about a factor of 100, or 20 dB, in certain ranges of the audio spectrum. Achieving a better result than this is difficult for several reasons, including direct bone conduction of sound to the middle ear. In noisy environments, plenty of noise enters your ears in spite of the noise cancellation. Contrary to rumor, noise cancellation earphones are not degraded for safety reasons, although that might be a good idea if they could in fact attenuate by 40 or 60 dB. — Eric J. Heller, Why You Hear What You Hear- An…, 161

Categorization

• XREF: Connects to One Square Inch of Silence work on noise and to the reader’s soundscape books; ANC is a direct application of the superposition principle from paragraph 160.
• SHARE: The 20 dB ceiling and bone-conduction limit on ANC is a nerdy fact an acoustics-interested friend would enjoy.