High-frequency noise resists cancellation because its wavelength approaches headphone size

Evidence

High frequencies are problematic for noise cancellation headphones. There are at least two reasons for this. First, the electronic circuitry and the headphone speaker have to react very fast to changes in sound pressure at high frequency. Second, high frequencies (say, above 3000 to 4000 Hz) start to have wavelengths comparable to the interior of the headphones, making cancellation more difficult, since the sound pressure becomes different in different places inside the headphones. In principle, the best cancellation technology is achievable with earbuds that seal off the outer ear: one can combine passive cancellation (itself good for about a – 20 dB attenuation) and active cancellation. — Eric J. Heller, Why You Hear What You Hear- An…, 163

Categorization

• DEFINE: Explains why spatial variation of pressure inside headphones defeats blanket cancellation at high frequencies.