It's clearly a typo. Should be "This works for wavelengths larger than the cavity size." A long wavelength implies that the phase of the reference input and fe

It's clearly a typo. Should be "This works for wavelengths larger than the cavity size." A long wavelength implies that the phase of the reference input and feedback input are similar, which is what allows you to just subtract them. Your processing delay, the distance between the speaker and the microphone, and the speed of sound are all fixed, so as the wavelength gets shorter, the relative phase difference between the two signals you're trying to subtract increases. At a certain point, you run out of phase margin and can no longer just subtract them anymore. If you were designing an ANR system, you'd probably want to put a low pass filter somewhere around this point to prevent your feedback system from oscillating.