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Active noise reduction headphones why the noise reduction effect is not good?

The first question can be either technical reasons or technical reasons. Why is the technical reasons: the market active noise reduction headphones noise reduction band is indeed concentrated in the 1kHz below the low frequency range, the general effective range of hundreds of Hz, so cover the incomplete voice band, the noise noise reduction effect cheap beats is not good. The basic principle of active noise reduction is to use the phase difference between the phase of the noise signal 180 degrees beats earbuds and the original signal to cancel the offset, so sensitive to the phase. And in the narrow space formed by the earphone and the ear / ear canal, it is inevitable that the phase difference is caused by individual differences or differences in the human beats by dre on sale ear. Note that the phase of the low-frequency component with longer wavelengths is more stable than the high-frequency component The In addition, the phase difference of less than 1%, but for the wavelength of more than 30cm 1000Hz noise signal and the reverse signal, a difference of 3cm (3cm), the difference between the two signals is less than 3% That phase difference of nearly 10%, if the deviation is greater or the frequency of some higher, it is entirely possible to design the signal in the reverse but under the bias into the same phase of the signal, into a noise reduction. Therefore, the more high-frequency components more prone to diminished by the situation. This is why the active noise reduction does not go to the high frequency to do the most important reason. Of course, active noise reduction in fact there is no need to beats by dre wireless do high-frequency, because the noise in the traditional passive way to do just fine, the effect is not bad, do not have to take the initiative to noise reduction. Interested to be able to check their own inner fidelity and other sites to do their own test data, like ear ear plug this, the structure itself passive sound effects in the high frequency can easily have 30 ~ 40dB.

In addition, the voice signal itself can quickly change the characteristics of one of the reasons, but personally think that only count the secondary problem, because the voice signal changes not as fast as the pulse (gunfire sound hit the door) as Active noise reduction circuit too late to respond. Why is it not technical reasons: If you can for everyone according to the ear canal structure custom custom exclusive custom debugging, the specific structure of the high frequency phase to do a precise estimate, that high frequency want to do active noise reduction is not a problem. The second problem, now on the market with almost all analog circuits, feedforward / feedback in two ways, or both mixed hybrid way, no use of digital circuits. Because the digital circuit there is delay, AD / DA noise and other issues. High-frequency effect is not good and digital circuit or analog circuit has nothing to do.