Active noise reduction (ANR) earmuffs have microphones mounted inside the earcup to monitor incoming noise.

That signal immediately passes through a circuit that reverses the phase of the noise,

effectively creating an “anti-noise” signal. This reverse-phase signal is then sent to a speaker within the earcup.

When the unmodified noise combines with its own reverse-phase mirror image,

there is a cancellation effect that results in an overall reduction in the noise level.

ANR circuitry is impressive to a listener: Incoming noise levels can be reduced with the push of a button.

But ANR circuitry has its limitations. ANR works well when the incoming noise is low-frequency,

but is relatively ineffective against the shorter-wavelength high frequencies (most ANR circuits have little effect over 500 Hz).

There is also a slight time delay for the circuit to activate,

meaning ANR is ideally suited for continuous noise, not intermittent noise or speech.

ANR circuitry also interferes with an earmuff’s ability to attenuate noise in two significant ways. First,

the addition of any electronics makes an earcup less efficient in blocking noise.

The passive attenuation rating of an ANR headset will always be lower than its comparable non-ANR counterpart.

And second, ANR circuitry generates a small amount of internal noise, heard by the user as a high-frequency hiss.

Because of these limitations, ANR has been marketed more for annoyance noise than for hazardous occupational noise.