When designing an earphone or headphone to have Active Noise Cancellation (ANC), many elements will affect the degree of noise cancellation that can be achieved in practice. This page describes the working principles of ANC, the limitations and which Ole Wolff receivers to use for outstading ANC perfomance.
An ANC circuit is basically a loop controlled circuit where one or more microphones, an earphone (receiver) and an ANC controller circuit are involved. A simplified diagram of this is shown in Fig.1
An electrical signal (speech or music) is fed into the ANC controller circuit and an electrical noise signal is picked up by a microphone, phase shifted by 180 deg. and fed into the ANC controller circuit as well.The microphone is positioned in a place where best pickup of background noise can be achieved. By phase shifting this noise signal 180 deg., the background noise will ideally be cancelled out and only the pure speech or music signal will reach the earphone and thereby no noise will reach the listeners ear.
While the basic ANC concept is the same, it can be implemented in three different ways in earphones/headphones. Each principle has advantages as well as disadvantages as described below:
In the feed-forward design the microphone is placed outside the ear cup. Here the microphone detects the noise before the listener does.
The ANC circuit processes the noise and makes the anti-noise signal before sending it to the earphone.
There will be no stability problems, because no feedback loop is involved.
The Feedforward microphone can be used for "outside listening" as extra feature.
The above mentioned ANC principles would work in a very broad frequency range if all conditions were ideal. In practice there are many factors limiting the ANC performance at higher frequencies.
When processing signals in the digital domain, there will be some delay issues, which will disturb the ANC performance. Ideally the processing should be with zero delay, but this I not possible. Such a delay will degrade high-frequency ANC the most due the shorter wavelengths of the signal frequencies involved.
For these reasons, the ANC performance is often by purpose band limited to around 1.2kHz. This also makes sense since the mechanical structure of the earphone/headphone gives a solid passive background noise attenuation above 1.2kHz making ANC obsolete over this frequency range.
There are three major elements in the ANC loop:
If the ANC performance should be able to handle really high noise sound pressure levels up to ex. 140dB SPL (traffic noise, flight take offs, shouts, building construction machine activities etc.), the receiver should be able to handle these SPLs as well without significant distortion and knowing that the noise signal must be reproduced in counter phase according to ANC loop. This is only possible with a receiver allowing a very high stroke of the membrane without entering a compression mode. The smaller the receiver is in diameter, the harder the need will be for a high membrane stroke.
The following parameters for the receiver application are of secondary importance, because they are strongly related to the amount of filter orders available in the chosen ANC digital system. Almost all of the following parameters can be compensated for, by having unlimited access to the number of filters, but the tradeoff is the cost for the ANC circuit as well as an increased power consumption. Therefore a good engineering practice is to optimize the following parameters before relying on a higher number of ANC filter orders.
The Damping Ratio, ζ (zeta), for the whole receiver application should be tuned to around 0.7 for achieving best reproduction of the electrical signal into acoustic signal (best mirroring of electrical signal). The best mirroring of the electrical signal into an acoustic signal (reproduced by the receiver) is very important for assuring the highest linearity of the ANC loop and thereby best ANC performance. A Damping Ratio of around 0.7 is the best compromise between fast enough rise time and minimized post ringing at the system`s resonance frequency. (See fig.6)
When looking at the Frequency domain, we normally express a Quality Factor (Q factor) for the resonance system. The relation between the damping ratio and quality factor is given by the equation (1)
It can be calculated using equation 1, that a damping ratio around 0.7 will result in a Q factor of 0.7 (called critical damping). This means that in the frequency domain you would need a critical damped system for achieving best ANC conditions (see Fig 7).
Fig. 7: Phase and frequency response vs. Q factors
The total Q factor for the system is not the same as the total Q factor for the receiver involved. The receivers total Q factor must primarily be controlled by a very strong magnet system which will lower the Q factor down towards the desired value of 0.7, but normally this is not enough for reaching the critical damped factor of 0.7. The remaining lowering of the system Q factor will be done by mesh tuning on rear side of the receiver and/or mesh covering a rear vent in the earphone/headphone. So when a Q factor of approx. 0.7 is tuned in, the Damping ratio will also be approx. 0.7 according to equation 1.
The receivers phase response up to around 1.2 kHz must have minimum phase shifts with a smooth response for the ANC processor to be able to handle the signal. Therefore resonances in this frequency range must be kept at a minimum. Preferably only the mechanical resonance determined by the moving mass (coil & membrane) and the compliance of the membrane. This resonance is unavoidable in a moving coil receiver but must have as low a resonance frequency as possible for being positioned outside of the pass band. Fig. 7 shows the phase response with different Q factors. It can be seen that a damping ratio of 0.7 (equation 1) will result in the most smooth phase response and thereby best ANC counter phase correction conditions.
The conclusion is that a system quality factor of 0.7 ensures the best conversion of the electrical signal to the acoustic signal out of the receiver, which is needed for best linearity of the system and the flattest phase response ensuring best counter phase conditions for the ANC loop.