Nowadays the trend for music headsets has shifted towards large over-the-ear headsets, whereas five years ago in-ear systems were the most popular. Consumer preferences have obviously changed in the last couple of years and besides the original use case (listening to music), headsets have turned into a fashion statement.
Developing an ANC headset requires considerable know-how, especially if you want to combine a piece of art with modern ANC technology. No matter in what kind of headset ANC is implemented (in-ear, on-ear or over-the-ear headsets) there are basically three different concepts to tackle the ambient noise.
The most common is the feed-forward topology (see Figure 1), in which a microphone exposed to the exterior senses ambient noise, and the ANC circuit generates an anti-noise signal that the speakers reproduce (together with the user's audio playback signal). This headset type consists of four blocks: the speakers, battery, ANC circuit, and ANC microphones for the left and right channels.
Figure 1: ANC Feed Forward Block Diagram
The feed-forward topology is typically used in communication headsets such as Bluetooth headsets because of its wide ANC bandwidth. Such systems can cancel noise up to 3 kHz with properly designed acoustics.
The higher frequencies in particular help to improve speech intelligibility if you are making phone calls since this is the typical frequency range of human voice. Another important advantage of a feed-forward ANC system is that there is absolutely no influence on the audio signal path.
Figure 2: Feed Forward Block Diagram
Figure 2 shows the signal flow of a standard feed-forward ANC headset. The "Dff" block represents the feed-forward delay caused by the speaker due to the conversion from electrical impulses to air pressure waves. The noise signal that is picked up by the microphone is treated with a gain and phase compensation filter G(w) and mixed together with the sound signal. The music playback path is completely independent from the noise reduction path.
A further mode that is commonly used in ANC headsets is the monitor mode. In this mode the ANC microphone is, when required, used to actively amplify the ambient noise. In this special mode the gain and phase compensation filter G(w) is bypassed and the microphone is connected directly to the speaker amplifier. This helps to overcome the passive attenuation of a headset when having a conversation with your neighbour or flight attended in a plane without removing the headset. Typically this mode can be activated by pressing a push button on the headset.
A disadvantage that comes with feed-forward systems is that they are susceptible to wind noise if the electronics and the acoustics are not properly designed.
A solution to overcome the wind noise issue is to use a different ANC topology. The second topology in the ANC industry is the feedback topology. This topology makes use of the same hardware blocks that are present in the feed-forward applications. The only difference is the location of the microphones, which are inside the ear capsule. This means the headset is no longer susceptible to wind noise.