What is ANC?

Ambient Noise Cancellation (ANC)is a technology that allows headphone users to enjoy superior audio performance and increased comfort, by electronically attenuating surrounding ambient noise. This is very useful in a number of applications where the user is subjected to long-term, fatiguing sound sources. A very common scenario would be that of a frequent flyer who may choose to listen to the plane's in-flight entertainment system through ANC-enabled headphones to remove the low-frequency drone of the engine.

ANC Theory

The theory behind ANC is extremely simple.

Sound from the outside world is captured by a microphone before it reaches the ear. The resultant signal is then processed and inverted to create an "Anti-Noise" signal. This signal is then fed to the the headphone loudspeaker in time to coincide with the direct sound which has continued on its way towards the inner ear. If the filtering is correct then the "Direct" sound and the "ANC" sound should perfectly cancel at the ear drum.

Design Challenges - Filtering

The theory is very simple but can prove to be very difficult to implement effectively.

The concept is made more complicated when we take into account the way the direct sound is modified as it passes through the physical structure of the headphone. In fact, it is this "modified" sound that we are trying to cancel, and so the electronic filtering must mimic these changes with great precision.

Different materials will attenuate particular frequencies more than others, and so the phase alignment of the frequencies that make up our "direct" ambient sound will have been completely altered. This means that unless the electronic filtering is absolutely perfect, the "anti-noise" signal will no longer be "in-sync" with the "direct sound" when combined at the eardrum. Indeed, there is a far greater chance of making things worse - i.e. amplifying the ambient noise!

Low Voltage, Low Cost... High Quality

Today's headphone loudspeakers are super efficient, which is great as it reduces the power constraints placed on the active circuitry. However, this tends to show up any short comings in the electronic design, as in low noise environments as each active stage is constantly operated near the limit of it's signal-to-noise capability. This results in audible "hiss". This problem is particularly apparent in ANC systems where the circuitry must retain a large dynamic range as the ambient sound-source in question could be a foot or 100 feet away from the user. The circuit must not clip when a loud sound source is experienced (such as a bus engine or a police siren), also there must not be any audible hiss when the user is sat in a quiet environment such as a library or office. The circuit will include microphone pre-amplifier as well as filters followed by more amplification stages - all designed to a ultra low price point - there is much scope for problems to arise.

Low voltage op-amps tend to be noisy or else expensive. Discrete transistors offer the best solution as they are small and cheap, and if utilised correctly will easily match the performance of any IC device. The effectiveness of such a design is entirely down to the skill of the designer.

MP3 Players, Mobile Phones, an EMC nightmare!

Portable digital devices such as media players and mobile phones represent troublesome sources of electro-magnetic interference. No doubt most people have experienced the effects of a mobile phone transmission on a nearby sound reproduction system. This problem is many times worse for ANC systems as they are not only used in very close proximity to such devices, but are actually intended to be directly plugged into these EM emitting devices. In the worst case, the ill-effects of a non-EMC-treated design can be heard across a quiet room by someone not even wearing the headphones!

Fortunately, these problems can be addressed in the circuit design stage, as well as by careful PCB layout and lastly shielding. However, this still represents a real challenge.

Experience

Simon Keats has been involved in ANC for a number of years, having previously been employed by Wolfson Microelectronics as sole analogue designer in this field. Wolfson are currently a leader in this area.

Simon was responsible for developing a low cost, high quality, discrete analogue circuit from the ground up.

Simon was employed as ANC electronics-design engineer for the following projects: Audio-Technica ATH-ANC3 QuietPoint Active Noise Cancelling Headphones and more recently the Nokia BH-905 Bluetooth Headset. These products were designed by the manufacturers in question in conjunction with Wolfson Microelectronics.

The photos shown on this page show these two products, and act as an effective comparison between a Pad-On-ear and In-Ear system.

Please get in contact for more details or to learn about the ways in which Keats Audio can help you develop your company's ANC products.