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What is active noise cancellation and where is the difference to noise isolation?

Why are wired earphones better than wireless ones?

 

Noise isolation vs noise cancelling

Active Noise cancelling (ANC) is an a great technology that has value for certain applications such as working / communicating in noisy environments. Particular Pilots or soldiers benefit from ANC. Nowadays, many consumers pay premium prices for headphones that feature this technology in order to block out ambient noise. Lets have a look at the technology and compare it with.

In ANC setups, integrated microphones record ambient noise, a dedicated circuit digitalizes and analyzes the sound and synthesizes an appropriate phase-shifted counter-sound and mixes this into the output driver. The phaseshift ideally results into a deletion of the ambient noise before it reaches the users ear(drum). Sounds simple enough? 

Well you might ask yourself how well ANC system can record and reproduce noise. Then, how well does the digital circuitry work? After all its a chain of recording the sound, converting it to digital signal, processing the signal using fourer analysis, analysing the resulting subfrequencies, potentially filter after one of the previous steps, generating and timing the phasesshifted signal, amplify appropriately for each frequency bracket and transfer to the transducer.

A great overview of the technology is given in this patent which provides a glimpse on how complex the circuitry and software is in ANC technology. As the title implies, such complex ciruitry generates a delay or latency for the antiphasic signal. As you know, sound travels with 343 m/s or 343 mm / ms so in one ms a sound wave may travel 34 cm. So if you want to intercept a signal like this ANC circuity must be able to process noise and generate a counter signal within the 1 - 10 microsecond domain. 

Consequently, noise cancelling technologies don't suppress ambient noise equally along the entire audible frequency spectrum (which is roughly between 20 - 20000 Hz, but for adults its more narrow like 50 - 15000 Hz).

In some cases / frequencies, audio reproduction kit with ANC technology even boost noise along certain frequencies rather than attenuate it. Also, the added frequency do add noise to music that is reproduced. Overall noise cancelling performance varies greatly with model (not only manufacturer but also type like circumaural or supraaural models) and technology used therein.Read this paper or this to get concrete performance data on a number of ANC models. 

Last but not least, when you ask yourself how accurately your headphone or speaker can reproduce music you might as well as the same question about accurate reproduction of a complex antiphasic "counter-noise".

In defence of ANC, there is a lot of interesting innovation going on. A recent patent by Cirrus logic, a chip maker, discloses adaptive filtering by injection and measurement of reference frequencies. Another one, also from CL even can adapt to the spatial sound location. Still it does not address the matter of latency or accurate counter signal reproduction. 

In contrast noise isolation is simply a creation of a airtight barrier around the transducer (speaker) and the acoustic transmission volume until the sound reaches the eardrum. Fitting suitable foam tips on in-ear earphones is a simple way to achieve noise isolation. Another is to use circumaural headphones. 

Advantages of noise isolation:

1) blocks ambient sound (in some cases as good as ANC)

2) no power or batteries needed, no recharging required

3) no negative impact on sound quality

4) reduction of ambient noise allow lower listening volumes and thus less risk for hearing loss.

 

Wireless vs wired headphones

Yes, we live in a world where wireless devices are ever more common. Wireless devices obviously appeal to many consumers since the offer less hassle. However, less tangle come with disadvantages: a lot of electronics move from music players to the earphone itself: Music or voice signals are transmitted as a digital information stream via radio frequency from music player to the wireless headphones. Besides radio frequency transmission modules, Digital - analogue conversion and amplification circuitry are required to generate sound in the headphone. A battery is also needed with sufficient capacity to power the device for hours. Obviously there is only limited space in headphones and in particular in-ear models which require miniaturisation of the circuitry.  This is of course possible, but will impact sound quality. Electrical engineers will agree that miniaturisation inevitable will have require ommiting certain parts or optimizations as can be found in large scale equipment.

To put it in a nutshell, there is ample doubt that wireless devices sound reproduction maybe inferior to wired devices.

Advantages of wired headphones:

1) no batteries, recharging needed

2) no negative impact on sound quality

3) you can tune sound quality by using different DACs, amplifiers or digital audio players of your choice.

 

Wearing cables over the ear

 


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