compression and rarefaction. Play the same
sound at the same time and you amplify that
original waveform... but switch the phase- invert the second waveform, turning its
peaks into troughs and vice versa – and the
two waves begin to counteract each other.
Get the amplification of the inverted
waveform correct and you can effectively
cancel any unwanted sound by opposing it.
That’s the simple overview, but the chaos
of reality isn’t quite so kind. Inverting a live
waveform is fundamentally very difficult,
since to do it perfectly you’d need absolutely
no latency between the sounds of the real
world and what’s pumped out of that
speaker. Most often, ANC won’t generate an
exact opposite of the sound it’s blocking out,
instead creating an approximate muddling
tone – this is why noise cancelling headsets
are great at blocking out droning sounds
that don’t change frequency much, like road
noise or aircraft hum, but can’t deal quite as
well with sudden sounds.
It’s also hard to cancel noise without
overwhelming any other signal you might
want to put through that speaker. For this
reason, most ANC focuses on lower end
frequencies, those which are most likely to
penetrate the closed backs of cans or ear
buds, and relies on isolating materials to
squash the rest of the sound.
There’s also, despite the broad term
‘Active Noise Cancelling’ (also referred to, in
some cases, as ‘Active Noise Reduction’) not
one single form of the technology. In fact,
there are three. Feedforward, the most
common variety, places a microphone on the
outside of the earcup or earpiece, catching
the noise before it reaches your ear. It then
pipes that signal into the ANC processor,
before spitting out the antinoise within the
ear cup. This has its advantages – notably
that brief microsecond extra it has in which
to perform its duties – but also has a couple
of disadvantages. Improper angling of the
microphone could, in some cases, cause the
ANC engine to actually amplify unwanted
sounds rather than muting them, and
captured wind noise can be problematic.
Feedforward also has no ability to correct
itself – it must make assumptions about how
to work with that outside noise, without
knowing what real effect its actions are
having on the sound inside the ear cup.
Feedback ANC moves the mic inside the
earpiece. This certainly solves the self-
correction issue (the microphone cannot
help but pick up what’s coming out of the
loudspeaker) and means the ANC engine can
adapt itself to precisely what the user is
hearing, even if the headset is sitting at an
odd angle. It does require very precise
calibration of the ANC engine, though, as
overzealous cancellation can cut frequencies
from the music playing through the
loudspeakers or – as its name might suggest- cause a painful feedback loop.
Hybrid ANC combines the two, with a
pair of microphones working in tandem: one
outside the earpiece, one inside. This allows
the system to pick up exact frequencies from
both the real world and the tiny ecosystem
close to the ear and react on both, but it
requires strong processing and a very exactly
tuned algorithm to get the ANC right
Overzealous
cancellation can
cause a painful
feedback loop
The top three...
WIRED ANC
WONDERS
On a budget? No problem!
Here’s how to get in-line
noise cancelling without
breaking the bankTAOT R ON I C S AC T I V E N OI S E
CANCELLING HEADPHONES
For this price, don’t expect high-end build
quality or insane noise cancelling. Do expect
good build quality, remarkably effective
ANC, and a battery that keeps things quiet
for a solid 15 hours on a full charge.
£29, amazon.co.ukBEOPLAY E4
Expect quality sound here, with B&O’s
earphones offering great music playback and
a quick transparency mode to let the outside
in. The ANC box is a little large, though, so
look elsewhere if you want to run.
£134, bang-olufsen.comRAZER HAMMERHEAD
USB-C ANC
An absolutely superb set of earphones
which power their ANC-capable DAC (and
pull in audio) from a USB-C connector. Not
so great for iPhone users, perhaps, but if
you’re on Android they’re an incredibly
effective way of cutting out the noise.
£79, razer.com02
03
01
60 T3 MARCH 2020The complete guide to...