Stepping back from ANR

"Marc J. Zeitlin" wrote

According to:

http://www.telex.com/aircraft/produc...ratus50Digital

that's what they do. Never used one - I have no idea how well it
works, if at all. I have a Bose, a Lightspeed 15XL, and a Clarity Aloft.

Which is about what I was saying in my post, back a bit. I thought they all
worked that way, but I learned something new, in this thread.

I did not think that the electronic switching reactions to sound would be
quick enough to cancel a rising sound pressure, as it happens, by applying
an immediate 180 degree out of phase correction. I thought the correction
was applied to the next pulse, as it would happen, if it occurred again.
--
Jim in NC

On 30 Aug 2006 12:10:35 -0700, "nrp" wrote:

Not all ANR headsets work on the same principle.

I understand that most of them are digital, in that they look for
repetitive noise & then use digital algorithims to s-l-o-w-l-y cancel
out the repetitive components of any noise signal. This has the
advantage that they can work to higher frequencies, but limited in that

But you don't need to have them work at higher frequencies and I
absolutely do not want them to work at higher frequencies. I can hear
the wind noise albeit weakly. I can hear the accessories on the
engine and I can hear voice through the set very well. Not too bad
even from some one beside me as the noise is only coming from one
direction and is not completely canceled by the ANR function.

they only work with repetitive (i. e. cyclic) signals. They are unable
to cancel random noise.

To me, those would be the ones to stay away from.
I see it as a case in simple is better.


Some (the Headsets Inc version and maybe others) are a broad band
analog bucking system. They do not cancel just a repetitive
excitation, but can also cancel most of any random noise sensed by the
internal microphone system. The disadvantage of this type is that it
can work only at frequencies below typically about 300 Hz. Above this,

These including my Telex work very well up through the voice
frequencies. Don't believe what you hear about 300 Hz. If this were
true I'd hear a lot of prop noise at full RPM and I don't. After all
the prop makes more noise than anything else on, or in the plane. They
work well up to around 3000 Hz. At least all I've tried have.

and they must rely on the passive noise rejection of the headset cups -
which is pretty good at high frequencies. It is low frequencies that
passive systems have the poorest atenuation.

Anyone else with more of the puzzle?
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

On 30 Aug 2006 18:10:23 -0700, "nrp" wrote:

On inquiry, I was told by technical people (I suspect it is a
near-garage shop operation) at Headsets Inc that their rejection system
is all analog. Being somewhat familiar with analog and digital noise
cancelling systems, I can verify that from the HInc system that I have.
Oddly enough the final performance seems somewhat similar.

It is easy to talk about a 180 degree cancelling pressure waveform
generator (per Roger above), but that is an over-simplification of the
problem that isn't acheiveable from a real control loop stability
standpoint.

True it is a bit of an over simplification and it doesn't give the
ultimate noise cancellation of digital overall or predictive over
specific ranges.

OTOH it does work very well as any user who has turned off their
analog ANR while in flight and then turned them back on. The
difference is astounding and it is substantial so the analog phase
canceling works. Also this is not an area where you are looking for
control loop stability, but rater effectiveness and they are not the
same.


Digital systems get around this by a slow optimization in the frequency

Remember the add hype and the claims that the new Telex has this
exclusively. We have to remember too that when you look at the
difference in db it doesn't appear much different. OTOH db is a ratio
and every 3 db is a doubling, or cutting in half depending on the
direction you are headed.

domain over a wide (i. e. hi) frequency range at the expense of being
able to cancel random noise, whereas analog does it in the time at the
expense of bandwidth, limiting it to a few hundred Hz, but making

My day-to-day experience says the bandwidth limitation is not nearly
that drastic, but is on the order of one full magnitude wider at 3,000
Hz where the fall off becomes noticeable.

analog better at random uncorrelated noise rejection. The Headsets inc
website posted the rejection capability of their system At low
frequencies it is about as good as the best digital system.

At high frequencies (above say 300 Hz), the passive rejection
capability of a good headset (mine is a David Clark H10-40) is more
than adequate to the task for my ears.

I've been trying to get HI interested in random motorcycle helmet noise
cancellation (generally under 100 Hz) but they don't seem interested -
or they may know why the analog control system can't be made stable.

A system that works on random noise by nature isn't stable, or we are
using the word differently. However a low frequency repetition rate
does not necessarily mean a low frequency sound. The bark of an
exhaust, supersonic prop tips, or a gun shot are all sounds with very
short rise times which makes them a high frequency sound, but with a
low repetition.

From my engineering experience, a 300 Hz analog response loop is an
impressive acheivement.

They sure do a good job on that big Continental exhaust bark at low
RPM. All you hear are the accessories running.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com