Bose Headsets

Thomas Borchert wrote:

/snip/. But ANR is not really effective at
high frequencies while passive isn't at low frequencies. So the headset
needs a balance between the two.


Thomas,
Not necessarily true, at all. The ANR function can be tuned to any
frequency, as desired. The Telex ANR-850's that I use in the jet are
FANTASTIC at reducing high-frequency wind noise, but are totally useless
in any propeller driven aircraft.

Happy Flying!
Scott Skylane

Scott,

The ANR function can be tuned to any
frequency, as desired.


Within the limits of physics.

--
Thomas Borchert (EDDH)

Scott Skylane wrote:

Not necessarily true, at all. The ANR function can be tuned to any
frequency, as desired. The Telex ANR-850's that I use in the jet are
FANTASTIC at reducing high-frequency wind noise, but are totally useless
in any propeller driven aircraft.

While there is SOME ability in digital ANR headsets like the Telex to
do some SLIGHT tuning of the ANR central frequency, it is completely
incorrect to state that it can be "tuned to any frequency, as desired".

ANR works by measuring the noise with a tiny microphone and then
injecting the inverse of the noise signal into the speakers. At low
frequencies (in the 50 - 300 Hz range, or so), the wavelength of the
sound is long enough that the sensing microphone and your ear only
need to be CLOSE to each other, but not coincident, in order for the
injected signal to approximately counteract the noise.

At high frequencies (above 1000-2000 Hz), the wavelength of the sound
starts becoming very small. This implies that for the microphone to
measure (and hence cancel) the noise that the EAR is hearing, the
microphone would have to be essentially inside the ear, or else it
would be canceling something other than what the ear perceives.

Now, that's not to say that SOME amount of ANR can be done with
digital techniques at somewhat higher frequencies than analog ANR can
do with some sophisticated algorithms, but it's hardly "any
frequency". I'd be very surprised if the "wind noise" you mention is
more than a few hundred Hz, and if, in fact, the 850's are "totally
useless" in any propeller driven aircraft, then they've got pretty
poor ANR capabilities. I've never worn them, so I can't comment on
them directly.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2006

Marc J. Zeitlin wrote:

/snip/
Now, that's not to say that SOME amount of ANR can be done with digital
techniques at somewhat higher frequencies than analog ANR can do with
some sophisticated algorithms, but it's hardly "any frequency". I'd be
very surprised if the "wind noise" you mention is more than a few
hundred Hz, and if, in fact, the 850's are "totally useless" in any
propeller driven aircraft, then they've got pretty poor ANR
capabilities. I've never worn them, so I can't comment on them directly.


Mark,

Per the specs from the Telex 850 manual, they provide 12db of active
attenuation between 100 and 2000 Hz. Looking at the Lightspeed 30G
specs, their max attenuation of 36db is centered at 150 Hz, and the 12db
threshold is reached at only 400 Hz. A significant upward shift in
attenuation range for the 850's, IMHO. Looking at the design of the
relative ear pieces, I would guess the Telex microphones sit
considerably closer to the ear than the Lightspeed's.

To appease the pedants running rampant in this group, when I said ANR
could be tuned to *any* frequency, I was incorrect. What I meant to say
was that ANR can apparently be tuned to any frequency desired, within
the realm of human hearing, specifically those frequencies that are
introduced in various aircraft cockpits as "noise". This statement was
to correct a blanket assertion by Mr. Borchert that "ANR is not really
effective at high frequencies..." He is wrong. ANR can be very
effective against the high frequency noise present in some cockpits, if
the headset designer chooses.

Happy Flying!
Scott Skylane
N92054

("Scott Skylane" wrote)
To appease the pedants running rampant in this group...


I've never seen it used like that before, so I had to look it up.

ped·ant (n)

1. One who pays undue attention to book learning and formal rules.
2. One who exhibits one's learning or scholarship ostentatiously.
3. (Obsolete) A schoolmaster.


Montblack :-)

"Montblack" wrote in message
...
("Scott Skylane" wrote)
To appease the pedants running rampant in this group...


I've never seen it used like that before, so I had to look it up.

ped·ant (n)

1. One who pays undue attention to book learning and formal rules.
2. One who exhibits one's learning or scholarship ostentatiously.
3. (Obsolete) A schoolmaster.

Then there's the manager who wanted to have someone fired for being
"pedantic", confusing it with the word "pedophile". (Somewhere on the Scott
Adams/Dilbert website)

Scott,

"ANR is not really
effective at high frequencies..."


You know, looking at you own post and comparing the 36 dB reduction you
mention for a low frequency and the 12 dB reduction you quote for a
high frequency, I would say my statement still stands. Measured in dB,
there are worlds between 36 and 12.

--
Thomas Borchert (EDDH)

Thomas Borchert wrote:


You know, looking at you own post and comparing the 36 dB reduction you
mention for a low frequency and the 12 dB reduction you quote for a
high frequency, I would say my statement still stands. Measured in dB,
there are worlds between 36 and 12.

Numericaly, of course, you are correct. Subjectively, sitting in the
cockpit of a jet airliner, I say that 12 db of high frequency
attenuation is *very* effective.

Tell you what: Next time you are flying in the cockpit of a jet
aircraft, try on a pair of the 850's, and let us know what you think.

Happy Flying!
Scott Skylane
N92054

"Marc J. Zeitlin" wrote:
At high frequencies (above 1000-2000 Hz), the wavelength of the sound
starts becoming very small.

The wavelength at ~2000 Hz is still about ~6 inches.

This implies that for the microphone to
measure (and hence cancel) the noise that the EAR is hearing, the
microphone would have to be essentially inside the ear, or else it
would be canceling something other than what the ear perceives.

Two limiting factors would seem to be the distance between the external
microphone and the headphone, and the frequency response of the feedback
electronics. If your feedback response is slow, it might actually pay to
design it so the external microphone is farther away from the headphone.

Since sound approaches from all directions, and the microphone and
headphone must accomplish their tasks in only a one to two-dimensional
space, a three-dimensional "cone of silence" device is best. Fortunately
such a device was invented back in the 1960s; here's some pictures:

http://www.oneeyedman.net/cone_of_silence.jpg
http://www.hojohnlee.com/weblog/wp-c...of-silence.jpg

Jim Logajan wrote:

The wavelength at ~2000 Hz is still about ~6 inches.

Closer to 7", but even so, but you need to have the microphone and the
node at which you're trying to cancel the noise VERY close together in
relation to the wavelength. Remember, 1/4 wavelength will be 90
degrees out of phase - you need something on the order of 1/16
wavelength or less to keep the phase mismatch reasonable, and ensure
that the microphone is picking up the same sounds as the ear picks up.
So even at 2K Hz, the microphone needs to be no further than 3/8"
from the ear - that's not going to happen - that's pretty much inside
your ear.

That's why the ANR rating of the Telex that's been discussed
previously is only 12 dB in the 2K Hz range, whereas the ANR rating of
the Bose/Lightspeed/etc. is in the mid to high 30 dB range at the
lower frequencies - as Thomas B. pointed out, that's a HUGE difference.

Two limiting factors would seem to be the distance between the
external microphone and the headphone, and the frequency response
of the feedback electronics.

Ummm, the external microphone (the one you speak into) is NOT the
microphone that's used for ANR sensing. There is a small microphone
INSIDE each earcup - as close to the ear as it can be placed. Using
the external mic. for sensing would be completely useless.

..... If your feedback response is slow, it
might actually pay to design it so the external microphone is
farther away from the headphone.

Again, putting the external mic. anywhere other than right near the
ear would make it impossible to do any sort of ANR.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2006