(Active) Noise canceling headphone questions

[Mr. Civility]

I thought I knew how these things worked, but after looking at a few in
the local CompUSA store and on-line, I'm puzzled.

For reference, I'm talking about consumer-grade active noise-canceling
headphones such as those on this page:
http://reviews.cnet.com/4520-3000_7-1017728-1.html[1]

So, you have a microphone on the outside of the headphone speaker and
the speaker points into your ear. The mic signal is fed 180 degrees out
of phase to the speaker and voila, some of the external noise is
cancelled (limited by multiple sound paths, phase delay differences, use
an adaptive filter DSP if you're really gung-ho, yada yada).

What bothers me is that these things seem to generally run on one AAA
cell, which doesn't seem to be much if you're planning to power two
headphones at any volume for any length of time. (I haven't found
longevity specs on the battery for any of the headphones.) This assumes
that the input signals and the noise error signals are combined
electronically and the outputs (of some chip) drive the speakers. Even
with class D audio amp chip efficiencies, I don't see a lot of lifetime.

Is is possible that the error amp is driving a separate winding in the
speaker, i.e. that custom multi-winding speakers are being used?

Or perhaps the battery just powers the microphone and its amplifier only
and the output is simply coupled into the speaker signal via a
transformer or low-impedance active output?

Or is it really just that a single AAA probably *will* power two power
amplifiers for an acceptably long enough time, given typical music
volumes, headphone efficiencies, etc.?

Can anyone clue me in?

[1] Note that the Panasonic one doesn't use a battery at all and yet
claims to be "active" noise-cancelling! (Or perhaps that's just the
category that web site put them in.)

 

[Rich Grise]

I thought I knew how these things worked, but after looking at a few in
the local CompUSA store and on-line, I'm puzzled.

For reference, I'm talking about consumer-grade active noise-canceling
headphones such as those on this page:
http://reviews.cnet.com/4520-3000_7-1017728-1.html[1]

So, you have a microphone on the outside of the headphone speaker and
the speaker points into your ear. The mic signal is fed 180 degrees out

At what frequency? ;-)

No, I'm just hassling you - the thing is, it's simply an inversion.

of phase to the speaker and voila, some of the external noise is
cancelled (limited by multiple sound paths, phase delay differences, use
an adaptive filter DSP if you're really gung-ho, yada yada).

What bothers me is that these things seem to generally run on one AAA
cell, which doesn't seem to be much if you're planning to power two
headphones at any volume for any length of time. ....
Or is it really just that a single AAA probably *will* power two power
amplifiers for an acceptably long enough time, given typical music
volumes, headphone efficiencies, etc.?

Can anyone clue me in?

I can try - when you look at the SPL of the noise without the noise-
cancellation feature, inside an ordinary pair of headphones, you don't
need very much.

When I was in the USAF, we wore "ear defenders" - same form factor
as stereo headphones, but with nothing inside but some foam rubber,
and they blocked out enough noise that you could comfortably work
next to a running jet engine. Stick a mic element on the outside,
run it through an inverting amplifier, and I'd be surprised if a
milliwatt wasn't almost too much. Of course, if you wanted to
talk with anybody, you had to scream your lungs out!

Cheers!
Rich

 

[Ban]

I thought I knew how these things worked, but after looking at a few in
the local CompUSA store and on-line, I'm puzzled.

For reference, I'm talking about consumer-grade active noise-canceling
headphones such as those on this page:
http://reviews.cnet.com/4520-3000_7-1017728-1.html[1]

So, you have a microphone on the outside of the headphone speaker and
the speaker points into your ear. The mic signal is fed 180 degrees out

At what frequency? ;-)

No, I'm just hassling you - the thing is, it's simply an inversion.

of phase to the speaker and voila, some of the external noise is
cancelled (limited by multiple sound paths, phase delay differences, use
an adaptive filter DSP if you're really gung-ho, yada yada).

What bothers me is that these things seem to generally run on one AAA
cell, which doesn't seem to be much if you're planning to power two
headphones at any volume for any length of time. ...
Or is it really just that a single AAA probably *will* power two power
amplifiers for an acceptably long enough time, given typical music
volumes, headphone efficiencies, etc.?

Can anyone clue me in?

I can try - when you look at the SPL of the noise without the noise-
cancellation feature, inside an ordinary pair of headphones, you don't
need very much.

When I was in the USAF, we wore "ear defenders" - same form factor
as stereo headphones, but with nothing inside but some foam rubber,
and they blocked out enough noise that you could comfortably work
next to a running jet engine. Stick a mic element on the outside,
run it through an inverting amplifier, and I'd be surprised if a
milliwatt wasn't almost too much. Of course, if you wanted to
talk with anybody, you had to scream your lungs out!

Cheers!
Rich

these ones use 2AA cells and last 80 hours on alcalines.
http://www.sennheiserusa.com/newsite/pdfs/hdc451.pdf
94dB nominal SPL is quite loud.

ciao Ban,
Apricale, Italy

 

[Mr. Civility]

What bothers me is that these things seem to generally run on one AAA
cell, which doesn't seem to be much if you're planning to power two
headphones at any volume for any length of time. (I haven't found
longevity specs on the battery for any of the headphones.) This assumes
that the input signals and the noise error signals are combined
electronically and the outputs (of some chip) drive the speakers. Even
with class D audio amp chip efficiencies, I don't see a lot of lifetime.

Thanks for the responses.

I missed a big clue. Most of these headphones say they work as normal
headphones when the battery is removed. So, obviously the speakers
aren't being driven by power-hungry power amps. The inverted noise
signal is being slipped into the circuit another way, perhaps with
transformer coupling or a low impedance semiconductor device in series
with the speaker or extra speakers or extra windings on the speaker coils.

Since this is a design group, how would *you* do it? (Spec: 1 AAA
battery, "acceptable" lifetime.)

 

[John Woodgate]

I read in sci.electronics.design that Mr. Civility
about '(Active) Noise canceling headphone questions', on Sun, 9 Oct
2005:

Since this is a design group, how would *you* do it? (Spec: 1 AAA
battery, "acceptable" lifetime.)

Microphone, hearing-aid type audio amplifier, probably
transformer-coupled to the transducers in parallel with the signal from
whatever is feeding the headphones.

 

[riscy]

This is interference cancellation rather than noise cancellation, since
noise modem is totally random. Interference is predictable signal that
may be elimiated.