Frequencies covered by noise cancellation

[Harlan Messinger]

I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
great job of removing the low, rumbling sounds, like the office copier
or the continuous roar on a plane. On the other hand, on a plane, they
do virtually nothing about the higher-frequency airplane sounds
(hissing, whining), and they add their own hiss.

I asked someone selling expensive Bose headphones at Denver Airport
about whether their phones cover the higher frequency ranges, but he
turned out not to be informed on such a technical topic.

Is this issue with my Sony phones normal--are only the lower frequencies
targeted by today's noise-canceling products? Or are there products that
I can rely on to cancel a wider range of frequencies and, ideally don't
introduce significant noise of their own?

[Don Pearce[_3_]]

On Thu, 04 Jun 2009 13:04:08 -0400, Harlan Messinger
> wrote:

>I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
>great job of removing the low, rumbling sounds, like the office copier
>or the continuous roar on a plane. On the other hand, on a plane, they
>do virtually nothing about the higher-frequency airplane sounds
>(hissing, whining), and they add their own hiss.
>
>I asked someone selling expensive Bose headphones at Denver Airport
>about whether their phones cover the higher frequency ranges, but he
>turned out not to be informed on such a technical topic.
>
>Is this issue with my Sony phones normal--are only the lower frequencies
>targeted by today's noise-canceling products? Or are there products that
>I can rely on to cancel a wider range of frequencies and, ideally don't
>introduce significant noise of their own?

It is not a case of low frequencies being targeted, but really that
dealing with high frequencies is much more technically challenging. If
you really want good HF isolation, forget noise cancelling and buy
some passive ear defender types.

d

[Harlan Messinger]

Don Pearce wrote:
> On Thu, 04 Jun 2009 13:04:08 -0400, Harlan Messinger
> > wrote:
>
>> I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
>> great job of removing the low, rumbling sounds, like the office copier
>> or the continuous roar on a plane. On the other hand, on a plane, they
>> do virtually nothing about the higher-frequency airplane sounds
>> (hissing, whining), and they add their own hiss.
>>
>> I asked someone selling expensive Bose headphones at Denver Airport
>> about whether their phones cover the higher frequency ranges, but he
>> turned out not to be informed on such a technical topic.
>>
>> Is this issue with my Sony phones normal--are only the lower frequencies
>> targeted by today's noise-canceling products? Or are there products that
>> I can rely on to cancel a wider range of frequencies and, ideally don't
>> introduce significant noise of their own?
>
> It is not a case of low frequencies being targeted, but really that
> dealing with high frequencies is much more technically challenging. If
> you really want good HF isolation, forget noise cancelling and buy
> some passive ear defender types.

Really, it's more challenging? I would have thought one approach would
be applicable across the spectrum. Do you know of any resources I can
look at that will explain the technical details? Meanwhile, thanks for
the tip.

[Richard Crowley]

"Harlan Messinger" wrote ...
> Don Pearce wrote:
>> It is not a case of low frequencies being targeted, but really that
>> dealing with high frequencies is much more technically challenging. If
>> you really want good HF isolation, forget noise cancelling and buy
>> some passive ear defender types.
>
> Really, it's more challenging? I would have thought one approach would be
> applicable across the spectrum.

The fundamental theory is the same. But reconstructing a cancellation
signal at higher frequencies requires more processing horespower
(i.e. faster processors). And at shorter wavelengths it gets trickier
to deliver the exact cancellation waveform *at your eardrum* from
several mm away.

As Mr. Pearce suggests, passive isolation is still the more practical
way of dealing with HF noise toay.

[Don Pearce[_3_]]

On Thu, 04 Jun 2009 13:25:42 -0400, Harlan Messinger
> wrote:

>Don Pearce wrote:
>> On Thu, 04 Jun 2009 13:04:08 -0400, Harlan Messinger
>> > wrote:
>>
>>> I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
>>> great job of removing the low, rumbling sounds, like the office copier
>>> or the continuous roar on a plane. On the other hand, on a plane, they
>>> do virtually nothing about the higher-frequency airplane sounds
>>> (hissing, whining), and they add their own hiss.
>>>
>>> I asked someone selling expensive Bose headphones at Denver Airport
>>> about whether their phones cover the higher frequency ranges, but he
>>> turned out not to be informed on such a technical topic.
>>>
>>> Is this issue with my Sony phones normal--are only the lower frequencies
>>> targeted by today's noise-canceling products? Or are there products that
>>> I can rely on to cancel a wider range of frequencies and, ideally don't
>>> introduce significant noise of their own?
>>
>> It is not a case of low frequencies being targeted, but really that
>> dealing with high frequencies is much more technically challenging. If
>> you really want good HF isolation, forget noise cancelling and buy
>> some passive ear defender types.
>
>Really, it's more challenging? I would have thought one approach would
>be applicable across the spectrum. Do you know of any resources I can
>look at that will explain the technical details? Meanwhile, thanks for
>the tip.

No need, I can explain. It is all a matter of wavelength. Outside the
headshell is a microphone that picks up the environmental sound which
must be suppressed. An inverted version of that sound is played inside
the headphones along with the wanted signal. The idea is that the
inverted sound cancels out the actual sound. For low frequencies it
works nicely, because the inverted and direct sounds line up nicely.
But at higher frequencies the wavelength becomes sufficiently small
that alignment becomes impossible, For example at 10kHz the wavelength
is about one inch, so if the microphone is half an inch from the
speaker (almost inevitable), there will be a half wave error, and far
from cancelling the sound will actually reinforce. Below 10kHz, the
situation is not that bad, but bad enough that good cancellation is
impossible.

d

[Harlan Messinger]

Soundhaspriority wrote:
> "Harlan Messinger" > wrote in message
> ...
>> I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
>> great job of removing the low, rumbling sounds, like the office copier or
>> the continuous roar on a plane. On the other hand, on a plane, they do
>> virtually nothing about the higher-frequency airplane sounds (hissing,
>> whining), and they add their own hiss.
>>
>> I asked someone selling expensive Bose headphones at Denver Airport about
>> whether their phones cover the higher frequency ranges, but he turned out
>> not to be informed on such a technical topic.
>>
>> Is this issue with my Sony phones normal--are only the lower frequencies
>> targeted by today's noise-canceling products? Or are there products that I
>> can rely on to cancel a wider range of frequencies and, ideally don't
>> introduce significant noise of their own?
>
> Harlan,
> For the reasons given in this thread, there is absolutely nothing that
> will cancel higher frequencies. User satisfaction with these products is
> greatest with jet aircraft, because there is a specific low frequency
> content with which active noise cancellation is very effective. They are
> largely ineffective in other kinds of transport, such as trains and subways,
> where high frequency clattering and impact sounds are present.

Ah, OK. Thanks to all of you who replied for your helpful explanations.
I understand now where the shortcomings of noise cancellation lie. I
will switch my attention over to passive noise insulation.

[Mr.T]

"Don Pearce" > wrote in message
news:4a2c020b.558504156@localhost...
> On Thu, 04 Jun 2009 13:04:08 -0400, Harlan Messinger
> > wrote:
>
> >I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
> >great job of removing the low, rumbling sounds, like the office copier
> >or the continuous roar on a plane. On the other hand, on a plane, they
> >do virtually nothing about the higher-frequency airplane sounds
> >(hissing, whining), and they add their own hiss.
> >
> >I asked someone selling expensive Bose headphones at Denver Airport
> >about whether their phones cover the higher frequency ranges, but he
> >turned out not to be informed on such a technical topic.
> >
> >Is this issue with my Sony phones normal--are only the lower frequencies
> >targeted by today's noise-canceling products? Or are there products that
> >I can rely on to cancel a wider range of frequencies and, ideally don't
> >introduce significant noise of their own?
>
> It is not a case of low frequencies being targeted, but really that
> dealing with high frequencies is much more technically challenging. If
> you really want good HF isolation, forget noise cancelling and buy
> some passive ear defender types.


I don't own any NC headphones, but surely anything designed properly, rather
than just for marketing hype, would use a combination of both NC to combat
LF noise where simple isolation is difficult and less affective, and use
good insulation/isolation of the higher frequencies where NC is impossible.

I imagine there are some that do meet the criteria, but you may need to look
past Sony and Bose, and put up with bigger, heavier headphones.

MrT.

[Don Pearce[_3_]]

On Fri, 5 Jun 2009 14:55:55 +1000, "Mr.T" <MrT@home> wrote:

>
>"Don Pearce" > wrote in message
>news:4a2c020b.558504156@localhost...
>> On Thu, 04 Jun 2009 13:04:08 -0400, Harlan Messinger
>> > wrote:
>>
>> >I've got a pair of Sony MDR-NC6 noise-canceling headphones. They do a
>> >great job of removing the low, rumbling sounds, like the office copier
>> >or the continuous roar on a plane. On the other hand, on a plane, they
>> >do virtually nothing about the higher-frequency airplane sounds
>> >(hissing, whining), and they add their own hiss.
>> >
>> >I asked someone selling expensive Bose headphones at Denver Airport
>> >about whether their phones cover the higher frequency ranges, but he
>> >turned out not to be informed on such a technical topic.
>> >
>> >Is this issue with my Sony phones normal--are only the lower frequencies
>> >targeted by today's noise-canceling products? Or are there products that
>> >I can rely on to cancel a wider range of frequencies and, ideally don't
>> >introduce significant noise of their own?
>>
>> It is not a case of low frequencies being targeted, but really that
>> dealing with high frequencies is much more technically challenging. If
>> you really want good HF isolation, forget noise cancelling and buy
>> some passive ear defender types.
>
>
>I don't own any NC headphones, but surely anything designed properly, rather
>than just for marketing hype, would use a combination of both NC to combat
>LF noise where simple isolation is difficult and less affective, and use
>good insulation/isolation of the higher frequencies where NC is impossible.
>
>I imagine there are some that do meet the criteria, but you may need to look
>past Sony and Bose, and put up with bigger, heavier headphones.
>
>MrT.
>

Phones that isolate the HF well also tend to deal with the bottom end
without resort to active cancelling. Where active phones work well is
in situations like light aircraft where there is a great deal of throb
and drone to get rid of, which is well taken care of with FFTs and
multiple band generators. Then, because the headshells are lightweight
and not particularly padded, all the other external sounds that you do
want to hear - radios, beacons, your passenger etc, which are not
repetitive - are far better heard than without them. That, as far as I
can see, is the true raison d'etre for active phones.

d

[Mr.T]

"Don Pearce" > wrote in message
news:4a2fb739.604885062@localhost...
> Phones that isolate the HF well also tend to deal with the bottom end
> without resort to active cancelling.

Not so, LF is a lot harder to absorb than HF.

>Where active phones work well is
> in situations like light aircraft where there is a great deal of throb
> and drone to get rid of,

That *is* LF noise, and the main reason for NC headphones. The OP wanted
more HF reduction as well, which is beyond the scope of active NC without
placing your head is a vice!

> Then, because the headshells are lightweight
> and not particularly padded, all the other external sounds that you do
> want to hear - radios, beacons, your passenger etc, which are not
> repetitive - are far better heard than without them. That, as far as I
> can see, is the true raison d'etre for active phones.

That may be so for some, IF the manufacturers made that clear to the buyers.
Personally I'd want ones that block LF *and* HF however, which appears to be
what the OP wants as well.

MrT.
..

[Don Pearce[_3_]]

On Fri, 5 Jun 2009 20:02:27 +1000, "Mr.T" <MrT@home> wrote:

>
>"Don Pearce" > wrote in message
>news:4a2fb739.604885062@localhost...
>> Phones that isolate the HF well also tend to deal with the bottom end
>> without resort to active cancelling.
>
>Not so, LF is a lot harder to absorb than HF.
>
You don't absorb it, you block it, which is not the same. Stiffness
and a bit of mass does the job.

>>Where active phones work well is
>> in situations like light aircraft where there is a great deal of throb
>> and drone to get rid of,
>
> That *is* LF noise, and the main reason for NC headphones. The OP wanted
>more HF reduction as well, which is beyond the scope of active NC without
>placing your head is a vice!
>

I already went through that in my first post.

>> Then, because the headshells are lightweight
>> and not particularly padded, all the other external sounds that you do
>> want to hear - radios, beacons, your passenger etc, which are not
>> repetitive - are far better heard than without them. That, as far as I
>> can see, is the true raison d'etre for active phones.
>
>That may be so for some, IF the manufacturers made that clear to the buyers.
>Personally I'd want ones that block LF *and* HF however, which appears to be
>what the OP wants as well.

Yup, that has been dealt with to the OP's satisfaction - we've moved
on to another facet now. It isn't an LF/HF thing. It is a
repetitive/non-repetitive differentiation. You need to hear the
one-off events while blocking the background drone. That is where NC
phones score over block-everything passives.

d

[[email protected]]

On Jun 4, 1:32*pm, "Richard Crowley" > wrote:
> "Harlan Messinger" wrote ...
>
> > Don Pearce wrote:
> >> It is not a case of low frequencies being targeted, but really that
> >> dealing with high frequencies is much more technically challenging. If
> >> you really want good HF isolation, forget noise cancelling and buy
> >> some passive ear defender types.
>
> > Really, it's more challenging? I would have thought one approach would be
> > applicable across the spectrum.
>
> The fundamental theory is the same. But reconstructing
> a cancellation signal at higher frequencies requires
> more processing horespower (i.e. faster processors).

More "horsepower?" In the limiting case, all the
horsepower that's needed is inverting the phase
of the signal.

> And at shorter wavelengths it gets trickier
> to deliver the exact cancellation waveform
> *at your eardrum* from several mm away.

THAT'S the crux of the problem: the fact that
the microphone used to detect the original
noise signal and the transducer used to produce
the cancelling signal can not physically occupy
the same point. Further, the REAL point where
you want the cancellation to occur is in the ear
canal, where it is, at best, very inconvenient to
place either.

It works well at low frequencies because the
wavelengths are large (at 100 Hz, they're 11 feet
long), thus the difference in sound pressure between
the microphone and cancelling speaker (say they're
two inches apart) is small and the phase difference
is also small (on the order of about 6 degrees).

At 10 kHz, those wavelengths are on the order of
1.4 inches of an inches, substantially larger than
our hypothetical 2" separation.

[[email protected]]

On Jun 4, 1:40*pm, "Soundhaspriority" > wrote:
> For the reasons given in this thread, there is
> absolutely nothing that will cancel higher
> frequencies.

Absolutely false. There may not be anything
PRACTICAL or AFFORDABLE that can be used
in a consumer headphone product, but the
statement there is "absolutely nothing" is
absolutely nonsense.

[Richard Crowley]

dpierce wrote ...
> "Richard Crowley" wrote:
>> The fundamental theory is the same. But reconstructing
>> a cancellation signal at higher frequencies requires
>> more processing horespower (i.e. faster processors).
>
> More "horsepower?" In the limiting case, all the
> horsepower that's needed is inverting the phase
> of the signal.

At what sampling rate?

[[email protected]]

On Jun 5, 5:08*pm, "Richard Crowley" > wrote:
> dpierce wrote ...
>
> > "Richard Crowley" wrote:
> >> The fundamental theory is the same. But reconstructing
> >> a cancellation signal at higher frequencies requires
> >> more processing horespower (i.e. faster processors).
>
> > More "horsepower?" In the limiting case, all the
> > horsepower that's needed is inverting the phase
> > of the signal.
>
> At what sampling rate?

Irrelevant. Take a signal, run it through ANYTHING
that inverts the phase. A transformer, an inverting
op amp. No discrete sampling, no DSP of ANY
kind required.

[Ron Capik[_2_]]

wrote:
> On Jun 5, 5:08 pm, "Richard Crowley" > wrote:
>> dpierce wrote ...
>>
>>> "Richard Crowley" wrote:
>>>> The fundamental theory is the same. But reconstructing
>>>> a cancellation signal at higher frequencies requires
>>>> more processing horespower (i.e. faster processors).
>>> More "horsepower?" In the limiting case, all the
>>> horsepower that's needed is inverting the phase
>>> of the signal.
>> At what sampling rate?
>
> Irrelevant. Take a signal, run it through ANYTHING
> that inverts the phase. A transformer, an inverting
> op amp. No discrete sampling, no DSP of ANY
> kind required.
However, the driver/ear canal response and microphone
response need to match exactly (or at least be damn
close) for that to work over a wide band.


Later...

Ron Capik
--

[Richard Crowley]

dpierce wrote ...
> "Richard Crowley" wrote:
> > dpierce wrote ...
> > > "Richard Crowley" wrote:
> > >> The fundamental theory is the same. But reconstructing
> >> a cancellation signal at higher frequencies requires
> > >> more processing horespower (i.e. faster processors).
> >
> > > More "horsepower?" In the limiting case, all the
> > > horsepower that's needed is inverting the phase
> > > of the signal.
> >
> > At what sampling rate?
>
> Irrelevant. Take a signal, run it through ANYTHING
> that inverts the phase. A transformer, an inverting
> op amp. No discrete sampling, no DSP of ANY
> kind required.

But that's not how noise cancellation works. That method
would merely create acoustic feedback.

Note that consumer noise-cancelling had to wait for the wide
availabity of low-cost DSP computing horsepower.

[Mr.T]

"Don Pearce" > wrote in message
news:4a30f001.619413625@localhost...
> >> Phones that isolate the HF well also tend to deal with the bottom end
> >> without resort to active cancelling.
> >
> >Not so, LF is a lot harder to absorb than HF.
> >
> You don't absorb it, you block it, which is not the same. Stiffness
> and a bit of mass does the job.

Solid concrete headphones perhaps?

Fact is NC headphones were invented to reduce LF noise because other methods
weren't satisfactory for many people.

MrT.

[Mr.T]

"Richard Crowley" > wrote in message
...
> dpierce wrote ...
> > Irrelevant. Take a signal, run it through ANYTHING
> > that inverts the phase. A transformer, an inverting
> > op amp. No discrete sampling, no DSP of ANY
> > kind required.
>
> But that's not how noise cancellation works. That method
> would merely create acoustic feedback.

You do know the difference between positive and negative feedback right?
*Negative* acoustic feedback is actually what they are trying to achieve.
And Dick specifically said to invert the phase, i.e. negative feedback.

MrT.

[Don Pearce[_3_]]

On Sun, 7 Jun 2009 11:14:17 +1000, "Mr.T" <MrT@home> wrote:

>
>"Don Pearce" > wrote in message
>news:4a30f001.619413625@localhost...
>> >> Phones that isolate the HF well also tend to deal with the bottom end
>> >> without resort to active cancelling.
>> >
>> >Not so, LF is a lot harder to absorb than HF.
>> >
>> You don't absorb it, you block it, which is not the same. Stiffness
>> and a bit of mass does the job.
>
>Solid concrete headphones perhaps?
>

Stiffness is all that is needed - not concrete.

>Fact is NC headphones were invented to reduce LF noise because other methods
>weren't satisfactory for many people.
>
No, they were invented because someone could. They then fulfilled a
male need for gadgets. I haven't done the data mining, but I'm willing
to bet that a good closed back or ear bud (Etymotic et al) provides as
good if not better LF isolation than a NC.

d

[Mr.T]

"Don Pearce" > wrote in message
news:4a2d15ba.760130281@localhost...
> >> You don't absorb it, you block it, which is not the same. Stiffness
> >> and a bit of mass does the job.
^^^^^^^^^
> >Solid concrete headphones perhaps?
>
> Stiffness is all that is needed

And the "bit of mass" ?

> No, they were invented because someone could. They then fulfilled a
> male need for gadgets. I haven't done the data mining, but I'm willing
> to bet that a good closed back or ear bud (Etymotic et al) provides as
> good if not better LF isolation than a NC.

As I already said many posts ago!
However the NC type *may* be lighter than closed back types with good
insulation.
As always you pay your money and you make your own choice.

MrT.

[Peter Larsen[_3_]]

Don Pearce wrote:

||| You don't absorb it, you block it, which is not the same. Stiffness
||| and a bit of mass does the job.

|| Solid concrete headphones perhaps?

| Stiffness is all that is needed - not concrete.

My old gunshootinge bilsom muffs did a great job with LF impulse noise due
to their glycerin filled seals. And yes ... stiffness does work wonders in
this context as one knows when having worked on a blanket to blanket offset
web-press.

|| Fact is NC headphones were invented to reduce LF noise because
|| other methods weren't satisfactory for many people.

| No, they were invented because someone could. They then fulfilled a
| male need for gadgets. I haven't done the data mining, but I'm
| willing to bet that a good closed back or ear bud (Etymotic et al)
| provides as good if not better LF isolation than a NC.

Sennheisers model is/was advertized as designed for helicopter use and
is/was a rigid shell type, based on web site imagery, there is sense in
reducing the LF masking beyond what mere rigidity can give you, that one is
about increasing speech understandability, so it makes sense. But
considering real lf noise it it not about those latter ... my guess for
something probable is 8 dB, but is is a guess ... reduction, but about the
first 20, and rigid shell earmuffs with good seals do that very well.

|| d

Kind regards

Peter Larsen

[Richard Crowley]

Mr.T wrote:
> "Richard Crowley" wrote ...
>> dpierce wrote ...
>>> Irrelevant. Take a signal, run it through ANYTHING
>>> that inverts the phase. A transformer, an inverting
>>> op amp. No discrete sampling, no DSP of ANY
>>> kind required.
>>
>> But that's not how noise cancellation works. That method
>> would merely create acoustic feedback.
>
> You do know the difference between positive and negative feedback
> right? *Negative* acoustic feedback is actually what they are trying
> to achieve. And Dick specifically said to invert the phase, i.e.
> negative feedback.

Easy to do electronically, extraordinarily difficult to do accoustically at
short wavelengths. That is why modern consumer noise cancelling products
depend on inexpensive DSP. You cannot just take an acoustic signal from a
microphone "invert" it and try to cancel noise with it. You don't have to
take my word for it. Try it for yourself.

[David Nebenzahl]

On 6/7/2009 6:39 AM Richard Crowley spake thus:

> Mr.T wrote:
>
>> "Richard Crowley" wrote ...
>>
>>> dpierce wrote ...
>>>
>>>> Irrelevant. Take a signal, run it through ANYTHING that inverts
>>>> the phase. A transformer, an inverting op amp. No discrete
>>>> sampling, no DSP of ANY kind required.
>>>
>>> But that's not how noise cancellation works. That method would
>>> merely create acoustic feedback.
>>
>> You do know the difference between positive and negative feedback
>> right? *Negative* acoustic feedback is actually what they are trying
>> to achieve. And Dick specifically said to invert the phase, i.e.
>> negative feedback.
>
> Easy to do electronically, extraordinarily difficult to do
> accoustically at short wavelengths. That is why modern consumer noise
> cancelling products depend on inexpensive DSP. You cannot just take
> an acoustic signal from a microphone "invert" it and try to cancel
> noise with it. You don't have to take my word for it. Try it for
> yourself.

Just out of curiosity, why not? And no, not about to do this experiment
myself anytime soon.

Let me guess: it (inverting the signal) would work fine at low
frequencies, but progressively worse at higher frequencies?


--
Found--the gene that causes belief in genetic determinism

[Mr.T]

"Richard Crowley" > wrote in message
...
> >> But that's not how noise cancellation works. That method
> >> would merely create acoustic feedback.
> >
> > You do know the difference between positive and negative feedback
> > right? *Negative* acoustic feedback is actually what they are trying
> > to achieve. And Dick specifically said to invert the phase, i.e.
> > negative feedback.
>
> Easy to do electronically, extraordinarily difficult to do accoustically
at
> short wavelengths. That is why modern consumer noise cancelling products
> depend on inexpensive DSP. You cannot just take an acoustic signal from a
> microphone "invert" it and try to cancel noise with it. You don't have to
> take my word for it. Try it for yourself.


Please read what I wrote, I never said it was that simple. I simply said in
practice you are providing negative acoustic feedback.
HOW it is *effectively* achieved is another matter entirely.

MrT.



>
>

[Mr.T]

"David Nebenzahl" > wrote in message
.com...
> Let me guess: it (inverting the signal) would work fine at low
> frequencies, but progressively worse at higher frequencies?

*IF* you could provide a 100% accurate signal of the actual sound in the ear
canal, and reinject a perfectly matched inverted copy in exactly the same
spot, with no phase shift whatsoever, then the rest should be very simple
indeed :-) :-)

MrT.

[David Nebenzahl]

On 6/7/2009 6:59 PM Mr.T spake thus:

> "David Nebenzahl" > wrote in message
> .com...
>
>> Let me guess: it (inverting the signal) would work fine at low
>> frequencies, but progressively worse at higher frequencies?
>
> *IF* you could provide a 100% accurate signal of the actual sound in the ear
> canal,

Well, that's the hard part here, isn't it? But seemingly not impossible
(well, not 100%, but close enough).

> and reinject a perfectly matched inverted copy in exactly the same
> spot,

Trivially easy, no? A simple phase inverter oughta do the trick.

> with no phase shift whatsoever, then the rest should be very simple
> indeed :-) :-)

Since we're talking audio frequencies (< 8 kHz, I'm guessing), shouldn't
be a big deal.


--
Found--the gene that causes belief in genetic determinism

[Mr.T]

"David Nebenzahl" > wrote in message
.com...
> Trivially easy, no? A simple phase inverter oughta do the trick.
>
> > with no phase shift whatsoever, then the rest should be very simple
> > indeed :-) :-)
>
> Since we're talking audio frequencies (< 8 kHz, I'm guessing), shouldn't
> be a big deal.

YOU might think so, good luck :-)

MrT.

[Richard Crowley]

David Nebenzahl wrote:
> Mr.T spake thus:
>> and reinject a perfectly matched inverted copy in exactly the same
>> spot,
>
> Trivially easy, no? A simple phase inverter oughta do the trick.

No. Because it will almost instantly turn into a closed-loop
feedback oscillator and likely deafen the wearer. If it were
as easy as simple signal inversion, we would have had noise
cancelling headphones decades sooner.

[David Nebenzahl]

On 6/7/2009 10:04 PM Richard Crowley spake thus:

> David Nebenzahl wrote:
>> Mr.T spake thus:
>>> and reinject a perfectly matched inverted copy in exactly the same
>>> spot,
>>
>> Trivially easy, no? A simple phase inverter oughta do the trick.
>
> No. Because it will almost instantly turn into a closed-loop
> feedback oscillator and likely deafen the wearer. If it were
> as easy as simple signal inversion, we would have had noise
> cancelling headphones decades sooner.

Hmm; that seems counterintuitive. Not disputing you, but I thought that
only positive (i.e., same-phase) signals would cause that kind of
feedback. An inverted signal should (nearly) cancel the original signal,
n'est-ce pas? What am I missing here?


--
Found--the gene that causes belief in genetic determinism

[Richard Crowley]

David Nebenzahl wrote:
> On 6/7/2009 10:04 PM Richard Crowley spake thus:
>
>> David Nebenzahl wrote:
>>> Mr.T spake thus:
>>>> and reinject a perfectly matched inverted copy in exactly the same
>>>> spot,
>>>
>>> Trivially easy, no? A simple phase inverter oughta do the trick.
>>
>> No. Because it will almost instantly turn into a closed-loop
>> feedback oscillator and likely deafen the wearer. If it were
>> as easy as simple signal inversion, we would have had noise
>> cancelling headphones decades sooner.
>
> Hmm; that seems counterintuitive. Not disputing you, but I thought
> that only positive (i.e., same-phase) signals would cause that kind of
> feedback. An inverted signal should (nearly) cancel the original
> signal, n'est-ce pas? What am I missing here?

The space inside the headphone forms a resonant cavity
and a broadband microphone - amplifier-speaker system
would seek the most resonant frequency within milliseconds.
Anyone who has ever operated a sound reinforcement
(PA) system knows the effect.

Noise cancellation systems work by sampling the waveform
and independently synthesizing an inverted copy of the noise
waveform.
http://en.wikipedia.org/wiki/Active_noise_control

[David Nebenzahl]

On 6/8/2009 12:05 AM Richard Crowley spake thus:

> David Nebenzahl wrote:
>
>> Hmm; that seems counterintuitive. Not disputing you, but I thought
>> that only positive (i.e., same-phase) signals would cause that kind of
>> feedback. An inverted signal should (nearly) cancel the original
>> signal, n'est-ce pas? What am I missing here?
>
> The space inside the headphone forms a resonant cavity
> and a broadband microphone - amplifier-speaker system
> would seek the most resonant frequency within milliseconds.
> Anyone who has ever operated a sound reinforcement
> (PA) system knows the effect.
>
> Noise cancellation systems work by sampling the waveform
> and independently synthesizing an inverted copy of the noise
> waveform.
> http://en.wikipedia.org/wiki/Active_noise_control

Well, I don't trust Wikipedia as far as one can throw it, but I trust
you, so I'll take your word for it.


--
Found--the gene that causes belief in genetic determinism

[Don Pearce[_3_]]

On Mon, 08 Jun 2009 00:13:00 -0700, David Nebenzahl
> wrote:

>On 6/8/2009 12:05 AM Richard Crowley spake thus:
>
>> David Nebenzahl wrote:
> >
>>> Hmm; that seems counterintuitive. Not disputing you, but I thought
>>> that only positive (i.e., same-phase) signals would cause that kind of
>>> feedback. An inverted signal should (nearly) cancel the original
>>> signal, n'est-ce pas? What am I missing here?
>>
>> The space inside the headphone forms a resonant cavity
>> and a broadband microphone - amplifier-speaker system
>> would seek the most resonant frequency within milliseconds.
>> Anyone who has ever operated a sound reinforcement
>> (PA) system knows the effect.
>>
>> Noise cancellation systems work by sampling the waveform
>> and independently synthesizing an inverted copy of the noise
>> waveform.
>> http://en.wikipedia.org/wiki/Active_noise_control
>
>Well, I don't trust Wikipedia as far as one can throw it, but I trust
>you, so I'll take your word for it.

I've just been investigating the isolation performance (claimed) of
active phones vs Etymotic passive ear buds, and here is the result:

http://81.174.169.10/odds/isolation.gif

Particularly interesting is the fact that at very low frequencies the
actives actually make the noise a bit louder. Once you get beyond
1kHz, of course, the active cancellers do nothing at all, while the
passives just go on getting better.

d

[Arny Krueger]

"David Nebenzahl" > wrote in message
.com...
> On 6/7/2009 10:04 PM Richard Crowley spake thus:
>
>> David Nebenzahl wrote:
>>> Mr.T spake thus:
>>>> and reinject a perfectly matched inverted copy in exactly the same
>>>> spot,
>>>
>>> Trivially easy, no? A simple phase inverter oughta do the trick.
>>
>> No. Because it will almost instantly turn into a closed-loop
>> feedback oscillator and likely deafen the wearer. If it were
>> as easy as simple signal inversion, we would have had noise
>> cancelling headphones decades sooner.
>
> Hmm; that seems counterintuitive. Not disputing you, but I thought that
> only positive (i.e., same-phase) signals would cause that kind of
> feedback. An inverted signal should (nearly) cancel the original signal,
> n'est-ce pas? What am I missing here?


You're missing the fact that it becomes progressively more difficult to
control the phase of the cancellation signal as the frequency increases.

Elsewhere in this thread I think I saw a discussion of the fact that
wavelengths get shorter as frequency increases. As the wavelength gets
shorter, the harder it gets to control the phase of the wave at any point.
Think of picking up a needle with a small tweezers. Think of picking up a
needle with a large bolt cutter.

For example, at 100 feet, a sound wave is about 10' long. If the microphone
that picks up the noise that is to be cancelled out is a fraction of an inch
from the point where the cancellation is to take place, that fractional inch
distance causes very little error.

At 10 KHz, a sound wave is about an inch. Now, you have to somehow put the
microphone far, far less than one inch from the point where the cancellation
it to take place for equal effectiveness as you easily obtained at 100 Hz.

If you don't accurately control the timing of the cancellation signal, the
system oscillates.

So, the difficulties involved in canceling high frequencies are due to the
laws of physics. Part is due to wavelength effects, and part is due to the
requirements for a system that is based on negative feedback to be stable.

[Harlan Messinger]

Don Pearce wrote:
> On Mon, 08 Jun 2009 00:13:00 -0700, David Nebenzahl
> > wrote:
>
>> On 6/8/2009 12:05 AM Richard Crowley spake thus:
>>
>>> David Nebenzahl wrote:
>>>
>>>> Hmm; that seems counterintuitive. Not disputing you, but I thought
>>>> that only positive (i.e., same-phase) signals would cause that kind of
>>>> feedback. An inverted signal should (nearly) cancel the original
>>>> signal, n'est-ce pas? What am I missing here?
>>> The space inside the headphone forms a resonant cavity
>>> and a broadband microphone - amplifier-speaker system
>>> would seek the most resonant frequency within milliseconds.
>>> Anyone who has ever operated a sound reinforcement
>>> (PA) system knows the effect.
>>>
>>> Noise cancellation systems work by sampling the waveform
>>> and independently synthesizing an inverted copy of the noise
>>> waveform.
>>> http://en.wikipedia.org/wiki/Active_noise_control
>> Well, I don't trust Wikipedia as far as one can throw it, but I trust
>> you, so I'll take your word for it.
>
> I've just been investigating the isolation performance (claimed) of
> active phones vs Etymotic passive ear buds, and here is the result:
>
> http://81.174.169.10/odds/isolation.gif
>
> Particularly interesting is the fact that at very low frequencies the
> actives actually make the noise a bit louder. Once you get beyond
> 1kHz, of course, the active cancellers do nothing at all, while the
> passives just go on getting better.

What are the units of the vertical axis?

[Don Pearce[_3_]]

On Mon, 08 Jun 2009 15:44:45 -0400, Harlan Messinger
> wrote:

>Don Pearce wrote:
>> On Mon, 08 Jun 2009 00:13:00 -0700, David Nebenzahl
>> > wrote:
>>
>>> On 6/8/2009 12:05 AM Richard Crowley spake thus:
>>>
>>>> David Nebenzahl wrote:
>>>>
>>>>> Hmm; that seems counterintuitive. Not disputing you, but I thought
>>>>> that only positive (i.e., same-phase) signals would cause that kind of
>>>>> feedback. An inverted signal should (nearly) cancel the original
>>>>> signal, n'est-ce pas? What am I missing here?
>>>> The space inside the headphone forms a resonant cavity
>>>> and a broadband microphone - amplifier-speaker system
>>>> would seek the most resonant frequency within milliseconds.
>>>> Anyone who has ever operated a sound reinforcement
>>>> (PA) system knows the effect.
>>>>
>>>> Noise cancellation systems work by sampling the waveform
>>>> and independently synthesizing an inverted copy of the noise
>>>> waveform.
>>>> http://en.wikipedia.org/wiki/Active_noise_control
>>> Well, I don't trust Wikipedia as far as one can throw it, but I trust
>>> you, so I'll take your word for it.
>>
>> I've just been investigating the isolation performance (claimed) of
>> active phones vs Etymotic passive ear buds, and here is the result:
>>
>> http://81.174.169.10/odds/isolation.gif
>>
>> Particularly interesting is the fact that at very low frequencies the
>> actives actually make the noise a bit louder. Once you get beyond
>> 1kHz, of course, the active cancellers do nothing at all, while the
>> passives just go on getting better.
>
>What are the units of the vertical axis?

dB of isolation.

d

[Harlan Messinger]

Don Pearce wrote:
> On Mon, 08 Jun 2009 15:44:45 -0400, Harlan Messinger
> > wrote:
>
>> Don Pearce wrote:
>>> Particularly interesting is the fact that at very low frequencies the
>>> actives actually make the noise a bit louder. Once you get beyond
>>> 1kHz, of course, the active cancellers do nothing at all, while the
>>> passives just go on getting better.
>> What are the units of the vertical axis?
>
> dB of isolation.
>

What does it mean, respectively, for there to be negative and positive
dB of isolation? I would have understood positive dB to mean that the
noise level was worse than without the "isolation".

[Don Pearce[_3_]]

On Tue, 09 Jun 2009 09:57:24 -0400, Harlan Messinger
> wrote:

>Don Pearce wrote:
>> On Mon, 08 Jun 2009 15:44:45 -0400, Harlan Messinger
>> > wrote:
>>
>>> Don Pearce wrote:
>>>> Particularly interesting is the fact that at very low frequencies the
>>>> actives actually make the noise a bit louder. Once you get beyond
>>>> 1kHz, of course, the active cancellers do nothing at all, while the
>>>> passives just go on getting better.
>>> What are the units of the vertical axis?
>>
>> dB of isolation.
>>
>
>What does it mean, respectively, for there to be negative and positive
>dB of isolation? I would have understood positive dB to mean that the
>noise level was worse than without the "isolation".

No, if you are talking isolation, then the more the better, Hence
positive dBs. The bit where the curve dips below the line (negative
isolation) is a frequency range in which the external sounds are
actually a little louder than they would be without the phones.

d

[Harlan Messinger]

Don Pearce wrote:
> On Tue, 09 Jun 2009 09:57:24 -0400, Harlan Messinger
> > wrote:
>
>> Don Pearce wrote:
>>> On Mon, 08 Jun 2009 15:44:45 -0400, Harlan Messinger
>>> > wrote:
>>>
>>>> Don Pearce wrote:
>>>>> Particularly interesting is the fact that at very low frequencies the
>>>>> actives actually make the noise a bit louder. Once you get beyond
>>>>> 1kHz, of course, the active cancellers do nothing at all, while the
>>>>> passives just go on getting better.
>>>> What are the units of the vertical axis?
>>> dB of isolation.
>>>
>> What does it mean, respectively, for there to be negative and positive
>> dB of isolation? I would have understood positive dB to mean that the
>> noise level was worse than without the "isolation".
>
> No, if you are talking isolation, then the more the better, Hence
> positive dBs. The bit where the curve dips below the line (negative
> isolation) is a frequency range in which the external sounds are
> actually a little louder than they would be without the phones.

Oh, OK, I get it now. Thanks!