Interesting artifact of Bose noise cancelling headphones

Hi,
I'm currently playing with noise cancelling headphones, mostly performing
frequency response measurements etc. I only have the QC15 here, which I
think are a few years old.
I noticed that if you try to listen to any pure sine wave frequency from
about 20-80Hz, you first hear nothing (the sine wave gets fully attenuated
by the ANC system), but after about 4-5 seconds, the amplitude of the sine
wave increases up to the loudest thing the speakers can produce - while
destroying my ears in the process:)

I wonder what this means about their control loop. I understand this isn't
such a common real life use case, but with the type of music that kids
listen to these days, I wonder why there hasn't been a lawsuit yet!!!

Thanks!
Dave

On 9/6/2011 5:06 PM, gretzteam wrote:
> Hi,
> I'm currently playing with noise cancelling headphones, mostly performing
> frequency response measurements etc. I only have the QC15 here, which I
> think are a few years old.
> I noticed that if you try to listen to any pure sine wave frequency from
> about 20-80Hz, you first hear nothing (the sine wave gets fully attenuated
> by the ANC system), but after about 4-5 seconds, the amplitude of the sine
> wave increases up to the loudest thing the speakers can produce - while
> destroying my ears in the process:)
>
> I wonder what this means about their control loop. I understand this isn't
> such a common real life use case, but with the type of music that kids
> listen to these days, I wonder why there hasn't been a lawsuit yet!!!

Why do you think that the headset isn't defective?

Jerry
-- 
Engineering is the art of making what you want from things you can get.

>> I wonder what this means about their control loop. I understand this
isn't
>> such a common real life use case, but with the type of music that kids
>> listen to these days, I wonder why there hasn't been a lawsuit yet!!!
>
>Why do you think that the headset isn't defective?
>

I didn't think that they could be defective because I use them all the time
and they sound great (at least to me). 

On Sep 7, 9:06&#4294967295;am, "gretzteam" <gretzteam@n_o_s_p_a_m.yahoo.com>
wrote:
> Hi,
> I'm currently playing with noise cancelling headphones, mostly performing
> frequency response measurements etc. I only have the QC15 here, which I
> think are a few years old.
> I noticed that if you try to listen to any pure sine wave frequency from
> about 20-80Hz, you first hear nothing (the sine wave gets fully attenuated
> by the ANC system), but after about 4-5 seconds, the amplitude of the sine
> wave increases up to the loudest thing the speakers can produce - while
> destroying my ears in the process:)
>
> I wonder what this means about their control loop. I understand this isn't
> such a common real life use case, but with the type of music that kids
> listen to these days, I wonder why there hasn't been a lawsuit yet!!!
>
> Thanks!
> Dave

Maybe because it's deterministic and modelling a sine-wave. There must
be large errors getting in
somehow. Write to the manufacturers..


Hardy

gretzteam wrote:

> Hi,
> I'm currently playing with noise cancelling headphones, mostly performing
> frequency response measurements etc. I only have the QC15 here, which I
> think are a few years old.
> I noticed that if you try to listen to any pure sine wave frequency from
> about 20-80Hz, you first hear nothing (the sine wave gets fully attenuated
> by the ANC system), but after about 4-5 seconds, the amplitude of the sine
> wave increases up to the loudest thing the speakers can produce - while
> destroying my ears in the process:)

Yesterday I was watching TV and poking ears with nail. Suddenly the 
sound dissapeared. Do you think what could be wrong with TV ?

> I wonder what this means about their control loop. I understand this isn't
> such a common real life use case, but with the type of music that kids
> listen to these days, I wonder why there hasn't been a lawsuit yet!!!

To my knowledge, there is no control loop in noise canceling headphones. 
It uses simple analog subtraction.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Vladimir Vassilevsky <nospam@nowhere.com> wrote:

(snip)
> To my knowledge, there is no control loop in noise 
> canceling headphones.  It uses simple analog subtraction.

I believe it is analog subtraction, but maybe not so simple.
One must do something to avoid the feedback problems that
would naturally occur.

I do know that they work less well at higher frequencies,
so I presume there is a filter in there somewhere.

-- glen

On 9/7/11 12:56 PM, Vladimir Vassilevsky wrote:
>
...
>
> To my knowledge, there is no control loop in noise canceling headphones.

you mean no loop at all?  no LMS nor NLMS (or whatever adaptive filter)?

> It uses simple analog subtraction.

how does it know what to subtract?  i would think that not only is the 
subtraction done analogly, but it's done acoustically (actually with a 
sign change and addition).  that's the whole point.  but i would think 
that in the headphone, close to the orifice of the ear canal (the 
closer, the better) is a tiny microphone that picks up the official 
difference signal (that we're trying to minimize) to feed back into the 
adaptive filter.

but, in fact, i have never ever ever done diddly with noise-canceling 
headphones, so i cannot claim to know diddly about it.  i just have an 
idea for how i would approach the problem.  i might suggest to the OP 
that adaptive filters usually work better on broadbanded signals and a 
single and pure sinusoid is what i would call "quite narrow banded".

-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

robert bristow-johnson wrote:

> On 9/7/11 12:56 PM, Vladimir Vassilevsky wrote:
> 
>>
> ....
> 
>>
>> To my knowledge, there is no control loop in noise canceling headphones.
> 
> you mean no loop at all?  no LMS nor NLMS (or whatever adaptive filter)?

No DSP, nothing adaptive. There is a couple of trimpots inside to adjust 
gains, that's it.

>> It uses simple analog subtraction.
> how does it know what to subtract?

A small mike on the outside of each earpiece.

> i would think that not only is the 
> subtraction done analogly, but it's done acoustically (actually with a 
> sign change and addition).

It is done electrically, in the headphone amplifier. The signal from the 
earpiece mike is subtracted from incoming audio. As noted by Glen, they 
knock the mic gain down at higher frequencies to avoid stability problems.

> that's the whole point.  but i would think 
> that in the headphone, close to the orifice of the ear canal (the 
> closer, the better) is a tiny microphone that picks up the official 
> difference signal (that we're trying to minimize) to feed back into the 
> adaptive filter.

That would be complicated, expensive and heavy and I am not sure if it 
would perform considerably better then a simple subtractor.

> but, in fact, i have never ever ever done diddly with noise-canceling 
> headphones, so i cannot claim to know diddly about it.  i just have an 
> idea for how i would approach the problem.  i might suggest to the OP 
> that adaptive filters usually work better on broadbanded signals and a 
> single and pure sinusoid is what i would call "quite narrow banded".

I suggest OP replacing the battery. It obviously drops under heavy load.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

>robert bristow-johnson wrote:
>> i would think that not only is the 
>> subtraction done analogly, but it's done acoustically (actually with a 
>> sign change and addition).
>
>It is done electrically, in the headphone amplifier. The signal from the 
>earpiece mike is subtracted from incoming audio. As noted by Glen, they 
>knock the mic gain down at higher frequencies to avoid stability
problems.

Its not just stability. Closed back headphones that fit snuggly to the ear
are pretty good at excluding higher frequencies. There are only low
frequencies which the electronics needs to remove.

Steve

> > Hi,
> > I'm currently playing with noise cancelling headphones, mostly performing
> > frequency response measurements etc. I only have the QC15 here, which I
> > think are a few years old.
> > I noticed that if you try to listen to any pure sine wave frequency from
> > about 20-80Hz, you first hear nothing (the sine wave gets fully attenuated
> > by the ANC system), but after about 4-5 seconds, the amplitude of the sine
> > wave increases up to the loudest thing the speakers can produce - while
> > destroying my ears in the process:)
>
> Yesterday I was watching TV and poking ears with nail. Suddenly the
> sound dissapeared. Do you think what could be wrong with TV ?
>
> > I wonder what this means about their control loop. I understand this isn't
> > such a common real life use case, but with the type of music that kids
> > listen to these days, I wonder why there hasn't been a lawsuit yet!!!
>
> To my knowledge, there is no control loop in noise canceling headphones.
> It uses simple analog subtraction.
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com

As Einstein pointed out, if it actually is complicated then it must be
treated as something complicated.

On the other hand always always always look for a more simple
solution.

�Perfection is achieved not when there is nothing left to add, but
when there is nothing left to take away�

� Antoine de Saint-Exupery

This needs to be posted in every engineering dept.


Bret Cahill


"Omit needless words."

-- Strunk & White