From: Neil <picdude3@narwani.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Thu, 19 Mar 2020 07:16:45 -0700
Subject: Re: [EE] Measuring speaker impedance
Thread-Topic: [EE] Measuring speaker impedance
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On 3/19/2020 3:39 AM, Sean Breheny wrote:
> As James says, there are some problems with this, but I think it could be
> done.
>
> What you have to realize, though, is that the whole concept of impedance =
is
> a small-signal one. A speaker only has a well-defined impedance for a fix=
ed
> frequency and a small amplitude of signal. Stray capacitance and the
> effects of air on the cone will cause the impedance to vary with frequenc=
y.
> Even at a fixed frequency, the speaker is fundamentally a non-linear devi=
ce
> and so the impedance will change with amplitude and even during each cycl=
e.

Didn't realize that amplitude would change impedance.  Need to research=20
that more.


> However, I suspect that you could make a mathematical model of the speake=
r,
> including some of the non-linear effects, and then apply the audio signal
> to both the speaker AND (virtually, in software) to your model. You would
> then compare the predicted current (from the model) with the measured,
> actual current, and adjust the model parameters until they match within
> some tolerance. This setup is known as an "observer" because it ends up
> "observing" the hidden parameters of the model based on a history of the
> model behavior versus the real system behavior.
>
> Once you had your model,. you could then compute an estimated small-signa=
l
> impedance for any given frequency.
>
> One open question is whether all of the relevant model parameters are
> observable from the data available to you (applied voltage and measured
> current). I suspect that they are as long as the music contains a wide
> enough frequency and amplitude content.
>
> This would be a moderately-challenging DSP project. I've done similar
> things with electric motors (estimate parameters while the motor is
> running). For some parameters, it took only a few days to get things to
> work. For others it took months to make it work.
Wow... a lot more involved that I imagined.  The concern now (other than=20
the significant effort here) is that he tinkers with car audio a lot and=20
always changing things, so he wants to be able to swap speakers and have=20
this device show him impedance curves, and modeling each speaker is not=20
going simple for him to do.  And there will be many other speakers in=20
the car, which I expect will need to be disconnected.


>
> Sean
>
>
> On Thu, Mar 19, 2020 at 1:17 AM James Cameron <quozl@laptop.org> wrote:
>
>> Problem foreseen is that the impedance you measure will then depend on
>> a set of simultaneous frequencies rather than a signal of just one
>> frequency, so using the impedance meaningfully is limited.
>>
>> The other thing is that the starting position of the cone would
>> matter; a low frequency signal within the mix of signals could cause
>> the cone to be in a position that may beat with the sampling.  So
>> despite doing it rapidly you could have the low frequency signal
>> imposed on the results.
>>
>> On Thu, Mar 19, 2020 at 01:02:18AM -0400, Neil wrote:
>>> Hi,
>>>
>>> I'm working on a project for a friend, which will measure speaker
>>> impedance over a given frequency range.  I'm thinking I'd use a functio=
n
>>> generator sweeping across frequency range, then measure current (prob
>>> with a clamp sensor) and voltage across the speaker and do the math fro=
m
>>> there.  No prob so far.
>>>
>>> But can how about doing this dynamically?  IE: play music and
>>> dynamically figure out speaker impedance from there.  He believes
>>> commercial devices do this.  So far, all I can think of is measuring th=
e
>>> voltage and current as before, but dynamically FFT-ing both and
>>> (rapidly) doing the math from there.  Does this seem workable?
>>>
>>> Cheers,
>>> -Neil.
>>>
>>>
>>> --
>>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
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>> --
>> James Cameron
>> http://quozl.netrek.org/
>> --
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>>

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