Reply
 
Thread Tools Display Modes
  #1   Report Post  
Svante
 
Posts: n/a
Default Voice coil inductance

I have been reading older posts in this group regarding the inductance
of the voice coil, and I realize that the inductance is far from
ideal, that its inductance drops with frequency (in one way of looking
at it). I also read an article by Marshall Leach (JAES no 6 2002, p
442-) which describes this phenomenon quite nicely and presents a
model that I have succeded in fitting very well to a small number of
real speakers I have had at hand. I actually carved the cone and coil
away from one element and measured the impedance curve again, and
indeed the impedance was quite different. Apart from the obvoius (the
peak corresponding to the mechanical resonance disappeared) the
inductance now looked like an inductance normally would, ie with a 90
degree phase shift and a +6dB/oct slope above some frequency (for the
disected loudspeaker, nb).

This brings up the question what the manufacturers say in their data
sheets. They mostly specify an inductance. They rarely specify the
"lossiness" (the n value in the article above) of this inductance or
the frequency at which the inductance was measured.

So, what does the "voice coil inductance" figure that the
manufacturers supply us with mean, really?

  #2   Report Post  
Dick Pierce
 
Posts: n/a
Default Voice coil inductance

(Svante) wrote in message news:hO3db.435049$Oz4.241458@rwcrnsc54...
I have been reading older posts in this group regarding the inductance
of the voice coil, and I realize that the inductance is far from
ideal, that its inductance drops with frequency (in one way of looking
at it). I also read an article by Marshall Leach (JAES no 6 2002, p
442-) which describes this phenomenon quite nicely and presents a
model that I have succeded in fitting very well to a small number of
real speakers I have had at hand. I actually carved the cone and coil
away from one element and measured the impedance curve again, and
indeed the impedance was quite different. Apart from the obvoius (the
peak corresponding to the mechanical resonance disappeared) the
inductance now looked like an inductance normally would, ie with a 90
degree phase shift and a +6dB/oct slope above some frequency (for the
disected loudspeaker, nb).


First of all, your data is inconsistant with Leach's and also with
Lip****z and Vanderkooy's, as well as my own, where it is the
mechanism of lossy eddy current generation in the pole piece and other
moederatey conductive parts in proximity to the voice coil that is
responsible for the non-ideal behavior of the inductance.

The phenomenon we observe is that as we go higher in frequency, we
see an increase in eddy current coupling in the metal parts, which
have a fairly high ohmic loss. The result is a decrease in inductance
and an increase in resistance that pretty reliably follows a 1/sqrt(f)
for inductance and sqrt(f) for resistance.

Now, there are secondary and tertiary effects due to cone resonances
and the like that can be visible, but they tend to be fairly narrow-
band phenomenon that do little to modify the overall trend of the curve.

If you want to experiment to verify what's being clained, try the
following: take your "modified" driver and remove the voice coil entirely
from the magnet and measure the inductance. Here, you'll find almost
perfect agreement with a simple series RL circuit model. Now, place
the voice coil back in its proper position in the gap, and fill the
gap with epoxy, solidly locking it in place, so it cab't move at all,
then measure the impedance again. You'll see the very strong non-ideal
behavior, with no cone resonances or other mechanical phenomenon.

One thing that is often done is to put a thin copper cap over the pole
piece. This acts as a shorted turn and prevents the generation of the
eddy currents in the iron pole piece and you see a more ideal inductance
behavior. I did a similar albeit pointlessly heoic experiment where I
had a pole piece sliced with a thin slitting saw about 4-5 mm apart
across its face, and saw a similar improvement in the bahavior
(improvement in the sense that I no longer saw the 1/sqrt(f) dependency).

(No, really, the guy running the machine shop owed me the favor,
I had access to raw speaker parts and assembly facilities, and
had several assemblies where I could swap out the basket/cone
assembly in one piece from magnet to magnet with little effort!)

This brings up the question what the manufacturers say in their data
sheets. They mostly specify an inductance. They rarely specify the
"lossiness" (the n value in the article above) of this inductance or
the frequency at which the inductance was measured.


That's because, as mentioned above, the "lossiness" is due to a
common mechanism for all drivers. They all suffer from these eddy
current losses, they all use low-carbon mild steel pole pieces
and fron plates which have the same bulk electrical/magnetic
properties.

So, what does the "voice coil inductance" figure that the
manufacturers supply us with mean, really?


In many case, since they are unable or unwilling to specify the
inductance any further, they're pretty useless. In general, most
manufacturers measure the inductance at 1 kHz. If you assume the
sqrt(f) behavior due to eddy current losses, you'll find that
predicting that the inductance at 10 kHz is about 1/3 that at 1
kHz (for woofers or for any other driver where we are well above
mechanical resonance) gets you real close.
  #3   Report Post  
Svante
 
Posts: n/a
Default Voice coil inductance

(Dick Pierce) wrote in message ...
(Svante) wrote in message news:hO3db.435049$Oz4.241458@rwcrnsc54...
I have been reading older posts in this group regarding the inductance
of the voice coil, and I realize that the inductance is far from
ideal, that its inductance drops with frequency (in one way of looking
at it). I also read an article by Marshall Leach (JAES no 6 2002, p
442-) which describes this phenomenon quite nicely and presents a
model that I have succeded in fitting very well to a small number of
real speakers I have had at hand. I actually carved the cone and coil
away from one element and measured the impedance curve again, and
indeed the impedance was quite different. Apart from the obvoius (the
peak corresponding to the mechanical resonance disappeared) the
inductance now looked like an inductance normally would, ie with a 90
degree phase shift and a +6dB/oct slope above some frequency (for the
disected loudspeaker, nb).


First of all, your data is inconsistant with Leach's and also with
Lip****z and Vanderkooy's, as well as my own, where it is the
mechanism of lossy eddy current generation in the pole piece and other
moederatey conductive parts in proximity to the voice coil that is
responsible for the non-ideal behavior of the inductance.

The phenomenon we observe is that as we go higher in frequency, we
see an increase in eddy current coupling in the metal parts, which
have a fairly high ohmic loss. The result is a decrease in inductance
and an increase in resistance that pretty reliably follows a 1/sqrt(f)
for inductance and sqrt(f) for resistance.


Yes, that is what I see too, approximately. So, I don't think our data
are that different.

However, I describe what happened when I cut a loudspeaker element
apart, removing the coil and cone from the magnet. When I did that,
the coil behaved very much like an ordinary coil, so my point was that
there IS a huge difference between the inductance of the normally
mounted coil and the "free-air" coil.


Now, there are secondary and tertiary effects due to cone resonances
and the like that can be visible, but they tend to be fairly narrow-
band phenomenon that do little to modify the overall trend of the curve.

If you want to experiment to verify what's being clained, try the
following: take your "modified" driver and remove the voice coil entirely
from the magnet and measure the inductance. Here, you'll find almost
perfect agreement with a simple series RL circuit model.


That's what I did.

Now, place
the voice coil back in its proper position in the gap, and fill the
gap with epoxy, solidly locking it in place, so it cab't move at all,
then measure the impedance again. You'll see the very strong non-ideal
behavior, with no cone resonances or other mechanical phenomenon.


Haven't done that (have you? ;-) ), but I agree that that should
happen.

One thing that is often done is to put a thin copper cap over the pole
piece. This acts as a shorted turn and prevents the generation of the
eddy currents in the iron pole piece and you see a more ideal inductance
behavior. I did a similar albeit pointlessly heoic experiment where I
had a pole piece sliced with a thin slitting saw about 4-5 mm apart
across its face, and saw a similar improvement in the bahavior
(improvement in the sense that I no longer saw the 1/sqrt(f) dependency).

(No, really, the guy running the machine shop owed me the favor,
I had access to raw speaker parts and assembly facilities, and
had several assemblies where I could swap out the basket/cone
assembly in one piece from magnet to magnet with little effort!)

This brings up the question what the manufacturers say in their data
sheets. They mostly specify an inductance. They rarely specify the
"lossiness" (the n value in the article above) of this inductance or
the frequency at which the inductance was measured.


That's because, as mentioned above, the "lossiness" is due to a
common mechanism for all drivers. They all suffer from these eddy
current losses, they all use low-carbon mild steel pole pieces
and fron plates which have the same bulk electrical/magnetic
properties.

So, what does the "voice coil inductance" figure that the
manufacturers supply us with mean, really?


In many case, since they are unable or unwilling to specify the
inductance any further, they're pretty useless. In general, most
manufacturers measure the inductance at 1 kHz. If you assume the
sqrt(f) behavior due to eddy current losses, you'll find that
predicting that the inductance at 10 kHz is about 1/3 that at 1
kHz (for woofers or for any other driver where we are well above
mechanical resonance) gets you real close.


I certainly agree it is pretty useless. Unless they specify at which
frequency the inductance is measured, the impedance cannot be matched
to the sqrt(f) behaviuor, and the figures cannot be used. But of
course, such a figure may not help the sales, since the speaker looks
more "non-ideal"... ;-)

I think you answered my question in this paragraph; "1 kHz" was what I
was looking for. Have you seen this frequency in datasheets or how do
you know this?

Thank you for your elaborate answer, I have appreciated your earlier
postings (quite a few!) too.
  #4   Report Post  
Svante
 
Posts: n/a
Default Voice coil inductance

(Dick Pierce) wrote in message ...
(Svante) wrote in message news:hO3db.435049$Oz4.241458@rwcrnsc54...
First of all, your data is inconsistant with Leach's and also with
Lip****z and Vanderkooy's, as well as my own, where it is the
mechanism of lossy eddy current generation in the pole piece and other
moederatey conductive parts in proximity to the voice coil that is
responsible for the non-ideal behavior of the inductance.

The phenomenon we observe is that as we go higher in frequency, we
see an increase in eddy current coupling in the metal parts, which
have a fairly high ohmic loss. The result is a decrease in inductance
and an increase in resistance that pretty reliably follows a 1/sqrt(f)
for inductance and sqrt(f) for resistance.


Pardon me for responding twice, but I thought you may be interested in
seeing the actual curves. The curves are available at
http://www.tolvan.com/disected_lsp.jpg
As I said I observe the same as you do, the red line is from the
disected loudspeaker, without any iron or magnet present, the black
curve is the impedance of a normal element of the same brand and
model, and the green dashed line is modelled data with a "n" value of
0.67. The model also includes the mechanical resonance, as you may
see. I think we agree that there is a huge difference between the
black and the red curves.
The model fitting was done manually by adjusting the parameters of the
model until the curve fitted well. I think the match between the green
and the black line is is pretty good. (Don't look to carefully at the
phase towards high frequencies, my sound card makes the phase values a
bit too low there)

  #5   Report Post  
Dick Pierce
 
Posts: n/a
Default Voice coil inductance

(Svante) wrote in message news:n3Idb.619108$uu5.100398@sccrnsc04...
(Dick Pierce) wrote in message ...
(Svante) wrote in message news:hO3db.435049$Oz4.241458@rwcrnsc54...
First of all, your data is inconsistant with Leach's and also with
Lip****z and Vanderkooy's, as well as my own, where it is the
mechanism of lossy eddy current generation in the pole piece and other
moederatey conductive parts in proximity to the voice coil that is
responsible for the non-ideal behavior of the inductance.

The phenomenon we observe is that as we go higher in frequency, we
see an increase in eddy current coupling in the metal parts, which
have a fairly high ohmic loss. The result is a decrease in inductance
and an increase in resistance that pretty reliably follows a 1/sqrt(f)
for inductance and sqrt(f) for resistance.


Pardon me for responding twice, but I thought you may be interested in
seeing the actual curves. The curves are available at
http://www.tolvan.com/disected_lsp.jpg
As I said I observe the same as you do, the red line is from the
disected loudspeaker, without any iron or magnet present, the black
curve is the impedance of a normal element of the same brand and
model, and the green dashed line is modelled data with a "n" value of
0.67. The model also includes the mechanical resonance, as you may
see. I think we agree that there is a huge difference between the
black and the red curves.
The model fitting was done manually by adjusting the parameters of the
model until the curve fitted well. I think the match between the green
and the black line is is pretty good. (Don't look to carefully at the
phase towards high frequencies, my sound card makes the phase values a
bit too low there)


I must confess as to having misread your first post. For some reason,
in a quick reading that was interrupted by my ISP going south for a
week I somehow thought you were saying that removing the cone gave the
data, when you said removing the cone and coil from the magnet gave the
data (the "data" being the near-ideal LR behavior).

Now that I have read it more carefully, your data is entirely consistent
with the current knowledge of the behavior, including my own measurements
and models.


  #6   Report Post  
Svante
 
Posts: n/a
Default Voice coil inductance

(Dick Pierce) wrote in message ...
(Svante) wrote in message news:n3Idb.619108$uu5.100398@sccrnsc04...
(Dick Pierce) wrote in message ...
(Svante) wrote in message news:hO3db.435049$Oz4.241458@rwcrnsc54...
The phenomenon we observe is that as we go higher in frequency, we
see an increase in eddy current coupling in the metal parts, which
have a fairly high ohmic loss. The result is a decrease in inductance
and an increase in resistance that pretty reliably follows a 1/sqrt(f)
for inductance and sqrt(f) for resistance.


Pardon me for responding twice, but I thought you may be interested in
seeing the actual curves. The curves are available at
http://www.tolvan.com/disected_lsp.jpg
As I said I observe the same as you do, the red line is from the
disected loudspeaker, without any iron or magnet present, the black
curve is the impedance of a normal element of the same brand and
model, and the green dashed line is modelled data with a "n" value of
0.67. The model also includes the mechanical resonance, as you may
see. I think we agree that there is a huge difference between the
black and the red curves.


Now that I have read it more carefully, your data is entirely consistent
with the current knowledge of the behavior, including my own measurements
and models.


So, what about the 1 kHz? Have you actually seen in datasheets that
manufacturers specify that they use this frequency when they measure
the inductance? What about tweeters, shouldn't there be be a great
interference from the mechanical resonance at 1kHz?

The reason that I ask is that I am writing a simulation software, and
I would like to use the inductance figure that one can find in the
datasheets. This is why its usefulness is of interest for me.
Reply
Thread Tools
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Wiring 2 ohm DVC sub, 2 channel or Bridged Wild Weasel Car Audio 18 July 13th 04 04:53 PM
Replacement voice coil for Dynaudio tweeter?? JeffM. Car Audio 1 March 20th 04 08:58 PM
Question about Dual Voice Coil Subwoofer Sped General 1 January 4th 04 12:06 PM
connect dual voice coil to 2 channels Johan Wagener Car Audio 10 December 22nd 03 12:26 AM
Only use one voice coil in a DVC sub? Scott Gardner Car Audio 11 December 11th 03 07:24 PM


All times are GMT +1. The time now is 09:50 PM.

Powered by: vBulletin
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AudioBanter.com.
The comments are property of their posters.
 

About Us

"It's about Audio and hi-fi"