Optimal Voice Coil Rest Position
AN 1
Application Note to the KLIPPEL R&D SYSTEM - Revision 1.2
The location of the voice coil in the magnetic gap is a very critical parameter of dynamic
transducers used in loudspeakers, shakers, headphones, etc. An offset from the perfect
symmetrical rest position in the magnetic field may produce unwanted signal distortion and
generate a dynamic DC-displacement, which degrades the stability of the driver by moving the
coil’s rest position towards the gap edges. As a solution, shifting the voice coil into the optimal rest
position in the magnetic field may fully or partially compensate for the asymmetries. The optimal
rest position may be found by measuring the symmetry of the force factor Bl versus displacement
x. The large signal identification module (LSI) determines the Bl(x) parameter dynamically by
operating the driver under normal working conditions. In addition, the LSI results include data
analysis tools to help assess the asymmetry in the Bl(x) curve and to find the amount of shift
required xB to obtain the optimal voice coil rest position.
Current Rest Position:
pole plate
magnet
Optimal Rest Position:
magnet
pole plate
uniform
magnetic field
concentrated
in the gap
Direction of Voice
Coil Shift
voice coil
pole piece
pole piece
x=0
Bl(x=0) < max
Equal Length Coil
x=xB
Bl(x=xB) = max
CONTENTS:
Measurement of the Large Signal Parameters ....................................................................................................... 2
Post Processing and Interpretation......................................................................................................................... 2
Examples ................................................................................................................................................................. 3
More Information ................................................................................................................................................... 5
Optimal Voice Coil Position
AN 1
Measurement of the Nonlinear Force Factor
Requirements
To measure the nonlinear characteristics of the force factor, the following hardware and
software is required:




Procedure
Hardware platform Distortion Analyzer (DA)
Software module LSI installed within dB-Lab on the PC
A driver stand or similar clamping
Laser displacement sensor (recommended)
1) Operate the DUT in free air (or in a box).
2) Create a new object Driver and add a new LSI operation based on the Default
template. Adjust the measurement set up parameters according to the
requirements of your selected DUT. Use caution not to overload the DUT. To
calibrate the displacement axis to the highest precision, import the force factor at the rest
position Bl(x=0) or the moving mass MMS from a previous LPM or other measurement.
3) Ensure that the DUT polarity and laser calibration are correct.
4) Start the measurement.
5) Open the results windows Bl(x) and Bl Symmetry Range.
Post Processing and Interpretation
Bl(x)
Symmetry
Point
The force factor Bl is not a constant as assumed in linear modeling but varies with the
voice coil displacement x. Clearly, Bl(x) decreases when the coil moves out of the gap. In
addition, there are symmetrical and asymmetrical variations of the Bl(x) curve. The
asymmetrical variations may be caused by an offset in the voice coil‘s rest position or by
an asymmetry in the magnetic B field. In the case of a voice coil offset, the asymmetries
can be fully compensated by shifting the voice coil into the optimal rest position.
However, when a magnetic field asymmetry exists, the asymmetry can only be partially
compensated with shift of the voice coil rest position. Finding the optimal voice coil shift
(in mm) can be tricky. For instance, the optimal voice coil shift is not always identical
with the maximum in the Bl(x) curve. A coil shift to the Bl(x) maximum may help at
smaller displacements but will make things worse at larger displacements. To assess the
asymmetry quantitatively and to find the optimal shift value, use the result window Bl
Symmetry Range as described in the “Examples” section of this app note.
The symmetry point xsym
in the
asymmetrical Bl(x) curve is the centre
point between two points having the
same Bl value for negative and positive
displacements xac from the symmetry
point:
Bl(xsym xac)+ xac) = Bl(xsym( xac)- xac)
6,0
Blmax
Bl
in
N/A
xa
xa
c
c
4,0
3,0
xsym
The displacement xac represents the
2,0
amplitude
of
sinusoidal
signal
generating the peak displacement
1,0
xsym(xac)+ xac and bottom displacement
xsym(xac)- xac. The force factor curve
0,0
would be perfectly symmetrical if the
-5,0
-2,5
0,0
2,5
5,0
symmetry point (xsym(xac) =const.) is
<< Coil in
X mm
coil out >>
constant for any amplitude xac. In
general, the symmetry point xsym(xac) depends on the amplitude xac as shown as the red
Page 2 of 5
Application Note KLIPPEL R&D SYSTEM
Optimal Voice Coil Position
AN 1
line in the lower diagram:
Xoff
5.0
5.0
Xac =0.5 mm
N/A
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
ABl  5 %
-7.5
-5.0
<< Coil in
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
ABl  5%
ABl  5%
-2.5
Xac = 5 mm
N/A
0.0
2.5
X
mm
5.0
7.5
ABl  5%
-7.5
coil out >>
ABl  5%
ABl  5 %
-5.0
<< Coil in
-2.5
0.0
2.5
X
mm
5.0
7.5
coil out >>
coil out
coil out
7.5
7.5
Xoff
Xoff
mm
mm
2.5
2.5
current rest position
rest position
0.0
0.0
symmetry region
-2.5
-2.5
xsym(xac)
symmetry region
-5.0
-5.0
-7.5
-7.5
coil in
coil in
0
0
1
1
2
2
3
4
5
6
3
4
5
6
Displacement Xac mm
Displacement Xac mm
7
7
8
8
Operating a transducer in the small signal domain where the amplitude AC signal is
negligible the symmetry point xsym(xac≈ 0) is identical with the location at maximum
force factor. However, the symmetry point xsym(xac≈xmax) measured in the large signal
domain where the amplitude is close to the maximum displacement xmax is more
relevevant for loudspeaker diagnostics and should be used for compensating an offset
in the voice coil rest position. For example, the left diagram shows a symmetry point
xsym(xac≈0.5mm) =3 mm deviating significantly from the current voice coil rest position
x=0. However, the maximum is on the plateau region of the Bl(x) where a constant
number of windings is in the gap and the large deviation of the symmetry point from
the current rest position is caused by the B field asymmetry and should not be
compensated by shift of the voice coil rest position. In large large signal domain the
symmetry point xsym(xac≈6mm) =1.6 mm is much closer to the current position. Here the
force factor curve has steeper slopes because coil windings leave the gap for positive
and negative displacement.
The “Bl Asymmetry” is an important characteristic for finding the optimal voice coil rest
Bl-Asymmetry
position by considering the symmetry point xsym and the steepness of the Bl(x) curve.
and Symmetry
This “Bl Asymmetry” defined as
Region
Bl x  x   Bl x  x 
A (x , x ) 
100%
Bl x  x   Bl x  x 
Bl
ac
off
ac
off
ac
off
ac
off
ac
off
depends on virtual shift Xoff of the coil and the amplitude displacement xac. If the Bl
Asymmetry |ABl(xoff ,xac )| < 5% than the offset between current rest position and
symmetry point is negligible. This case is represented by a grey symmetry region in the
upper diagram. In the small signal domain (xac≈0.5mm) the current rest position (Xoff=0)
is in the grey symmetry region and no correction of the voice coil position is required.
However, the Bl Asymmetry |ABl(xoff ,xac ) exceeds the 5 percent threshold at 2 mm
amplitude of the displacement. In the large signal domain (xac≈6 mm) the symmetry
region is far away from the current rest position (Xoff=0) and a voice coil shift inwards to
symmetry point xsym(xac≈6mm) =1.6 mm is recommended.
Examples
An equal-length configuration is very sensitive to an offset in the voice coil’s rest position.
Equal-length In most of these cases, the impact from the magnetic field asymmetries plays a secondary
Configuration role.
Page 3 of 5
Application Note KLIPPEL R&D SYSTEM
AN 1
Optimal Voice Coil Position
For ce f act or
Bl Symmetry Range
Bl ( X)
( 00: 08: 27)
- Xp r o t
<
X
<
Xp
X rp o
- t <
X
<
Xp +
5
Bl ( - X)
KLIPPEL
KL I PPEL
7
Symmetry Point
4
3
6
Coil out >>
2
4
Offset
B l [ N/ A ]
5
3
1
-0
Shift
coil
0.6
mm
-1
<< Coil in
-2
2
ok
-3
-4
1
-5
0,0
0
- 5
- 4
- 3
- 2
C o il in
- 1
X
0
[ m
1
m
2
3
4
0,5
1,0
1,5
2,0
2,5
3,0
Amplitude [mm]
5
]
c o il o u t
3,5
4,0
4,5
> >
As shown in the result window Force factor Bl(x), the overlay of the measured Bl(x) curve
(red solid line) with the derived Bl(-x) curve (grey dotted line) mirrored at x=0 reveals the
asymmetry
in
the
Bl-characteristic.
As shown in the result window Bl Symmetry Range, the shaded area is the range where
the Bl asymmetry is below 5% as a function of displacement amplitude (horizontal axis)
and voice coil offset xoff (vertical axis). The current rest position of the voice coil is
indicated by the voice coil offset zero reference (Y=0).
The displacement where the border of the shaded area crosses the zero reference in the
voice coil offset is an important value. It is the displacement where Bl(x) has decreased to
82 % of the static Bl value, which also corresponds to a THD level of 10 %. As shown in this
example, a displacement working range of +/- 0.8 mm satisfies this condition. This is a
very small displacement for a woofer application. To increase the displacement working
range, while maintaining the same distortion tolerance, it is recommended to have the
zero reference of the voice coil offset xoff=0 located completely within the Bl symmetry
range (shaded area).
This can be accomplished by assessing the Bl-symmetry point xsym, which is the red dashed
line in the result window BL Symmetry Range. Ideally, xsym should coincide with the voice
coil offset zero reference. In this example, xsym is +0.6 mm and it is constant over the
displacement range from 0 < xac <4.4 mm. Therefore, a shift of the voice coil 0.6 mm in the
positive direction (outwards) will completely compensate for the asymmetry in the Blcharacteristic and improve the stability of the driver, thereby reducing the generation of
DC displacement and distortion.
A large overhang of the voice coil gives more robustness against an offset in the voice
Overhang
Configuration coil‘s rest position but is more sensitive to asymmetries in the magnetic field.
Force f act or Bl (X)
- Xpr ot
KLIPPEL
KLI PPEL
15
10
12, 5
5
Coil out >>
15, 0
10, 0
Offset
[ N / A ]
Bl Symmetry Range
Symmetry Point
< X < Xpr ot
17, 5
B l
7, 5
Improve field
symmetry
ok
0
-5
-10
<< Coil in
< <
5, 0
-15
XBl-asym
2, 5
0,0
2,5
5,0
7,5
10,0
Amplitude [mm]
0, 0
- 15 - 10
< <
C o il
in
Page 4 of 5
- 5
X
0
[
m
5
m
]
10
12,5
15,0
17,5
15
c o il
o u t
> >
Application Note KLIPPEL R&D SYSTEM
Optimal Voice Coil Position
AN 1
In this example, an amplitude 6.5 mm of the AC displacement or less will result in
acceptable Bl intermodulation distortion corresponding to an Bl asymmetry |ABl(xoff ,xac )|
< 5 % . To increase the displacement working range it is recommended to improve the
magnetic field symmetry in the gap. Shifting the voice coil rest inwards by 2 mm will partly
compensate for the B field asymmetry up to 15 mm amplitude but not at larger negative
displacements where the Bl curve decays rapidly.
Note: a FEM analysis will provide further information regarding the cause of the stray
magnetic field.
More Information
Related
Application
Notes
"Separating Spider and Surround", Application Note AN 2
“Adjusting the Mechanical Suspension”, Application Note AN3
“Measurement of Peak Displacement“, Application Note AN4
Related
Specification
“LSI”, S1
Software
User Manual for KLIPPEL R&D SYSTEM.
References
W. Klippel, “Diagnosis and Remedy of Nonlinearities in Electro-dynamical
Transducers,” presented at the 109th Convention of the Audio Engineering Society, Los
Angeles, September 22-25, 2000, preprint 5261.
Find explanations for symbols at:
http://www.klippel.de/know-how/literature.html
Last updated: 08.01.16
Page 5 of 5
Application Note KLIPPEL R&D SYSTEM