Optimal Voice Coil Rest Position AN 1

Transcription

1 Optimal Voice Coil Rest Position 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 set from the perfect syetrical rest position in the magnetic field may produce unwanted signal distortion and generate a dynamic DC-displement, 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 asyetries. The optimal rest position may be found by measuring the syetry of the force ftor versus displement x. The large signal identification module (LSI) determines the (x) parameter dynamically by operating the driver under normal working conditions. In addition, the LSI results include data analysis tools to help assess the asyetry in the (x) curve and to find the amount of shift required x B to obtain the optimal voice coil rest position. Current Rest Position: Optimal Rest Position: magnet pole plate Direction of Voice Coil Shift magnet pole plate uniform magnetic field concentrated in the gap voice coil pole piece x=0 pole piece x=xb (x=0) < max Equal Length Coil (x=xb) = max CONTENTS: Measurement of the Large Signal Parameters... 2 Post Processing and Interpretation... 2 Examples... 3 More Information...

2 Measurement of the Nonlinear Force Ftor Requirements Procedure To measure the nonlinear charteristics of the force ftor, the following hardware and software is required: Hardware platform Distortion Analyzer (DA) Software module LSI installed within db-lab on the PC A driver stand or similar clamping Laser displement sensor (recoended) 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 cording to the requirements of your selected DUT. Use caution not to overload the DUT. To calibrate the displement axis to the highest precision, import the force ftor at the rest position (x=0) or the moving mass M MS from a previous LPM or other measurement. 3) Ensure that the DUT polarity and laser calibration are correct. 4) Start the measurement. ) Open the results windows (x) and Syetry Range. Post Processing and Interpretation (x) The force ftor is not a constant as assumed in linear modeling but varies with the voice coil displement x. Clearly, (x) decreases when the coil moves out of the gap. In addition, there are syetrical and asyetrical variations of the (x) curve. The asyetrical variations may be caused by an set in the voice coil s rest position or by an asyetry in the magnetic B field. In the case of a voice coil set, the asyetries can be fully compensated by shifting the voice coil into the optimal rest position. However, when a magnetic field asyetry exists, the asyetry can only be partially compensated with shift of the voice coil rest position. Finding the optimal voice coil shift (in ) can be tricky. For instance, the optimal voice coil shift is not always identical with the maximum in the (x) curve. A coil shift to the (x) maximum may help at smaller displements but will make things worse at larger displements. To assess the asyetry quantitatively and to find the optimal shift value, use the result window Syetry Range as described in the Examples section of this app note. Syetry Point The syetry point xsym in the asyetrical (x) curve is the centre point between two points having the same value for negative and positive displements x from the syetry point: 6,0 in 4,0 x a c max x a c (xsym x)+ x) = (xsym( x)- x) 3,0 xsym The displement x represents the 2,0 amplitude of sinusoidal signal generating the peak displement xsym(x)+ x and bottom displement 1,0 xsym(x)- x. The force ftor curve 0,0 would be perfectly syetrical if the -,0-2, 0,0 2,,0 syetry point (xsym(x) =const.) is constant for any amplitude x. In << Coil in X coil out >> general, the syetry point xsym(x) depends on the amplitude x as shown as the red Page 2 of

3 line in the lower diagram: A % X =0. A X % % A << in X coil out Coil >> A % X = A % % A << in X coil out Coil >> coil out coil out X X current rest position rest position - - syetry region xsym(x) syetry region coil in coil in Displement X Displement X Operating a transducer in the small signal domain where the amplitude AC signal is negligible the syetry point xsym(x 0) is identical with the location at maximum force ftor. However, the syetry point xsym(x xmax) measured in the large signal domain where the amplitude is close to the maximum displement xmax is more relevevant for loudspeaker diagnostics and should be used for compensating an set in the voice coil rest position. For example, the left diagram shows a syetry point xsym(x 0.) =3 deviating significantly from the current voice coil rest position x=0. However, the maximum is on the plateau region of the (x) where a constant number of windings is in the gap and the large deviation of the syetry point from the current rest position is caused by the B field asyetry and should not be compensated by shift of the voice coil rest position. In large large signal domain the syetry point xsym(x 6) =1.6 is much closer to the current position. Here the force ftor curve has steeper slopes because coil windings leave the gap for positive and negative displement. -Asyetry and Syetry Region The Asyetry is an important charteristic for finding the optimal voice coil rest position by considering the syetry point xsym and the steepness of the (x) curve. This Asyetry defined as A ( x, x ) x x x x x x x x 100% depends on virtual shift X of the coil and the amplitude displement x. If the Asyetry A(x,x ) < % than the set between current rest position and syetry point is negligible. This case is represented by a grey syetry region in the upper diagram. In the small signal domain (x 0.) the current rest position (X=0) is in the grey syetry region and no correction of the voice coil position is required. However, the Asyetry A(x,x ) exceeds the percent threshold at 2 amplitude of the displement. In the large signal domain (x 6 ) the syetry region is far away from the current rest position (X=0) and a voice coil shift inwards to syetry point xsym(x 6) =1.6 is recoended. Examples Equal-length Configuration An equal-length configuration is very sensitive to an set in the voice coil s rest position. In most of these cases, the impt from the magnetic field asyetries plays a secondary role. Page 3 of

4 For ce f t or ( X) ( 0 0 : 0 8 : 2 7 ) - X p r o t < X < X px rp o- t < X < X p + B l ( - X ) Syetry Point Syetry Range KLIPPEL 7 K L I P P E L B l [ N / A ] << Coil in Offset Coil out >> ok Shift coil ,0 0, 1,0 1, 2,0 2, 3,0 3, 4,0 4, Amplitude [] < < C o il in X [ m m ] c o il o u t > > As shown in the result window Force ftor (x), the overlay of the measured (x) curve (red solid line) with the derived (-x) curve (grey dotted line) mirrored at x=0 reveals the asyetry in the -charteristic. As shown in the result window Syetry Range, the shaded area is the range where the asyetry is below % as a function of displement amplitude (horizontal axis) and voice coil set x (vertical axis). The current rest position of the voice coil is indicated by the voice coil set zero reference (Y=0). The displement where the border of the shaded area crosses the zero reference in the voice coil set is an important value. It is the displement where (x) has decreased to 82 % of the static value, which also corresponds to a THD level of 10 %. As shown in this example, a displement working range of +/- 0.8 satisfies this condition. This is a very small displement for a woofer application. To increase the displement working range, while maintaining the same distortion tolerance, it is recoended to have the zero reference of the voice coil set x=0 located completely within the syetry range (shaded area). This can be complished by assessing the -syetry point xsym, which is the red dashed line in the result window BL Syetry Range. Ideally, xsym should coincide with the voice coil set zero reference. In this example, xsym is +0.6 and it is constant over the displement range from 0 < x <4.4. Therefore, a shift of the voice coil 0.6 in the positive direction (outwards) will completely compensate for the asyetry in the charteristic and improve the stability of the driver, thereby reducing the generation of DC displement and distortion. Overhang Configuration A large overhang of the voice coil gives more robustness against an set in the voice coil s rest position but is more sensitive to asyetries in the magnetic field. Force ftor (X) Syetry Range - Xpr ot < X < Xpr ot 17, KLI PPEL 1 Syetry Point KLIPPEL B l [ N / A ] 1, 0 12, 10, 0 7,, 0 << Coil in Offset Coil out >> ok Improve field syetry 2, X -asym 0,0 2,,0 7, 10,0 12, 1,0 17, Amplitude [] 0, X m < < C o il in [ m ] c o il o u t > > Page 4 of

5 In this example, an amplitude 6. of the AC displement or less will result in ceptable intermodulation distortion corresponding to an asyetry A(x,x ) < %. To increase the displement working range it is recoended to improve the magnetic field syetry in the gap. Shifting the voice coil rest inwards by 2 will partly compensate for the B field asyetry up to 1 amplitude but not at larger negative displements where the curve decays rapidly. Note: a FEM analysis will provide further information regarding the cause of the stray magnetic field. More Information Related Application Notes Related Specification Software References "Separating Spider and Surround", Application Note AN 2 Adjusting the Mechanical Suspension, Application Note AN3 Measurement of Peak Displement, Application Note AN4 LSI, S1 User Manual for KLIPPEL R&D SYSTEM. W. Klippel, Diagnosis and Remedy of Nonlinearities in Electro-dynamical Transducers, presented at the 109 th Convention of the Audio Engineering Society, Los Angeles, September 22-2, 2000, preprint 261. Find explanations for symbols at: Last updated: Page of