Motor Stability

3D-Distortion Measurement (DIS)

Measurement of


versus amplitude at 2fs

Driver Name:

 6 inch cross surround

Driver Comment:

 used as example in subjective and objective evaluation on website www.Klippel.de


 05. DIS: Motor Stability

Measurement Comment:



The movement of the  voice coil in a magnetic field can become unstable for excitation tones above the resonance frequency. The instability has the tendency to push the coil out of the gap. Using the DIS software module (3D distortion measurement) of the Klippel Analyzer System the most critical excitation frequency is determined in order to measure the corresponding dynamically generated DC displacement. Various ways for improving the stability of the driver are discussed.   

Instability of the Motor


The ‘Achilles' heel of the electro-dynamical transducer is the stability of the coil above the resonance frequency. We face this problem on many drivers using a short coil and a soft suspension. The natural decay of the force factor begins already at small positive and negative displacements if the overhang of the coil is low. A small coil offset or even some small disturbance produces a substantial asymmetry of the Bl(x) curve for the coil. Due to the phase relationship between current i and displacement x a new DC component will produced by the driving force F=Bl(x)*i . It increases the asymmetry of Bl(x) and pushes the coil more and more out of the gap. This unstable process continues until the suspension has produced a restoring force large enough to stop it.

Critical Frequency

The most critical excitation frequency fc =2*fs is about twice the resonance frequency. Here both the magnitude of current and displacement is high and the phase of the displacement is lagging by more than 90 degree behind the current.

Critical Ratio

The ratio between DC displacement and magnitude of the fundamental Displacement


is a critical measure for the stability of the driver. If the value of IDC < 10 % the driver is sufficient stable.


There are some ways to improve the stability of the driver

1)       1)       First of all we have to avoid any voice coil offset and have to reduce an asymmetrical geometry of the induction field B in the gap. The measurement of the large signal parameters in connection with a good FEM program for modeling the motor structure will lead to success eventually.

2)       2)       If we can not avoid the generation of a DC force we may reduce the generated DC displacement by using a suspension with a high stiffness. However, this remedy goes not to the root of the problem and will increase the resonance frequency of the driver. In many application this is not acceptable.

3)       3)       Reducing the creep factor of the suspension is a good way to have maximal stiffness at very low frequencies while having at the resonance frequency minimal stiffness.

4)       4)       Mounting the driver in a "sealed" enclosure will shift the critical frequency range to higher frequencies where the displacement is usually smaller.

Related Application Notes

Dynamic Generation of DC Displacement, Application Note AN 13, Klippel Analyzer System

Checking for Compliance Asymmetry, Application Note AN 14, Klippel Analyzer System


Method of Measurement


Loudspeaker Setup

The driver is mounted in the driver stand and the laser sensor is adjusted to the diaphragm. A dot of white ink may be used to increase the signal to noise ratio of the measured displacement signal.

Resonance frequency

If the resonance frequency fs of the driver is not known from the module Linear Parameter Measurement (LPM) than the fundamental response of the current is measured by using the 3D Distortion Measurement (DIS). The minimum of the current response shows the resonance frequency of the driver.


At the frequency fc a series of measurement with varied amplitude is performed to determine the DC and the fundamental component of the displacement and to provide the ratio IDC.


Using the 3D Distortion Measurement (DIS)


The following hardware and software is required for assessing Xmax

·          ·          Distortion Analyzer 1 + PC

·          ·          Software module 3D Distortion Measurement (DIS) + dB-Lab

·          ·          Laser sensor head and laser controller


Use new DIS operation based on the operation template "DIS Motor Stability"


Displacement Signal


After pausing the measurement the result window Y2(t) shows the displacement versus time. The peak and bottom value of the displacement shows DC offset of the coil.

Input signal Y2(t) vs time



The 2D-plot shows the displacement versus frequency at specified amplitudes on linear axes to show the compression effect. Only at sufficiently low amplitudes there is a linear relationship between input and output signals.

Fundamental component



  DC Component

The figure below shows the dc component in voice coil displacement for varied voltage U  of the sinusoidal excitation tone.

DC component

Voice Coil Temperature


The figure below shows the instantaneous voice coil temperature during the measurement.

Increase of voice coil temperature Delta Tv




This report is generated by the report generator in the frame software dB-Lab in the Klippel Analyzer System. Using an existing report as template, the old data are replaced  by the results of the current measurement automatically.










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