Measuring Loudspeaker Performance with VoiceCoil Offset
Enables fine-tuning of the voice coil position for optimum performance

Measuring loudspeaker performance related to voice coil offset (the amount that a voice coil is displaced from its resting position by a small DC or low frequency applied voltage) is useful for loudspeaker designers. It enables fine-tuning of the voice coil position for optimum performance, and also enables quantification of performance under offset voltages that may be applied by other components in the system such as a power amplifier.
This sequence tests the speaker under a range of 5 positive and negative offset voltages, and runs a complete loudspeaker test at each. The final display shows the comparison of five different offset levels for 2nd Harmonic (H2), 3rd Harmonic (H3) and Impedance. The sequence can easily be modified to display different results and to change the applied offset voltages. It can also be used for microspeakers, but will need to be modified so that offset voltages are in the mV range.
The relationship between Voice coil offset and DC Input Voltage can be determined through use of a mechanical displacement gauge or optical “contact-less” method. When using a mechanical displacement gauge, the offset in mm vs DCV should be measured after determining the optimal voice coil rest position from the sequence results.
For our example measurement a Tang Band model W4-654S loudspeaker was used. In this case, for each volt of DC Offset a 1 mm change in displacement occurred. Other loudspeakers may require less DC Offset voltage and smaller increments.
Hardware
- SCM-3 (or other reference microphone) (Listen p/n 4002)
- SC Amp audio test amplifier or equivalent (Listen p/n 4060)
- Listen SoundConnect 2 microphoone power supply (Listen p/n 4024/4025)
- DC Connect or other variable DC supply (Listen p/n 4030)
Software
- SoundCheck version 13 or higher
- 2000 – Limits
- 2001 – Harmonic Distortion
- 2018 – Stepped Sine
Sequence Operation
- You need to enter the driver model number in the Lot Number field.
- When running the sequence, you are prompted to enter the stimulus level for the sweep.
- The Auto Save and Print to Word steps use Lot Name and Stimulus Level to create the file name. This can be modified as needed.
The sequence is setup to run DC offset steps starting at -2 VDC and increasing to 2 VDC in 1 VDC increments. The sequence can be updated to run in smaller voltage steps by adding additional DC Connect Stimulus and Limit Steps as shown in Steps 3 through 12.
Note: Microspeakers will require a smaller DC offset voltage range, in smaller voltage steps. The sequence can be modified by changing the individual DC Connect Voltage steps.
Step Jumps – Conditional Branching
Loop Start Point is set to jump to the last DC Connect Stimulus Step, #34. This starts the increment count of the VDC value.
Step 34, DC Connect Reset, jumps back to the first VDC Limit Step, # 3 (VDC -2). The VDC value is checked in each Limit Step until a Pass state occurs and allows the following DC Connect Stimulus Step to run.
Each DC Connect Stimulus Step (Steps 4, 6, 8, 10) is configured to jump to the Measurement Section after the new DC Offset level is applied to the loudspeaker. Step 12 does not need to be configured.
See Sequence Logic in documentation.
Measurement Section
This part of the sequence was built using the Complete Test Sequence as a base. All of the Complete Test settings are left intact so that you can choose what information to keep for the final display.
The settings in the limit steps are arbitrary since the sequence is not driver specific. Useful limits will need to be set around a specific device.
The display shows the comparison of five different offset levels for 2nd Harmonic (H2), 3rd Harmonic (H3) and Impedance.
See Example Report below.
Download Sequence (zip file)
Download Sequence documentation (PDF)
To run the sequence in SoundCheck, please save the zip file to your hard drive, then follow the instructions in ‘Sequence Editor: Exporting/Importing a Sequence’ section of the manual.