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.