Current drive of speakers and
2. Measurement of distortion of speaker current
3. Measurement of speaker distortion with voltage and current drive
4. Measurement of tweeters and midrange speaker
3. Measurement of speaker distortion under voltage and current drive
3.1. Speaker with vented port
After we have seen that there is much higher distortion of speaker current with voltage drive, compared to distortion of the driving voltage, it is logical to measure with microphone at acoustical side of the speaker, to confirm or disprove an assumption that linear current drive should result in decreased speaker distortion. The amplifier was prepared that is easy to be switched between voltage and current mode.
Distortion test was performed at speaker terminals voltage 2Vrms. The voltage was checked before every measurement. 3-way vented port speaker was measured, at 30Hz, 40Hz, 61Hz and 100Hz. Impedance response of the speaker can be seen in the next image.
First, harmonic distortion of the amplifier used was tested, in a current mode, voltage mode, at all frequencies and always with 2Vrms at speaker terminals. THD of the amplifier was between 0.001% and 0.006%, depending on driving mode and frequency. Harmonic components had fast decay and all of them were far below speaker distortion.
The measurement was performed with ECM8000 microphone placed in 9cm distance from the woofer membrane. All measurements were repeated to check their credibility. Measurements in higher distance were also done to exclude influence of microphone distortion.
At 30Hz, 100Hz and 200Hz the speaker distortion was also same with voltage drive and current drive, with the vented box. Ratio of individual harmonics was the only difference. At 40Hz, distortion with voltage drive was lower, 0.887% compared to 1.30% with the current drive. At 61Hz, at resonance, distortion with the current drive was lower, 0.292% compared to 0.545% obtained with voltage drive. These were the biggest differences measured with the vented box.
40Hz - voltage drive
40Hz - current drive
61Hz - voltage drive
61Hz - current drive
200Hz - voltage drive
200Hz - current drive
3.2. Measurements with closed port
Further set of measurements was done with closed bassreflex port. This has changed the impedance response, see below. With closed port, the amplitude of the speaker membrane displacement is higher, so there is an assumption of possible higher impact of the linear current drive.
40Hz voltage drive
40Hz current drive
61Hz voltage drive
61Hz current drive
80Hz voltage drive
80Hz current drive
We can see that with closed port, distortion was decreased at all frequencies of measurements, 40Hz, 61Hz and 80Hz. At 100Hz, both methods have shown the same distortion.
The benefit of the current drive can be seen at higher amplitude of membrane displacement, when the deviation from linearity of the speaker parameters is higher.
continue - measurement of tweeters and midrange speaker
Pavel Macura, 01/2015