Distortion comparison: a 3.5" midrange unit
In the following are some comparisons taken from quite a typical 3.5" cone midrange driver that has been used in Infinity Reference series speakers. The unit has a conventional ferrite magnet motor structure (with double magnet) and is 6 Ω in impedance.
Below is an amplitude spectrum plot of the modulation distortion that occurs in the V/I conversion of the driver. The test signal consists of two sine waves at frequencies 450 Hz (f1) and 3.75 kHz (f2) with voltage proportion 5:1. (Due to impedance variation, the proportion of currents differs from this a little.) The signal level was set for 2.83 Vrms which yields peak values of 4.7 V.
It is seen that the 3750 Hz tone current becomes quite badly distorted in the presence of the 450 Hz tone. The modulation product f2-2f1 (2850 Hz) alone is 1.9% (-34.5 dB) in magnitude with respect to the original 3750 Hz tone. In addition to f2+2f1 (4650 Hz), the products 2f2-3f1 (6150 Hz), 2f2-f1 (7050 Hz), and 2f2+f1 (7950 Hz) are also very significant.
The corresponding acoustic distortion spectrum on voltage drive is seen below. The modulation components match well to the current distortion above, indicating that the V/I conversion indeed is the principal source of nonlinearity.
The most material of these components are the f2-2f1 (2850 Hz), 2f2-f1 (7050 Hz), and 2f2+f1 (7950 Hz), while some of the lower peaks may also be audible. By calculating the power sum of all the significant components (excluding the harmonics), the total modulation distortion of the original 3.75 kHz tone is 3.5%.
The corresponding result on current-drive is seen above. Most of the modulation products are reduced by at least 13 dB; and the highest one at 2850 Hz by 17 dB. By calculating the sum as before, we get as the total modulation distortion 0.79%, that is, an improvement by a factor of 4.4.
Below are the results on 2nd and 3rd harmonic distortions (H2 and H3 respectively) in the useful range of the driver, taken at 2.83 Vrms level. The test has been carried out point by point, taking care that the signal level remained the same in both modes. The reddish line denotes voltage drive and the blue current-drive.
Below 1000 Hz, the 2nd harmonic is reduced remarkably on current-drive. At high mids, the level of this harmonic is low enough in both operation modes. The main improvement is reaped, however, in the 3rd harmonic that drops at frequencies above 700 Hz by 10-15 dB.
At middle frequencies, a -50 dB level (0.3%) of 3rd harmonic distortion is still audible (i.e. not masked by the fundamental). Thus, the benefit especially at and above 1000 Hz is substantial.