Current drive of speakers and
2. Measurement of distortion of speaker current
3. Measurement of speaker distortion with voltage and current drive
Electrodynamic transducer – speaker is
designed to be driven from ideal voltage source, to get most flat
amplitude response at acoustical side.
But when we look at lumped parameters model of the speaker, Fig. 1, we can see that elements at electrical side are connected in series and they share the same driving current.
Because values of the elements of the model are nonlinear with membrane displacement (Le, R1, L1, Bl and Cms) and with temperature (Re), even with ideal voltage source drive the current that flows through the speaker voice coil is distorted. This introduces inherent nonlinearity into the speaker driving signal, the speaker current. We can deduce a suggestion then, to use a linear speaker current as a driving signal. We will need an ideal current source, better say ideal V/I converter with voltage input and current as an output signal. But if the speaker is driven from ideal current source, then its amplitude frequency response will copy an inversion of speaker impedance. Thus we need to pre-filter the input signal for the V/I converter in order to get flat amplitude response from the speaker.
This pre-filter would have frequency
response that is an inversion of speaker impedance frequency
The principle of the V/I converter is shown in the Fig. 3.
continue - measurement of distortion of speaker current
Pavel Macura, 01/2015