Loudspeaker operation: The superiority of current drive over voltage drive (Part 2)
Note: This is Part 2 (see Part 1) of an overview of the destructive effects that voltage drive has on the performance of electrodynamic loudspeakers . A more comprehensive treatment of the subject can be found in the book Current-Driving of Loudspeakers: Eliminating Major Distortion and Interference Effects by the Physically Correct Operation Method by Esa Meriläinen.
Microphone action of the voice coil
A moving-coil drive unit is a bidirectional device that functions as a microphone all the time, whether it is wanted or not. Thus, all the noise that is generated inside the speaker cabinet, and much of which typically passes out through the woofer cone, is picked up by the voice coil as a microphone EMF. This cabinet noise EMF summates to the total motional EMF voltage, Em, accordingly modulating the flow of current and hence the very drive signal of a voltage-driven speaker. Thus, there is established a poor-quality feedback loop that circulates an even poorer quality reverberation signal.
The effect can be investigated with a cabinet housing two identical drivers; one is used as the speaker and the other as the microphone EMF indicator.
At lower mid-frequencies, the magnitude of this cabinet noise EMF component is, even in a low-sensitivity hi-fi driver, typically several percent of the driver's terminal voltage and increases with increasing sensitivity, so that even at values below 95 dB/W the proportion of this noise EMF is well beyond 10 percent of the terminal voltage. This also helps explain why PA speakers sound as they do; but even in domestic equipment the magnitude of the interference is simply unacceptable.
Cabinet-noise passage through the cone at lower mid-frequencies is in itself a serious yet widely neglected problem in all contemporary enclosed speaker designs. It can only be addressed properly by stuffing the cabinet tightly with relatively heavy damping material, such as cotton cloth, but for vented designs this is not possible. (Note: the electrical damping is not of any bearing here, as we are dealing with frequencies above the resonance region.)
It can be quite easily inferred that in a typical middle-sized hi-fi loudspeaker with a 6.5-inch woofer and even with comparably good damping, the magnitude of the cabinet noise penetrating the cone is in the 300-Hz region less than 20 dB below the directly radiated sound's magnitude. However, by heavy and careful stuffing it is possible to improve this figure to around 40 dB.
Voltage-driven tweeters are also subject to similar microphone EMF feedback interference due to their back cavity reflections. Only the frequencies affected are about 20-fold that of woofers.
Outward microphone interference
Microphone coupling also occurs outwardly between adjacent drivers, the induced disturbance EMF being roughly inversely proportional to frequency. The magnitude of this effect is lower than that of the inward coupling but extends higher in frequency.
In coaxial drivers, quite popular today, microphone coupling between the low- and high-frequency units is also remarkable. The magnitude of this disturbance EMF depends, again, on the sensitivity properties of the interacting units but additionally is also strongly modulated by the cone position.
Indefinite EMF generation due to mechanical non-idealities
Extraneous and indefinite EMFs are also generated by the driver itself, without external pressure. All these EMFs also sum to the total motional EMF, Em, and are reflected as such to the current of a voltage-driven speaker.
At least the following non-idealities are able to introduce in the voice coil parasitic mechanical vibrations, regardless of signal level:
- Reflections returning from the cone rim.
- In a dome diaphragm, the returning of the mechanical wave back to the coil former joint
- Loose mass and reflection effects of the cone's inner suspension
- Modification of the effective mass due to waving and disconnection of the diaphragm.
- Bell modes developing in the cone at certain frequencies, causing the diaphragm to deform and divide into sectors that vibrate in different phases.
- Air currents through a perforated coil former and through the air gap of the magnetic circuit
- Stirring of ferrofluid around the voice coil
- Flexing of the voice coil adhesives and coil former. At the high end of the frequency range, cone travels are so tiny that even a slight compliance in the glue layers can introduce response alterations and even hysteresis.
- Air loading required for acoustic radiation. Especially on the part of backward radiation, this loading may include vague attributes.
These interferences are of course harmful in themselves, regardless of the driving mode, but in voltage drive the damage becomes yet circulated in the upcoming signal.