Its been done. Even commercially.
https://reverb.com/item/1041525-servodrive-contrabass-14hz-115db-subwoofer-carpet-black
https://reverb.com/item/1041525-servodrive-contrabass-14hz-115db-subwoofer-carpet-black
I'm aware it's been done for subwoofers. In addition that design seems a bit ridiculous. Why use a motor and not an actuator? I've spoken with the owner of Rythmik Audio extensively. I should be more specific the interest applies to drivers which can play at various frequency bands or full range, not just subwoofers. The advantage is simple -- greater and in fact direct control over the diaphragm if pistonic from a given source signal -- unlike a traditional voice coil diaphragm magnet which is a less controlled design. Similarly, a direct relationship of voltage gain to output, rather than an electromagnetic correlation seems superior with less margin for loss. It's in some ways similar to an electrostatic signal path, though instead of a capacitive diaphragm, the diaphragm itself would not factor into the circuit electrically.
Perhaps magnetic voice coils are more robust? I don't know the limits of actuators as related to friction though I wonder if they are able to withstand longterm use at high frequency and high volume output.
Perhaps magnetic voice coils are more robust? I don't know the limits of actuators as related to friction though I wonder if they are able to withstand longterm use at high frequency and high volume output.
"Actuator" doesn't tell us what kind of actuator you are talking about or whether it's remotely feasible. There are all kinds of actuators, most of which do not run at remotely relevant frequencies for audio.
How much moving mass is involved? How does that affect high frequencies and transient abilities?
Do you have any evidence of poor diaphragm control in good voice-coil based drivers?
Without specific information, we're just reading between the lines and speculating.
How much moving mass is involved? How does that affect high frequencies and transient abilities?
Do you have any evidence of poor diaphragm control in good voice-coil based drivers?
Without specific information, we're just reading between the lines and speculating.
A linear actuator is what I had in mind though I suppose a rotary actuator or servo motor could yield interesting results with some creative thinking. I assume that is the case with most actuators -- they aren't built to withstand operating at audiorate speeds. No clue about moving mass at this point though I presume this is less of a constraint compared with magnets given the limitations of flux density and maxwell strength correlated with cost and size. I don't have evidence of a poor diaphragm control though I understand traditionally voicecoils excite a diaphragm and a wave travels up the diaphragm from the voice coil simultaneously with some degree of excursion at the surround. Contemporary drivers (eg. purifi midbasee drivers, seas aluminum woofers, carbon fiber drivers, etc.) overlook this original design and are fully pistonic due to their rigid diaphragm reducing harmonic distortion though perhaps at the cost of transient response. I figure if possible feasible... if one is designing a pistonic style driver, why not opt for an electronic servo motor of some variety which should yield greater or in theory absolute control over the diaphragm (similar to an electrostatic driver) rather than a magnetic voicecoil which yields an approximation of control. That said I have no clue of such an implementation is feasible hence my asking.
In addition to the previous thoughts, I wonder if todays digital stepper motors would allow for new and more efficient digital audio signal paths from source to driver -- perhaps a digital signal could drive the driver without the need for analog conversion and traditional analog amplification? This is what I was envisioning in the earlier post regarding greater control, without being able to articulate exactly why or how.
here is some discussion: https://www.diyaudio.com/community/threads/very-long-throw-drivers.330366/
a non voice coil actuator (E.G linear motor) would be advantageous for high excursion drivers. Low excursion drivers have the majority of the voice coil sitting in a strong magnetic field (1-2.2T dependent on driver type/quality) whereas for subwoofers the majority of the coil is above and below the top plate and so not subject to the peak magnetic field. Increasing the excursion capability involves making the coil longer which reduces the percentage of the coil in the peak field region, reducing efficiency. A linear stepper motor would allow arbitrary excursion from the motor without efficiency reduction, if the motor was of the moving magnet type heatsinking the coils would also be easier. There are however a number of engineering issues:
1) this motor requires an accurate absolute high speed positioning sensor, these are available but expensive. (or some combination of hall effect and relative)
2) there are sliding parts (unless you want the additional complication of magnetic levitation as is done by semiconductor positioning stages), this results in wear and noise.
3) Surrounds are only capable of excursions around what current subs manage as soon as you exceed this you will need new surround tech
4) cogging!
5) instantaneous force demand can be very high due to the moving mass and acceleration required
a non voice coil actuator (E.G linear motor) would be advantageous for high excursion drivers. Low excursion drivers have the majority of the voice coil sitting in a strong magnetic field (1-2.2T dependent on driver type/quality) whereas for subwoofers the majority of the coil is above and below the top plate and so not subject to the peak magnetic field. Increasing the excursion capability involves making the coil longer which reduces the percentage of the coil in the peak field region, reducing efficiency. A linear stepper motor would allow arbitrary excursion from the motor without efficiency reduction, if the motor was of the moving magnet type heatsinking the coils would also be easier. There are however a number of engineering issues:
1) this motor requires an accurate absolute high speed positioning sensor, these are available but expensive. (or some combination of hall effect and relative)
2) there are sliding parts (unless you want the additional complication of magnetic levitation as is done by semiconductor positioning stages), this results in wear and noise.
3) Surrounds are only capable of excursions around what current subs manage as soon as you exceed this you will need new surround tech
4) cogging!
5) instantaneous force demand can be very high due to the moving mass and acceleration required