"...supposed to be more linear (lower distortions), especially for big excursions."
I do not agree that underhung motors are more linear for large excursions as a blanket statement; imo the situation is more complicated than that. Yes they are somewhat more linear as x-max is approached, but typically underhung motors have less linear excursion (x-max) than overhung motors, so for a given SPL it depends on the specifics. And when x-max is exceeded with an underhung motor, it doesn’t take much overshoot for a large portion of that short coil to exit the gap, which causes distortion to rise rapidly.
An underhung motor can achieve higher efficiency because the moving mass is usually reduced by the voice coil being shorter and therefore lighter.
On the other hand that short voice coil typically has less thermal mass than the much longer voice coil of an overhung motor, so in general underhung motors are more susceptible to thermal compression. (The short voice coil being typically surrounded by thermally conductive metal tends to lessen but not reverse this disparity.)
The inductance of that short underhung voice coil will typically be less than the inductance of a longer overhung voice coil, unless the overhung motor includes Faraday rings.
I have used woofers with both types of motors and if we’re comparing high quality, roughly equal-cost woofers, it has not been obvious to me that underhung motors inherently outperform overhung ones. Perhaps one reason why the theoretically improved linearity of underhung motors wasn’t obvious lies in human hearing: The low-order distortion that woofer motors produce is not highly audible to begin with.
If there were more underhung motor woofers out there today I might be using one, assuming it did what I wanted better than its overhung competition.
Duke
I do not agree that underhung motors are more linear for large excursions as a blanket statement; imo the situation is more complicated than that. Yes they are somewhat more linear as x-max is approached, but typically underhung motors have less linear excursion (x-max) than overhung motors, so for a given SPL it depends on the specifics. And when x-max is exceeded with an underhung motor, it doesn’t take much overshoot for a large portion of that short coil to exit the gap, which causes distortion to rise rapidly.
An underhung motor can achieve higher efficiency because the moving mass is usually reduced by the voice coil being shorter and therefore lighter.
On the other hand that short voice coil typically has less thermal mass than the much longer voice coil of an overhung motor, so in general underhung motors are more susceptible to thermal compression. (The short voice coil being typically surrounded by thermally conductive metal tends to lessen but not reverse this disparity.)
The inductance of that short underhung voice coil will typically be less than the inductance of a longer overhung voice coil, unless the overhung motor includes Faraday rings.
I have used woofers with both types of motors and if we’re comparing high quality, roughly equal-cost woofers, it has not been obvious to me that underhung motors inherently outperform overhung ones. Perhaps one reason why the theoretically improved linearity of underhung motors wasn’t obvious lies in human hearing: The low-order distortion that woofer motors produce is not highly audible to begin with.
If there were more underhung motor woofers out there today I might be using one, assuming it did what I wanted better than its overhung competition.
Duke