Could someone please point me to a link/thread that details the decisions a driver designer might go through when trying to design a woofer, mid-range driver, or tweeter, specifically regarding whether to use an overhung or underhung voice coil? I have an intuitive understanding that it can involve a tradeoff between moving mass and stationary mass, but there are obviously a lot of subtleties involved. Thanks.
This is mostly woofers but I think it can help it gives you a general idea.
https://www.lansingheritage.org/html/jbl/reference/notes/tech1-3a.htm
Rob 🙂
https://www.lansingheritage.org/html/jbl/reference/notes/tech1-3a.htm
Rob 🙂
Thank you, at first glance that confirms my thoughts (which have mostly been confined to woofers).
Do you know if there is a preferred configuration (overhung/equal/underhung) for voice coils in midrange/tweeter drivers?
Do you know if there is a preferred configuration (overhung/equal/underhung) for voice coils in midrange/tweeter drivers?
You cannot have equal as far as voice coils go. Just over or under hung.
In the case of a woofer where you want lots of power handling you usually choose overhung since the voice coil is larger.
In the case of a mid-woofer, midrange, or tweeter you want to avoid excessive moving mass so you sacrifice power handling for a reduction in moving mass.
In the case of a woofer where you want lots of power handling you usually choose overhung since the voice coil is larger.
In the case of a mid-woofer, midrange, or tweeter you want to avoid excessive moving mass so you sacrifice power handling for a reduction in moving mass.
Why is it not possible to have a voice coil with an axial length equal to that of the pole piece?
This is an excerpt from a conversation with Per Skaaning (Audiotechnology)
"The overhung utilizes the whole magnet strength, as where the underhung only part of the magnetic strength.
Green represents the typical behavior of underhung and red the overhung. Underhung, somewhat suppressed midband and overhung the opposite.
Some people find the underhung more pleasant to the ear, as where the overhung faster sounding."
"The overhung utilizes the whole magnet strength, as where the underhung only part of the magnetic strength.
Green represents the typical behavior of underhung and red the overhung. Underhung, somewhat suppressed midband and overhung the opposite.
Some people find the underhung more pleasant to the ear, as where the overhung faster sounding."
If the voice coil and pole piece were the same length then the force (BL) would decrease with voice coil motion in either direction. For overhung, the voice coil is longer than the pole piece on both sides by your desired Xmax. The down side is BL has to drop as you extend the voice coil past the pole piece.
For underhung, xmax is determined by the distance from the voice coil to the end of the pole piece.
For underhung, xmax is determined by the distance from the voice coil to the end of the pole piece.
Well for a compression driver or tweeter where you want maximum efficiency and have minimum excursion I would think either slightly under-hung or same. You will also get the best heat transfer to the pole piece with an under-hung.
Rob 🙂
Check out this link (see posts #3 and #6): https://www.diyaudio.com/community/threads/overhung-or-underhung-voice-coils.184216/
My understanding is that the underhung coil is entirely contained within the gap, with none of the wire is outside the gap when the coil is at rest. Highest efficiency is obtained because all turns of the coil exert full force when energized because all are located where the B-field is strongest. Problem with this approach is that many turns move out of the gap when the coil moves., and this generates a displacement vs. force variation that causes distortion and compression. The overhung coil is much longer than the gap thickness, so the same amount of wire is submerged in the B-field whenever the coil moves within its specified excursion limits. Force vs. displacement is closer to constant since the same amount of wire as always in the gap as the coil moves. Since a lot of the wire in an overhung coil is not in the gap, that wire does not add any force to move the coil, so any power dissipated in that section of coil is wasted, producing only heat and little or no useful force on the coil. Efficiency is poor as a result of that.
You can still do it. it's just not common. I seem to recall reading somewhere that older pro audio drivers did have a greater tendency to have equal-hung voice coils, as it generally maximizes efficiency at the expense of excursion.
Many turns can move out of the gap when the coil moves. But if the coil is sufficiently underhung relative to the actual excursion (i.e. if the excursion is low enough relative to the amount of under-hung-ness) then the coil still stays entirely within the gap. This is a bit of an oversimplification though because a magnetic system can be designed so that the B stays constant, or even is a bit stronger relative to center position, at the upper & lower (physical) extents of the gap itself.
Good thoughts, keep 'em coming. It all jibes with my general expectation regarding linearity VS heat/efficiency.
For most magnets (with possible exceptions for some rare-earth versions) the pole piece is a conductor, and that turns the inductor into a transformer with a short-circuited secondary coil. This would be fine, if not for the variable airgap that changes dynamically with the movement of the voice coil.
As the wire turns move further away from the pole piece, the inductance in that part of the coil goes up, so they basically hog more voltage than the equivalent number of turns that are located closer to the pole piece. So there's a lot happening, especially at the h.frequency end of the spectrum.
As the wire turns move further away from the pole piece, the inductance in that part of the coil goes up, so they basically hog more voltage than the equivalent number of turns that are located closer to the pole piece. So there's a lot happening, especially at the h.frequency end of the spectrum.
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So I think there are a few things designers probably look at:
-good symmetry of the motor. The changing flux curve on the 'outside' matches the inside, so most of the distortion is cancelled out, as long as the VC stays near the middle.
-I've heard that strong saturation is important, but I have a hunch that de-tuning a motor to lower strength might linearise the inductance along the length of the VC. So the non-magnetised iron core adds inductance in the middle to match the stray inductance at the ends of the overhanging coil. This might go some way to explaining the magic sauce behind old alnico designs. A wider airgap could also smooth out the flux curve, while also reducing the parasitic load from the pole piece. However, it would probably be prone to damage or other distortion mechanisms if overpowered.
-good symmetry of the motor. The changing flux curve on the 'outside' matches the inside, so most of the distortion is cancelled out, as long as the VC stays near the middle.
-I've heard that strong saturation is important, but I have a hunch that de-tuning a motor to lower strength might linearise the inductance along the length of the VC. So the non-magnetised iron core adds inductance in the middle to match the stray inductance at the ends of the overhanging coil. This might go some way to explaining the magic sauce behind old alnico designs. A wider airgap could also smooth out the flux curve, while also reducing the parasitic load from the pole piece. However, it would probably be prone to damage or other distortion mechanisms if overpowered.