Back more on topic, I've recently been fiddling with a large-diameter VC design that's a bit off the beaten track--a 12" horn-loaded planar-magnetic ring radiator. I conceived it as a way to get a large surface area to horn load up to a high frequency and down relatively low, too, in a short horn.
Can't say much more about it--just throwing it out as an example of other directions big-VC thinking can take you...
Can't say much more about it--just throwing it out as an example of other directions big-VC thinking can take you...
IMHO...
Upside of large diameters:
- More thermal dissipation
- Larger glue area for stronger joints
Downside of larger diameters:
- More mass
- Less efficient use of flux (BL becomes proportionately more L, less B)
- More inductance
- Higher cost
Personally, I like to use as small of a voice coil as possible, since it seems we're always fighting for more efficiency and better dynamic stability of parameters, both of which are compromised with larger diameter voice coils.
Dan Wiggins
Adire Audio
Upside of large diameters:
- More thermal dissipation
- Larger glue area for stronger joints
Downside of larger diameters:
- More mass
- Less efficient use of flux (BL becomes proportionately more L, less B)
- More inductance
- Higher cost
Personally, I like to use as small of a voice coil as possible, since it seems we're always fighting for more efficiency and better dynamic stability of parameters, both of which are compromised with larger diameter voice coils.
Dan Wiggins
Adire Audio
Hi, Dan -
JBL has had a highly successful 15" woofer on the market for nearly two decades with a large 4" vc and only 55g Mms. Plus, everything else being equal, wouldn't a 12" woofer with a 4" vc start breaking up at about the same frequency as a 9" woofer with a 1" vc? Also, the reduction in thermal compression with larger vc's with their improved heat dissipation is not audibly negligible at realistic levels.
IMO, if you're willing to pay, you can never have too much flux🙂
JBL has had a highly successful 15" woofer on the market for nearly two decades with a large 4" vc and only 55g Mms. Plus, everything else being equal, wouldn't a 12" woofer with a 4" vc start breaking up at about the same frequency as a 9" woofer with a 1" vc? Also, the reduction in thermal compression with larger vc's with their improved heat dissipation is not audibly negligible at realistic levels.
IMO, if you're willing to pay, you can never have too much flux🙂
sreten said:
Why? To get a stronger magnetic gap you'd basically just have to make the pole plate thinner, or use a stronger magnet (or both). For a wider v.c. diameter you get a higher inductance per unit of v.c. height (assuming the same type of windings). This means that you can make the pole plate thinner to maintain the same motor strength, while also making the voice-coil a smaller height. Obviously an underhung voice-coil is not a good idea due the outrageous magnetic requirements, and there is a physical requirement for a tall v.c. former so the dome doesn't crunch against the pole plate.
However there are lots of other issues to consider. Not all of the magnetic flux stays in the cylindrical gap of a speaker's magnet. Some of it "leaks" on both sides of the pole plate. If the pole plate is made thinner, then the saturated parts close to the centre won't carry much more flux, and there'll be more flux in the surrounding air. That leakage may not be symmetrical on the inside and outside of the magnet, this will produce distortion unless the motor design is optimised to make it equal on both sides. To minimize excursion-related distortion, the voice-coil should be very long, so that the Bl product doesn't vary depending on its position.
The thickness and layout of the coil's windings may influence distortion at higher frequencies. If there is a sharp transition between the flux density in the magnetic gap and the flux density just outside of the gap, then distortion can be generated from changes in the number of windings in the gap. A low impedance v.c with very few windings would probably be more affected than a high impedance coil with many, thin windings. A good design would have a specially shaped pole piece and pole plate to provide a smooth transition in flux density.
A large diameter v.c. has mechanical advantages by pushing up the cone's break-up mode to a higher frequency. The break-up frequency can be increased if the mass is well pivoted between the centre and edge of the cone, and this is one advantage that inverted dome tweeters have over ordinary dome tweeters.
CM
thoriated said:
You can always find examples that run counter...😉
As far as the JBL 2226s, they're definitely woofers, but not subwoofers (they just don't play low with authority). For a high efficiency requirement, you have to make that tradeoff, and a single layer voice coil can do that - large format and light weight (Eminence has a bunch of lightweight large drivers, too, but they don't play low either).
With that design comes a LOT of problems, though - lack of low end response (high BL to Fs ratio) and high price (single layer means pretty much all your BL is from B, which is quite expensive) are probably the biggest two.
As far as breakup modes, shape has a LOT to do with the stiffness of the cone when operating the driver. A shallow cone will break up earlier in frequency than a deep cone. And of course, for a basket, the deeper the cone, the smaller the voice coil must be (and still maintain clearance). So going to a larger diameter forces you to a less stiff geometry. And the hunt for lower Mms also means thinner material, again meaning breakup issues. Take a look at the datasheet for the 2226 - it starts to break up at 400 Hz, indicated by the rapid rise in 3rd order distortion.
Now, compare that with our Maelstrom - an 18" driver with a 3" voice coil. Mms is quite high (about 2.5X higher than the 2226), and the cone is pretty deep in angle (we have a smaller diameter voice coil) and thicker. Less efficient, about the same BL, and the Fs is a full OCTAVE lower. First breakup mode is at 900 Hz, and the driver is quite clean to 1.2 kHz. We're talking "audiophile" clean, not PA clean... Larger diameter driver, small diameter voice coil, but less breakup issues! It's because of the geometry of the cone, as well as the cone thickness (stiffness).
IMHO, if you want a super strong motor, you're gonna need a very heavy mass to get decent bass extension out of the design. You can have too much flux, just like you can have too little flux. It's a balancing act. Flux is a good thing, but too much flux can definitely hurt you - it depends upon what you're looking to do. As with beer and wine, all things in moderation...😀
Dan Wiggins
Adire Audio
- Status
- Not open for further replies.