Newbie Q:Importance of voice coil inductance in Direct Radiator design.

[asberrys]

What are the variables for Voice coil inductance in speakers? greater length of VC equals increased excursion but greater inductance hence loss of transient response? Ie.,it won't produce the higher frequencies well? Is that it?
Also does a wider diameter VC equal lower inductance but less strong B field?

I notice visually the mid/woof in the Magico mini seems to have a much smaller VC than the famous Scanspeak midwoofs. Is this part of the reason it has stat like clarity? Superb transients? I know it is underhung whereas the Scanspeaks are overhung I think.
Is VC inductance the most important parameter when designing a Direct Radiator?
Thanks in advance.

[speaker dave]

N squared A over L is the approximate formula.

N is the number of turns and is squared, so double the number of turns will give 4 times the inductance. A is the area of the circle incribed by the coil diameter (pi r squared), and L is length from top winding to bottom.

B (actually motor strength, B is magnet strength) is primarily related to the volume of copper in the gap. You can slice the copper thin for high DCR or thick for low DCR, but more copper is better.

In the end inductance prevents current from flowing at high frequencies, so it is one of the natural limits causing HF rolloff. Note that slicing the copper thinner will increase R and L at the same time. I think DCR and L would go up in proportion so the cut-off point wouldn't change. It is the ratio of inductance to DCR that sets the cutoff point.

Finally, the voice coil inductance includes a loosely coupled iron core so it gradually rises rather than having a straight 6dB per octave rise. This also means the rolloff effect is gradual.

David S.

[Bill F.]

greater length of VC equals increased excursion but greater inductance hence loss of transient response? Ie.,it won't produce the higher frequencies well?

Yes, in an overhung motor topology where winding height is greater than gap height, increased winding height means more Xmax. Of course, the opposite would be true in an underhung motor where a shorter winding height would allow greater Xmax for a given gap height.

Le is most strongly affected by the presence or lack of shorting rings in or near the gap than by winding height

Also does a wider diameter VC equal lower inductance but less strong B field?

Achieving high flux density across a gap of any significant height requires a substantial cross-section of steel to conduct to flux. This usually means larger diameter pole pieces which of course require larger diameter voice coils (and more Le). If cost is no object, you can cheat on pole diameter a bit by using cobalt steels with high saturation points.

I notice visually the mid/woof in the Magico mini seems to have a much smaller VC than the famous Scanspeak midwoofs.

Viewed from the front, those drivers are hard to judge because I believe they use a single-piece diaphragm, so the small dimple you see in the middle is not necessarily indicative of the VC diameter.

Is VC inductance the most important parameter when designing a Direct Radiator?

I wouldn't say so. It's just one of many important parameters that must be balanced to achieve a desired outcome within a constellation of other system variables (amplifier, crossover, etc). For example, hitting a low Le is probably much more critical when designing a tweeter than a subwoofer.

[buggsson]

Originally Posted by Bill F.

Achieving high flux density across a gap of any significant height requires a substantial cross-section of steel to conduct to flux. This usually means larger diameter pole pieces which of course require larger diameter voice coils (and more Le). If cost is no object, you can cheat on pole diameter a bit by using cobalt steels with high saturation points.

I would think there are more tricks at hand to manage the inductance in conjunction with large VCs as can be seen below.

6" driver, 1 kHz, 0.52mH and a large 75mm VC.

http://www.morelhifi.com/products/pd...20EW%20638.pdf

Tighter gaps is said to increase flux, and square threads will allow more thread material for a given VC length. Now my memory is blank, but I think I recall there are even more to the issue than this.

Wasn't it Dan Wiggins who did a test and concluded that VC inductance is an important parameter in controlling the transients of a driver?

And if I may borrow the thread, for an inductance question? When can inductance be considered to be low, an in a low inductance driver?