Driver Conversions

This page details the conversions and estimations to fill in the gaps where the supplier doesn't publish the full specs for a driver, particularly for different arrangements of coils in dual voice coil drivers

T/S Parameters

Single Coil Driven
Both coils in series
Both coils in parallel
Re
Re *2
Re / 2
Le
Le * 2
Le / 2
Bl
Bl * 2
Bl
Qes
Qes / 2
Qes / 2
Qms
Qms
Qms
Power sensitivity ( dB per watt@1m )
+ 3dB
+ 3dB
Voltage sensitivity ( dB per 2.83V 8ohms @1m )
The same
+ 6dB

 

  • Manually re-calculate Qts = Qms*Qes/(Qms+Qes)

  • If SPL is expressed as Voltage sensitivity (2.83v into 8ohms @ 1m) , the figure can be converted to Power sensitivity (1watt @ 1m) by subtracting 10*log( 8 / Re)

My thanks to contributors at the diyAudio subwoofer forumLink opens in external window and HTGuide forumLink opens in external window for their help assembling this table

 

Resistive braking - one coil shorted

When driving a Double Voice Coil driver via a single coil, the second coil can be shorted or have a resistor fitted. This causes resistance to movement which produces a modified value for Qms. Several manufacturers use this method:

  • Ascendant AudioLink opens in external window used to supply their "Atlas" driver, however it's no long available and the details of RDO operation have also been removed from their current website. It was originally shown with a 4 ohm driving coil and a 2 ohm control coil. They used either a dead short or a 1.5 ohm resistor across the control coil

  • Searching some forums, I came up with a posting by Dan WiggensLink opens in external window of Avatar Audio, who states that for a driver with two coils (presumably identical), shorting the second coil causes the Qms of the driver to change to a value equal to that of Qes. Avatar Audio is now Adire Audio.

Quoting from the post:

"When you run a single voice coil, it has the nominal single-coil Qes of 0.8.

So, with the second coil open, you'd have a Qes of 0.8, and a Qms (unchanged) of 6.5, for a Qts of ~0.7.

Now, when you short the second (unused) coil, it essentially acts as an electric brake.

This will manifest itself by lowering the Qms to the Qes of a single coil: 0.8.

So, you'll have a driver Qes (of the coil you're driving) of 0.8, and a Qms (of the shorted coil) of 0.8, for a combined Qts of ~0.4.

One coil driven, other open, Qts = ~0.7. One coil driven, other shorted, Qts = ~0.4.

Dan Wiggins

President Avatar Audio"

 

It is this last method I explore with some of the DVC drivers. The T/S specs for this arrangement are identical to a single coil arrangement except that Qms=Qes and Qts is re-calculated as per the formula at the top of this page.

On the subject of alternative arrangements for DVC drivers, you can also run the driver on just one coil. There are opposing views on the subject, although the consensus is that you can do it, but generally you would be better to use a SVC driver of the required impedence and power rating.

There's a great tutorial on DVC driversLink opens in external window at JL Audio, where they also point out the change in T/S parameters and power handling if only one coil is used.

You can see drawings of these different configurations, along with the conversions for one-coil-shorted and RDO, at f4ier Link opens in external window

 

Cone Area

Where Cone Area (Sd) is unknown, following estimations can be used

Driver size
Cone Area (cm squared)
08 inch
220
10 inch
346
12 inch
500
15 inch
750
18 inch
1200

 

 

Efficiency Bandwidth Product

EBP can be used as a predictor of which box type is mosted suited for a particular driver.

EBP = Fs / Qes ---- if Qes is unknown, approximate as Fs / Qts

EBP
Usage
50 or less
Best in sealed enclosure
50 - 90
Good all-rounder
over 90
Best in ported enclosure

 

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Last update to this page 1st July 2007

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