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Conversions and estimations

Sensitivity conversion

Sensitivity can be expressed as Power sensitivity, which is what WinISD uses for modelling, or a Voltage sensitivity.
Convert between them as follows:
Power sensitivity = Volage sensitivity - 10 * log( 8 / Re )
going the other way....
Voltage sensitivity = Power sensitivity + 10 * log( 8 / Re )

DVC 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)
My thanks to contributors at the diyAudio subwoofer forum and HTGuide forum 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: 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 drivers 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

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

 

Last update to this page 8th April 2009