- 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 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:
- Ascendant
Audio
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 Wiggens
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 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
