Shiva sealed design converted to vented problems

[jquin]

Hi Guys/Girls

I am new to this obviously.
Some years ago I bought a Shiva Mk1 and built
the 88.5 Litre sealed reference design by adire.
I was not impressed by the bass so I ported it
with a 4" port, length calculated by WinISD.

Now all I designed the port length for was max flatness.
Measurements have backed up the port length I used by changing
it and remeasuring.

So this is not a recognised alignment like SBB4.
Now it sounds much better than the sealed but is this subwoofer suicide?
Should I be rebuilding it to get "so much more" out of the driver.

[JinMTVT]

i just built a sub with the new Shiva driver in 12"
and a 300W Bash amplifier
using a 95liters sealed enclosure...

and i have to say WOAAAAA

this cheap driver reallly rocks
are you sure that you got ur stuff working correctly with the sealed box ?

what amplification are you using ?

[TheFettler]

Now it sounds much better than the sealed but is this subwoofer suicide?

that kinda depends...

by changing to a ported box you have altered the way the driver is loaded. In a sealed box much of the 'springing' responsible for restraining the cone is provided by the air volume in the box.
In a vented or ported box that pretty much vanishes, and the restraint is provided by the physical construction of the driver - the spider, surround, the pole pieces etc.

When you whack a dollop of bass through the driver in a sealed box, the cone moves, but nothing like as much as it can move in a vented box.

You need to make sure you do not exceed the Xmax parameter ( the quoted maximum linear displacement of the cone).

The real issue with a vented box becomes apparent when you get towards the resonant frequency of the box , and worse - the driver.
At this point the damping becomes dangerously low and the impedance of the driver goes ballistic. This is where a lot of speaker damage is done as the voicecoil slams against the pole-piece , or the suspension gets ripped out.

So , provided you keep power down to a safe point then you will never risk the speaker, but by changing from a sealed box to a vented box you have lowered the power limit safe point.
A driver that can take 200Watts in a sealed box may self destruct in 60Watts in a vented one.

The up-side is the extra 3dB in gain and the lower roll-off


When you look at driver specs, the Xmax and Qts/Qes figures are good for a rough guide as to what type of enclosure you are best using for that particular driver - clearly you woun't get much usable power in a vented box with a driver that specs a Xmax of 1.2mm...


Bootnote:

Having just looked up the specs for the Shiva , with a quoted Xmax of almost 16mm I would not have too many concerns about using a vented box !! , although I would suggest possibly a bigger enclosure than you currently have , possibly 100 litres or so.
One other thing you might consider is driving only one coil , and using a resistive damper on the other to adjust the Qts to suit your needs.