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.
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.
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.
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.
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