sealed box design made simple

[Ianm2]

I have just been reading a book which makes this pretty straightforward.

For those with no idea at all about the myriad thiele parameters, here is 95% of what you need.

ok, it starts off with Q, Q is to do with resonance, aka ringing and the opposite is damping, or friction.

Your car springs 'ring' and need damping to stop them wobbling, oscillating forever.

Its to do with electrical filter theory, filters have a number of parameters to describe them

1/ type, lo cut, hi cut
2/ frequency at which its 3 decibels down
3/ how steep it cuts off, ie slope
4/ shape of curve

right, Q to get a flat response, is 0.7, less is overdamped and gives weaker bass, more is underdamped and gives boomier bass

check your Q on your woofer, it may be around 0.2.

So your target Q is anywhere from 0.5-1.2, no more or it'll sound too strong.

lets say we want 0.8.

Times the 0.2 by 4

Next we want to know our freqency.

AS Q rises, so does frequency.

By the same amount, so times the Resonant freqency of your woofer by 4.

That's where it starts rolling off. so a 20hz woofer will roll off at 80 hz.

(in filters we want to know where its 3dbs down, a little more involved)

finally what should the box size be?

For this we want the VAS, the suspension stiffness( to do with the rubber surround outer suspension and the spider (inner susp.)

suppose its 225 litres, your size is given by this.

size= VAS divided by (your q factor or 4) squared minus 1

eg box volume = 225 divided 16-1

or 15 litres

easy eh?

(interestingly these are the holy trinity of thiele parameters for a sealed box, Q, res. freq and VAS, here's where it gets interesting)

res. freq. is to do with suspension stiffness (VAS) and the moving mass, ie all that moves, that is, the coil former, the coil, the glue, the dust cap, the cone and half the suspension.

heavier mass=lower res freq.
lighter=higher

VAS is the suspension stiffness, stiffer raises the res freq.
looser lowers it.

Q is a kind of friction, it consists of mechanical fricition (Qm) and electrical(Qe) and stops it resonating or ringing on.

So we have

1/ total Q of driver
which consists of electrical Q and mechanical Q

2/ and Q when you put it in a box, the box air being less volume is stiffer than your big room, so the res. freq goes up as res freq is part to do with stiffness.

Its all interlinked.

Even more interesting, you know you put a capacitor in series with the tweeter to give a lo cut filter, and talior the cross over point, and shape the curve?

Well the box acts as an acoustic capactor to 'roll in' the woofer( visualize the woofer curve as a tweeter curve and its the same thing, only done by the box instead of a capacitor.

[Ianm2]

oh and if you use a computer program, then you don't need to understand the above.

for the benefit of those who don't fully understand speakers and boxes, including me!! the theory is truly horrid