Postby whome » March 13th, 2006, 2:03 am
Efficiency Bandwidth Product =EBP (Fs/Qes) is indeed a good guide. However you can get drivers which can go either way depending on what transfer function you're looking for and your intended application.
For the newbie, woofers have different "behavior patterns" depending on a lot of factors when it was designed/made. These factors/specs describe the woofer in "action".
Qts which is called total Q combines an electrical factor as well as a mechanical factor. (magnets and suspension have major roles here)
In addition the woofer has a springiness. This spinginess is expressed as a function of something like a "compression ratio". If you take a piston and compress it in a long cylinder it will fell soft. Take that same piston and compress it in a short cylinder and it will fell hard. If the speaker soft, it will have a larger Vas than one that is hard.
The weight of thespeaker cone etc will then want to bounce up and down at a certain frequency that is the resonant frequency. That is called Fs. Put all of these things down and you get general Thiele Small parameters. Throw them into some math equations and you get speaker simulation at SMALL SIGNAL Levels. Generally this will give an idea of where you're going. At large power levels, things get a little more complicated because things get NON LINEAR and distortion start happening. Thisngs like the size of the port, the suspension travel etc.... come into play.
To get an idea of a sealed box, imagine your car and its suspension. The wheel, the coil and shock absorber. The spring is like the size of box you use on the speaker. So you have a car and you can't change the shocks but you can play with the springs. What happens when you put on real hard springs. The car bouncing around up and down like crazy - like some people with chopped springs. That fellas is a very small sealed box. The air spring in the box too hard for the shocks in the speaker. The car bounces a lot a high frequency ...... boomy bass. But the car could take a lot of licks without bottoming........high power handling.
Now put on some soft springs and keep the same shocks. the car is now tight and the shocks holding the car good. That is like a big sealed box. The spring is soft and the shocks keeping the bass tight. BUT the car spring so soft it keep bottoming......low power handling.
Now what is that ported box like on a car. Imagine you use TWO different springs in a line on the car AND you use TWO shocks in a line too. Like TWo short springs to make one long one and two shocks to make one long one. Now try to imagine what happens. It's not that simple to predict right? Well that is something like a ported box. it's a lot more complicated and if you don't get the spring and shocks right, the susepnsion does a lot of funny things.....like a badly tuned ported box. ( Actually this model is missing a weight at the joint of the springs and shocks.)
Now imagine how complicated a bandpass gets!!!!!!! Three springs in line!!!!!!
That is why sealed boxes is the friend of the builder who doesn't have instruments to tune the ported box. It's hard to go badly wrong and you have an idea of what type of change to expect when you reduce or increase the size of the box. Go smaller, get boomier bass higher infrequency and higher power handling. Go bigger, get tighter bass, lower notes but reduced power handling.
I highly recommend Brian Steele's site as someone else has indicated. Once you start modeling The graph of SPL, excursion and power is invaluable to see why some megawatt subs are destroyed in ported boxes with as little as 20 watts.
