You know, if we make the port long enough, it would exhibit quarter-wave resonator effects...which could be a bonus depending on how you look at it (would throw off the tuning, but if we modeled it as a tline...)
So, double or single?
Or even Double 18?
My favourite is the one in the middle.
I should have been clearer with the specs - flat as high as humanly possible, yet ideally crossed over at 120-50hz (numbers pulled out of my hat).
Edited by: Deadbeat
\'bits of ply round a driver\'
You stated that you want high efficiency, how high? 103/104? that could be doable, but what would the power handling of the cab be like? should we say it has to reach X watts with out over excursion, or do we want to aim for an overall dB value from one cab @ max power?
Bass horns generally exhibit resonant pipe behaviour at low frequencies. This is because the impedance match between the horn mouth and airload degrades, permitting reflections. This effect is incorporated in most bass horn designs already - theresponse peakresulting from the quarter-wave resonance is designed to occur just as the horn gain is rolling off and acts to extend the flat response down a bit (with a steeper rolloff below).
One can liken such a bass horn to a fourth-order bandpass in its low-end behaviour - it only differs above about the half-wave frequency. The so-called "bandpass horn" just pulls down the resonant frequency by adding additional air volume at the throat.
A typical ported horn that is ported to the outside can be likened to a parallel or type A 6th order bandpass. With the port being like the lower tuned chanber and the horn like the upper tuned chamber.
A throat-ported horn simply resembles the series-tuned (type 6th-order bandpass. The benefit is that the resonant-pipe behaviour of the horn provides additional gain over what the port produces alone. The disadvantage is that some of the higher frequencies will tend to disappear through the ports back into the cab instead of coming through the horn. So you get more output at the low end in exchange for less output higher up.
I also believe it should be easier to get port output in phase over a greater range of frequencies. This is because standard ported reflex theory applies when the port output is adjacent to the driver. If ported to the outside, you have to compensate for the horn path delay.
2 drivers need twice the volume in ported chambers and twice the horn throat area (hence twice the volume in the horn). So the cabinet gets to be twice as big. But it will be 3dB more efficient, so you *may* be able to claw back some volume by reducing horn gain, port resonant gain etc.
Don't worry about the port dimensions at this stage. The design should confer a resonant frequency for the port, and max power output considerations should confer a minimum cross-sectional area. The length should be found by experimental tuning on a prototype. Edited by: jsg
Whatever the resonant chamber technology is it obviously has the effect of extending the low end of a reflex, while this might not be the most efficient setup in the world it fulfills the initial criteria of
103/104dB efficient (although the more the merrier!)
30/35Hz to ~130Hz
61cm x 122cm x 81.3cm
Do ya know... If I knocked 20litres off my ported horn design then it would be exactly half the size you specify, (its probably quite doable, just cut 5.5 cm off the front) and it could well be efficient enough and maybe (with a more hardcore driver) drop low enough.....