look what i found here in the UK guys, its a pretty good breakdown for the Laymen (like me..lol) of how bandpass works....
thought it might be quite useful ...... then again it might not see what you think. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
thought it might be quite useful ...... then again it might not see what you think. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
Well, there has been an increasing interest in bandpass boxes as of late.I undertstand that finding information is difficult and at the best of times, most of it seems to be talking in an unknown language.
Now my knowledge of 4th order greatly outceeds that of 6th, so i'll focus on 4th.
So here's my take on the designs. It's not Gods gospel, neither is it by any means a replacement for trial and error (the only practical method for ANY box design).
It may be of little use to the SPL orientated guys out there, but to fellow streetbassers/ bassracers/ Or just genuine bass lovers, it's a rough guide.
So first off, what is a bandpass box?
A bandpass box is a box which cut's away unwanted frequency's. That is it in it's most simple term.
Just like an electronic crossover, it removes frequencies (well attenuates them) which you don't want.
Oftenly this is done to favour a greater SPL. The good old bandwidth vs gain. I.e. Reducing bandwidth (available frequency range) = Increased gain (more SPL), or visa-versa.
What i've been paying particular intest to is try and get as big a practical bandwidth as possible, for as high a gain as possible. So playing maybe 25hz - 65hz with over 140db.
It's hard, very hard to do, I have been able to get over 140db, low power 4 12's, 28-50hz, so far.
However!!!! MANY PEOPLE NEGLECT THIS!
Not all drivers are suitable for a 4th order bandpass box. There is an easy way to work out if it is/or isn't suitable (Thanks to Mr singh for pointing this one out to me when I was learning).
So simply take the Fs of the sub and the Qes of the sub (available from any good manufactures T/S parameters).
Now fs/Qes = a number... (EBP)
If EBP is less than 70, the driver would like to be used in a sealed box. If it's great than 70, use a ported rear cab (6th order).
So a GT4 for example is a perfect 4th order sub. However a BTL for example is a perfect 6th order sub
Ok Smartie pants, how does it look on paper?
Well, the same as an electronic crossover would.
The slope is determined by the 'order' of the box.
Simply put, the low end response (high pass part), on a 4th order (sealed rear) is very similar to that of a sealed enclosure.
A 6th order, is similar to that of a ported enclosure.
The 'operating style' can be considered the same as their 'rear' boxes too. SO just like you wouldn't play a ported box at full volume under tuning, you shouldn't play a 6th order bandpass box below tuning (high pass point).
This isn't too much of a problem with 4th orders as, just like sealed enclosures, you can play pretty much anything (there is no 'high pass' part to a 4th order as such.
However, this is the part which catches people out. What is the point of this great wopping box on front of the conventional boxes?
Well this is where the magic happens.... whoooooo.
Ok so we now have the low response part, but how is this steep part of the response coming about on the upper frequency?
Well first off consider the front enclosure. The front box is the 'gain cabinet'
You know when you run into a tunnel and shout HELLO. Your voice sounds louder in there than it does if you weren't in the tunnel, well thats what this box does, it makes a 'room' in which waves are allowed to form and 'add together' or for you clever cloggs, it's called constructive sound wave interference.
Along with this the port act as a spring, which in turn modifies the returning wave, back to the speaker, thus modifying the speakers response.
The port then filters out higher frequencies as they cannot form properly inside the front box, giving the low pass filter characteristic.
So here you can see the 2 things which control the response of the upper frequency, box volume and port size.
Now it works like a ported box, but the slope is backwards. So a bigger box = lower tuning, however increased gain (think of it as move waves can add together in a bigger space).
Then port is as ever, larger mouth = higher tuning, longer length = lower tuning.
Ok, shutup about all the theory, give me some numbers!
Before notes:
1. Get the 'mouth' as big as possible. Seriously, bigger mouth = less noise. All that bass has to come through a port now, not straight off a speaker.
2. Use the correct sized rear box. For 4th order, simply use a generic (or if there's published) box size. So for an 8" driver, around 0.5cu. 10" - .8cu, 12" - 1cu.
3. Design the box so if it's possible you cannot see the subwoofer through the port, (hang on a minute that's different to what you did alex... ), the idea is, to allow a clean 'loading wall' for the speaker(s) to push against.
Now what's this nonsense about ratio's?
Ok simply put, the bigger the front cab, the greater the gain. So 6:1 = maximum gain, little bandwidth. 1:1 = superb bandwidth, but doubtful you will see any gain at all from a conventional box.
Now, multiple woofers can make place better than a bigger cab. What? again please? Yes, 16 woofers inside a 2:1 ratio, may very well be louder than 8 woofers in a 3:1.
So pick your front cab size carefully. Many opt to spec the rear size as recommended, then see what room they have left for the front cabinet. It's probably the best way if you already have your kit and are insistent on using all 512 gt5's you've collected... (Lozzy.... )
Having said that, I like 3:1. It's enough gain to say wow, this bandpass stuff is good! Yet i've still had a very good response curve from them.
For those out there wanting to try and use 1 or 2 woofers, I would recommend at least using 2.5:1. I say this as I personally have never tried small ratios. But more importantly, from what I have read, and understand, is that the gain presented in a large enclosure may be 8db. Yet in a small enclosure, you may receive little at all. I don't want people thinking pfft. Bandpass is rubbish, it is rubbish... If you only gain 0.5db but lose twice the boot space as you did with your sealed box! However if you loose 3 times that space and gain 4db's, you're going to be grinning like the Cheshire Cat right?
Now mouth area. Open wide sunny Jim!
As above, as big as possible, but here's some numbers to help you
Minimum port area (in square inches) should be 18x the volume of the front cabinet (in cubic feet).
Maximum port area (in square inches) should be 25x the volume of the front cabinet (in cubic feet).
It's a rough guide, yet it's good enough for me!
Port length, excuse me, i'm happy with 4 inches!
Yes, keep length down to as small as possible, I'd prefer to see a tiddly port but a small length than a great huge port, with some 20" length
Aim for wood thickness (1") to 6" maximum (unless you're going for a single sub then you may need a little more length...)
Now finally tuning. Where exactly and how exactly do I figure all this out. Well the easiest way (the way I do it), is to use bbp6. Or ask someone who has it, to....
Choose bandpass single tuned box (unless you're going 6th order) in which case, go for series double tuned. I recommend you DON'T try parallel tuning, as it really gets fun then .
Now input you box volumes (which you worked out above).
Now go to port and put in your area (as you worked out above). Then try a length of say 4".
See the tuning, 50 - 70hz... are you mad? No. Don't forget that it's a LOW pass, so anything above this wont play.
So low tuning = better low end response. Higher tuning = better fart box.
Final notes:
So i'd pick around 55-60hz.
Set subsonic to high pass frequency (if 6th order)
Set low pass to tuning frequency (4th or 6th) - this is imperative as with lots of power you will easily destroy the driver.
Also, keep in your minds at all times, hearing distortion inside a bandpass enclosure is nigh on impossible. I cannot do it, if you hear distortion, the speaker is probably already destroyed. So play within the frequency range you have made the box to play within! Also ensure there is NO CLIP!
Finally for those with an idea of how it works... you'll notice that the front cabinet acts like a spring, the same as the rear cabinet, we all know that a better spring means higher jumping, but it's easier for big people right? Yeah, so a bandpass enclosure will take more power, a fair bit more power than a ported or sealed box might, as it's got the 2 chambers acting as a dampener instead of just 1.
I hope that lot has helped you folks out a little
