Recommended size for a bassreflex port

[moses pray]

Hi everybody,

since nobody else - not the driver's manufacturer, nor the music dealer - could/would/did answer my request to my satisfaction, i would be truly grateful for any help on this subject.

I have an old, closed back, 66,05 litre bass (guitar) cabinet, which is hopefully soon to be equipped with a new 10" bass woofer speaker.
To enhance the bass, there is a rectangular 18 x 6 cm bassreflex port already cut into the 2.1 cm front panel (gives it a volume of 0,2268 litres, right?). I don't know the original speaker the cabinet was built for - looks like a homemade job but it's quite sturdy, well-done - so i can't figure out if the bassreflex ports size has been calculated properly or just been cut by rule of thumb ... (?)
The Thiele/Small parameters to the speaker i've already picked (an Eminence 'Legend BP102-4', small but very powerful) are luckily provided along with other technical data on the manufacturer's website.

My problem: in question is if i can use the already-cut bassreflex port as is, or if i have to alter its size - ? Therefore i would have to know how to calculate its ideal dimensions, or maybe a range of possible sizes - unfortunately, although i found lots of 'cabinet size calculators' on the net it seems, there is not one out there that allows for this special job ...

Using online calculators or simulation softwares for cabinets seem to either work to create virtual cabinets from scratch only, impossible to type in already existing dimensions(?) - or they come up with data results i do not understand, like abstract curves, frequency stuff and the likes - instead of (what i was expecting - might be a bit naive) a more clear range of possible sizes for optimum speaker operation.
My misconception? ...

I really don't expect anybody to do calculation stuff for me (!) - i just really need some hint on how to proceed:
does anybody of you guys have an idea on how to exactly calculate a bassreflex port - say, which formula to use? Which of the Thiele/Small parameters to put in use here -
and then: if using a software - how to interpret (resonant?) frequency results and such stuff to turn this abstract nonsense, ah, excuse me - this doctoral thesis into a worke horse for a working musician? ...

The general goal is to make sure the speaker is able to deliver as much bass frequency and power as possible, for it will be in use for bass amplification as a second, 'woofer' cabinet along with a powerful, midrangy 2x12" box. Both will be fed through one main amp respectively.

I would be really happy if i would hear some recommendations.
Sorry, i'm only a musician who just wants to change one bass driver ... i really didn't expect having to get a doctor's degree on physics first ... quite an unexpected challenge.

Many, many thanks for any help,
all the best, mo
__________________________________________________________________________________________________________________________________________________

Edit: Sorry, if i sub-posted this under an obviously wrong category! Didn't get this concept - i assumed "post new thread" means posting an independant thread ... my fault. damn ...

 

[Steve Williams]

No problem. Let me move it to a more appropriate forum and welcome to WBF

 

[Duke LeJeune]

I have an old, closed back, 66,05 litre bass (guitar) cabinet, which is hopefully soon to be equipped with a new 10" bass woofer speaker.
To enhance the bass, there is a rectangular 18 x 6 cm bassreflex port already cut into the 2.1 cm front panel (gives it a volume of 0,2268 litres, right?)...
The Thiele/Small parameters to the speaker i've already picked (an Eminence 'Legend BP102-4', small but very powerful) are luckily provided along with other technical data on the manufacturer's website.

First, thank you for providing so much information.

In my opinion that's a very large box for a 10" woofer, even a long-excursion one like the BP102-4. You'll get a very big low end, but may well find that it farts out at a lower power level than you'd expected. So assuming you want to press ahead with your project, be prepared to either use a protective high-pass filter or dial in some low-end cut with your tone controls or EQ.

Actually, that's a big enough box for two BP-102 woofers. Is there room for another one on the front baffle?

I assume the rectangular port is only as deep as the plywood (maybe 18 mm). If so, that port will tune the cab to the mid to upper 70's, and that's way too high for electric bass. The low end will be boomy, pitch definition will be poor, and the woofer will tend to fartout from over-excursion because there will be too much energy below the tuning frequency.

Before suggesting a tuning frequency and port dimenions, let me ask you a few questions:

1. What's your lowest string tuned to? Low E? Low B? Something else?

2. Does your amp (or preamp) have a protective high-pass filter? IF so, do you know what frequency it kicks in at?

3. That rectangular port - where is it? I'd like to know which, if any, port edge is right up against a wall of the cabinet, because that needs to be taken into account in calculating the port length, if we keep the rectangular shape but make it longer.

4. Would you be comfortable with sealing up that rectangular port and simply installing round ports? If so, what size (inner diameter) plastic pipes do you have access to, preferably in the plumbing supplies department of your local hardware store? I'm not familiar with metric sizes for pvc plumbing pipes.

5. Do you anticipate playing in a wide range of different rooms? For instance, if you occasionally have to deal with very boomy rooms, we should take that into account, which will be easier for us to do with round ports.

 

[moses pray]

Hi there Duke -

thanks for your great input! If i'd only had that kind of info before (tried to get it - but to no avail, seeked the wrong places i guess) i'd probably come up with something completely different!
Large box - i was told(!) to *get a big one* to support power + low end of this driver... (!) ...so i turned around to this old project again, a cabinet lying about - yes, of course originally for a 15" - ! - i already installed a fitting new 10" baffle, cleaned everything up... and btw no, there's no way to put in a 2. driver, even if i wouldn't mind the weight.
To keep cost and carrying weight low - the empty box is quite heavy already - thought this small driver would be phantastic, read some reviews... originally thought of a 12" but wouldn't find one fitting descriptionwise - i really like the Eminence Legends for guitar, even use 2 V12 guitar ones for bass(!) - just this time wanted to add a small woofer for a bit more low end for live, and i also have a second power amp section which i carry around anyway and which remained unused, so i thought i'd use this for a woofer...
I know this is quite unusual, but i really like the tone of the guitar speakers, recorded lot's of stuff with nobody ever complaining about low end missing!

I'm more of a vintage style player, also soundwise, crossover/funk days are over;-) - so: nope, no low B string, E string is the lowest here.
Yep, the rectangular port is as deep as the baffle, it's 2.1cm deep, lengthy side ca. 3-4cm from the wall... right, i didn't think of this either, good point.
I could close the damned thing by attaching some wood like i did for the new speaker baffle hole to either make it smaller or to glue in a new, round tube... But there's a piece of reinforcement wood between the speaker hole and the port, so i can't place it anywhere else - like more to the middle, if that makes sense - means i have to go with the placement quite close to the wall...

Ok, i was thinking of a protective high-pass filter myself... but again, couldn't get any possitive thoughts on this.

Now that i already worked on this box - after some positive commends to do so (Jeeez...), i'd like to go ahead... maybe thinking of getting a different speaker for the job, i didn't buy the 10" Legend yet ... (please, not a 15" again!;-))

Hm. I have to think about it... i'm busy right now, let's see tomorrow...

Again - many thanks + respect!
Best, stay tuned
Mo

 

[Duke LeJeune]

Thanks for the additional information.

If we don't have to worry about low-B, that makes life a lot easier on the cab, as far as fartout resistance goes.

Okay, I suggest keeping the rectangular port shape that you have, and building a four-sided extension inside the box that adds about 18 cm of depth to the port (bringing the total depth to about 20 cm). This will drop the tuning frequency to the lower 40's, so cone motion at the fundamental of low-E (41 Hz) will be low, and your fartout resistance should be pretty good.

This cab would only be down about 4 dB at the fundamental of low-E, which is rare extension for a bass cab. You may find that it's too much low end, competing with the kick drum and/or muddying up the mix... or you and your band mates may love it.

If you would like a "tighter" low end, let me know how deep the cab is on the inside and I'll revisit the port. The BP102 isn't an especially "tight" woofer to begin with, but we can push things in that direction, at the expense of some fartout resistance.

If you let me know the approximate internal dimensions of the cab, I can offer some thoughts about damping material.

Finally if you're thinking of changing drivers, let me know what brands you have access to, along with what sort of tone you're looking for (smooth or grindy upper mids, for example).

 

[moses pray]

Hi Duke - thanks again.

Ok, please alow a few (loud) thoughts...
> You may find that it's too much low end, competing with the kick drum (...)

... at the moment i'm only playing together with drums when recording (where i, depending on the project, use either my old Fender valve amp/dionsaur 15" Jensen / or the guitar 2x12" Eminence rig *plus DI* anyway)...
The only live gig with bass i have at the moment (i play guitar otherwise) is a small 'Afrobeat'/Singer-Songwriter group with a percussion player - congas, hi-hat, cajon - that's looking forward to play on festival stages soon besides the club scene. Anyway - no real 'kick drum competition' here.

Otherwise, i plan to use the existing 2x12" through one main-amp channel as the main sound, only to add(!) the woofer channel/kind-of balancing it to the mix. The woofer isn't meant as a stand alone solution anyway: just to add what might be missing, depending on the situation.
Also, i guess i do already have enough of some nice 'upper mids' via the 2x12" - the woofer does not really need to provide them.

Btw i've had lot's of occasions of 'mud bass' horror just when playing with a few drummers on a medium size stage - no matter what *i* have been using and *if* i did anything - just because of a boomy bass drum and ridiculous monitor levels, even when i used kind-of bass-free(!), sometimes even guitar equipment - and when this happened, it happenend most of the time *even when i did not play a note*;-) ...
In my experience it might help a bit to tighten the low end, but sometimes that's just beyond your control anyway, besides that i'm not an especially loud player.
Now i would say: Let's go for the fun sound;-)

To summerize my weird explanations: it might be cool to have that bit of low end in reserve - as long as i can control it with a knob. - ?
No, the preamp doesn't come with a really effective eq/ nor high-pass filter to safe the driver.
So - i guess i still should still add one, right? Guess at around 40Hz, or did i get this wrong? ... (have no experience on that, but if you could recommend a frequency of transition or what it's called, all i have to do is visit the electronics guy around the corner, who could help me on this for just a few bucks.)

> The BP102 isn't an especially "tight" woofer to begin with, but we can push things in that direction, at the expense of some fartout resistance.
/
> Finally if you're thinking of changing drivers (...)

... i think i try the Legend... somehow it feels right, if you know what i mean (before i even got/heard it - stupid me).
Well, it depends on what you mean by 'tightness': some amount of "tightness" might be cool to add a bit of punch to those 'african 16th' (kind-of those very early Tower Of Power lines - yeah), but
1. i don't want to lose possible 'real big fatness' for the vintage soul stuff... also on quite quick notes, if that makes sense;
2. i would rather not risk the "at the expense of some fartout resistance" thing... better keep the resistance. Otherwise - i don't know if the high-pass filter would be enough to do the safety job, when altering the port's length for a tiny bit "more tightness"? ...


Well, in my case - maybe your suggestion for the plus-18-cm's-port-length is just the exact right way to go?

Anyway, you asked for the internal dimensions - thanks! Here we go;-): the inner depth from baffle to back is about 27cm = 10 5/8", height is 18", length 23". Minus lot's of bandings and reinforcement beams and the compensation piece of wood that will hold the 10" on the former, 15" hole.
The whole cabinet, incuding the back, is filled with glued on damping mineral wool. Do i have to remove any of it, maybe near the port?

So, at the moment i'm considering buying 18cm wide plywood material to cut and mount a port extension - until the next change of plan;-)

Thanks and best! Mo

 

[Duke LeJeune]

For a protective highpass filter, I'd say your idea of 40 Hz sounds fine. Somewhere between 35 and 40 Hz.

I'm not sure this cab will do real well on fast notes... if you find this to be the case, I think we can add a variable tuning feature to the cab. You see, if we can lower the tuning frequency, we can probably tighten up those fast notes a bit. Here's one way: Cut a block of wood the same length as the port, but slightly less than half the cross-sectional area. Leave a little bit of "play" so that you can wrap it in electrical tape to get a snug fit and keep it from rattling. With this block inserted into the port, your tuning frequency will drop down to about 32 Hz, which will tighten up the low end a bit. Experiment with this in practice first to make sure you're still okay on fartout resistance (it will be reduced, but hopefully not too much).

 

[moses pray]

Duke - great stuff, thanks -

just want to let you know at the moment i'm assembling the port extension and build the highpass filter, while ordering the speaker ... might all'n'all take a few days, until i can take it to the rehearsal space.

Hope this thread won't be closed to soon ... i still might have another question or two once i've put all the pieces together and did a first soundcheck. Maybe not - we'll see.

Until then - best, Mo