So after contemplating, I'm wondering what drawbacks there might be to building a pair if 8-9' cabs. 22X12X57 internal vol before any bracing, fill, or driver displacement. Probably around 8' net. I would mount a 2269 on the 22" 'side'. One each behind my Acoustat Monitor 3..(perfectly hidden), after I may be done listening to the York U15s whose bottom end needs extension, imo. How much does it matter the dimensions and/or placement of the driver? TIA.
If you're going with a 2269 why not copy the SUB18 box?Originally Posted by gasfan
So after contemplating, I'm wondering what drawbacks there might be to building a pair if 8-9' cabs. 22X12X57 internal vol before any bracing, fill, or driver displacement. Probably around 8' net. I would mount a 2269 on the 22" 'side'. One each behind my Acoustat Monitor 3..(perfectly hidden), after I may be done listening to the York U15s whose bottom end needs extension, imo. How much does it matter the dimensions and/or placement of the driver? TIA.
It is a simple square box nothing special about it, at about 11.25 cubic feet - if you're considering 9' ...
When doing boxes you must consider the ports - number of and length - as this will influence the size (depth) of the box and achievable tuning frequency. Ports also take up baffle space - more than you think.
You don't want to find yourself in a situation where you have to consider bending a port as the length to the desired tuning frequency is longer than the depth of the cabinet.
I think the point is to avoid typical large looking big square boxes... I know I would hate a pair of big square boxes in my listening room.
Regarding the tube length, the port tubes can equally go on any side of the six sided cabinet. I have not looked into this, but if the length is too long for the 12" deep side, it could go on the 22" deep side, or even the 8' plus side, so I don't think a curved tube will be needed.
That said, you do need to follow the basic rules about being near adjoining sides or the rear of the cabinet's interior.
Widget
Did I calculate wrong? Height would be 60", 57" internal. 22X12x57 internal yields 8.7'. So with a bit of eq i should be able to get them down to 22hz with room gain? I have a Driverack 4820 to mate with the 2269s and the U15s.
No, well, maybe, but I didn't bother multiplying your dimensions. I must have mistaken your 8 cu ft for an 8' tall column.
Regardless, the answer is essentially the same. You can make them any dimensions that work for you as long as the volume is correct. There are basic rules about standing waves etc., so avoid cubes and simple relationships of even multiples. You don't want a 1' by 2' by 4' interior dimension if you can help it.
Widget
Remember to account for the volume occupied by the port inside the enclosure.
You may have accounted for the following. But l will put the following out there for the uninitiated.
I have not simulated this particular driver.
If you look at the original JBL drawings for the 2245H Subwoofer Project that would be a good starting point to realise a practical box. You could put the woofer on one end of the enclosure or one of the sides. Mounting the port on one end will possibly give you enough port length with three 4 inch ports.
Those ports will use up in the order of 18L of enclosure volume. The woofer volume is about 8L and the bracing can be assumed to 24L. So your gross enclosure volume of say 250L ends up being 50L less of used volume. That leaves 200L of volume remaining behind the woofer.
Then allowing for the fibre glass wadding on the enclosure walls this will add around 5- 10% volume or 20L to the internal volume of THD enclosure. The net enclosure volume might look like 220L. But that’s 30L less than your target volume.
Box losses can also unfortunately impact on your net enclosure volume.
A unity box loss call QL=7 on your simulator assumes a fairly rigid box with no leaks. The rule of thumb is the bigger the enclosure the more it will typically flex, shake or leak and dissipate energy as a loss. However to much bracing will make the box impracticable to lift and move.
So your best option is to oversized the enclosure by 10-15 % and tune the ports according to your modelling for the desired low end F3.
This is just a FYI to take on board after looking at your simulations. It used to do my head in working this out. Simply over volume your box a bit after running your simulation to be on the safe side!
Be careful on the depth of the cabinet - looking at the 2269 specs in the library the depth of the driver is 12.5"
I hadn't paid attention to that either. It will probably still fit since part of that 12.5" depth will be taken up by the baffle. That said, I would never have the back of a woofer even remotely close to the rear of the cabinet as the reflected midrange (upper bass) will come back through the cone.
If this was something I was serious about, I would build a test cabinet before diving in.
Widget
A PICTURE IS WORTH A THOUSAND WORDS... Might be the best one i saw yet to show this specific aspect.
Installing a woofer exactly in the center of the baffle may look better to some but it promotes standing waves inside the box.
This is a picture from the speaker building material of former British manufacturer McKenzie Acoustics (integrated into Fane Acoustics if i remember well). Its a generic box drawing, no proprietary info given, simply to illustrate what is referred to in the phrase under the sketch. That phrase is what matters about where to put the woofer.
Although the numerous box dimensions in the document (not shown here) are in the Golden ratio as indicated, McKenzie engineers still recommended asymmetrical woofer placement on the baffle. Again to minimize standing waves in cab. So here, a proper box ratio + off-center driver = the belt and the suspenders.
In a subwoofer cab its often easier to locate the woofer off-center since there's usually no other driver sharing the baffle space, contrary to a multi-way system.
I try to take best practice from each manufacturer i come across. You can increase the pic's size at will its clear. Please don't repost it on other web site(s).
Richard
POWERED BY: QSC, Ashly, Tascam, Rolls Mosfet, NAD, and Crest Audio
Hello gasfan;
Since the modal calculation rules of acoustics applies to all spaces large and small we find what plaques us in small rooms benefits us in low frequency enclosures. Where the length of frequencies in subwoofer enclosures are almost always longer than the enclosures internal dimension we can often get away with murder, so to speak. We can build cubes with no negative consequences.
Where the oft quoted golden ratio or other acoustic ratios are applied we find even distribution of modes ie the golden ratio divides standing waves on 2/3 octave intervals. Another often used enclosure ratio is 0.7937 X 1 X 1.2559 divides standing waves at 1/3 octave intervals and these ratios avoid mode stacking as well.
The math on your enclosure with the longest dimension of 57” dictates the enclosures first standing wave mode will be about 118Hz. That is likely outside of the passband of most subwoofers and while there will still be some energy at 118Hz with a say an 80 Hz crossover, with the high aspect ratio of your proposed enclosure there will not be another mode anywhere close. Everything below 118Hz will be pressure modulation only.
As has been stated, be sure the motor has adequate venting distance to the back of the enclosure.
I say build it and brace well.
Barry.
If we knew what the hell we were doing, we wouldn't call it research would we.
Right. Those dimensions will fit neatly out of the way. It's about space. The Sub18s are just too massive for my space.
You need to like the box shape and its fit in your home, something you can live with under different circumstances since lifestyle changes over time.
Golden ratio cabinet typically leads to what some call a shoebox type cab, e.g. 5 cu.ft. = 33.2 X 20.52 X 12.68"
As for the Acoustic ratio the box usually isn't as tall, but deeper type, e.g. 5 cu. ft. = 25.85 X 20.52 X 16.29", this assumes woofer assigned to baffle in the same manner as above.
Another cab building rule. Even though one has respected a proper box ratio (Golden, Acoustic, etc), therefore shouldn't have to bother further about this, there's yet more to reinforce the prevention of standing waves in the cab: woofer placement off-center vertically and/or horizontally!
JBL 1983 2245H original article, extract page 5, picture. There's another pic about this i'll post it tomorrow. Gotta go now.
Richard
POWERED BY: QSC, Ashly, Tascam, Rolls Mosfet, NAD, and Crest Audio
In addition to post #6, JBL also has a rule that any cab dimension must not be more than three times any other. Moreover, high power drivers must have sufficient clearance behind woofer back vent and rear of enclosure. The latter also applies to vent clearance vs rear of box.
That kind of kills your proposed dimensions, sorry. I think you will have to use one of the usually accepted box ratios (proportions), such as the Golden ratio, the Acoustic ratio, or one of the other such ratios mentioned in Dickason's Loudspeaker Design Cookbook, and in Weems' Designing Building ... Speaker Systems.
The rest of the phrase on the pic is: three times greater than the shortest dimension. (JBL document)
Richard
POWERED BY: QSC, Ashly, Tascam, Rolls Mosfet, NAD, and Crest Audio
What would/could the consequence be? The driver on the 22" side 10" up from the bottom, the ports up top.
"I could be arguing in my spare time"
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