8-inch ported bass speaker¶
Francis Deck
Created Nov. 11, 2023
Updated Dec. 18, 2023
I plan on rebuilding this thing, probably in the spring, to further address the port air speed issue.
This is my 8-inch ported bass speaker design, using a Faital Pro 8PR200 lightweight driver. I've been using it for several years, and have finally gotten around to writing it up. At present, I'm using it for all of my gigging on upright and electric bass. It's not perfect, as I'll discuss, but quite a capable speaker for its intended uses.
Builders should note that my construction "plans" are going to be quite sparse, because I don't work from plans. I write down the dimensions that I've worked out in my modeling program, then I go into the garage and start cutting plywood.
Here's the modeling data¶
Motivation and design goals¶
My approach to speaker design is mainly to wait for the right driver to come along. Sometimes I wait a long time. I take note of drivers that have caught the interest of builders, and plug them into my software. If a driver looks like it can be turned into a promising design, then I take it further. This is what happened with my 12" design, based on an Eminence 2512-ii driver, and now with the 8PR200.
The 8PR200 is a lightweight 8" driver with respectable characteristics: A low resonant frequency means that it can be designed into a relatively compact bass speaker, perhaps at the expense of overall sensitivity. Generous Xmax of 8 mm means it will produce a fair amount of sound. I don't have an eye for the Q specs, so I just plug drivers into my program and see what happens.
I play upright and electric bass, mostly jazz. I still have the 12" speaker at my disposal, but if a jazz gig needs more sound then the 8"er can put out, it's too loud.
A widespread rule of thumb is that the port air speed should be kept below 10% of the speed of sound, i.e., 0.1 Mach. We've got a problem. The 8" design is too close to 0.1 Mach for comfort.
The port air speed could be reduced with a larger port area or diameter, but only by increasing its length to an extreme leading to impractical box dimensions. This is what I call the "small box crisis." Small box plus large Xmax leads to a port that can grow to the size of the box itself. Big box negates the benefit of a small driver.
At the very least, the box requires careful port design. A respected builder on TalkBass, Duke LeJeune, recommends cutting the inside end of the port at an oblique angle as a sort of poor man's flare. The outside should also be flared if possible. But at the end of the day, this box will probably require a high pass filter (HPF) to be functional at higher volumes. I don't plan on ever pushing it beyond 100 Watts.
I feel kind of guilty about that, because I sell HPF's, and seems unfair to share a box design that forces you to buy my stuff. But it's something that you need to be aware of.
Next comes excursion. This plot shows cone excursion at 100 Watts:
The excursion limit for the 8PR200 driver at 100 Watts is a generous 8 mm. My design will handle 100 Watts without difficulty, especially given that the bass signal is a mixture of harmonics. Why did I choose that power level? It's consistent with the output voltage of my amplifier when it's driven with a sine wave to the threshold of its built-in limiter, so it's a realistic maximum for my typical use.
Besides, port air speed is also a power limit for the design. The impact of the "small box crisis" is that the port becomes the factor that limits power. Small speakers have no secrets that allow them to bypass the laws of physics.
Now, how's it sound? Here's the response curve:
There's not much to say, except that with a little bit of equalization, the speaker should have respectable bass response, but it won't ever be super loud. It's actually a fairly deep sounding speaker.
Construction in general¶
The internal dimensions of the box are 12 x 9 x 9.5 inches, and the port is given as 5.5 cm square. My program calls for a roughly 10 inch port, depending on how you account for the volume displaced by the driver. I got away with a trick with my 12" speaker, which is that the port terminates within a couple inches of the baffle, so it is effectively a bent port. I did the same here. Measurement of the port frequency will tell me how close I got!
I've played with a couple of tricks to make this port work. It is technically a shelf port, but with a square cross section. It opens to the side of the box, so it can run along the longest internal dimension. I was able to fashion this port from scrap wood without buying a commercial port tube. I wasn't able to clamp the pieces very well, but it's also not structural in this design.
I built it from 1/2 inch "BC sanded" plywood. This is not the highest grade of lumber, but seems to work just fine for smaller cabinets. I don't have access to a local source of high grade plywood that is not in 4 x 8 foot sheets. This box can be made from a 2 x 4 foot sheet. I used thinner plywood from my scrap pile for the inside walls of the port.
Now for my construction method¶
Here are the drawings.
The rules of my method are:¶
- Work safely. If you're not proficient at safe use of the tools you've got, just stop. It's not worth getting injured.
- Choose the straightest possible plywood panel you can find. You might have to pick through the pieces at the store.
- Verify my math. You need to be able to prove to yourself that my dimensions are correct! In fact you need to "own" the design. That's why I've shared everything I know.
- Cut one panel at a time, or at most two if they have identical dimensions. The idea is that if you wait to cut more panels, you can correct for minor dimensional errors as you go. Do some planning:
- You don't want to run out of wood. So you should at least lay everything out on a piece of grid paper and plan your cuts
- If you have a saw with a fence, cut all of the identical dimensions at the same time, for instance the 9 inch dimensions of panels A, A', B, and B'.
Glue the pieces together one at a time. This minimizes the number of clamps and fixtures that you need to own. Verify square with a square.
When you're ready to cut another piece, measure what you've got, in case you need to make minor changes to the dimensions.
Glue the next piece on, repeat until done.
The steps are:¶
Cut panel A. Cut the hole.
Cut panels B and B', which should be identical. You can cut the hole for the port in one of the panels at this point.
Glue panels A and B
Glue panels A and B', making sure everything is square and parallel.
Cut and glue panel A' onto the edges of B and B'. Now you have the skeleton of the box.
Cut and glue panel C (the bottom), but not C' yet. This will give you access to build the port.
Cut and glue D. This is not a structural part.
Cut and glue E. This is not a structural part.
Finally, cut and glue panel C'.
Trimming and finishing:¶
My philosophy is to avoid buying piles of store-bought trimmings. They really add to the cost, and I just don't need them. My gear doesn't get thrown around by roadies, or taken on tour. It doesn't need to look "professional," just unobtrusive. Jazz players and audiences are mostly blind to gear.
I rounded all of the edges with sandpaper on a large sanding pad that's sold for drywall finishing, one of my best tool purchases ever.
I rounded the edges of the port hole with a router bit, but this could also be done by hand with a wood rasp and sandpaper, or a chisel. I would not go out and buy a router just to make this speaker. And routers have their own safety issues.
As you can see, the feet are pads of plywood, glued on, with additional self adhesive rubber feet that I had on hand.
For the connector, I drilled part way into the plywood with a hole saw, then cleaned out the hole with a chisel, and drilled the smaller hole all the way through. The hole saw is from fitting a new door to an old house. As you can see in the above picture, I put the connector on the top of the box, so it doesn't interfere with anything when I carry it around "combo style." Likewise, putting the port on the side of the box means that I can shove it up against a wall, or the back of a chair, if I want to.
Black Duratex, but ouch the price, even for the small quantities. Today, I'd be looking for something else, even black spray paint. I still have most of a gallon left from when it was cheaper.
The speaker is held in by drywall screws into pilot holes. I don't remember if the driver came with a gasket, or I had to make one out of weatherstripping.
The grille is actually a "bug eye" grill for an 18 inch speaker. I cut it down to size, then re-used the rubber gasket from the 18-inch grill, just to hide the cut edges.
The internal wiring is whatever I had on hand. You could pull apart an old computer power cord (unplug it first), and use its conductors.
The handle is a scrap of nylon webbing, held on the sides by more drywall screws and washers. Again, I didn't want to pay for a handle. The strap is long enough to go over the amp head, which is held down by a piece of bike innertube.
Testing¶
With a mic close to the cone, I ran a crude response test. The "dip" in the curve is the cone resonance, at roughly 45 Hz. Close enough!
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