In my most recent speaker project, I set out to build a single driver transmission line design. The transmission line is a design of speaker which uses the back pressure of the speaker driver through a long continuous chamber to enhance the lower frequency output. The advantage of a transmission line over a typical ported design (slim desktop speakers, stereo speakers) is a better control of resonances and potential for lower frequency tuning.
My aim was to get as much sound out of a four inch full range driver as possible, using this box design to extend the otherwise lacking bass. I was also keenly interested in small footprint designs at the time, and combined the two to make an ultra small footprint, full range transmission line. The resulting speaker impressed me, with the box design providing a deep and smooth output as intended. Below is my documented design and assembly process.
The project began with acoustic calculations. I began with some rules of thumb since the full model of this design is not popularly documented. The design is based on the length of the chamber being a quarter the wavelength of the target resonant frequency. This could be the driver's free air resonance, Fs, but I decided to tune lower as transmission lines are able to lower the Fs of the driver. Other parameters include the cross sectional area of the chamber, the taper ratio from start to end, the lining and stuffing. The cross sectional area should be no smaller than that of the speaker's radiating surface area, and having it narrow towards the opening can improve bass responsiveness. I experimented with different lengths and tapers and decided to aim below the drivers Fs by just less than an octave. I found this to produce a smoother response than resonating at the driver's Fs. A taper ratio of 1:1.25 was used also used, with the opening equal to the driver's radiating surface area.
I used these dimensions and restrictions to calculate the rest of the box geometry. But first I needed to decide how to arrange the chamber length. A continuous line would be too tall, and leads to standing waves which introduce unwanted resonances. I used Solidworks to model the speaker using both two and three fold designs and decided on the former, using the Solidworks sketches as cutting plans.
I used 12 mm hardwood plywood to construct the enclosures for its rigidity and ease of working and assembly. I hand sawed all the pieces making sure each side aligned perfectly. I used clamps, belts, and nails to hold the box together while the PVA glue dried. The combination of nails and glue makes for an incredibly rigid and strong box. The important factor for speaker design is to minimise enclosure flex, as any flex means energy lost. The internal wall to create the folds also acts as bracing to reduce flex. The panels were also hot glued along their seems to eliminate air leaks, another important speaker enclosure requirement. The speaker driver is mounted and wired to the terminals at the back. However, the side panel is not mounted yet. This is so that testing can be done and changes made inside, namely, fine tune the amount of stuffing and lining and its placement. Stuffing and lining help reduce standing wave resonances, reduce higher frequency output, and reduce internal sound reflections being emitted through the speaker surface, all of which distort the sound.
Test and Refine
Different placements and amounts of lining and stuffing were tried and recorded, the side panel was held on as tightly as possible with clamps and belts throughout testing. The control test "0g Stuffing & No Lining" shows that without any treatment the box behaves as expected, with uncontrolled resonant peaks. The roll of the stuffing and lining is to control these peaks and create a smooth output. I found that there was a trade off between control and output power; the more damped the system the lower the output power. Since output power is important for efficiency (especially with such a small driver) and makes a more noticeable difference to the bass frequencies, it was important to preserve some power. I found that stuffing was most effective when directly behind the driver, meaning a smaller amount could be used to maintain power with good enough peak damping effects. The lining always seemed to help control the smoothness without reducing output so was left included completely. I settled with 36g of stuff placed in the immediate path from the rear of the driver.
I then finished the side panel assembly and speaker enclosure. I used a wood fill to cover over the breaks in the plywood edges and smooth over imperfections, and sanded the box down. I repeated this process until a very smooth finish was achieved. Next I painted on a few coats of white primer and sanded between applications. Finally, a medium gloss white paint was applied. I also added an aluminium mesh to the slot opening to hide the internal fabric lining and stop objects falling inside. I found out at this point that fabric covers are not suitable for covering transmission line openings as they restrict the air flow too much.
The speakers are surprisingly powerful. While they won't be replacing a subwoofer, when positioned in the corners to take advantage of room gain they are powerful enough for all applications.