The Transmission Line.

       Our project is based upon a transmission line (TL) and is similar in design to the TL1 as described in our products section.
Here we explain the basic ideas behind its operation, detail the construction with comprehensive drawings and incorporate hints, tips and suggestions for optimising performance. As levels of ability vary in woodworking and electrical skills etc, all panels, parts and components can be obtained separately.

The Transmission Line Explained.

       So what is a TL? Well, most loudspeaker chassis require an enclosure / cabinet to live in such that they can give of their best performance. There are a number of different styles of cabinet available, closed box, reflex, horn, open baffle, transmission line, panel etc, all of which have their own characteristics, just like the neighbours, and, like the neighbours, they can shuffle about from one house / cabinet to another. Of course some are more suited to a particular environment and therefore interchange between them is on a limited basis. So.... a transmission line is just nothing more than one of many differing styles of cabinet enclosure to house a loudspeaker chassis although it does have its own particular method of operation and characteristics.

       The driver is essentially mounted at one end of a long tube. The length and cross section of the tube being related to the physical properties of the driver. This system of driver loading (enclosure) is made more user friendly by folding it into the shape of a box. How many folds depends upon the length required and the acceptability of the final shape but two or three folds is usually sufficient. The cross-section is similar to the driver Sd (cone area) and can be a straight tube or tapered to reduce resonances. The tube length approximates to a quarter wavelength of the drivers Fs, (resonant frequency) although other ratios 1/2 and 3/4 wavelengths can be used resulting in longer / larger enclosures.
        Selective stuffing is used to both absorb the higher frequencies and increase the acoustic path length. This means you can use a shorter tube. The effect varies with the stuffing density and material, and is an inexact science. The art of stuffing application is called tuning the system. As we're more interested here in the practice than the theory the finer details are omitted other than where covered in the general scheme of events.

General Comments.

       TL's are well known? for three basic parameters, copious bass, fine midrange and excessive cabinet size. In our project both bass and size have been restricted by driver choice to provide a satisfactory / balanced acoustic, (our opinion), whilst leaving the midrange clarity untouched. The driver line-up uses two well-known manufacturer's units, a Focal dual-wound mid-bass unit partnered with a Morel MDT30S tweeter along with crossovers incorporating air cored coils and polypropylene capacitors.
       Please don't ask if you can use other drivers, bits etc., of course, all things are possible, but if you bend it out of shape the performance may suffer and the design becomes yours not ours, and you're then on your own, OK so......nuff said.