Transmission line loudspeaker

A transmission line loudspeaker is a loudspeaker enclosure design (topology) that uses an acoustic transmission line within the cabinet, compared to the simpler enclosures used by sealed (closed) or ported (bass reflex) designs. Instead of reverberating in a fairly simple damped enclosure, sound from the back of the bass speaker is directed into a long (generally folded) damped pathway within the speaker enclosure, which allows far greater control and use of speaker energy, and the resulting sound.Inside a transmission line loudspeaker, is a (usually folded) pathway into which the sound is directed. The pathway is often covered with varying types and depths of absorbent material, may vary in size or taper, and may be open or closed at its far end. Used correctly, such a design ensures that undesired resonances and energies, which would otherwise cause undesirable auditory effects, are instead selectively absorbed or reduced (""damped"") due to the effects of the duct, or alternatively only emerge from the open end in phase with the sound radiated from the front of the driver, enhancing the output level (""sensitivity"") at low frequencies. The transmission line acts as an acoustic waveguide, and the padding both reduces reflection and resonance, and also slows the speed of sound within the cabinet to allow for better tuning.Transmission line loudspeakers designs are more complex to implement, making mass production difficult, but their advantages have led to acclaim for a number of manufacturers such as IMF, TDL, PMC, KVART & BØLGE and the like. As a rule, transmission line speakers tend to have exceptionally high fidelity low frequency response far below that of a typical speaker or subwoofer, into the infrasonic (British company TDL's studio monitor range from the 1990s quoted their frequency responses as starting from as low as 7 Hz depending upon model) at sensitivity 96 dB, without need for a separate enclosure or driver. Acoustically, TL speakers roll off slower at low frequencies and provide better driver control than standard reflex cabinet designs, are less sensitive to positioning, and tend to create a very spacious soundstage. Modern TL speakers were described in a 2000 review as ""match[ing] reflex cabinet designs in every respect, but with an extra octave of bass, lower LF distortion and a frequency balance which is more independent of listening level"".Although more complex to design and tune, and not as easy to analyze and calculate as other designs, the transmission line design is valued by several smaller manufacturers, as it avoids many of the major disadvantages of other loudspeaker designs. In particular, the basic parameters and equations describing sealed and reflex designs are fairly well understood, the range of options involved in a transmission line design mean that the general design can be somewhat calculated but final transmission line tuning requires considerable attention and is less easy to automate.