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Concept
Transmission line loudspeaker
Summary
A transmission line loudspeaker is a loudspeaker enclosure design which uses the topology of 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 (TL) 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, and it 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 commercial success for a number of manufacturers such as IMF, TDL, and PMC. As a rule, transmission line speakers tend to have exceptionally high fidelity low frequency response far below that of a typical speaker or subwoofer, reaching into the infrasonic range (British company TDL's studio monitor range from the 1990s quoted their frequency responses as starting from as low as 17 Hz depending upon model with a sensitivity of 87 dB for 1 W @ 1 meter), without the need for a separate enclosure or driver.
Official source
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