Acoustic transmission line
Transmission line cabinet (short: TL or TML ) are a special type of loudspeaker cabinets. This speaker principle uses standing waves in a delay line to improve the reproduction of low frequencies. Despite similarity to the bass reflex speaker, the active principles of TML and bass reflex differ significantly from each other.
Operation
In a simple transmission line, a loudspeaker excites a tube open on one side at or near the closed end to vibrations.
The shaft, which runs towards the pipe end, is reflected for the most part due to the different impedances of the radiation pipe and the outside air and is formed in the tube, a standing wave. The fundamental wave is formed in tube length = lambda / 4, the harmonics in the odd-numbered multiple of lambda / 4 One therefore speaks of quarter-wave resonance enclosures. Particularly at these frequencies, sound energy is then discharged via the open tube end primarily in the environment, while the diaphragm of the speaker is in the rest position. The length of the tube is often (but not necessarily) chosen so that its lambda/4-Resonanz approximately coincides with the resonance frequency of the speaker:
Length = speed of sound / (4 x speaker resonance frequency)
By resonating at the two ends of the air mass, the pipe behaves as if it was longer by about the diameter thereof.
It must be prevented that higher-frequency components of the music signal radiated by the Rohröffung except the lambda/4-Resonanz, which would by interference- induced comb filter characteristic lead to a very unpleasant " tube sound ". A suitable design is, for example, the arrangement of a properly sized air volume than 6 dB low pass between the speaker and tube ( little clear-cut ).
__________ | | _______________________________________________________ \ \ Coupling / volume / _______________________________________________________ | __________ | An effective way to counter the severity of individual pipe resonances, is to position the speakers in the antinodes of a resonance. The elimination of the second harmonic wave the loudspeaker chassis would be arranged ( as seen from the closed end ), for the second harmonic at a fifth of the length of one third of the length.
The effect can be observed in addition to acoustic measurements also at the frequency response of the impedance.
______________________________________________________________________ | \ ____ / | D D D D D D D D D D D D D D I__I | ______________________________________________________________________ 1/3 D damping material .... For the practical implementation of TL housings are circulating numerous, partially false or outdated rules of thumb and design requirements. A realistic prediction of the acoustic behavior of a loudspeaker drivers in a transmission line cabinet allows the meantime simulation software, the most common programs are AkAbak and AJHorn, and the Mathcad Worksheets by Martin J. King.
The attenuation of higher resonances porous or fibrous damping material is usually introduced into the pipe. Is too much damping material in the tube, the tube behaves like an acoustic swamp and not as a transmission line.
The damping material is to be determined by appropriate measures in place. It also results in a reduction of the speed of sound, whereby a lower constructional length is required. The slight lowering of the speaker resonant frequency by resonating insulation is hardly relevant for the dimensioning.
Again, the impedance response provide evidence whether sufficient and was attenuated in the right places. In the field of speaker resonance frequency is a maximum impedance.
A transmission line pipe is due to its length, for example, about two meters for 40 Hertz tube resonance, in living rooms often difficult to accommodate. Therefore, and because material also can be saved, usually folded transmission lines to be constructed.
Characteristic is caused by destructive interference between the direct sound of the speaker chassis and the signal emitted by the tube opening sound amplitude frequency response sink in the upper bass range ( 80 to 120 Hz). Depending on the housing design and damping show up to higher frequencies more interference and pipe resonance effects in the amplitude frequency response.
Although bass reflex enclosures are to construct more efficient and easier to control, some prefer ( especially English ) designers and music lovers this construction.
---- ---------- . | | | | | | | | | | | | | | | | | | | | | / | | | | # # / | | | | # # < | | | | # # \ | | ' | \ | | | | | ' ------ ' ------ ' ---------------- For transmission lines of the speakers should have a relatively low resonance frequency.
The influence of Bedämpfungsmaterials on the mechanical quality factor and subsequently also the overall quality factor can be helpful when using the speaker chassis with low power and high overall quality factors as advantageous, with the strong mechanical damping is always accompanied with an increased risk of non-linear distortions. Cheap speakers that due to high quality factors for other housing types less suitable, are therefore often used in transmission line enclosures. Contrary to outdated design guidelines a high overall quality factor, however, is not a prerequisite for a functioning transmission line.