What's an Ml-Tl ?
Some transmission line purists hold fast to a position that there is only one “true” transmission line configuration, one in which the line has a constant cross-sectional area along its whole length, where the ¼-wavelength resonant frequency determined by its length is the sole tuning component, where the line contains fiber stuffing usually throughout its whole length, and where there is only one peak in the impedance characteristic curve. Ignoring any disagreements about what a “true” TL is, we’ve established what a traditional or classical TL is. The main disadvantages to a traditional TL are that it must be rather long for an F3 in the 20-Hz range and its overall response will have significant peaks and dips at the odd-harmonics of its ¼-wave resonant frequency. For instance, if its ¼-wave resonant frequency needs to be in the 20-30 Hz range, it has to be 110-170 inches long. Unless most, if not all of its length contains stuffing, one has to live with those odd-harmonic dips and peaks in the response, but when those are adequately smoothed over by the stuffing, there is little, if any, contribution to the system’s overall output from the line’s exit (terminus), ultimately limiting useful bass output.
Many of the disadvantages of a traditional TL can be minimized by making the line have a decreasing area, tapering, going from the largest area at the driver end of the line to the smallest area at the terminus end of the line. Tapering makes the line’s effective length longer than its actual length and reduces the magnitude of the response ripples. For instance, a line having a 10:1 taper needs to be only ~60% as long as a non-tapered line having the same ¼-wave resonant frequency. Fiber stuffing is still needed, but it’s no longer necessary to stuff the whole line, typically just the first 40-60% of the line, in order to adequately minimize response ripples. For the same F3 (which is determined by the volume contained in the line and the specific driver’s abilities), a tapered line needs to contain the same volume as in the equivalent traditional line. The terminus’ output in a tapered line also provides quite a bit of contribution to the bass output.
A mass-loaded TL is very similar to a tapered TL in terms of its performance and usually has a constant cross-sectional area along its whole length, plus exits via a port tube instead of just a hole in the cabinet wall (terminus). An ML-TL can also have a deceasing taper or an expanding taper. In this latter case, it would then be called an ML-TQWT, for mass-loaded, tapered, quarter-wave tube, and the driver would be located at ~half way along the line. A good starting point for designing an ML-TL with a constant area is to make its length equal to the length of a 10:1 tapered TL that has the required ¼-wave resonant frequency. Initially the port’s area can be chosen to be ~10% of the line’s cross-sectional area, or the nearest “standard” tube diameter. In modeling the line, its length can be changed and the port’s dimensions and location can be fine-tuned to provide optimum results.
In an ML-TL, then, the system’s overall tuning frequency is partially determined by the ¼-wave resonant frequency of the line’s actual length and partially by the port’s diameter and length. The system’s bass reach (F3, F6 and F10) for the specific driver is primarily determined by the volume in the line, so increasing or decreasing the cross-sectional area of the line will subsequently increase or decrease the line’s volume, followed by tweaking the port’s dimensions if necessary. Stuffing is still needed, but usually not more than in the first 50% of the line’s length, with lighter densities sufficing.
The mass-loading port’s location can also be adjusted to optimize the system’s overall response flatness, with the port usually being located several inches from the end of the line opposite the driver end. In each of these two ML-TL woofer enclosures for the Philharmonic speakers, the line’s ¼-wave resonant frequency determined by its actual length falls into the 50-60 Hz range, while the mass of air in the port loads the end of the line at and around the port’s resonant frequency, thus lowering the overall system tuning frequency to the 25-30 Hz range needed for optimum system response with the specific woofer. Like the terminus in a tapered TL, the port in an ML-TL significantly contributes to the system’s bass output. Some people consider an ML-TL to “just” be a tall, or long, bass reflex or vented system with no advantages. That’s simply not true because an ML-TL will have a lower tuning frequency, due to its ¼-wavelength resonance, compared to a more cubic-shaped bass reflex box of the same volume and port size.
Whether traditional, tapered or mass-loaded, these TLs share a major common trait; all use the ¼-wave resonant frequency of a longish column to provide bass support.
The two bass modules for the three Philharmonic speaker models have constant-area, mass-loaded transmission lines, with the lines folded into three segments to achieve the desired line lengths.
Philharmonic Speaker Systems
Concept, driver selection and crossover designs by Dennis Murphy.
Transmission line designs by Paul Kittinger.