ML-TL: a line that ends in a port

An ML-TL, or mass-loaded transmission line, is a straight line that does not end in an open end but in a short port. The port adds air mass at the end of the line and so tunes it lower. The line can therefore be shorter and the speaker lower. Martin J. King calculated and measured this type in detail.

Section through a column with an ML-TL. A straight line with no baffle, a closed top with the driver close to it, damping only in the upper part and a short port near the bottom.

How it works

An ML-TL is something between a transmission line and a bass reflex. At the tuning the port plays as in a bass reflex, higher up the line resonances come into play. The driver goes close to the closed end, at about a fifth to a quarter of the length. The damping belongs only in the upper part; the port must stay clear.

What the calculation shows

All charts in this series on enclosure types are calculated by Loudsmith for the same 6.5" woofer, the SB Acoustics SB17NRX2C35-8 (Fs 36.5 Hz, Qts 0.43, Vas 27 l), so they can be compared. The 0 dB level is the driver's level in the midrange. An ML-TL with a 75 cm long line, a volume of 35.4 l and a port of Ø 40 mm and 40 mm long, tuned to 36 Hz.

TotalConePortSealed box 13.6 l for comparison

ML-TL 35.4 l. The −3 dB point is at 35 Hz, similar to a bass reflex. Above the tuning the outlet is damped.

The impedance has two main peaks, 33 Ω at 24 Hz and 31 Ω at 57 Hz, like a bass reflex. The small peaks at 237 and 463 Hz are line resonances.

Pros and cons

Pros: deep bass like a bass reflex, but with less box resonance in the mid-bass, because the line damps the standing waves. It is shorter than a transmission line and simple to build: a column with no baffles.
Cons: the box is bigger than a bass reflex, and the short port makes noise in strong bass if it is narrow. The amount of damping has to be fine-tuned.

Which driver and which room

It suits both full-range drivers and woofers, and floorstanding speakers. A port near the floor should stay at least a few centimetres away from it.

Well-known designs

Martin J. King: Lowther DX in an ML-TL and Fostex FE-206E. This is a 107 cm tall column, a Ø 4" port and damping of 0.25 lb/ft³ only in the upper part. For the Fostex King gives a tuning of 32 to 34 Hz; Loudsmith calculates 33 Hz.
Planet10 and Scott Lindgren published complete ML-TL tables for small Fostex and Alpair full-range drivers.

In Loudsmith

Loudsmith calculates an ML-TL as a straight line with a port at the end. The line is the inside of the column, and the driver position is taken from the drawing. It picks a port from the shop's range so that the air in it does not exceed 17 m/s.

Enclosure types series: sealed · bass reflex · passive radiator · aperiodic · transmission line · TQWT · ML-TL · open baffle · back-loaded horn. You will find the formulas in the article Speaker box calculations step by step.