Why I Decided Not to Build an MTM Transmission Line

A practical reflection on MTM, impedance, and listening reality During the exploration of a new loudspeaker project, I was presented with a transmission line (TML) floorstanding design based on Scan-Speak Revelator drivers, configured in a D’Appolito (MTM) layout and with a nominal impedance of 4 ohms. On paper, this design is highly attractive. I have built several Scan-Speak–based loudspeakers in the past, including the Ekta Grand, and I greatly admire the brand’s consistency, low distortion, and tonal neutrality. The transmission line principle itself also appeals to me: the promise of controlled, extended bass without the typical reflex artifacts is compelling. However, after a deeper technical and practical analysis, I decided not to proceed with this MTM TML design — and this decision was not driven by a single flaw, but by the combination of design choices.

The D’Appolito (MTM) Principle in Practice

An MTM configuration places two identical midwoofers symmetrically above and below a tweeter. The theoretical goal is controlled vertical dispersion and reduced floor and ceiling reflections. When executed correctly, this can result in excellent focus, precision, and reduced distortion.

The Lautsprecher Berlin TML Emotion Berlin II is an example of this design.  

Bausatz TML Emotion Berlin II (Paarpreis)

However, the practical implications are significant:

  • Very narrow vertical listening window
    The tonal balance changes noticeably when listening above or below the tweeter axis. This makes the design highly sensitive to seating height and speaker positioning.

  • Crossover sensitivity
    MTM designs require precise driver spacing and steep, well-controlled crossovers. Small deviations in construction, baffle layout, or component tolerances can become audible.

  • Listening position discipline
    These speakers reward a fixed, seated listening position, but are far less forgiving in real-life living spaces.

While none of this is inherently negative, it defines a speaker that prioritizes precision over flexibility.

4 Ohms vs 8 Ohms: Why It Matters

The nominal 4 ohm impedance of this design is another critical factor.

Although often misunderstood, a 4 ohm loudspeaker is not simply “twice as loud” or “more powerful” than an 8 ohm design. In practice, it means:

  • Significantly higher current demand from the amplifier

  • Greater thermal stress on output stages and power supplies

  • More sensitivity to impedance dips and phase angles

  • Reduced amplifier headroom under dynamic conditions

Even with a technically capable amplifier, a 4 ohm MTM design often feels more demanding, less relaxed, and more dependent on amplifier quality than an equivalent 8 ohm system.

This is especially relevant when combined with:

  • moderate sensitivity (≈85 dB / 2.83 V / 1 m),

  • parallel midwoofers,

  • and a complex crossover network.

MTM + TML + 4 Ohms: A Cumulative Effect

Individually, none of these choices is problematic:

  • MTM can be exceptionally precise

  • Transmission lines can produce superb low-frequency quality

  • 4 ohm systems can offer excellent control and low distortion

But together, they form a loudspeaker that demands:

  • a powerful, high-current amplifier,

  • meticulous construction,

  • careful listening geometry,

  • and a listener who values precision over ease.

In contrast, my reference speaker — the Ekta Grand bass-reflex design — offers:

  • higher perceived efficiency,

  • greater tolerance to listening height and position,

  • a more relaxed and dynamic presentation,

  • and a sense of musical “flow” that requires less effort from the system as a whole.

The Key Insight: It’s About the Combination

This experience led me to an important conclusion:

The issue is not the transmission line principle itself, but the cumulative effect of multiple demanding design decisions.

A TML with:

  • a single midwoofer (MT),

  • or a 2.5-way topology,

  • larger drivers,

  • and an 8 ohm impedance
    could very well align with my preferences.

What I chose to step away from — for now — is the specific combination of MTM + 4 ohms + moderate sensitivity, despite its undeniable technical elegance.

Final Thoughts

DIY loudspeaker building is not about chasing complexity or theoretical perfection. It is about coherence between design, room, amplifier, and listening habits.

Walking away from a technically impressive design is not a rejection of its quality — it is an affirmation of one’s own listening priorities.

In this case, choosing not to build turned out to be just as valuable as choosing to build.