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.
Technology
Design Summary – Cabinet Height & Bass Reflex Tuning For this loudspeaker design based on the Accuton C168-6-890 midwoofer and Mundorf AMT25CS2.1-R, careful attention was paid to cabinet proportions and low-frequency tuning. An external cabinet height of 105 cm was chosen to place the tweeter’s acoustic centre close to seated ear height, ensuring optimal tonal balance and stable imaging. Taller cabinets (e.g. 120 cm) tend to position the tweeter too high when mounted near the top. Using Troels-inspired proportions with 22 mm MDF, the cabinet provides a net internal volume of approximately 42–44 litres, which is well suited to a single C168-6-890 in a bass-reflex alignment. Low-frequency loading is handled by a single rear-firing Ø80 mm bass reflex port. For a tight and musically balanced low end, a tuning frequency of approximately 42 Hz is recommended, corresponding to a physical port length of about 14–14.5 cm. Final tuning should always be verified by impedance measurement. This combination of proven cabinet geometry and measurement-based optimisation offers an excellent balance between depth, control and efficiency, forming a solid foundation for a high-performance DIY loudspeaker.
CLIO Pocket – Summary The CLIO Pocket measurement system is fully sufficient for designing a passive two-way loudspeaker such as the Inn Mk II using an Accuton midwoofer and a Mundorf AMT tweeter. It provides all essential measurements, including impedance (Fs, bass reflex tuning Fb, Zobel and baffle-step effects), quasi-anechoic frequency response (on- and off-axis with time gating), and phase/timing analysis needed to verify crossover integration and stepped-baffle alignment. While CLIO Pocket does not offer full anechoic conditions or very low-frequency distortion analysis, these limitations are irrelevant for serious DIY loudspeaker development. With correct setup and careful interpretation of the data, CLIO Pocket enables precise crossover tuning and reliable optimisation of cabinet and port design, making it an ideal tool for high-quality DIY loudspeaker projects.
The CLIO Pocket measurement system is entirely sufficient for designing a passive two-way loudspeaker such as the Inn Mk II using an Accuton midwoofer and a Mundorf AMT tweeter. It provides all essential measurements, including impedance (Fs, bass reflex tuning Fb, Zobel and baffle-step effects), quasi-anechoic frequency response (on- and off-axis with time gating), and phase/timing analysis needed to verify crossover integration and stepped-baffle alignment. While CLIO Pocket does not offer full anechoic conditions or very low-frequency distortion analysis, these limitations are irrelevant for serious DIY loudspeaker development. With proper setup and careful data interpretation, CLIO Pocket enables precise crossover tuning and reliable optimisation of cabinet and port design, making it an ideal tool for high-quality DIY loudspeaker projects.