PMC Prophecy1 – Timing Beast!

Something about ATL - Advanced Transmission Line

What PMC is all about, of course, is the ATL system: Advanced Transmission Line with the Laminary X flow technology which controls the airflow through this transmission line. PMC has been working on this for more than 30 years.

A transmission line is substantially different from a bass reflex system. A transmission line is basically a long pipe with a bass unit at the end. This ‘tunnel’ attenuates high and mid tones but does allow the lower frequencies to pass through. The area in which this ‘tunnel’ works is much larger than in bass reflex which is often tuned to a single frequency.

However, the proper design of a transmission line is much more complex, expensive and takes up more space (i.e., the cabinet must also be larger). Although the transmission line is designed around a certain frequency, it works much more broadly than a bass reflex system (the attenuation is also never 100%, allowing harmonics to come along as well, which is mostly beneficial for natural reproduction).

However, a tranmission line does stand 90 degrees out of phase. The playback at the rear of the unit is 180 degrees out of phase, the transmission line is usually 1/4 of the wavelength, so 180 90 degrees = 270 degrees, resulting in -90 degrees out of phase.

This is not an immediate problem, because the whole system is thus much more broadband, which means that the energy in the bass contributes longer and more broadband. This compensates for the part around the tuning frequency, which is out of phase. This is in contrast to a bass reflex system where, although the port is ín phase, it cuts off much more abruptly outside the tuning frequency. (This is actually always noticeable and audible, your author thinks. Especially at softer volumes).

Examples – Transmission Line, Bass Reflex, Closed

Here are some examples of amplitude and phase progression of a closed system, a bass reflex system and a transmission line. Which is better or more favorable is always up to the manufacturer’s use a design.

• Theoretical models – amplitude

  • Bass reflex (orange) gives a clear peak around tuning.
  • Transmission line (blue) runs smoother, with deeper response.
  • Closed box (green) falls off nicely by 12 dB/octave.

• Theoretical models – phase

  • Bass reflex: sharp turnaround around tuning.
  • Transmission line: around -90°, but spread out wider.
  • Closed box: predictable and gradual to -90°.

• Practical example 8″ woofer

  • Closed (20 L): tight, but little output below ±60 Hz.
  • Bass reflex (30 L, fb=40 Hz): more punch around 40 Hz, but abruptly tapering off below that.
  • Transmission line (60 L, fb≈35 Hz): larger cabinet, but smoother and deeper low end.