@ chrisj1948, @iron, and anyone else interested. I’m going to take a stab at describing the differences and potential benefits of 1/4 wave/transmission line loading vs bass reflex or sealed box loading. I will preface what I’m about to write with the fact that I’m a keen layperson and past casual home brew speaker designer, not an audio professional so if I miss something obvious, I’m happy to have any more knowledgeable member here chime in.
Like the air spring of the enclosed volume of an acoustic suspension (sealed box) loudspeaker, or the port tuning of a bass reflex design, the primary job of a Transmission line’s 1/4 wave tapered tunnel length is to damp the woofer’s motion at low frequency system resonance. This prevents the woofer from gyrating wildly at resonance, protecting it from damage due to over excursion and reducing distortion.
Like a bass reflex design (whether ported or passive radiator), a 1/4 wave/transmission line offers the additional benefit of using the port output of the line termination to reinforce the woofer’s fading response at the bottom of the speaker’s passband. Some keen AV gearheads should now be asking, “Well if the port of a transmission line speaker is essentially doing the same job as the much simpler port or passive radiator in a bass reflex speaker, why go to all the effort and expense of building much more complex transmission line enclosures???” Great question! Read on…
Knowledgeable fans of transmission line loading will point out that a tapered and optimally stuffed transmission line provides damping of the woofer over a broader frequency range than does the tightly tuned port or passive radiator resonance of a bass reflex speaker. This should result in superior control of woofer motion resulting in “tighter” lower distortion bass. If the transmission line designer got it right, the benefits will be measurable in a few ways.
Compared to bass reflex designs or big woofer/small sealed box acoustic suspension designs, the physically larger transmission line speaker should exhibit much smoother electrical impedance at low frequencies. A smoother less reactive impedance curve makes the transmission line an easier load for an amp to drive. The acoustically measured port output of a transmission line is lower Q- the output will be a broader shallower hump than that of a bass reflex speaker. Finally when compared to the port or passive radiator output of a bass reflex speaker, the port output of a transmission line remains largely in phase with that of the woofer. However some experts claim that the audibility of gradual changes in acoustic phase is negligible, particularly down low.
I’m going to wrap this up with the fact that in my 40+ years experience as a keen audio hobbiest, transmission line loading is not the only road to great bass. Yes I’ve heard some truly great, tight, extended, and authoritative bass produced by Transmission lines from IMF, Fried, and PMC over the years. But I’ve also experienced equally great bass from the coffin sized sealed enclosures of the Dunlavy SC4, SC5, and SC6. And then there would be the incredibly propulsive, low distortion bass of active bass reflex ATC loudspeakers from their SCM50ASL model on up. Or the tight, tuneful low frequencies Bobby Palkovic (RIP) of Merlin Music Systems was able to coax from the single 7" pulp cone mid-woofer in his compact floor standing VSM bass reflex design. Great subwoofers add another dimension to what is possible yet. There is clearly more than one road to bass perfection.
A final sort of related thought (really this time). In my experience the killer low frequency prowess of all the specific loudspeakers I mention above can be completely ruined by poor placement and/or the unfortunate acoustics of re-purposed domestic living spaces. Optimal placement in a well designed and treated room trumps all differences in loudspeaker design. My largest listening let-downs have all involved hearing mega-buck flagship loudspeakers and systems in rooms where acoustics and placement were not a priority.
but I can say what they can add to a great system can be jaw dropping when properly set up! I picked my v1’s up for bargain basement price and would never think about paying $10k for subs but when I hear what it can do in a good system it justify’s the price.
