Battle of the boxes

March 5, 2023
 by Paul McGowan

In my post about Dr. Suess and HiFi, I mentioned the work on loudspeaker enclosures by British engineer Leslie Bucknell while at the company my father worked for, Stromberg Carlson.

Bucknell’s approach to loudspeaker design was to create a speaker enclosure that would eliminate distortions that occur because of cancellations and additions due to standing waves internal to the cabinet. By carefully controlling the way that the sound waves travel through the speaker enclosure—routing them through a complex maze of tuned baffles—he felt that his Labyrinth design was the cat’s meow.

During this same time period, another British-born engineer, Arthur Bailey, was taking a slightly different tack to speaker design called the Transmission Line.

The transmission line uses a long narrow folded duct behind the woofer. The woofer’s output travels through this unimpeded maze until it exits out of the port. This technique differs from the traditional port (basically, a tuned hole in the speaker enclosure) because it is carefully calculated to arrive in phase and add to the low frequency performance of the speaker. In addition, as sound waves travel through the transmission line, they are gradually damped and absorbed, which helps to eliminate resonances and other distortions.

Of the two approaches, it is the work of Arthur Bailey that lives on today in speaker designs like those of PMC, Martin Logan, Audio Physics, and Zu Audio, who all (best I can tell) still use the transmission line approach to making better bass.

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30 comments on “Battle of the boxes”

  1. Ah…*there’s* the inner-bond that wasn’t there in yesterday’s internal cabinet design.

    “…transmission line approach to make better bass.”
    Better than what?
    Better than ABRs like the ones that are used in the aspen FR20 & FR30??
    I don’t believe that transmission-line cabinet design gives you “better bass”.
    However, it might give you more bass from a certain size of loudspeaker cabinet, than a sealed or ported cabinet design of the same volume/size.

    ‘Royal Ascot Mike’ went from a pair of PMC – ‘EB1’ (transmission-line) floorstanders, that he had for 15 years, to a pair of Spendor – ‘Classic 200’ (acoustic suspension/sealed box) floorstanders & I’m very sure from what he has told me that the bass is ‘better’ delivered by the Classic 200’s.

    1. I hate to see comments like ‘acoustic suspension/sealed box’ as it implies(and most people seem believe) they are the same thing. They aren’t. Acoustic suspension is a paticular form of sealed box. All sealed boxes are not acoustic suspension. And I suspect the Spendor Classic 200 is not acoustic suspension.

      The loss of the correct definition probably occured because in the 50s and 60s acoustic suspension was the dominant form of bass loading. This changed with the advent of Thiele/Small who developed the math for ported speakers so it was no longer a guessing game. Probably because porting is more efficient and more importantly for sales sounds more bassy in real world designs ported designs took over almost completely and with time almost everyone assumed that no port meant acoustic suspension when it only guaranteed sealed box.

      1. hahax,
        You are correct.
        I should have qualified.
        However, in this instance I typed, “acoustic suspension/sealed box” because I was unsure as to exactly which one applies to the
        Spendor – ‘Classic 200’ floorstander.
        Yes, acoustic suspension is a type of closed box design, but not all closed box designs are of the acoustic suspension type.

        If anyone reading this wants to understand the exact differences between the two types of design, a great explanation is available
        on Wikipedia – ‘Loudspeaker enclosures’ ✌

        1. “not all closed box designs are of the acoustic suspension type.”

          Oh? I suppose the next thing you’ll be claiming is that not all Greeks are Socrates.

          1. scottsol,
            Well, I *was* going to claim that not all Jacuzzi’s
            are hot-tubs or not all Hoover’s are vacuum
            cleaners, but, meh, what the hell 😉

    2. indeed I did Martin and the EB1’s (enormous box) served me well for over 15 years powered by Bryston amplification in my large listening room along the long wall…..a change was needed/desired firstly replacing my aging Bryston kit with a Musical Fidelity M8xi integrated which actually did relieve a little “edge” from the EB1’s. I then auditioned at home some very expensive modern speakers from Sonus Faber and Magico also in store the PMC Fenestria…….i was looking for detail and transparency together with all the usual other traits of a good speaker but without setting my teeth on edge and the Spendor classic 200 delivered just that; now the rest of the Spendor range including the classic 100’s are ported whilst the 200’s are sealed……now for the more and perhaps surprising bit the bass, oh the bass!! which is in a different league and dependant on completely different placement. My PMC’s sounded best well into the room 8-9ft apart with near zero toe in…….the Spendor’s however after much moving around need to be placed much closer to the front wall (front baffle currently 40 inches) 10/11ft apart and toe’d well in!……the bass is so full and exceptionally fast as a 100 plus db listen to Dave Brubeck’s 1963 recording at Carnegie Hall “Castillian Drums” verifies……Now i’m not a technophobe but I have now after 50 years plus of audio found what I consider to be mighty close to what I’ve been searching for……..I will add to this that a stereo pair of brand new REL S812’s run on high level have been relegated for movie use only on low level connection. No matter where i place them in the room, what phase setting, close to either inside or outside of speakers, in the corners, behind me or diagonally across and even setting them at ridiculously low levels in both crossover and gain they 100% muddied and slowed the bass! go figure that one out (like i say I’m no techy)……..its all in the bass..NAH!! its all about the phase! Don’t be phased by the phase IMHO.

      rock and bloody roll
      Baldy Bloke.

  2. If you rip open a Bose all in one crap box and try to figure out why they are so expensive and you see this mousetrap air way they called it

  3. A transmission line is nothing more than a long port that allows the rear wave of the woofer to be in phase with the front wave. A more accurate port with less cancellation. Acoustic suspension is never to my knowledge ever used to describe a ported or transmission line, the term used is reflex or bass loaded. A sealed box using dynamic drivers is referred to as acoustic suspension.

    1. You also don’t tend to get the port fart from a transmission line. I was going to add this but got caught by the 10 minute to edit rule. Not sure why that’s not at least an hour or more or gone completely. People should be able to edit or delete their posts at anytime in my opinion. That is without effecting replies. Only reason i can think of is it might be harder to understand the replies if the main post is edited or deleted, Can I lobby for a 30 minute edit timeline? 🙂

    2. Seriously Joe?

      As ‘hahax’ & I have loosely explained above, that a sealed box
      design is NOT necessarily an acoustic suspension design.
      Check on-line to educate yourself.

  4. Seems to me that standing waves have to be dealt with both in the enclosed speaker design and then again in the room. You could eliminate the speaker ‘dealing with’ by going to an open baffle design…. And deal with standing waves once.

    “Making better bass”… is as controversial & subjective as is “making better mid range or highs.” Poetic License? 🙂

    There’s design advantages and disadvantage while making ‘better’. The choices are made in the design phase… the implementation of the design…. Aesthetics and pricing all play a roll… and of course the how well will it sell / profit margin factor.

    1. Open baffle speakers… a favorite of audio designer NP, reviewers Steve Guttenberg and Guido (last name unknown), and photos of music systems that I see online all the time. Seems like open baffle speakers are making a comeback.

  5. Let’s also consider the related tapered tube loading designs of Laurence Dickie, starting with the B&W speakers, and now with his own brand, Vivid Audio. He uses tapered tube loading to eliminate back wave problems from all the drivers, apparently reducing distortions. I cannot say this design is responsible, but the Vivid Audio speakers are remarkable and some of the very best I have ever heard. Vivid Speakers exhibit a midrange, especially, which is clean, clear and as uncolored as any I have ever heard-listen to them any chance you get. We at Sonore used the Vivid Kaya 45 as our demo speakers one year at RMAF, and listening to my Girlfriend’s singing voice through them on a live in studio rough track (with little post processing) was amazingly like the real thing in the same room.

    1. Backwaves emerging from the speaker cabinet will always cause lobing, transient smearing, and temporal distortion*. When transmitted through the cabinet walls or reflected back through the diaphragm they will also have spectral distortion and probably harmonic and inter-modulation distortion.

      The tapered exponential cone with sound absorbing stuffing is a good mechanical design for absorbing the backwave, creating increasing acoustic impedance as the strength of the enclosure increases with decreasing diameter.

      There is another benefit: reducing the Q of the electro-mechanical system at the resonance point (i.e. where the response drops off at the lower frequency limit). These resonances (which are inherent in electro-dynamic drivers) always ring on transients, including the envelope transients of notes which start and stop abruptly (percussion, etc.) I was working on this problem when I re-invented the back wave absorber, only to be reminded that B&W already had one on the market. I still have a prototype cabinet built for my bass guitar which has a folded backwave chamber tapering to a point made with internal baffles, like a transmission line that goes to zero cross-section.

      I later built a backwave absorber for my listening tests implemented in a folded cylinder using proprietary internal construction that did not violate any speaker patents. The difference in lower bass transient distortion was obvious to non-audiophiles, surpassing bandpass, vented, aperiodic, transmission line, passive radiator, sealed cabinets and even true “infinite baffle”.

      * dipoles are appropriate when reproducing double sided sound boards like piano, trap set, and orchestral strings. The dipole backwave is not so good for representing winds (including voice), brass, and closed back sound boards like guitar, lute, and harpsichord.

  6. While I personally favor sealed enclosures I think the biggest challenge in bass performance is whether or not to use a subwoofer and, if you do making it seamless with the main speakers

  7. There are a boatload of math & calculations in port design. I did enclosure design in car for years. You need the woofer’s specs; Vas, Qts, Qes, fs, to properly calculate box size & port size. Until enclosure design software came out, it was quite a pain in the patootie to design a good box. And you can’t design a perfect box and then just replace the woofer with a different one – even if it appears identical. The great Howard Doctor designed a fantastic 4 port 6th order bandpass sub design that was (is) absolutely magnificent. His 6 inch driver version would rival dual 10 inch subs in a basic enclosure. And at that level of enclosure engineering, port length being out by millimeters is catastrophic. I used to just laugh at the pre-fab ported enclosures you could buy and even bandpass versions with cool plexiglass fronts – just add you own woofers!

    I’ve a pair of DCM Timeframe TF1000 transmission lines and they very often produce bass notes that STILL impress my ‘wow’ lobe. I can’t even begin to imagine the calculations involved in designing great transmission line tuning.

  8. Mass loaded transmission lines were and are a nice variation. Thanks to math CAD & pioneering Work by ML King. And recent passive radiation designs, we have a one third of good boxes. Then of course is Bo less.

    Like that Dave Mason song (paraphrasing) , there is no good guy there is no bad guy we just disagree.

  9. Transmission lines are rarely based on an accurate model of fiber behaviors as described by LJS Bradbury:

    This early researcher found that appropriate fibers undergo a transition in performance based on the acoustic resistance and compliance of the fiber matrix. Above the transition frequency, the fibers do not move, and offer static resistance to the velocity component of the sound generated by the woofer, suppressing the organ pipe resonances of the transmission line and secondary radiation of higher ferquencies. Below the transition frequency, the fibers move with the air particle movement, and so increase the effective medium density several times, which decreases the speed of sound in the line. The typical “rule of thumb” of making the line 1/4 wavelength long effectively makes it 1/2 wavelength because the speed of sound is halved. This provide the 180° reversal for the backwave of the woofer, and a +6dB boost.

    To stuff a transmission line properly, one first must measure the transition frequency of the fibers used (which also changes somewhat with stuffing density); and then the stuffing has to be performed to yield uniform density with the right amount of stuffing.

    I built an apparatus to measure the acoustic properties of fibers, and discovered that polyester pillow stuffing can’t produce the proper transition. Dow-Corning Miraflex (TM) worked, as does long fiber lamb’s wool – although the latter ages poorly. “Acousta-Stuf” is polyamid fiber re-packaged at a substantial markup, and has some utility – but every fiber has a different transition frequency, which has to be matched to the woofer and cabinet design.

    Note that good transmission line stuffing can also be used to increase the effective size of sealed boxes as well as damp higher frequency internal acoustic resonances.

  10. I don’t understand the statement that the output from a transmission line design is “carefully calculated to arrive in phase and add to the low frequency performance of the speaker”. Wouldn’t that only happen at one fundamental frequency, depending on the acoustic delay time of the transmission line (and the acoustic distance between the driver and the “port”), where the signal would actually be shifted by 360 degrees (thereby appearing to be in-phase for the case of a continuing signal), with its harmonics similarly shifted by integer multiples of 360 degrees (and thereby also appearing to be in-phase) – but with all other frequencies being output at different phase shifts spanning the entire range between 0 to 360 degrees? This would cause some frequencies to totally reinforce, some to totally cancel, and others to fall somewhere in between – right? Isn’t that what happens with a “bass reflex” design, whether using a basically open port or (with different details) a passive radiator? I thought that the ideal intent of a transmission line woofer design is to totally absorb the rear radiation of the woofer cone, such that phase considerations of the duct/maze/port acoustic output would not have any significant relevance.

    1. You are correct. Passive radiators and ports including transmission lines can only add less than an octave to the low frequency response. A well tuned resonance is necessary to extend even that far – most systems only go half an octave below the woofer resonance.

      This may not seem like much – a true infinite baffle will go almost as deep; however, a very important function of secondary acoustic resonance extensions is reduction in Doppler Inter-Modulation Distortion (DIMD). For every octave the bass is extended, the woofer cone has to move twice as far. The velocity of these movements cause the other frequencies radiated by the woofer to rise and fall in frequency – and anharmonic frequency modulation distortion is far more audible than harmonic distortion. Note that Doppler distortion is limited by Physics – no unobtainium, secret sauce, or gaggle of patents will reduce it, only larger surface area or attenuated bass.

      Secondary acoustic resonances load the woofer and reduce the travel in the active band for a significant reduction in audible distortion. This is more important the smaller the woofer. In my world, 8″ and 10″ drivers are midrange – like the 3″ x 10″ midrange in the FR20 & FR30, which is roughly equivalent to an 8″ cone in surface area, but with better transient response and wider horizontal dispersion. The 4×8″ woofers in the FR30 are equal to a single 15″, and likewise narrower and faster; and the 4×10″ PRs are equivalent to a 21″ subwoofer.

  11. I may be missing something even after reading the wikipedia entry, but if I understand correctly the difference between a sealed box and acoustic suspension is that in the latter the air behind the driver is supposed to act like a spring. This is easy to understand. But how does one design a sealed box without the air in the enclosure acting like a spring?

    1. Willem,
      You make the box (loudspeaker cabinet) so big, relative to the air moved by the driver, that the ‘air-spring’ effect is negligible, that’s how 😉
      For a proper, working, air suspension design to work as intended, the maths co-efficient between driver air displacement & box volume must be correctly ‘worked out’ otherwise there will be very little ‘air-spring’ effect, or, conversely, to much ‘air-spring’ effect.

      1. Fat Rat this makes perfect sense. But it means that acoustic suspension speakers are simply sealed enclosures with a volume calculated to provide optimal spring effect. To me this means all sealed enclosures are acoustic suspension some just more effective than others, because physics demand that you will always have spring effect

        1. Willem,
          Yes, however, if the driver is relying primarily on the spider & surround rather than the spring effect of the trapped air then
          it’s not going to be a ‘proper’ acoustic suspension design.
          Best that you read Wikipedia – ‘Loudspeaker enclosures’
          And please note the correct title, ie. it’s *not* ‘Speaker enclosures’ 😉

  12. During his time at the helm of Meadowlark Audio, Pat McGinty was a fan of transmission line designs. Though I don’t listen to them often these days, I still have, and love, a pair of his second-generation “Kestrel 2” floorstanders. On his current website, you can see a photo of him holding one before its right side was installed:

    1. You are correct – for an empty transmission line, or one that is under-stuffed, or utilizes the wrong fiber.

      Transmissoin line efficacy requires a stuffing that acts as an acoustic resistance that attenuates ALL harmonics of the line length, but cuts the speed of sound in half with minimal attenuation of the desired reinforcement frequencies. This is why amateur implementations rarely succeed. You have to follow the formula precisely – identical fiber properties, identical driver, identical isotropic stuffing density, identical geometry.

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