In another thread, the TRW sub concept from Bruce Thigpen of Eminent Technology was supported by a very thorough examination of WHAT IS WRONG with ported subwoofers. Since a full range system must have a woofer that includes much of the range that a subwoofer is supposed to augment, its a useful discussion for stand alone "big" speakers as well.
Its a long read, but worth it. While I don't agree 100%, he's got the gist like few other designers I know or have read about.
http://iar-80.com/page142.html
Think of it this way; There is no way that the air in a port can PREDICT that the backwave of the woofer diaphragm needs to push it forward in time with the innitial transient of a low frequency envelope of sound, such a drum whack or bass string pluck. Thus, instead of SUPPORTING this first cycle of sound energy, it CANCELS it from entering the air load outside of the box, storing it in the resonant system that is the 4th order bass reflex, adding the mass/spring system behavior of the driver to that of the Helmholtz resonator of the ported enclosure. The touted efficiency so long expressed by enthusiasts of ported speakers is largely illusory, since THIS EARLY TRANSIENT PORTION is missing at the critical tuning frequency, and below that frequency even sustained sine waves suffer the same fate. Careful examinations of area-under-the-curve during transient events (90% of music) for sealed vs./ ported designs show that gains are far less than the 6dB theoretical advantage. Yes, certain organ-type sounds will seemingly not be harmed by a ported design at first listen, but their PACING will be off, in terms of aligning their tempo with the rest of the voices in a performance.
Since there is no precession function in the port air, and no direct signal to it from the amplifier, it's behavior is only "loosely" based on the original waveform. Interestingly, a sealed box woofer design that has sufficient extension into the lowest octave commonly associated with the "bottom" of human hearing (20Hz - 40Hz; see Thigpen) actually uses a high mass cone and underdamped woofer motor system, for a high Qts of well over 0.4. Otherwise, the ratio under a Qb of 0.707 for a 2nd order Butterworth (B2) alignment is too high, and the resonance requirement of the woofer driver will become ludicrously low, and the Vas will become equivalent to the interior of a city bus. Suspensions this loose tend to have a hard time staying centered or linear. Energy storage of such high mass systems also present a problem. You simultaneously have weak brakes from the driver motor and a big chunk to slow "downhill" into the source- intended decay of the transient.
My solution has been a proprietary analog active circuit (from design work in 1993) that provides minimum phase compensation in front of the power amplifier for earlier than desired roll-off of extremely low Qts drivers sporting massive magnets. Imagine a minimum phase design that uses a single differential gain block and 8-12 passive parts, that can give arbitrary extension below F-3 for any sealed woofer! It's even skimpier than Linkwitz's design... This circuit opens the door to the use of high acceleration capable drivers that are highly overdamped in the eyes of standard T/S models. A specification used by Audax of France, the calculated GAMMA values of some of my subwoofer drivers exceed 1000. I've read carefully the proposition to rate driver acceleration using voice coil CURRENT exclusively, and it holds no water in my estimation. Bl is calculated against earth's gravitational pull at a given input current. The mass of the diaphragm, statically or dynamically loaded (turns out not to matter much due to the partial inclusion or exclusion of the surround/spider and or air load) , gives the other half of "power to weight ratio". When rating a sports car's performance, the 0-60 time is given by the same factors. Worrying about crankshaft torque but ignoring the mass of the car is missing the question. Thus GAMMA is the ratio:
GAMMA = Bl/Mms(or Mmd)
For proper scaling, mass is in kg.
Thus a really good sealed design for a SUBWOOFER will begin its 12dB/oct attenuation at 60-120Hz, and the output will occur BELOW SYSTEM RESONANCE. There are high mass designs that go for this, and our brother Bob Carver tried to take some credit, but I and others were using similar practice for decades before. I remember in the mid 80's using a Henry Kloss driver from the mid SEVENTIES, that had a crazy magnet on a long throw 7" woofer, that was originally used in a sealed box with an outboard analog processor.
So why a GAMMA that high? Because the brakes are a secondary function of the back EMF absorption as provided by the output impedance of a power amplifier, coupled with the damping rate of the driver motor. If the damping factor is poor on the amp, or the driver has a wimpy magnet (to get a good F(-3) in a full range box), the GAMMA will be in the 100-200 range, and thus the ACCELERATION PROVIDED TO SHUT THE DRIVER BABBLE UP will be lacking the ability to do so with authority. What you get is a speaker that goes "BONG" after each time it is asked to move, and it bongs at system resonance, and it bongs AUDIBLY and it bongs for a LONG TIME for small magnet drivers. Sure, you can make the bong thing better (higher Bl and lower Qts) with a shorter voice coil, but then you get less excursion, and you're arguing about a midrange speaker being a good subwoofer. Also a shorter coil leads to lower moving mass, thus higher Fs, and you lose the 20-40Hz battle.
In short, no ported speaker can be considered "high fidelity" from a waveform measurement standpoint. For a long time, the step function test was ignored as a measure of system performance because so many designs measured so poorly against it, that it was too glaring a repudiation of the bass reflex approach.
I'm surprised no one mentioned ADS as a candidate for classical speakers that were SEALED. Now, take the 8" or whatever woofer out of an ADS speaker and eyeball it. The magnet on it will be a TINY 12-16oz. The Qts will be very high. And the bass, while better time-wise than a ported design, will still be sickly sounding and unconvincing.
If the silences between notes are filled with the complaining of the speakers trying to play precisely nothing, the dynamics of 24bit recordings will be reduced, just as surely as by a high signal noise floor. There is hugely important timber information at low levels following large signal transients, and if these aural clues are buried under the struggle to regain composure of a typical sealed box design, significant enjoyment is lost.
In conclusion, look for any loudspeaker that uses a sealed box, seemingly stupidly large magnets, and an outboard bass processor. You'll be on the right road...
