There are several ways to achieve low frequency extension. The most commonly used methods are:
A) using some type of vent i.e. port, passive radiator, tuned slot, aperiodic ( vario-vent ) etc.. This combines the backwave of the woofer with the front wave of the woofer. The vent is tuned lower than the resonance point of the woofer in the box. This approach typically extends bass response appr 1 octave lower ( strictly a ballpark figure ), reduces cone excursion for a given power level, increases sensitivity and spl capability, etc...
The drawbacks to this approach are that the woofers become far more susceptible to bottoming out when fed a high level signal below the frequency that the vent is tuned at. This is one of the reasons that "rumble filters" were invented, as record warps play MAJOR havoc on larger vented speakers.
In a direct comparison with a properly designed sealed box of reasonable Q, the transient response of a vented box will always be slower, sloppier and suffer from more ringing. In effect, we've picked up low frequency extension and spl capability at the expense of definition and attack / decay characteristics. Sealed vs vented designs would be a perfect example of the old "quantity vs quality" dilemma.
B) active equalization is applied to the driver. The rate of boost and frequencies that are effected are based on the specific speaker and box being used. While this approach has merit, the most common problem is an increase in driver doubling ( distortion ) and a massive increase in power requirements. This approach is most succesful with sealed designs due to their shallower roll-off rate as compared to a vented design. Since most sealed designs already require higher power amps to get the woofer "strokin", adding a few hundred more watts is not really a big deal : )
For the record, this is the approach that Bag End took with their electronically equalized subs. Bobby P's "Merlin" speakers also use this approach when using his "BAM" system. Keep in mind that this approach WILL sacrifice ultimate SPL capabilities and potentially increase distortion. It can also damage the driver due to "force feeding it" increased power within a frequency range that it would not normally handle in the same non-equalized situation.
C) speaker placement is designed to take advantage of room reinforcement and room nodes. While the Klipschorn uses this method to some extent, i am primarily talking about smaller boxes that need to be corner loaded to achieve the results that they do. In most of these designs, quality of sound typically plays second fiddle to quantity.
In this case, i am primarily speaking of the drastic differences of arrival times of the soundwaves generated by the various drivers being used within the system at the listening point. This results in a "mish-mash" of sound with a lack of coherency or unified presentation. While there is nothing wrong with taking advantage of room acoustics and "room gain", most designs lack the amount of research and knowledge to fully accomplish such a feat.
While i did somewhat exclude K-horns in the placement category, i would include them in the "time delay" category. This is due to the various lengths of horns used within the different frequency ranges and the paths traveled before the sound makes it to our ears*.
D) use drivers with a large radiating surface, long excursion capabilities and a large box. Obviously, this is not an approach that most folks are willing to live with. However, it is about the only approach that works well with the least amount of electro-acoustic drawbacks. The only real drawbacks in such a design is that of physical size and the associated placement issues and the potential for larger than average amounts of power.
Most any other design other than the "King Kong" approach becomes a juggling act. The designer has to choose which of the "necessary evils" he is willing to accept in order to achieve the goals he has set forth within the constraints he has to work with.
Given all of the above info, we are currently stretching many speaker designs ( especially small two ways ) to a point that was unimaginable 25 - 30 years ago. Most of this has to do with the advances in driver design, materials used, increases in technology and research and computer optimization. This is not to say that all newer designs are better than all of the older designs. As a general rule though, i would say that it is not far from that point. As usual, there are always exceptions to every rule or generalization and i'm sure that someone will bring up a few of them : ) Sean
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* As Bear stated, i am not "against" horn designs. I have owned many pairs of horn based speakers and still do. However, "facts is facts" and one should not present information without presenting both sides of the coin. As such, a bass horn would have to be HUGE to actually go down to what most of us would consider "sub" territory.
