Just for reference, it's spelled "ya'll". Keep in mind that "ya'll" is singular. The plural of "ya'll" is "all ya'll"
1. The purpose of a passive radiator is to utilize the negative movement [negative meaning in the direction towards the back of the speaker] of the main driver in producing sound. A port can also be used to do this, but problems arise with things like port noise. A passive radiator design will be able to produce higher SPL than a sealed enclosure or infinite baffle design with the same driver and amplifier, but still not to the same extent a vented enclosure can.
2. Yes, but that's the purpose! A passive radiator (for this example, positioned in line with the active driver but on the opposite side of the speaker cabinet) will create somewhat of a dipole effect, where it moves out from the cabinet when the other driver movies in, and vice versa. It is essentially an extension of the active driver. In a design like the BIC DV-84 speaker, with two active drivers and two passive radiators all placed on the front of the cabinet, the idea is to increase the SPL and bass response without porting the speaker. Keep in mind that a passive radiator design does attenuate bass response and decrease sensitivity, but not to the same degree that a totally sealed enclosure would.
On a side note, many people often prefer the sound of sealed enclosures or those with passive radiators because they feel the sound is
subjectively "tighter", "punchier", and "more defined". I have nothing concrete to back up these statements, but I thought I'd throw them out there as food for thought.
3. Loudspeaker quality [in a traditional boxed enclosure with conical drivers] is divided between the cabinet design, driver design, and crossover design. Any of the three elements can greatly influence the sound, for better or for worse depending on the design.
Cabinets are generally designed to be inert as possible [this is of course dedicated by budget]. An inert design will stop the influences of resonance within the cabinet and thusly allow for lower distortion and more linear movement of the driver elements. Some designers, however (such as Sonus Faber and Boesendorfer) design speaker cabinets with real wood like one would design a musical instrument. Real wood, unlike MDF, has specific resonant properties that, when excited, will add harmonics to the sound that will often color it in a pleasing way [this indicates the presence of even-order harmonics].
A good driver design will have linear motion, fast transient response, and low distortion. No matter how high the quality of the other parts, a bad driver will yield "sloppy" [think Bose trying to do midbass and bass], distorted sound. When pushed with some higher power, the driver might bottom out, or start to resonate.
A proper crossover design is also integral in creating a quality loudspeaker with cohesive, low distortion sound. The crossover determines what frequencies are sent to which driver and can also alter the phase of the incoming electrical signals. An improperly designed crossover will result in poor driver integration and/or over-emphasis of certain frequencies. For instance, trying to send a 2kHz signal to a tweeter that is only flat down to 8kHz will yield bad results because it is being asked to reproduce a signal 2 octaves below its optimum operating range. The crossover controls this.
Using high quality components in a crossover is also important. When a passive crossover design is implemented (the kind one sees in almost every loudspeaker), the components must have a high-power handling to allow for uncompressed playback. The components must have a fine tolerance so the impedance doesn't vary too much and the function of the crossover doesn't change with use. The capacitors, inductors, resistors, and other devices used can determine the difference between a good design and a great design.
This does not mean that they need to use Vishray resistors, Black Gate capacitors, silver wire, and WBT silver solder with WBT terminations on the input board. If this is what you're getting at, then the idea that special "audio-grade" components are better is often BS. Vishray resistors are very high quality with tolerances, but this is why they work well in high-end designs. An example of this is the difference in the S/N ratio of 9dB or better when used in the RE Designs LPNA 150 monoblock.
4. The rolloff in speaker designs is determined by the crossover (as you rightly thought). However, a driver will have a frequency response, notated as one of the T/S parameters. This response dictates where the rolloff starts when a crossover is being designed. Take for instance the response chart for the
Dayton 6" Reference Woofer:
http://www.partexpress.com/pdf/295-362g.pdf
5. No, this is a design choice that is often found in higher-priced speakers like Vandersteens, Dunlavys, Wilsons, and others. The design is a two part effort. First is the positioning of the drivers in the cabinet to allow for proper dispersion and in-phase radiation, and second, the crossover design. I know for a fact that Dunlavy and Vandersteen use a first-order crossover design that preserves electrical phase as it routes signals. Many people feel that this produces a very "complete" and "musical" sound. The downside is that a first-order crossover can only roll off frequencies at 6dB/octave, which means that the drivers used in such a design must have a wide usable bandwidth in order to accomodate signals that they do not usually encounter.
Keep the good questions coming, Robbie!
