Are suspended floors something that I'd only find in purpose-built home theater rooms?
I'd be concerned about the floor vibrating with some errant frequencies; I'm not sure what the technical term for that is. EDIT: The technical term might include the word 'resonate', but I still can't remember the exact technical term.
A "suspended" floor is a floor not connected to the earth. IOW, a first floor over a basement or crawl space would be "suspended." The suspension system would be the floor joists, subfloor and flooring. Since these have no immediate contact with the foundation of the earth, they have the ability to move. "Resonate" is the correct term. "Shake" is the other correct term.
So you think that carpet is better than plain hardwood or otherwise reflective flooring? That's helpful, because the room that I'm considering moving my A/V equipment to is carpeted, and when I look at other places to live, I can try to find a place with a multipurpose room or dedicated theater room that has carpeted flooring.
Reflective flooring causes... reflections. Those reflections of the soundwaves can combine and interfere with the direct soundwaves causing reinforcements and cancellations, which change the sound that arrives at your ears. Floor bounces are generally not considered beneficial, and absorption of them is usually desired. A thicker cut-pyle carpet will absorb more sound than a thin berber style carpet. The pad can also be sound deadening. The thicker the carpet/pad system is, the lower in frequency it will absorb.
I think that "boundaries" means the floor, walls, and ceiling. So you're saying that how the walls, floor, and ceiling were built, and what materials they were built with, will impact the modal response. That makes sense. What materials are best?
This is a complex subject as there are many different construction techniques, even within the range of "studs and drywall." Plus, there are two dissimilar goals for wall construction techniques, sound absorption and sound proofing. Some walls flex and the flexing absorbs acoustic energy. Other don't flex nearly as much and reflect more energy. Concrete walls don't flex at all and reflect virtually all the acoustic energy that hits them. I don't want to get into all that but if you want more info, try googling "wall construction for sound absorption."
Bottom line, for in-room sound treatment, most people recommend flexible interior wall like your standard stud walls with 1/2" drywall, glued and screwed.
What are "room modes" and "bandwidth"?
Room Modes: http://www.acousticfrontiers.com/room-modes-101/
Bandwidth: the total band of frequencies of interest. In the context I used it, I meant the modal frequencies, usually from about 20 Hz to about 200 to 400 Hz, depending on the size of the room.
Does that mean entry and exit, as in 'the location of the doors will affect where people enter and exit the room'?
Yes. The location of the door(s) will impact where people will enter or leave, but will also impact the acoustics, speaker placements, furniture placements, ability to place acoustic treatments, etc. Likewise with windows.
Hotel California allows its residents to check out anytime they like, even though they can never leave.
If I were building a room from scratch or were remodeling a room, could I design it to be acoustically perfect and still be a 'living' room? I guess that the seating is the main problem: if the seating is arranged such that everyone can see each other, then not everyone is facing the screen, and not everyone is inside of the acoustic sweet spot. What other trade-offs are there when it comes to choosing a living room over a dedicated theater room? On the other hand, what trade-offs are there when it comes to choosing a dedicated theater room instead of a living room?
A living room or family room is a place for socialization. A dedicated theater is a space with one and only one purpose: viewing movies and television.
In a living room, many, many things affect the ability to implement a "hypothetical room deasign for ideal acoustics": seating arrangements, visibility of speakers, subwoofers and electronic equipment, (aesthetics and "WAF" or "Wife Acceptance Factor), placement of video screens, (i.e., above the fireplace is BAD), selection of flat panel TV vs projector and screen, etc, etc, etc. Many, many things can be optimized in a dedicated theater that must be compromised in a living room. The number and magnitude of the compromises will vary depending on how the living/family room is utilized, and some of those compromises can also carry over to dedicated theaters, but in general, it is far easier to optimize a dedicated space than to try and cram a Home Theater in to a living/family room. Nonetheless, many people do it... and thoroughly enjoy the results.
And what about furniture in a living room? What impact does furniture have on the acoustics? I understand that more furniture = more acoustic reflections, but how do different types of furniture and the materials that they are made out of impact acoustics? 100% wooden furniture (such as a table) vs a couch with cushions?
Soft cushions with padding can work quite well as acoustic absorbers and be benficial for acoustics. More reflective surfaces, like leather, wood, glass, porcelin, etc, will add some reflections, and they may, or may not be diffuse. If diffuse, they're beneficial. If they cause direct reflections, like off of a wooden/glass coffee table situated between the front speakers and the listener, they can cause acoustic interference, which is not beneficial. Also, speaker mounted in "entertainment units generally don't benefit from the internal reflections off the unit. Dedicated Home Theaters would not use an entertainment console.
Is there a general rule for how much space should exist between the front speakers and the side walls, and how much space should exist between the rear speakers (if present) and the side walls? I understand that ideally, I'd know the dispersion patterns of all of my speakers and choose the amount of space between them and the side walls accordingly, but a general rule would be helpful for those of us that don't know the dispersion patterns of our speakers.
You've answered your own question. This will depend ENTIRELY on the speakers, so find out what the dispersion characteristics are. Having said that, there are some good guidelines in the link I posted previously summarizing Dr. Toole's book. I would encourage to read that entire website. It has the answers to most, or all of your questions.