Accuracy, Soundstage, and Ambience
The three basic characteristics of what you hear when you use your high fidelity sound system are accuracy, soundstage, and ambience. (One can find dictionaries which have dozens of words to describe the audio experience, but let's keep things simple.) Accuracy is the ability of the system to reproduce sounds precisely so a violin sounds like a violin, a trumpet like a trumpet, a piano like a piano, and so on. Secondly, it allows you to distinguish such subtleties as the fact that two violins are playing a melodic line, rather than just one violin. With good accuracy, you can hear the mallet striking the skin of the drum, and not just the boom of the drum. A guitar pick touching the string is heard, and not just the note being played. One speaker can produce this characteristic, so stereo is not necessary.
The soundstage is the left to right, and front to back (depth) placement of the instruments or voices. At least two speakers and separately recorded channels (stereo) are necessary for a soundstage (unless you can be satisfied with a soundstage as wide as only one speaker, if you only have just one). The left to right concept is easy enough to understand, since the sound is being reproduced through at least two microphones, one each for the stereo channels. Front to back is a little more complicated. This phenomenon (front to back soundstage) is a result of how close the microphones were to the instruments, and the relative volume (loudness) that different frequencies are reproduced by the speakers. If the speakers have a strong midrange, then instruments in that frequency range (clarinets) would sound "forward", whereas if the speakers are very strong at the high end of the spectrum, piccolos and cymbals would sound forward. If the midrange frequencies are not as strongly reproduced, then the clarinets would sound "recessed" into the background. Imaging is a term used to describe the placement of the instruments on the soundstage.
The third characteristic, ambience, is a factor not only of the speakers, but the room in which they are placed. Ambience is essentially made of echoes (reflections or reverberations), and it gives you a feeling that "you are there". If you listen to a concert live, the ambience is a result of the sound of the instruments bouncing off the ceiling, walls, floors, other people, and anything else in the symphony hall. This goes onto the recording along with the original sound of the instruments. In your home, the original ambience comes through, filtered somewhat by the fact that you are listening to the music through two windows (stereo speakers), but it is there nonetheless. The ceiling, walls, floor, doors, windows, and furniture in your listening room add to this ambience, which can make up for some of the loss caused by having only two speakers. Ambience is also emphasized in surround sound. In fact, that is one of its main functions. You can get this ambience in a Dolby Surround system, and you can also achieve it to a certain degree by simply adding two speakers in the rear that duplicate the front stereo speakers, without using any surround processor. Further ambience is achieved by using dipolar or bipolar speakers (enclosure has drivers on opposite sides) which aim the sound in several directions, bouncing it off numerous surfaces, totally enveloping you with sound. Many rear channel surround speakers are of the dipolar or bipolar design, and you should audition some of them. Digital surround sound, such as Dolby Digital (DD, AC-3) and DTS, have the potential for tremendous ambience, since each channel is discrete from the others and they are all full range, except for the Low Frequency Effects (LFE) channel.
Once you have purchased the speakers, experiment with their placement before you mount them permanently. Speakers not designed specifically for home theater front channels radiate their sound vertically, so that the sound bounces off the ceiling and the floor. Typically, you need to sit directly in front of them to hear all the frequencies clearly. Home theater speakers made for the front channels are designed to radiate the sound side to side. This is so that you can sit anywhere in the room to watch a movie, and hear the sound clearly from all the speakers, even if you are off to one side. However, these side to side radiating speakers will produce less room ambience than vertically radiating speakers (because vertically radiating speakers bounce sound off the ceiling and floor), and some people prefer the room ambience over the side to side clarity. Thus, you should audition both types, using not only some movie segments, but CD music as well. Then decide. Speakers designed for home theater rear channel surround sound (dipolar and bipolar) radiate in numerous directions, so that ambience is emphasized. Rear surround speakers that have drivers only on the front should be aimed in whatever direction you find pleasing. Don't rule out using dipolar speakers in the front. This can add even more ambience. With digital surround sound, the use of the same speakers in the rear as are in the front works very well. In fact, because of the full range nature of digital surround sound in all the channels, it's very useful to have big speakers all the way around.
Every speaker model has an electrical specification called the "nominal impedance". Usually, it is 8 Ohms, which means that, not including the speaker cable, the amplifier is presented with 8 Ohms of resistance (impedance) in passing its electrical signal through the speaker. The word nominal means that 8 Ohms is the average impedance. Actually, it varies depending on the frequency of the music, and may range from 3 Ohms up to 20 Ohms across the auditory spectrum. Some speakers have a nominal impedance of 4 Ohms, and others, 6. This is important, because, as the impedance drops, the demand for current to be delivered by the amplifier increases for a specific amount of watts. While the current goes up, the voltage goes down in delivering those watts of power. This is defined by "Ohm's Law" which states that E = I R, or Voltage (Volts) = Current (Amps) x Resistance (Ohms), and the Power Function which states that P = E I, or Power (Watts) = Voltage (Volts) x Current (Amps). You can see from Ohm's Law that, for a specific amount of voltage, the current will go up if the resistance drops. So, if you change from an 8 Ohm speaker to a 4 Ohm speaker, the current doubles when the voltage is held constant. Then, by the power function, if you want to deliver 100 watts to the 4 Ohm speakers as you did when you had 8 Ohm speakers, the voltage would drop by a factor of Voltage/1.414, while the current would increase by a factor of Current x 1.414, in order for the power to remain at 100 watts. Therefore, if you choose a low impedance speaker (4 Ohms nominal), make sure that your amplifier can supply the current that will be necessary to drive the speaker at high listening levels.
The impedance of the speaker will also affect what is known as the "damping factor". This is defined as the ratio of the impedance of the speaker to the output impedance of the amplifier. Thus, if the speaker impedance is 8 Ohms, and the amplifier output impedance is 0.05 Ohms, then the damping factor is 8 divided by 0.05 = 160. High damping factors usually mean that the bass response will be well defined ("tight"), whereas a low damping factor will result in a loose sounding bass. Tight or loose bass is a matter of preference; one is not necessarily better than the other. Tube amplifiers often have low damping factors, for example, 10, compared to solid state, which may contribute to their typically loose bass response (tube amplifiers are often described in terms of "warmth" or "looseness", and it can be a very pleasant effect). In any case, the specification sheet for the amplifier will sometimes list the damping factor, so if you choose a low impedance speaker, a higher damping factor on the amplifier may be necessary if you like the "tight" bass sound (dropping the speaker impedance from 8 to 4 Ohms would reduce the damping factor by 50% for any given amplifier).