Copied from a car audio thread.
A little different but worth reading.
Let's begin with definitions of some aspects of sound.
Tonality:
Commonly referred to as tonal accuracy and spectral balance, tonality is that quality of a system that gives the musical instruments their natural sound. If a saxophone is played, for example, it sounds exactly like a real saxophone, and you can tell it is not a trombone, french horn, tuba, or any other brass instrument. Likewise, any instrument has its own characteristic sound, and a system with good tonality will allow the listener to differentiate the instruments being played. According to the Official IASCA (International Auto Sound Challenge Association) rulebook of competition, Tonal Accuracy and Spectral Balance is a combination of six characteristics---loudness, pitch, timbre, modulation, duration, and attack and decay. I won't go in-depth on each of these, but will give a brief definition:
Loudness - the magnitude of the sound.
Pitch - the quality of a sound that determines its position on a musical scale.
Timbre - harmonics that give a sound its sonic signature.
Modulation - changes in amplitude, phase, or frequency that occurs in a sound.
Duration - length of time a sound is heard.
Attack and decay - the time it takes a sound to build-up (attack) and die-down (decay).
Listening Position relative to the perceived sound stage:
Basically, in a concert, the musicians are on a stage in front of you, and likely they are well in front of you as you'll be sitting in the audience. Good SQ systems will give the illusion of this same perceived stage being well in front of you. Systems with poor "listening positions" will make you feel like the musicians are in your face, around you rather than in front of you, or even worse, behind you. Remember the goal is to simulate watching and listening to a live performance, and as such there are no musicians beside or behind you. The best systems will give you the impression of the stage being a considerable distance in front of you, as if you were sitting a few rows back in the audience.
Stage Width:
How wide is the sound stage? Car interiors are horrible for good sound reproduction, but we will cover this later on. However, a wide sound stage is an important factor. Bad systems will have almost no width, sounding as if only a center channel speaker is playing. Better systems have well defined left and right stage boundaries, but these boundaries will stay well within the interior of the vehicle.
The best systems will have stage boundaries, which extend BEYOND the physical area of the vehicle, with noticeable sounds that seem to emanate from a location outside the car (like from the side mirrors). The key to getting good width is to do so WITHOUT affecting center imaging, creating a "sonic hole" in the middle of the stage. We shall cover this later.
Stage Height:
Simply, how high is the sound stage? Additionally, is the stage height stable, meaning even height from right to left? When we sit in an audience, we IDEALLY see the musicians slightly above us. Thus, when we try to reproduce the "live performance" feeling in a car, the stage should seem to be in front of us, at eye level OR slightly higher. Too low, and it doesn't seem real, as our focus is skewed downward and we end up looking at the floor to envision the performance. Too high, and we feel like we are "star-gazing" and again, end up focusing on the sun visors or sunroof, again, it is unnatural. Getting the stage height to be stable means you get the picture of, for example, a guitarist at far left, a drummer in the center, and a harmonica far right. The key to a "stable" stage is to get these locations to project at exactly the same height. Often times, a car with an unstable height will portray the center image nicely, but the left and right images will be very noticeably lower, giving us the "rainbow" effect, and thus a mediocre stage height score. Likewise, some cars have frequency-dependant stages, where the high frequencies from the tweeters might be at eye level, the midrange frequencies seem slightly lower, and the midbass and sub bass appear to be coming from the floor. This is also unacceptable, and many factors affect this phenomenon. Some of them are speaker location, resonation of speaker enclosures, coloration caused by standing waves inside the enclosures, and poor equalization and sonic balance. We shall cover each of these in depth shortly. So, we want to see a "stable" sound stage projected at eye level in a proper SQ set-up.
Stage Depth:
Depth and Position to Soundstage ore often mistaken for one another. As discussed, Position to Sound stage pertains to how far in front of the listener the sound stage actually is. Stage Depth pertains to the placement of musicians on the sound stage either in front of or behind one another. Often times, for instance, the drum set will be located "behind" the guitarists, and the vocalist will be in front of these instruments, front and center. Systems with no stage depth will portray a flat sounding, one-dimensional stage where every musician appears to be side by side. Systems with excellent stage depth will give the listener a sense of space between the performers, and you should be able to tell that instruments are being played at different distances from you.
Ambience:
Ambience is that phenomenon that gives you a sense of space around an instrument.
Many people confuse ambience with artificial "surround-sound" echoes meant to add the illusion of music all around you. Every recording contains ambience of some sort, be it a sense of "air" around the instrument or the "sound" of the room the track was recorded in. Using reference recordings where the actual recording room characteristics are known ahead of time, SQ judges can score ambience based on whether the performance "seems" to be played from inside this actual room. For example, there is a track on the '97 IASCA SQ competition CD that was recorded by a single microphone at a level of approximately 20' above the stage, INSIDE a large auditorium. With proper ambience, it should seem like you are in that large auditorium when you listen to this track. THAT is proper ambience, not artificial ambience. Tuning and speaker quality have the most impact on ambience, followed closely by controlling sonic reflections and resonations in the vehicle and speaker placement with proper x/o selection. Some people use ambiently tuned rear fill speakers to try to accomplish this aspect, but in a properly designed and tuned system, rear fill is NOT necessary to acquire excellent ambience.
Imaging:
The sound stage should be looked at in 5 separate sections: Left, left center, center, right center, and right. These are the most common locations on the stage used to evaluate the imaging characteristics of the car. By using reference material, the judges know exactly where the different instruments should be "located" on the stage. A system that images well will have well-defined and focused instruments located exactly where they should be on the sound stage. These images will not wander from their locations, and you could easily close your eyes and point to where each instrument actually is. Many systems exhibit frequency-dependant imaging where as the frequency changes in pitch or scale, the image will move accordingly. This is not good imaging. Likewise, some systems have good left and right imaging, but fail to get a center-stage focus at all. Too many, the center image is the most important location on the stage, and it is easy to tell which cars have good centers and which don't. Speaker placement, path length differences, and proper equalization vastly affect imaging, and to a lesser degree, resonations, reflections, standing waves, and uneven interior surfaces play a role in the imaging characteristics of a system. Again, we will get to these aspects shortly.
Sound Linearity:
How well balanced does the system sound at low, moderate, and high volume? A system with good linearity will sound equally balanced at all three loudness levels, remaining accurate tonally and free of distortion of any kind. This depends mainly on proper gain settings and equalizer tuning, though other factors can possibly affect it.
Absence of noise:
A good SQ system will be a symbience of many factors coming together to provide a performance free of unwanted noises such as speaker pops, alternator whine, ground loop noise, additional noise floor in the form of extra hiss, on/off thumps or pops, and any other form of unwanted noises.
All recordings exhibit a noise floor in the background, it is a byproduct of the recording process that cannot be overlooked and for this reason, only the noise level present in the original recording will be acceptable. Again, several factors affect system noise, and an "avoiding noise" section will deal with them all.
Dynamic Impact:
A superior system should be capable of reproducing the proper "feel" of the music. We need to consider the fact that music has two dimensions --- that which we hear, and that which we feel. If we were to go to a rock concert, sitting front and center, and the drummer decides to go-off on an improve solo while the rest of the band grabs a cold one and some ho-hoes, we are treated to a barrage of dynamic percussion sounds. When facing the stage, we feel the sound waves both in our chest and abdomen as well as on our skin. The bass drum obviously will be the most prominent; however the toms, snare, and even hard cymbal strikes can be felt, and felt easily if the guy is REALLY going off. We can even plug our ears to make this effect more pronounced. Sound is emitted in waves. These waves possess energy levels that are dependant on amplitude (loudness) and the proximity of the listener to the source, thus, we can feel the sound waves as they interact with the touch-receptors in our bodies. Ever flinch or blink upon hearing an abrupt, loud sound? Well, I am not certain of precise figures, but I'll bet it is about 50% due to what we hear, and 50% due to a subconscious physical reaction to "feeling" the sound wave, causing sudden stimulus in our nerve endings. The best car systems are capable of re-creating this sense of dynamics, as it should be "felt" live. And they should do so without feeling percussive waves emanating from rear mounted subs (Yet another topic we will discuss), making us hear the bass player up front, but he feels like he is behind us at the concession stand getting a sausage dog or something.
