THE "SOUND-FLOOR"-THE ULTIMATE KEY
The term I formerly used, "noise-floor", has been in use in the audio world for a number of years now, and while I do recognize the vital importance of the underlying concept, I also felt that the choice of words ("noise-floor") was very unfortunate, because it has proved to be a confusing term for many audiophiles, even veterans.
A much better term, in my opinion, is: "Sound-Floor"
I feel this way because: The expression "noise-floor" has Nothing to do with traditional "noise".
"Traditional" meaning the measurable noises (hum, thermal hiss, mechanical buzzes etc.) that emanate from all active, electronic components; such as amplifiers, preamplifiers, motors and even CD players. While loudspeakers, which are a passive component, have no "traditional noise".
In contrast, all audio components, passive AND active, including loudspeakers, have a "sound-floor". And that is not all...
So does the actual software; records, tapes and CDs, and in this instance, I am, once again, not referring to their background "hiss".
For a better or different perspective, the reader must realize that...
The key word in this expression is not "sound" or "noise", but "Floor". The word "floor", in this instance, is an indication of the "lower limit" of a particular capability of that component.
The "sound floor" (or "noise floor") can be best described as: The "lower limit" of an audio component's capability to reproduce (or pass) softer and softer sounds.
Put in another manner, the "sound-floor" can be described as: The softest sound that can be heard or sensed through that component (or system).
(And that by definition means) Any sound that is lower (or softer) than the "sound floor" Must be Inaudible.
Analogy- It is the audio component's (or system's) direct equivalent of the listener's ability to sense or hear "soft sounds".
THE PRACTICAL IMPLICATIONS OF THIS ESSENTIAL CAPABILITY
A component with a high "sound-floor" will obscure and mask a large amount of audible sound (music), while a component with a very low "sound-floor" will reveal virtually everything about the sound (music). Unfortunately, "the weakest-link-in-the-chain" rule applies in this case. This means that if even one component has a high sound-floor, so must the entire system.
This is the reason why systems that have a high sound-floor will be played at a higher volume, usually without conscious awareness, in an attempt to hear what is missing.
WHY AND HOW LOW-LEVEL INFORMATION IS LOST
Why do certain components have a higher or lower "sound-floor"? That is not entirely known. What is known is that components that use overly complex circuits and layouts, longer signal lengths and poor quality passive components (wire, resistors, inductors, capacitors, speaker drivers and vibrating cabinets etc.) generally have a higher "sound floor" than those components which avoid their use. Poor execution will also compromise the "sound floor".
Also, everything being equal, tube preamplifiers and power amplifiers will almost always have a lower "sound floor" than their transistor equivalents. Ironically, this is true even though their actual "noise" (hiss, hum) will usually be measurably higher. Speculation about this phenomena has focused on the greater simplicity of most tube circuits (especially single-ended-triode designs) and the fact that the actual amplification occurs in a vacuum, not silicon or some other material.
So to summarize, there are four requirements in order for a component to have an exceptionally low "sound floor":
1. A simple, though highly competent, design
2. The use of the best quality parts, both active and passive, within the component
3. The finest execution of the above, both in build quality and in close attention to (small) details
4. The shortest signal length(s) possible
If any of the four requirements are compromised, the "sound floor" will rise accordingly, and the recorded sounds (and the music) will be permanently lost. Sadly, only a few rare and outstanding components meet all the requirements. Searching for them, and hearing them, one way or the other, should be high on the list of a serious audiophile's priorities.
THE SOUND-FLOOR AND "LISTENING FATIGUE"
There is also a relationship between a system's sound-floor and listening FATIGUE.
When a system has a high sound-floor, meaning more of the musical information is missing, the listener will then (automatically) attempt to fill in "the missing parts" with his brain.
This continual effort, usually unconscious, will eventually cause "listening fatigue". The existence, and even the degree, of the fatigue is dependent on the previous experiences, and expectations, of the listener.
For example, digital recordings and sources are known to have a higher sound-floor than good analog. This is the reason why some listeners, who are used to analog, may experience fatigue with digital, despite digital's other sonic advantages over analog.
While other listeners, who are used to primarily digital recordings, do not appear to suffer the same fatigue.