Transparent, My Dear Watson
Rather more difficult to grasp, though, are those terms that describe things added to the sound by the reproducing system, because understanding this terminology requires enough audio listening experience to have encountered, first-band, the effects that they describe. One such term is transparency (footnote 1).
The aim of stereo is to create the illusion of listening in on the concert hail through a large window, extending between and perhaps even beyond the space between your loudspeakers. Ideally, that window should be perfectly clear, or "transparent," just as a window for viewing a visual scene should be perfectly transparent. Anything that tends to obscure the view is going to impair the window's transparency. Two kinds of imperfection can do this: texturing or hazing (fogging).
Texturing is best understood by analogy with photography. Consider for a moment an extremely fine-grained black-and-white print. To the naked eye, the print appears to have no texture at all. Grays appear absolutely uniform and featureless, blacks are solid. There are no tiny stipples, dots, or whatever to disturb the illusion of smooth continuity from one area of the picture to another.
Let us think of this photograph as the sound that's recorded on an LP. If we view the photo through a perfectly clean, flat sheet of glass, it will appear unchanged. We are viewing it through a "transparent" medium, which has no effect on it whatsoever.
Now, let's think of system amplification as the equivalent of taking another photo of that original photo and making a samesized print from it. What will the new print look like?
Well, it will have more grain. Film grain consists of randomly-spaced speckles, and when one set of such random speckles is superimposed over another, some speckles will abut with others, while others will not. The areas where they abut (clump) will appear as larger grains. In other words, the composite of those speckles will appear to be grainier—coarser in texture.
If the grains are still too small to see (or hear), we can still consider the reproduction to be absolutely "transparent." If the grains reach the point of visibility, then "transparency" is replaced by "texturing"—which is its opposite.
Sound is essentially analog in nature—that it is made up of smooth and continuous variations in air pressure. The purest sound, a sinewave (fig.1), looks like a drawn graph of a mathematical function, which in fact is what it is. (I won't elaborate on that.) The line is smooth and sinuously curving, without any discontinuities whatsoever; it also never occurs in nature.
Fig.1 Pure sinewave.
Now imagine, if you can, what that sinewave might sound like if it contained tiny, random irregularities (fig.2). Instead of a pure tone, the sinewave would contain what sound like discontinuities. It would have a coarseness or roughness to it—a texture. If the irregularities were close together, the texturing would be "fine," like barely visible grain in a photo. If they were farther apart, and larger, they would become both more audible and longer in duration; that is, the texturing would be "coarse."
Fig.2. Sinewave with added random noise.
One rather messy way to get an aural handle on texturing is by pouring things down the surface of a sheet of light cardboard. (This is best done over the kitchen sink.)
First, round up a box of rice (uncooked, of course), a cup of flour, a container of salt with a pour spout, and a measuring cup half full of water.
Now, holding the cardboard sheet at an angle of about 30° from vertical, with its bottom resting in the bottom of the sink, cascade some rice grains down the surface of the card and listen to theft sound. Note that the sound. of rice has a very "coarse" texture in which you may even be able to distinguish individual clicks. Do the same thing with the salt; that will produce a relatively "fine" texture. Next, the flour. This produces no clearly perceptible texture as such, but the dull rushing sound that it; makes is. obviously, coming from something dry. And "dry" is how we would describe it.
Finally, the water. Although it may gurgle, this makes no noise at all travelling across the card. This is the ultimate "transparency," or freedom from texture: liquidity. Now, reinforce what you ye just heard by feeling the same things. Take a smidgeon of each of the aforementioned materials and rub them between your fingertips. You will note that they feel the way they sound. Rice feels "gritty" (almost "pebbly"), salt feels "fine-grained" or "sandy;' flour feels "dry" and "chalky" (like chalk dust), and water has no perceptible texture at all.
To pound this whole business into the ground, take a fine-grained photograph (that framed portrait of Aunt Beth and Uncle Ezra will do) and sprinkle it, as evenly as possible across its surface, with each of these materials. You can skip the water if you wish. It will ruin the photo, but you can guess what effect it will have on transparency: none. That's called "liquid transparency," or just "liquidity."
Then, put about 2" of water in the measuring cup, drop a coin into it, and note the change in its appearance. It looks even more sharply etched than when dry. Add a single drop of milk and stir the water. The outline of the coin is still sharp; that is, the "focus" is still good, but the contrast—the range between light and dark—has been diminished. This is the visual equivalent of the loss of silence or space between sounds when the system's rendering of detail is poor. The aural equivalent of the result is what is called "haziness" or "veiling" or "fogging."
Footnote 1: Harry Pearson, in his essay on transparency in Hi-Fi News (January, 1986), included depth, breadth, perspective and focus in his explanation of tnnsparency. They are related to transparency, insofar as you cannot get them without it, but you can also get transparency without all, or even most, of them. (The photo, for example.) They are separate aspects of the reproduction, and must be assessed independently of it. Otherwise, to say that a system is "very transparent" would automatically imply that all of those other things are right-on, which is not necessarily the case.