The sound quality of a horn is as much mental experience as knowable acoustic phenomenon. Ask horn players to describe their favorite horn sound and you get everything from "Heroic LA Gold" to "Powerful Chicago Edge". Everyone has an ideal sound in their head and everyone conceives and executes the production of that sound differently. As a horn builder, turning someone's description of their perfect horn sound into reality is, truthfully, impossible. Getting very close is difficult, but it can be done, and that pursuit is what keeps my techniques and ideas ever evolving and improving.
"Bright" and "dark" as descriptors of sound quality are useless in my opinion, and I encourage the horn playing community to reject these and choose more descriptive terms, especially when talking with horn builders or resellers. I have noticed, in talking with clients, that the two terms have very loaded meanings and are more often used to differentiate between the Geyer and Kruspe schools of thought than anything actually sound related. In fact, many people's usage of "bright" and "dark" are reversed from one another. Some consider the great Dale Clevenger to have been very bright and Richard King to be very dark. Others believe that Chicago style brass Geyer wraps are dark and Cleveland nickel silver 8D's are bright. I speculate that the difference may be on which horn they were taught that "dark" was preferable. The "other side" is surely bright and wrong... As Bob Ward in San Francisco (a great inspiration to me and my concept of sound quality) writes, "you must train yourself to accept a sound from your bell that is brighter than you think you ought to play" (read the rest of his excellent thoughts here)
So what is a better way to communicate sound qualities?
A graphical representation may be more descriptive because different people are able to understand the concept intrinsically without having to first define definitions or agree on which is the correct ideal. Consider the following three "sounds" (fig. 1). On the far left, we have what I tend to think of as the classic Chicago Geyer horn sound. This sound is mostly core and has the ability to produce high amounts of volume and "burn" (or "zing" "edge" "color" whatever you call it) without shredding or breaking apart. Some have described this as the "laser beam" sound. It is compact, dense, and very efficient; the solid white circle illustrates this intuitively. Next to that is what many consider the ideal nickel silver Kruspe studio horn sound. This sound has much less core and is very large and deep, dissipating into nothingness at the edges. This sound can vibrate an entire small room or recital hall. Finally is what I consider to be a good balanced sound and where I try to fit my horns. The core is expanded enough to project well, without becoming too brittle, and there is a "glow" around the sound to give warmth to the tone. The glow is contained at the edges however and is just enough to color the core with some nice rich tones but not enough to disappear into the rafters of a hall.
A second way is to describe horn sounds as vowel sounds. Do you prefer an "ehhhhhh" sound, a "ooooooo" sound or an "ahhhhhh" sound (my favorite)? This question conveys so much more information that discussing brightness and darkness. Its even possible to "graph" the distribution of harmonics of different vowel sound. By balancing the dense compact core with a surrounding "glow" or "roundness", I can create a sound which is pleasant enough for solo work, but still has enough power available to project through an orchestra.