What is a "Sound Adjustment?"
The simple answer is: The art of obtaining the best sound possible from your instrument by optimizing each part, in concert with all of it's other parts, all organized to produce full tone, maximum power, and sensitive response to your bow. There are many parts of your instrument involved, and many ways to optimize them - just look at your own instrument. Virtually every part you observe has some influence on the sound and may be improved.
Could your instrument be improved to sound better?
In a Sound Adjustment, we focus first on improving these stringed-instrument fundamentals of great tone production - 1. Volume overall and the level of power, or sound energy projected, 2. Sustain, when played pizzicato 3. Timbre, or the dark or bright, warm or nasal 'character' of the sound, and 4. Evenness of volume from note to note, as well as balance register to register.
Every instrument can realize improvement to it's tone, sometimes dramatic improvement - in some or all of these fundamental areas, and this is where each of the individual parts of the instrument come into play. The strings, the bridge, the sound post, the saddle, the nut, the endpin, the tailpiece & tailgut - are all of the removable parts of the instrument that can be optimized in many different ways to improve your sound.
But the most noticeable improvement available for pretty much any stringed instrument is made by adjustments to 1) the soundpost, and 2) the bridge, along with a good new set of strings custom-selected for the type of sound improvement we intend to achieve.
The soundpost is our starting point for any kind of sound adjustment, as well as for doing setups in general. The most critical aspect is how well-fitted the sound post is when placed in the correct position with respect to the bridge. If you notice the curve of the top and back of your instrument - that curve is also duplicated on the inside of the instrument, and the sound post end is cut to match that inside curve, so that there is a perfect transfer of sound energy into the sound post from the top to the back of the instrument.
The soundpost also has the structural job of supporting the violin top against the tremendous downward pressure from the strings, so the length of the finished soundpost is also a key factor. It should just barely be able to stand up alone by friction, in position touching the top and back inside with no bridge yet in place. If the soundpost is too long, even by just fractions of a millimeter, the top and back can't vibrate freely. It's like putting your thumb on a speaker cone while it's playing - you'll hear an unpleasant degrade in sound if you impede the vibration of that speaker cone. The vibrating violin top responds the same way to having a sound post that's too long. A too-short soundpost is just as problematic - it allows the downward pressure of the strings on the bridge to warp the top plate, also introducing tension and impeding the free vibration of the top, and also leading to cracks and other structural problems over time.
Because these measurements and adjustments are measured in the thousandths of an inch, and your instrument changes its physical size with every seasonal humidity and temperature change, there are many professionals with very sensitive instruments who request a 'winter' soundpost and a 'summer' soundpost. As the instrument shrinks in the cold, less humid winter season, the soundpost does not shrink in nearly the same way, as it's wood grain is perpendicular to the rest of the instrument. So when the humidity drops, the wood shrinks and the soundpost becomes 'tight', thereby impeding the sound, calling for the shorter, winter soundpost to be installed.
One thing I learned working on violin instruments in the extremely dry desert conditions of Las Vegas - you cannot fight Mother Nature. You can only adapt your instrument to the highly predictable seasonal changes that occur with the calendar, like clockwork. This is why the setup and sound of your instrument changes through the year, some by a little and some by a lot, but it is always changing with the weather, noticeable or not.
The second most-sensitive part of your Sound Adjustment, is the bridge. We start with the fundamental location of the bridge, with respect to the center line of the instrument and fingerboard, and then ensure that it is aligned properly with the small notches carved into the F-holes in the top of the instrument. The location of the bridge may also be micro-adjusted within limits to achieve a brighter or darker quality to your sound, or to better line up with a neck that is slightly off-center.
But the most critical aspect of the bridge is how well-fitted the bridge feet are to the curvature of the violin top. This is a very sensitive, precise, meticulous process of carving the bridge feet to achieve perfect, full contact with the top of the instrument, so that 100% of the sound energy of the strings is transmitted into the instrument. Poorly-fitted bridge feet can drastically impede the sound in different ways, and since all woods are constantly moving and changing size with the seasons, over time this sensitive fit can even degrade on its own, and it is recommended to get a new bridge every 10 years or so in any case. Over time, the tremendous downward pressure of the string tension on the bridge compresses the wood in the bridge, making it less able to transmit the full range of sound energy from your strings.
The thickness of the bridge is also taken into account - in general, a thicker bridge may achieve a darker sound while a thinner bridge can produce a brighter sound. However, there are pragmatic limitations - a bridge carved too thin cannot transmit warmer tones well and is subject to early structural failure, like warping, strings cutting-in, etc. A too-thick bridge with too much mass will impede the sound energy from the strings from reaching the body, and may sound 'dead' or too dark, with reduced sustain and power.
Technological Advances Aid the Sound Adjustment
Of course the human ear is a wonderfully sensitive tool to analyze sound, and is the very first tool used in a Sound Adjustment. My routine is to always play for a few minutes on a new arrival to the shop, and get a subjective sense of what our starting point is - Is the instrument too bright? too dark? a little strident? How's the balance and evenness? etc.. Then, I 'take a picture' of the sound that becomes a resource for my ear.
This image is called a spectrogram, or what I refer to as a 'sound print.' Each column is one pluck on a bass string, left to sustain for about 8 seconds. Each horizontal bar is the next frequency present in the overall harmonic structure of the tone - from bottom to top, the fundamental note, then the first harmonic, the second harmonic, and so forth.
As you can see, the G and D strings have pretty clear and consistent harmonic structures, while the A and E strings have more 'unfocused' harmonics, and shorter sustain on the fundamental.
With this visual reference, I play the instrument again not just to confirm the spectrograph for my own ear, but to make a judgement as to what adjustments might make improvement, and to what degree may be possible for the given instrument and all of it's own unique qualities.