Have you ever really thought about what the tone knob on your guitar does? If you’re like most players, you've probably got the general idea that it makes your guitar's tone more or less “bright."
Of course, “tone” can be a bit of a vague concept, one which different musicians interpret and describe differently. Generally, guitarists use the term “brightness” to refer to tone that could be described as thin, light or twangy. The opposite of a “bright” tone would be a deeper, richer tone.
So when most guitarists adjust their guitar's tone knob, they think in these terms: turning it "up" results in a "brighter" tone, while turning it "down" results in a less bright tone. That might be the effect of adjusting the tone knob, but what does it actually do?
Simply put, the tone knob works by removing the high frequencies that account for "bright" tone.
In this sense, the entire notion of turning the tone up to make it brighter is more than a little misleading. And that's because, when adjusting the tone knob, you're never adding any additional frequencies to the sound. Instead, turning the tone knob down removes higher frequencies, making the tone sound effectively less bright (though no bass frequencies are actually added). In other words, "10" (or "11," if you're Nigel Tufnel) is the baseline; it's this setting that lets through the sound your pickups naturally produce. Only by turning the knob down do you alter the tone.
So, from level 10, you can turn down the knob to take away some of these higher frequencies, often known as “rolling off the treble.” With the reduced frequencies comes a deeper sound, bereft of the brighter, higher tones. In this case, we are not adding any deeper tones to the sound, but simply removing the higher frequencies. To bring those frequencies back, it’s as simple as turning the knob back up to a higher level.
When do you want high or low frequencies?
The next question is, how do you know how you should set your tone knob? This question is really asking us what sort of frequencies we want to include in the sound coming from our guitar. And the answer to this will depend on the sound you want to produce. If you’re playing a song that requires a brighter, twangier tone, you may want to include these higher frequencies and thus set the tone knob to a higher level.
On the other hand, if you’re playing music that requires deeper, richer tones, it might be best to turn the tone knob down and take out those higher frequencies that add brightness thin out the sound. Really, it’s about experimenting with the sound, understanding what type of sound and tone you want to produce, and playing with what types of tones work well in different parts of your music.