Have you ever noticed how certain notes (for example all A notes or D# notes) sound richer and sustain better on a particular guitar? You can sometimes find certain notes on that guitar that are brighter and just seem to ring, while others are "not quite as good" or maybe even seem a just little dead? Every object has certain frequencies that it wants to vibrate at and others that it doesn't.
An example close to home is a guitar string.
If you pluck a perfectly tuned low E string, it will vibrate back and forth 82 times per second.
As it moves back and forth, it disturbs the air 82 times per second.
And, like the ripples in a pond, these sound waves propagate through the air and strike our ear drums 82 times per second and we "hear" an E note.
Frequencies that an object wants to vibrate at (or is sympathetic to) are called resonant frequencies.
Just like a pendulum likes to swing back and forth at a specific rate or frequency; a guitar string, a guitar body or a guitar neck vibrates back and forth at certain frequencies much better than others.
82 times per second or 82 Hertz, is a properly-tuned low E string's resonant frequency.
We pluck it to get it started.
After that, it finds the 82 Hz frequency on its own.
It does this naturally-every time-because 82 Hz is its resonant frequency.
In general, larger objects tend to vibrate or oscillate at lower frequencies than smaller objects.
That's why bass guitars are larger than six strings.
Bass guitars hold longer and larger strings than standard guitars.
And these larger strings like to vibrate at lower frequencies (tones) than shorter guitar strings.
Also the base body and neck likes to vibrate at (resonate with) lower frequencies than a guitar does.
And when you fret a note, you are in actuality, shortening that string.
This makes it smaller so that it can vibrate faster and create a higher tone or pitch.
This phenomenon is also, by the way, the exact reason why we can hear the bass and kick drum best when the neighbor turns the music up too high in the next apartment.
The wall (a very large object) likes to resonate at those very low frequencies.
The higher frequencies just can't move such a large object, so they don't come through.
It's the same with your guitar.
Certain pitches are naturally stronger because they are resonant frequencies.
Others sound dead because they are not.
Resonant frequencies tend to be amplified better by the guitar and are sustained for longer periods of time.
This is why we hear them as being stronger or richer.
Non-resonant frequencies tend to be absorbed or damped by the guitar and we hear them as being weaker.
All objects have certain frequencies at which they naturally tend to vibrate.
These natural frequencies are called resonant frequencies.
Resonant frequencies are determined by the geometry, weight and structure of the object.
In general larger objects have lower resonant frequencies than smaller objects.
The weight, shape, size, materials and construction of your guitar all determine the resonant frequencies of your guitar and thus, the general sound quality of your instrument.