The back plate is much less important acoustically for most frequencies, partly because it is held against the player's body. The sides of the guitar do not vibrate much in the direction perpendicular to their surface, and so do not radiate much sound. It is worth making it clear that the body doesn't amplify the sound in the technical sense of amplify. An electronic amplifier takes a signal with small power and, using electrical power from the mains, turns it into a more powerful signal.
In an acoustic guitar, all of the sound energy that is produced by the body originally comes from energy put into the string by the guitarists finger. The purpose of the body is to make that conversion process more efficient. In an electric guitar, very little of the energy of the plucked string is converted to sound. The air inside The air inside the body is quite important, especially for the low range on the instrument.
It can vibrate a little like the air in a bottle when you blow across the top. In fact if you sing a note somewhere between F 2 and A2 it depends on the guitar while holding your ear close to the sound hole, you will hear the air in the body resonating.
This is called the Helmholtz resonance and is introduced below. Another way to hear the effect of this resonance is to play the open A string and, while it is sounding, move a piece of cardboard or paper back and forth across the soundhole.
This stops the resonance or shifts it to a lower frequency and you will notice the loss of bass response when you close up the hole. The air inside is also coupled effectively to the lowest resonance of the top plate. Together they give a strong resonance at about an octave above the main air resonance. The air also couples the motion of the top and back plates to some extent. The Helmholtz resonance of a guitar is due to the air at the soundhole oscillating, driven by the springiness of the air inside the body.
I expect that everyone has blown across the top of a bottle and enjoyed the surprisingly low pitched note that results. This lowest guitar resonance is similar. Air is springy: when you compress it, its pressure increases. Consider a 'lump' of air at the soundhole. If this moves into the body a small distance, it compresses the internal air.
That pressure now drives the 'lump' of air out but, when it gets to its original position, its momentum takes it on outside the body a small distance. This rarifies the air inside the body, which then sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring. The hole is normally round and centered, but F-shaped pairs of holes, as in a violin, are sometimes seen.
Attached to the soundboard is a piece called the bridge , which acts as the anchor for one end of the six strings. The bridge has a thin, hard piece embedded in it called the saddle , which is the part that the strings rest against. When the strings vibrate, the vibrations travel through the saddle to the bridge to the soundboard. The entire soundboard is now vibrating. The body of the guitar forms a hollow soundbox that amplifies the vibrations of the soundboard.
If you touch a tuning fork to the bridge of a guitar you can prove that the vibrations of the soundboard are what produce the sound in an acoustic guitar. The process in an electric guitar is completely different, as described later in this article. The body of most acoustic guitars has a "waist," or a narrowing. This narrowing happens to make it easy to rest the guitar on your knee. The two widenings are called bouts. The upper bout is where the neck connects, and the lower bout is where the bridge attaches.
The size and shape of the body and the bouts has a lot to do with the tone that a given guitar produces. Two guitars that have different body shapes and sizes will sound a bit different.
The two bouts also affect the sound: If you drop a pick into the body of a guitar and rattle it back and forth in the lower bout and then the upper bout, you will be able to hear a difference.
The lower bout accentuates lower tones and the upper bout accentuates higher tones. The face of the neck, containing the frets, is called the fingerboard. The frets are metal pieces cut into the fingerboard at specific intervals. By pressing a string down onto a fret, you change the length of the string and therefore the tone it produces when it vibrates. We'll talk a lot more about frets and specific fret spacings later on.
Between the neck and the head is a piece called the nut , which is grooved to accept the strings. From a musical standpoint, the saddle and the nut act as the two ends of the string. The distance between these two points is called the scale length of the guitar.
The strings pass over the nut and attach to tuning heads , which allow the player to increase or decrease the tension on the strings to tune them. In almost all tuning heads, a tuning knob turns a worm gear that turns a string post. The guitar is a musical instrument, so its goal in life is to make music.
Music is the arrangement of tones into patterns that the human brain finds pleasing or if not pleasing, then at least intriguing. In order to better understand music, let's start at the beginning: "What is sound? Sound is any change in air pressure that our ears are able to detect and process. For our ears to detect it, a change in pressure has to be strong enough to move the eardrums in our ears. The more strongly the pressure changes, the "louder" we perceive the sound to be.
For our ears to be able to perceive a sound, the sound has to occur in a certain frequency range. For most people, the range of perceivable sounds falls between 20 Hertz Hz, oscillations per second and 15, Hz. We cannot hear sounds below 20 Hertz or above 15, Hertz. A tone is made up of one frequency or a very small number of related frequencies. The alternative to a tone is a combination of hundreds or thousands of random frequencies.
We refer to these random-combination sounds as noise. When you hear the sound of a river, or the sound of wind rustling through leaves, or the sound of paper tearing or the sound made when you tune your TV to a nonexistent station, you are hearing noise. A musical note is a tone. However, a musical-note tone comes from a small collection of tones that are pleasing to the human brain when used together. For example, you might pick a set of tones at the following frequencies:.
This particular collection of tones is known as the major scale. Each tone in the scale is multiplied by a certain fraction to come up with the next tone in the scale. Here's how the major scale works:. Why are these particular fractions chosen in the major scale? Simply because they sound pleasing. The names are totally arbitrary, as with the fractions. It just turns out that they have a pleasing sound to human ears. One thing to notice is that the two C notes are separated by exactly a factor of two -- is one half of This is the basis of octaves.
Any note's frequency can be doubled to "go up an octave," and any note's frequency can be halved to "go down an octave. You may have heard of " sharps " and " flats. The scale of tones shown above is "in the key of C" because the fractions were applied with C as the starting note.
If we were to start the fractions at D, with a frequency of , then we would be "tuned to the key of D" and the frequencies would look like this:. The E note in the key of D at The same applies for the A note. F and C, however, are distinct in the two keys. Note that F sharp is also known as G flat, and C sharp is also known as D flat. If you apply the fractions to several different keys, merge together all the identical and pretty-close notes and then look at the unique sharps that fall out, you realize that you need A , C , D , F and G to handle all the keys.
You can see that, with all of these mergings of keys, the major scale can leave you with some pretty arbitrary decisions to make when you tune an instrument. For example, you can tune the major notes to the key of C, and then the sharps for F and C to the key of D, and the sharps for D and G to It can get pretty messy. Over time, most of the musical world came to agree on a scale called the tempered scale , with the A note set at Hz and all of the other notes tuned off of that.
Musical Instrument Guide. The Structure of the Acoustic Guitar How a guitar makes sound. String vibration resonates throughout the body Guitars consist of two sections: the neck and the body.
Learn the parts of the acoustic guitar Acoustic guitar parts. The top is extremely important! Picks used for steel strings.
Structure How a guitar makes sound Six strings, each with a higher pitch The rule of strings and pitch Key points in determining reverberation [Experiment1]Sound hole size [Experiment2]Changing the material of the top; [Experiment3]Changing the material of the saddle and nut.
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