An acoustic guitar is a musical instrument in the string family. When a string is plucked, its vibration travels from the bridge to the top of the guitar. This vibration also moves to the sides and back of the instrument, causing the air inside the body to vibrate. Sound is then released through the sound hole. The original name for this instrument is "guitar," but the term "acoustic guitar" is used to describe the steel-string version, which is different from an electric guitar that uses electronic amplification. The body of a guitar is shaped like a sound box. The top of the box acts as a sound board, which helps strengthen the vibrations from the strings. In standard tuning, the six strings are tuned (from lowest to highest) E2, A2, D3, G3, B3, and E4.

Guitar strings can be plucked one at a time using a pick or fingers, or they can be strummed to play multiple notes at once. When a string is plucked, it vibrates at a main pitch determined by the string's length, thickness, and tightness. Additional vibrations, called overtones, also occur and are related to the main pitch. These vibrations cause the sound board and the air inside the sound box to vibrate. Because these parts have their own natural vibrations, they amplify certain overtones more than others, which changes the overall sound quality of the instrument.

History

The guitar probably began in Spain around the early 1600s. It came from an earlier instrument called the guitarra latina. During the Spanish Middle Ages, small, plucked instruments called gitterns were made. These had a round back, similar to the lute. Guitar-shaped instruments with modern designs were not made until the Renaissance period, when their bodies and sizes started to look more like today’s guitars.

In Spanish music, the earliest string instruments linked to the guitar were called vihuelas. These were common in the 16th century during the Renaissance. Later, Spanish writers divided vihuelas into two types. One type, the vihuela de arco, was similar to a violin. Another type, the vihuela de Penola, was played with a plectrum or by hand. When played by hand, it was called the vihuela de mano. This instrument was very similar to the Renaissance guitar because it used hand movements near the sound hole to make music.

By 1790, most vihuela guitars had six courses, which are pairs of strings tuned to the same note. These became the main type of guitar in Spain. Older 5-course guitars were still used but were changed to have six courses. A book by Fernando Ferandiere, published in 1799, described the standard Spanish guitar of that time as having seventeen frets and six courses. The first two strings were made of gut and tuned to the note "G." At this time, the acoustic guitar began to take the shape of modern acoustic guitars. Over time, the use of paired strings decreased, and single strings became more common.

Around 1850, Spanish guitar maker Antonio Torres Jurado created the modern guitar’s design. He made the guitar body larger, changed its shape, and used a bracing pattern called fan bracing, which had first appeared in guitars made by Francisco Sanguino in the late 1700s. Bracing refers to the way wood is arranged inside the guitar to support the top and back and keep the instrument strong. Torres’ changes improved the guitar’s sound, making it louder and clearer. His design has remained largely the same since that time.

Acoustic properties

The soundboard, or top, of an acoustic guitar plays an important role in how loud the guitar sounds. Woods like spruce are often used for the soundboard because they help sound travel well. No outside energy is added to make the sound louder, as would happen with an electronic amplifier. All the energy comes from the strings being plucked. Without a soundboard, the strings would vibrate in the air without moving it much. The soundboard increases the area that vibrates, a process called mechanical impedance matching. Because the soundboard is large and flat, it moves air more easily than the string alone. This improves how efficiently energy is transferred, making the sound louder.

The hollow body of the acoustic guitar also helps. The air inside the guitar’s cavity vibrates along with the strings and soundboard. At low frequencies, the cavity acts like a Helmholtz resonator, which can increase or decrease the sound’s volume depending on whether the air inside the cavity moves in the same direction as the strings or opposite to them. When the air moves in the same direction, the sound becomes about 3 decibels louder. When it moves in the opposite direction, the sound becomes about 3 decibels softer. The air at the opening of the guitar vibrates either in or out of phase with the air inside the cavity and the strings. These vibrations either strengthen or weaken certain sound frequencies, changing how the guitar sounds. The vibrations from the cavity eventually travel through the sound hole to the outside air. Some guitars have f-holes, like certain electric guitars, instead of a single sound hole. This coupling is very efficient because air is pushing air directly.

An acoustic guitar has several ways sound is transferred: from the strings to the soundboard, from the soundboard to the air inside the guitar, and from the soundboard and cavity air to the outside air. The back of the guitar also vibrates slightly, influenced by the air inside the cavity and by the guitar’s structure. The guitar shapes the sound by how it creates and emphasizes different harmonic tones and how it transfers energy to the surrounding air, which we hear as loudness. However, better sound transfer can cause the sound to fade faster because the string’s energy is released more quickly. Solid-body electric guitars, which have no soundboard, produce very little volume but often have longer sustain.

The way air moves and the natural vibrations of the guitar’s parts are key reasons why different guitars sound unique. The sound is a mix of many harmonic tones that give each guitar its special sound.

Amplification

Classical gut-string guitars did not produce enough sound to replace banjos until new improvements made them louder. Two major changes were made by the United States company C.F. Martin: the use of steel strings and the expansion of the guitar top area. The popularity of Martin's larger "dreadnought" body size among acoustic musicians is connected to the louder sound it created. These changes allowed guitars to compete with and often replace banjos, which had been the main instruments in jazz bands. The steel strings added more pressure on the guitar neck, so Martin added a steel truss rod inside the neck to keep it stable. This design became standard in later steel-string guitars.

Acoustic guitars can be made louder using pickups or microphones. However, making acoustic guitars louder often caused problems with unwanted noise called feedback. In the 1960s, Ovation introduced guitars with parabolic bowls, which greatly reduced feedback and allowed acoustic guitars to be amplified more effectively. In the 1970s, Ovation created thinner soundboards using carbon-based materials layered with birch in its Adamas model. This design is considered one of the most unusual in acoustic guitar history. The Adamas model replaced the single sound hole of traditional guitars with 22 small sound holes in the upper part of the guitar, increasing volume and reducing feedback during amplification. Another way to reduce feedback is to place a rubber or plastic disc inside the sound hole.

The most common types of pickups used to amplify acoustic guitars are piezo and magnetic pickups. Piezo pickups are usually placed under the bridge saddle of the guitar and can be connected to a mixer or amplifier. A piezo pickup made by Baldwin was built into the body of Ovation guitars instead of being attached by drilling through the body. The combination of the piezo pickup and the parabolic ("roundback") body helped Ovation succeed in the market during the 1970s.

Magnetic pickups on acoustic guitars are typically placed inside the sound hole and are similar to those found on electric guitars. An acoustic guitar with pickups for electrical amplification is called an acoustic-electric guitar.

In the 2000s, manufacturers introduced new types of pickups to better amplify the full sound of acoustic guitars. These include body sensors and systems that use an internal microphone along with body sensors or under-the-saddle pickups.

Types

Historical and modern acoustic guitars come in many different designs and styles. Some of the most important types include the classical guitar (also called the Spanish guitar or nylon-stringed guitar), the steel-string acoustic guitar, and the Colombian tiple.

Nylon or Gut-Stringed Guitars: These include the vihuela, gittern, charango, cuatro, ukulele, Baroque guitar, Romantic guitar, classical guitar, and the modern version of the original guitar. Some models have extra strings, such as seven-string, eight-string, nine-string, ten-string, eleven-string, thirteen-string, and the Flamenco guitar. Other related instruments include the lute.

Steel-Stringed Guitars: These are also called western, folk, or country guitars. They include the twelve-string model, the Colombian tiple, the resonator guitar (like the Dobro), the archtop guitar, the Selmer/Maccaferri (Manouche) guitar, the Battente guitar, the lap steel guitar, the lap slide guitar, the parlor guitar, and the lyre-guitar.

Other Variants: These include the harp guitar, the Pikasso guitar (a type of harp guitar), the contraguitar (a Viennese version of the harp guitar), the acoustic bass guitar, and the banjo guitar.

Common body shapes for modern acoustic guitars, from smallest to largest:

Range: This is the smallest common body shape, sometimes called a mini jumbo. It is about three-quarters the size of a jumbo guitar. The rounded back helps the smaller body project sound better. The smaller size and shorter scale make it easier for players who find larger guitars difficult to handle.

Parlor: Parlor guitars have small, compact bodies. They are known for a bright, punchy sound and a delicate tone. They usually have 12 open frets. The small size makes them more comfortable for players who find larger guitars uncomfortable.

Grand Concert: This is a mid-sized body shape. It is not as deep as full-size guitars but has a full waist. The smaller size helps control the sound, making it useful for recording.

Auditorium: This shape is similar in size to the dreadnought but has a more pronounced waist. It is sometimes called an "Orchestra" style guitar, depending on the manufacturer. The waist design changes the sound to stand out. This is a newer body shape compared to others like the dreadnought.

Dreadnought: This is the classic body shape. It was designed by Martin Guitars to create a deeper sound than older-style guitars, with strong bass tones. The body is large, and the waist is less pronounced than in the auditorium or grand concert shapes. Many variations exist, including the Gibson J-45.

Jumbo: This is the largest standard body shape. It is bigger than the auditorium but has a similar shape. It was designed by Gibson to compete with the dreadnought, offering more space for sound and greater volume. The Gibson J-200 is a famous example. Most guitar makers also produce at least one jumbo model.

Playing techniques

The acoustic guitar is used in many types of music, and each style uses different ways to play it. Some common techniques include:

Strumming uses a back-and-forth motion of the picking hand (right hand for right-handed players; left hand for left-handed players) across the strings, while the other hand holds down the strings to form chords. This can be done with or without a guitar pick, depending on whether the player wants a clear or softer sound. Strumming patterns vary based on the song’s beat. Simple strumming on the main beats is usually the first technique guitarists learn. Players can also change patterns or emphasize certain beats to create rhythm and style. An example of a song with strumming is "Free Fallin’" by Tom Petty, where you hear full chords being strummed.

Fingerstyle, also called fingerpicking, uses the picking hand to pluck individual strings in a pattern. This method focuses on playing specific notes in a melody rather than full chords. Guitarists use their thumb, index, middle, and ring fingers, which are labeled as "p" (thumb), "i" (index), "m" (middle), and "a" (ring finger) in sheet music. These letters help players know which finger to use for each note. Plucking the strings downward creates a clear, musical sound. A variation called "percussive fingerstyle" adds rhythmic taps on the guitar body to create a drum-like effect. An example of fingerstyle is "Landslide" by Fleetwood Mac, where you hear individual notes being plucked instead of full chords.

Slide guitar is a technique used on acoustic, steel-string, and electric guitars. It is often found in blues, rock, and country music. Players wear a small metal, glass, or plastic tube on a finger of their fretting hand and slide it along the fretboard instead of pressing on individual frets. The picking hand strums or plucks the strings normally. This creates a smooth, flowing transition between notes, called glissando. An example of slide guitar is "For Emma, Forever Ago" by Bon Iver, where you can hear a smooth sliding melody throughout the song.

Gallery

• Gittern (1450)
• Lute (17th century)
• Romantic guitar (around 1830)
• Classical guitar
• 10-string guitar
• Steel-string acoustic guitar
• Resonator guitar
• Chitarra battente
• Lyre-guitar
• Pikasso
• Steel guitar (around 1920)
• Harp guitar
• Mexican vihuela
• 12-string guitar
• Acoustic bass guitar