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Sound Theory
Sound is any change in air pressure that our ears are able to detect and process. If the change in pressure is strong enough, the eardrum can pick up sound waves and it then vibrates, and this vibration is translated by the brain into sound. The greater the pressure change, the louder we perceive the sound to be.
Sound Waves
A sound wave is introduced into a medium by the vibration of an object following an input of energy. The air molecules surrounding the vibrating object are forced to compress and expand, creating a pressure disturbance consisting of this alternating pattern of areas of compression and rarefaction. The disturbance then travels through the medium by the transportion of energy from particle to particle. This energy originates from the vibrating object. The more energy input into the object, the greater the amplitude of vibration and thus the more energy imparted to the medium. The amplitude of vibration of the particles of the medium is therefore increased.
Aristotle believed that sound and light were carried through air, but could not travel through a vacuum. It was not until the seventeenth century that this theory was proved correct, by Robert Boyle, an Irish scientist. Boyle placed a ticking watch into a jar from which he pumped air, observing that as the amount of air diminished, the ticking sound could no longer be heard.
We now know that sound does indeed require a medium to travel through - this can be gases, solids or liquids. Sound travels in air at approx. 330 m/s.
Frequency and Intensity
For us to be able to hear sound, it has to fall within a certain frequency range. Frequency is measured in Hertz (Hz) and tells us the number of sound waves given off by a vibrating object per second. For most people, the perceivable range of sounds falls between 20 Hz and 15 000 Hz.
The intensity of sound is measured in decibels (dB). The lowest possible sound that can be heard is called the threshold of hearing. The threshold of pain measures at approx. 120 dB, although any sound above 85 dB can damage a person's hearing. Here are some levels of sound intensity encountered in everyday life:
Near total silence - O dB
Acoustic Guitar
A guitar is made up of three parts - the hollow body, the neck (which holds the frets) and the head (which contains the tuning pegs). The soundboard of a guitar is a wooden piece mounted onto the front of the guitar's body, and it is this that amplifies the sound. Attached to the soundboard is the bridge in which the saddle is embedded, against which the strings rest.
When you pluck or strum the guitar strings, it vibrates at its natural frequency and resonance occurrs, increasing the amplitude. These amplified vibrations are submitted from the saddle to the bridge to the soundboard, which in turn vibrates. The entire body of the guitar forms a hollow soundbox that amplifies the vibrations of the soundboard, and the sound comes out of the sound hole (a large hole in the soundboard).
The vibrating strings of a guitar generate tones (a sound repeated at a certain, specific frequency). The frequency of the vibration of a string under tension is controlled by the length, weight, and springiness of the material of the string, as well as the amount of tension on the string. On a guitar, the first string is the thinnest whereas the sixth string is much heavier and thicker. The tension on the strings can be controlled by the tuning pegs. The frets are metal pieces cut into the fingerboard (the face of the neck), onto which a string can be pressed, altering the string's length and, therefore, the tone it produces when it vibrates. The frets are spaced out in such a way as to allow the proper frequencies to be produced when the string is held down at each fret.
Electric Guitar
Magnetic Pickups
Most electric guitars are completely passive (ie. they consume no power). The vibration of the strings produces a signal in the coil of the magnetic pickup, an electromagnet consisting of a bar magnet wrapped with up to 7 000 turns of fine wire. The magnetic pickup, which is located under the steel strings, senses these vibrations electronically and routes an electric signal to an amplifier and speaker. The pickup coil sends these signals through a simple circuit:
In an electromagnet, electrical energy can be turned into motion by the coils and the magnet, and vice versa, as in the case of the electric guitar. The vibration of the strings produces a corresponding vibration in the magnet's magnetic field and, therefore, a vibrating current in the coil. A basic pickup isa single magnetic bar under all six strings, although it is possible to have a separate polepiece for each string as below.
Many electric guitars have two or three pickups located at different points of the body, each with its own distinctive sound. Pickups can also be paired to produce additional variations.
Tone, Frequency & Amplitude
Several resistors are used to control the tone, frequency and amplitude. Tone is adjusted by the upper variable resistor, high frequencies can be cut out by using a resistor (~500 kilo-ohms) and a capacitor (0.02 microfarads), and the amplitude of the signal that reaches the jack is controlled by a second resistor. From the jack, the signal runs to an amplifier, which drives a speaker.
Amplifiers and Distortion...
A typical amplifier consists of a pre-amp, a power amplifier and a speaker. Some also include effects and reverb circuits. As it is passive, the electric guitar does not have enough power to drive the power amp directly, and so the boosts the guitar's signal enough to make this possible.
The electric guitar amp is actually part of the instrument, and, unlike a stereo amp which is designed to reproduce and amplify sound with the least distortion possible, for musicians the option of distortion - which occurs when the signal in an amp's circuitry is too powerful - is actually extremely desirable. Depending on the design of the amp, the level of distortion can be controlled by the guitarist. Vacuum tubes can also be used to create distortion patterns and create specific sounds.
Another concept unique to the electric guitar is that of feedback loops between the amp and the guitar. When the sound coming out of the amp and speaker is loud enough, it can cause the guitar's strings to vibrate so that when the musician hits a note with the guitar that string can continue vibrating indefinitely.
Psychology behind the musician...
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