AMPLIFICATION

Amplification, along with recording, has had a greater impact on music in the 20th century than any other technical innovation. The process of making sound signals louder has changed significantly the way music is made. For the listener, amplification at home enables control of the level of sound, while in an auditorium amplification allows for greater clarity and detail of sound within a large space. Amplification also affects the way a musician plays and how a composer orchestrates—instruments that were rarely heard before amplification can now be highlighted. Perhaps most importantly, some forms of music, such as rock, are inseparable from high sound volumes.

The basic principle of the electronic amplifier was established in 1907, when Lee De Forest invented the audion tube: a small signal voltage was fed through the tube producing a larger signal voltage with an identical pattern of variation within the tube’s circuit.

THE PROCESS OF AMPLIFICATION

Sound is acoustical energy, and a device called an electroacoustic transducer is needed to transform the acoustical energy into electrical signals that can flow through wires and electrical equipment. A microphone is the most popular electroacoustic transducer, containing a membrane, called a diaphragm, that vibrates when sound is directed towards it—the higher the pitch of the sound, the faster the microphone’s diaphragm vibrates. Through these vibrations, the microphone generates an electrical signal that is converted back into acoustical energy, sounding exactly like the original sound.

The microphone is a vital component in the amplification process, and there are many different types. Microphones vary in their ability to pick up sounds from different directions. An omnidirectional, or nondi-rectional, microphone responds equally to sounds arriving from all directions. Such microphones are ideal when the application calls for all sounds in a vicinity to be detected equally. Directional microphones, on the other hand, attenuate (diminish) unwanted sounds— sounds arriving from directions other than the direction of interest. Such a directional microphone is perfect for amplifying the voice of the singer and not the band.

Once the microphone has converted sound into electrical signals, it channels this energy into an electrical component called a pre-amplifier. A preamplifier produces a larger electrical voltage and allows the user to control equalisation of tone and other enhancements. In most audio applications, the term “pre-amplifier” is actually a misnomer, and refers to a device more properly called a control amplifier. An integrated amplifier is an amplifier containing the pre-amplifier and the power supply in one unit. The signal then goes from the pre-amplifier to the power amplifier, which magnifies the signal and powers the loudspeakers. The loudspeakers convert the electrical energy back into mechanical sound energy.

LOUDSPEAKERS

Loudspeakers are technically called electromechanical transducers, and they contain an electro- or permanent magnet that produces a magnetic field. A voice coil situated within the magnetic field is energised by the electrical signal coming from the amplifier. The magnetic field generated within the coil varies with the frequency of the electrical signal, attracting and repelling the coil to the magnet. This coil is linked to a diaphragm, which vibrates and causes the surrounding air to vibrate, recreating the original sound.

Each loudspeaker usually consists of two or more connected individual speakers, also called drivers, that are enclosed in a wooden box. The enclosure protects the mechanics of the speaker system and helps to project the sound. Each of the individual speakers reproduces a part of the sound spectrum. The woofer, for example, reproduces the lower bass sounds, while the tweeter reproduces the higher frequencies. A mid-range speaker is often used to reproduce the wide range of middle frequencies. A special electronic circuit called a crossover directs each of the frequencies to the relevant speaker.

When selling a stereo system, a salesperson often refers to speakers as “two-way” or “three-way.” These terms refer to the number of individual frequency drivers contained by each speaker. Speakers are also classified by the way the woofer is positioned in its cabinet, such as acoustic suspension, bass reflex, or open-baffle. Each position of the woofer is appropriate for different sound applications.

The journey that a sound signal makes—when it is converted via a transducer to electricity, processed through an amplifier and driven to loudspeakers where the reconversion to mechanical sound energy takes place—is called the signal path. The refinement of the signal path to maximize the amplified sound is the job of stage managers, sound engineers, and other professionals within the music industry.

Sound equipment intended for a special purpose, such as a public-address system, may have a built-in volume level or frequency range limitation. Equipment intended to reproduce music, however, is rated according to the accuracy, or fidelity, of its reproduction. Thus “high fidelity” sound systems are those that reproduce the original sound of music, authentically re-creating its full frequency range, dynamic range (loud/soft contrast), and timbre (tone quality). Such “hi-fi” sound systems are commonly found in home stereo systems.

MUSICAL INSTRUMENT AMPLIFICATION

A musical instrument amplifier—often called “amp”— works in a similar way to other forms of amplification. For guitar amps, the guitar’s pickup acts as an electroa-coustic transducer. The signal flows through the pre-amp and power amp and is often processed by internal equipment that can equalise the signal before it is sent to the loudspeaker to be reconverted to sound energy.

The most common misconception about guitar amplifiers is that they are designed to reproduce the raw, acoustic sound of an electric guitar. For nearly all styles of electric guitar amplification, however, amps are used as an integral part of sound creation. Factors that influence how the music will sound as it becomes amplified include the musical instrument itself, the pre-amplification (such as equalisation and sound effects), the use of the power amplifier with varying amounts of overdrive, and even the selection of amp speakers.

Modern amps often have special circuitry called a digital signal processor (DSP). These manipulate the signal as it passes from the instrument through the amplifier. For example, a distortion effect can alter a signal, changing its sound. Outboard effects units that sit on the floor and are activated by foot switches can create almost any effect, including automatically transposing music into different keys. A chorus effect multiplies the number of signals occurring simultaneously, resulting in a multi-instrumental sound from a single instrument. Multi-effect racks and floor units are designed specifically for rock, jazz, blues, and other musical applications. Distortion (hard-clipping) causes changes in a signal that involve the addition of spurious tones at frequencies not present in the original sound. There is also digital overdrive, where the guitar signal passes through an analog-to-digital converter, to software emulation of amplifier overdrive, and finally to digital-to-analog conversion. Other similar effects can make a guitar sound like a full string orchestra or even a Harley-Davidson motorcycle.

FEEDBACK AND OTHER TECHNIQUES

Early experiments in using amplification as part of the music-making process have resulted in some innovative techniques. The use of feedback, for instance, most likely started when a guitarist stepped too close to a speaker. Feedback occurs when an artist uses the acoustic energy of the speakers with the vibrating strings to produce either an endless sustain or the controlled feedback of harmonically related notes. Other experiments included tearing the loudspeaker diaphragm to produce a “fuzz” effect, which can now be created with effects processors.

Such unique effects play a vital role in contemporary music, and are as much a part of the identity of a song as the melody. Legendary rock guitarists such as Jimi HENDRIX and Jimmy Page earned their place in music through the manipulation of amplification. While it is probable that early overdrive, distortion, and other amplified effects were merely discovered in attempts to play as loudly as possible, there is also little doubt that this approach has produced some of the 20th century’s most innovative music.

James Tuverson