Claims About Tube Amps Part 2: Using The Speaker Cabinet To Add Tone Colouring

date: 10/30/2012 category: features
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Claims About Tube Amps Part 2: Using The Speaker Cabinet To Add Tone Colouring
Note to readers: the previous article was meant to generate thought and stimulate discussion, not to present fixed ideas. Significant improvement in any field only occurs when we loosen our hold on dogma and begin to try to pick out objectively what works and what doesn't and what has an effect and what is merely superfluous. While time-tested traditions have their place, there is no reason why these cannot exist alongside new innovations and progressive improvement. This is another such article. Wood has been used for centuries in musical instruments - in everything from flutes to horns to violins to pianos to guitars. It has long been recognized for its ability to impart warm and pleasing overtones to just about any musically generated sound. The most significant use of wood is in the sound box of stringed instruments to amplify and project the sound coming off the strings. In electrical instruments such as guitars and more recently, violins, the sound box is missing and along with it the pleasing, woody overtones. Here it is suggested to re-introduce the pleasant sound colouring of wood further down the line in the speaker cabinet. One might be tempted to ask, well, why not just amplify an acoustic instrument that already has a sound box? Anyone who's tried to do this knows how tricky it can be. The biggest problem is feedback and is the reason why solid-body electrics were introduced in the first place, and with considerable success. Probably the most effective way to amp an acoustic instrument is with a microphone. This is fine for the studio, but for live performances is less than ideal. Contact pick-ups tend to amplify string sounds - indeed, any sort of pick-up attached to the body often results in sounds that are considerably distorted and to not match the fullness of the unamplified instrument. Another reason to do this is that the design of instrument sound-boxes is limited by the needs of the human player. They cannot be too large or bulky or otherwise awkwardly shaped. By contrast, the sound-box hanging off a speaker cabinet can made any size or shape we desire. Meanwhile, the instrument actually held in the musicians hands can be considerably reduced in size so that it is lighter, easier to handle and more durable. The body of a solid body electric guitar is really just for show and could potentially be removed and replaced with nothing more than an elongated block of wood. To date, only a few guitars have been manufactured with such a design. Meanwhile, because the sound power is much greater at the speakers, the wood can be made much thicker and therefore more durable. Of course, speaker cabinets are normally already made of wood. In modern speaker design, however, at least in its theoretical ideal, the wood of the cabinet is not meant to be an active component, and indeed, very thick wood is typically used. The speaker cabinet is meant to approximate an infinite, rigid wall which separates the vibrations coming from the back of the speakers from those coming from the front, the rational being that the two waves are 180 degrees out-of-phase and therefore will destructively interfere. The simplest approximate realization of this ideal is that of the open-backed cabinets used in many guitar amplifiers. Many designs attempt to re-introduce the lost energy from the back of the speaker by inverting the phase and projecting it out the front. The simplest design to take advantage of wood for sound colouring would take the existing speaker design and simply add an extra, wooden cabinet out the front with a hole in it - just like the sound box of a guitar which approximates a Helmholtz oscillator. A more radical idea would be to use the speakers to oscillate the wood directly, either with a weight for opposition, or another part of the speaker. Here we are using the wood as a sound board - similar to an acoustic guitar - instead of a thin, paper membrane. This could potentially necessitate a significant redesign of the speakers themselves. The membrane could be removed and since wood is much more rigid than paper, the amplitude of the speaker oscillations would need to be reduced. A piezo-electric electric speaker, which generates oscillations through a voltage applied across a crystal, for instance, might be preferable to the more common dynamic speaker which uses electro-magnetic coils.
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