[ electroacoustic instruments ]
Any instrument can be transformed from a purely acoustic one into an electro-acoustic version. This requires the fitting of a transducer system, "Transducer" simply means energy converter. In this case, they convert acoustic energy into electrical energy, and they can take various forms . Life is full of greyness, and nowhere is this more true than in terms of instrument acoustics. The choices are full of pros and cons, trade-offs of all kinds. The only good guide is to hold on to what it is that you are trying to achieve. The perfect system has never been invented!
1. Internal microphones.
As the name suggests, these are usually omnidirectional miniature microphones fitted within the guitar body. Microphones can be active or passive. Passive ones work by generating a current through the movement of a coil in a magnetic field. They do not require power, but are difficult to miniaturise. The preferable type is the capacitor mic. This senses vibrations by detecting the change in distance between two charged pates. Since the mic can only work when an electrical charge is present, this type requires a power supply, which usually takes the form of a 9 volt battery, also mounted internally. However, it is often more practical to include an onboard pre-amplifier, so the battery can power both together. If the battery fails, there is no output! The advantages of pre-amplifiers are dealt with below. Although there are certain local effects within a guitar body, microphones give a very broad sampling, and tend to "hear" the full interaction of soundboard, body and internal air. This is usually perceived as a warm or full sound, and gives the most natural sound of the various types, but suffers from the disadvantage that a feedback loop is generated at low amplification levels. Typically for a classical guitar this resonance falls around G or G# on the bottom string, or a closely related harmonic. The most common way to limit the feedback is to balance more towards a different transducer that hears less of the body frequencies. There are two options.
2. Soundboard transducer.
This and subsequent devices work through the magic of piezoelectric crystals. See piezoelectric. Soundboard mounted transducers fall in the middle of the spectrum in terms of generating the full blend of tone. They colour the sound more to the soundboard resonance, which is quite pleasing to the ear. However, a single transducer samples over a very small area and so suffers from dead spots and sometimes erratic volume at different frequencies. Take, for example, the idea of measuring temperature on a summer day. On the shady side of the house, the thermometer may read 25�C. If you simply move to a sunny window, it may give a different result, say 32�C. The difficulty with very local sampling is that you do not necessarily get a clear overall picture. It is exactly the same with the output from a soundboard transducer.
3. Saddle transducers.
These either take the form of a continuous strip fitted under the saddle, or an integrally moulded piezoelectric element and saddle. Often, for best results, the elements are broken into six individual ones, located under each string. The strip type are comparatively easy to fit as a conversion to an existing acoustic bridge, however, you will by now realise that there can be associated problems. Firstly, the connections to the strip are very delicate and sometimes reduce the life of the transducer. Secondly, the strip will only balance well if the pressure on the strip is similar under each string. This requires a very flat bottom surface for the saddle, and a very flat slot in the wood of the bridge. In practice, this is quite hard to achieve, and there may be dead strings when the contact between the saddle and the strip is poor. Because the transducer is sensing at the nodal point on the string - the point where the string does not move, the harmonics are rather strange. This is the effect of playing "ponticello", sometimes referred to as a "metallic" tone. The saddle is not the ideal position to detect tone, however, a transducer placed here does not hear so much body noise or mechanical noise, and allows higher amplification levels before producing feedback.
All small preamplifiers are solid state. In other words, they are driven by transistors. Our world is full of electricity, mostly piped around in electricity mains at 50 or 60 Hertz. The electricity main is effectively a huge transmitter, causing mains hum in anything conductive, like other wires or even your own body. Since the transducer output is analogue, it is very susceptible to background noise. As cable lengths increase the output level falls, while the noise level rises, so the moral is to get an amplifier into the system as early as possible. Preamplifiers also give the ability to balance between different inputs, and usually incorporate filters, which may be split into narrow bands, in which case they become equalisers.
Whatever arrangement you choose, it is ultimately necessary to get the signal out of the guitar and into some external amplification system. This is usually done through a � inch (6mm) endpin jack. These jack sockets are usually mounted by drilling a 12mm hole through the bottom block of the guitar. Such sockets can also incorporate a button for a shoulder strap.