Just as they do for electric guitars and basses, acoustic pickups act as transducers, converting sound vibrations in electrical current, which is then passed to an output source, usually either a preamp or directly to the PA via a Direct Input (DI). Just as there are more than one type of acoustic guitar model, there are several types of acoustic pickups that each respond differently to the vibration of strings and soundboard.
ACTIVE VS PASSIVE
Most electric guitars use passive pickups, which pickup the sound and transmit it through the guitar cable to the amplifier without requiring any further alterations to the sound. While these are used in acoustic guitars too, it is also common to find active pickups, which, through the addition of battery power, are able to offer increased gain. Active pickups also often come equipped with built-in EQ controls, and some even have tuners.
Undersaddle pickups are the most common types found in acoustic guitars and are made of a small strip of piezoelectric crystals that sit underneath the saddle of the bridgeplate. The sound causes the crystals to vibrate and those vibrations are then transduced into electrical sound waves.
Piezo pickups are often accused of having a thin and artificial sound that fails to reproduce the instrument’s natural warmth and body. This is largely due to the fact the pickup does not touch connect with the soundboard and so only reproduced the sound of the strings and not the body of the guitar. To compensate for this, undersaddle piezo transducers are sometimes paired with other types of pickups or microphones to attempt to create a more balanced and true sound. One benefit of this system is that it allows the guitar to be played at louder levels without feeding back.
MAGNETIC AND SOUNDHOLE PICKUPS
If your acoustic guitar does not come with a pre-installed undersaddle pickup, one of the most common and easiest options to install are magnetic soundhole pickups. These fit directly into the soundhole of the guitar, sitting underneath the strings and consequently only amplify the strings and not the vibration of the top of the guitar. They are, however, very good at capturing the clarity of each string and individual pole pieces can be adjusted to manually create an even sound balance between each string.
Contact pickups are small passive sensors that are placed either underneath the bridgeplate or on top of the guitar. They work by amplifying the vibration on the top and body of the guitar and consequently have a much warmer, richer sound than some other pickup types as they pick up both the strings and the top vibrations of the instrument. However, because contact pickups function in a similar fashion to microphones, they are more prone to causing feedback than other types.
There are also guitars that utilise a combination of microphones and undersaddle pickups in order to capture a more balanced and natural acoustic sound. An example of a hybrid setup is the L.R. Baggs Anthem system, which features a condenser microphone installed inside of the instrument, with inbuilt noise cancelling technology to help cancel out phasing issues, and a Class A preamp with a preset crossover.