What is an "Active" Pickup System?
When talking about acoustic pickups, an "active system" means it's powered by an internal battery. This is technically called a preamp. Preamps are simply small powered circuits designed to make the pickup signal sound better before it leaves the guitar. A preamp adds frequency equalization (EQ) to give the pickup a good tonal character before it gets to your sound system. Most acoustic pickup systems are active. And those that aren't are always recommended to be preamp-ed outside the guitar before going into your amplifier or PA.
Three Basic Types
Piezo elements and transducers are essentially the same thing. These materials create a vibrating electrical current whenever they are placed under pressure and vibrated at an audible frequency. This alternating current is low voltage and must be manipulated to sound better. Some companies, like K&K are making transducer elements with a good natural tone by themselves even when running "passively." However, they don't perform at a professional level without a preamp. All transducers achieve the best sound possible with they are preamp-ed, or active.
The pickups you see mounted in the soundhole are driven by "magnetism" imparted on the strings. Without getting into the details of electromagnetic induction, just know that magnetic pickups use the same mechanics as electric guitar pickups to create signal. They will only work with steel string guitars. They will not work with classical or nylon string guitars. There are many different types, and only the active models are worthwhile in our opinion.
Internal and external mics are available for acoustic guitars as well. We aren't talking about recording mics on stands here. These are small scale condenser mics that get mounted on or in the instrument. L.R. Baggs makes a specially designed microphone that attaches to the bridge plate. This is the only standalone mic we recommend. Microphones are best used to supplement one or more of the pickup types discussed above. There are a few others we like that are used to blend with a transducer or magnetic pickup signal.
When you combine two of these signal sources together your acoustic tone will be transformed with depth and character. Multi-source systems combine and mix the signals of different pickups in the guitar. This is arguably to only way to get a professional acoustic sound. A system like this uses a preamp to do the mixing, and most players have units that allow control over the mix and EQ. Most professional preamps have a dedicated D.I. out as well. This allows you to send a clean balanced signal directly to a soundboard without any issues or worries about the sound guy.
What to choose?
We have a number of favorite acoustic systems that we prefer and recommend. The two most common pickups we install are the Fishman Matrix and the K&K Pure pickup. But we offer many different choices depending on your guitar, budget, and type of use. We will assess your needs and find the best pickup for your situation. Schedule a consultation to discuss options and types!