Like hi-fi speakers, public address cabinets are able to produce their audio by making use of an electromagnet that has a physical effect on a cone within them. Electrical energy is converted into audio via a pushing and pulling motion of the magnet and its cone; together called a driver. As the driver moves, so the air next to it is caused to vibrate. It is these vibrations in the air that we perceive as sound and, in the case of DJ equipment, as music.
Active or Passive Speakers?
Although some speakers are powered directly, others get their energy from an external amplifier. An integrated amp in a speaker cabinet is convenient, but not necessary. If you connect an audio source to an active amp or two, then the power rating of each will match the driver's capacity. In this way, it is easier to have differently rated amps and speakers dotted around a room. However, in most DJ set ups, you will want an equal balance of left and right, so a power amp which supplies energy to two, or more, passive speakers is often preferable. Furthermore, with a passive setup you don't need a power outlet at every speaker position, just one for the amp.
Focus on Drivers
As mentioned, drivers create a physical movement which translates into sound. There are three basic types of driver. These are specifically designed to create different types of audio. Woofers are the first. They create bass sounds and are an important part of any DJ set up because this is where the groove of the music is usually heard. Tweeters are the second type of driver which produce the high-end frequencies, such as those that need to be reproduced for good cymbal sounds on a track. The final type of driver is called a midrange which is for the in-between audio. The quality of the midrange driver is crucial because this is where most of the audio is made. Vocals, guitars, keyboards and wind instruments all occupy middle frequencies, so many sounds need to be made clearly and distinctly using just a single midrange driver.
Setting Up PA Speakers
Ideally, your midrange drivers will be set up at ear-level. This often means mounting your speaker cabinets onto a speaker stand so that when people are dancing they can hear the crucial middle frequencies. Bass drivers can be lower down because they get less absorbed by people moving around and dancing. Tweeters can go a little above head height, but angle these drivers downward a little so that their sounds reach the middle the of the dance floor.