Usually compact in size, and often confused with standard bookshelf style speakers, a studio monitor is a specialist piece of equipment.
Where Hi-fi and PA speakers are engineered to enhance a stereo spread, or add extra low end response and midrange, to cater for all environments and humans logarithmic hearing, these are not desirable or useful traits in a monitor system.
Often referred to as ‘near-field’ monitors, which means they are designed to be placed close to the listener so as to avoid the sound being affected by the room and its surroundings from natural reflection and reverb.
They are designed for accurate audio reproduction, for use in recording studios, television and radio studios, and for home recording purposes where a near flat ( linear ) phase and frequency response is required. This is to avoid certain frequencies being enhanced or subdued, so that the speaker reproduces the source signal with no added distortion, tonal enhancement or colouration, and that the phasing of a stereo signal remains true.
Dedicated monitors are also far more robust in their mechanical qualities than a hi-fi speaker of similar size. This is due to them being required to handle raw unmastered audio feeds which often feature sudden loud bursts of sound, widely varying frequencies and high volumes used in mastering, compared to a hi-fi unit that's only handling material that's been compressed, level adjusted and fed from a low level source.
Studio monitors are available as both passive and active units. Most modern studios, and home studio users will opt for active units, as they offer the convenience of placement, cabling, and that the on board amplifier and crossover system is specifically designed to work with the drivers and cabinet for optimum performance.
The crossovers in most powered monitors are also active, and the internal amplifier units split into two (bi-amp) or three (tri-amp) outputs, with each section powering either the woofer or tweeter driver through the crossover, which results in a much cleaner and defined reproduction.
Some engineers do still prefer a passive setup with separate amplification, as they like to change parts of the system depending on the job, or they believe that having passive speakers gives a more neutral response than a powered unit. These are specific decisions made from personal preference, but the modern market would suggest that active systems are now the industry standard, and are the best place to start if you are putting together a home or small professional studio.