Loudspeakers are critical to any audio system. From tweeter speakers to woofer speakers, loudspeakers are the components that provide movies, music, and sports with sounds that are often taken for granted.
Microphones convert sound into electrical impulses that can be recorded onto some form of storage media. Once captured and stored, it can be reproduced at a later time or place. Hearing recorded sound requires a playback device, an amplifier, and, most critically, a loudspeaker.
What Is a Loudspeaker?
A loudspeaker is a device that converts electrical signals into sound as the result of an electro-mechanical process.
Speakers typically incorporate the following construction:
A metal frame or basket, within which all the speaker components are placed.
A diaphragm that pushes air out through vibration. The vibration patterns reproduce the desired sound waves received by your ears. The diaphragm is often referred to as the cone. Although a vibrating cone is commonly used, there are some variations, which are discussed below.
An outer ring of rubber, foam, or other compatible material, referred to as a surround. Not be confused with surround sound or surround speakers, the surround holds the diaphragm in place while providing enough flexibility to vibrate. Additional support is provided by another structure, referred to as a spider. The spider makes sure that the vibrating speaker diaphragm and surround do not touch the outer metal frame.
A voice coil wrapped around an electromagnet is placed at the back of the diaphragm. The magnet or voice coil assembly provides the power to make the diaphragm vibrate according to the received electrical impulse patterns.
Cone speakers also have a little bulge that covers the area where the voice coil is attached to the diaphragm. This is referred to as the dust cap.
The speaker (also referred to as a speaker driver or driver) can now reproduce sound, but the story doesn't end there.
The speaker must be placed inside an enclosure so that it performs well and looks aesthetically pleasing. Most of the time, the enclosure is some type of wood box. Other materials, such as plastic and aluminum, are sometimes used. Instead of a box, speakers can come in other shapes, such as a flat panel or sphere.
Not all speakers use a cone to reproduce sound. Some speaker makers, such as Klipsch, use horns in addition to cone speakers. Other speaker makers, most notably Martin Logan, use electrostatic technology in speaker construction. Still others, such as Magnepan, use ribbon technology. There are also cases where the sound is reproduced by non-traditional methods.
Full-Range, Woofers, Tweeters, and Mid-Range Speakers
The simplest loudspeaker enclosure contains only one speaker, which reproduces all the frequencies sent to it. However, if the speaker is too small, it may only reproduce higher frequencies.
If it is medium-sized, it may reproduce the sound of a human voice and similar frequencies well and fall short in the high and low-frequency range. If the speaker is too large, it may do well with lower frequencies and, perhaps, mid-range frequencies, and may not do well with higher frequencies.
The solution is to optimize the frequency range that can be reproduced by having speakers of different sizes inside the same enclosure.
Woofer speaker 
A woofer is a speaker that is sized and constructed so that it can reproduce low and mid-range frequencies. Woofers do most of the work in reproducing the frequencies you hear, such as voices, most musical instruments, and sound effects.
Depending on the size of the enclosure, a woofer can be as small as 4 inches in diameter or as large as 15 inches. Woofers with 6.5-inch to 8-inch diameters are common in floor standing speakers. Woofers with diameters in the 4-inch and 5-inch range are common in bookshelf speakers.
Tweeter speaker
A tweeter is a specially designed speaker that is smaller than a woofer. It only reproduces audio frequencies above a certain threshold, including, in some cases, sounds that human ears cannot hear but only sense. There are ribbon tweeter, dome tweeter, and membrane tweeter, etc.  
Because high-frequencies are highly directional, tweeters disperse high-frequency sounds into the room so that the sounds are heard accurately. If the dispersion is too narrow, the listener has a limited amount of listening position options. If the dispersion is too wide, the sense of direction of where the sound is coming from is lost.
These are the different types of tweeters:
Cone: A smaller version of a standard speaker.
Dome: The voice coil is attached to a dome that is made of fabric or a compatible metal.
Piezo: Instead of a voice coil and cone or dome, an electrical connection is applied to a piezoelectric crystal, which in turn vibrates a diaphragm.
Ribbon: Instead of a traditional diaphragm, a magnetic force is applied to a thin ribbon to create sound.
Electrostatic: A thin diaphragm is suspended between two metal screens. The screens react to an electrical signal in such a way that the screens become out-of-phase. This alternately attracts and repels the suspended diaphragm, creating the needed vibration to create sound.
Mid-Range Speaker and full range speaker
A speaker enclosure may incorporate a woofer and tweeter to cover the entire frequency range. However, some speaker makers add a third speaker that further separates the low-range and mid-range frequencies. This is referred to as a mid-range speaker.
2-Way vs. 3-Way
Enclosures that incorporate only a woofer and a tweeter are referred to as 2-way speakers. Enclosures that house a woofer, tweeter, and mid-range are referred to as 3-way speakers.
The 3-way speakers may not always be better. A well-designed 2-way speaker can sound excellent, and a poorly-designed 3-way speaker can sound terrible. It's not only the size and number of speakers that matters. The sound quality also depends on the materials the speakers are constructed of, the enclosure's interior design, and the quality of the next needed component—the crossover.
Crossovers
You just don't throw a woofer and a tweeter into a box, wire them together, and hope it sounds good. When you have a 2-way speaker or a 3-way speaker in your cabinet, you also need a crossover. A crossover is an electronic circuit that assigns the appropriate frequency range to different speakers.
For example, in a 2-way speaker, the crossover is set at a specific frequency point. Any frequencies above that point are sent to the tweeter, while the remainder are sent to the woofer.
In a 3-way speaker, a crossover can be designed so that it has two frequency points—one for the point between the woofer and mid-range, and another for the point between the mid-range and tweeter.
The frequency points of the crossover vary. A typical 2-way crossover point might be 3kHz (anything above goes to the tweeter, anything below goes to the woofer). Typical 3-way crossover points might be 160Hz to 200Hz between the woofer and mid-range, and then the 3kHz point between the mid-range and tweeter.
Passive Radiators and Ports
A passive radiator looks like a speaker. It has a diaphragm, surround, spider, and frame, but it is missing the voice coil. Instead of using a voice coil to vibrate the speaker diaphragm, a passive radiator vibrates in accordance with the amount of air the woofer pushes inside the enclosure.
This creates a complementary effect in which the woofer provides the energy to power itself and the passive radiator. Although not the same as having two woofers connected directly to the amplifier, the combination of the woofer and the passive radiator produces more effective bass output. This system works well in smaller speaker cabinets, as the main woofer can be pointed outward towards the listening area, while the passive radiator can be placed on the back of the speaker enclosure.
An alternative to a passive radiator is a port. The port is a tube placed on the front or rear of the speaker enclosure so that the air pumped out by the woofer is sent through the port, creating a similar complementary low-frequency enhancement as a passive radiator.
A port must be a specific diameter and tuned to the characteristics of the enclosure and woofer that it complements. Speakers that include a port are referred to as bass reflex speakers.
Subwoofer speaker
A subwoofer reproduces very low frequencies and is used mostly in home theater surround sound applications and high-end audio.
Examples where a subwoofer is desired include reproducing specific low-frequency effects (LFE), such as earthquakes and explosions in movies, and for music, pipe organ pedal notes, acoustic double bass, and tympani.
Most subwoofers are powered. That means that unlike traditional speakers, subwoofers have a built-in amplifier. On the other hand, like some traditional speakers, subwoofers may employ a passive radiator or port to enhance low-frequency response.
The Bottom Line
Loudspeakers reproduce recorded sound so that it can be heard in a different time or place. There are several ways to design a loudspeaker, including bookshelf and floor standing size options.
Before you buy a loudspeaker or a loudspeaker system, do some critical listening with content you are familiar with. CDs, DVDs, Blu-ray and Ultra HD Blu-ray Discs, or vinyl records would all work.