Woofers, Tweeters, and Crossovers - The Language of Loudspeakers

Example of Typical Loudspeaker Driver Construction
Example of Typical Loudspeaker Driver Construction. Klipsch
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What is a Loudspeaker?

Loudspeaker Driver Construction Diagram
Loudspeaker Driver Construction Diagram. Image courtesy of Amplified Parts.com

Sound is all around us in our daily lives. In nature, sound is generated by both natural forces and living things, and the vast majority of humans are able to hear sound through their ears.

Also, with our technological prowess, humans can also capture sound using a device referred to as a microphone, which converts 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. However, hearing recorded sound requires a playback device, an amplifier, and, most critical of all, a loudspeaker - which is the subject of this article.

A Loudspeaker is a device that converts electrical signals into sound as the result of an electro-mechanical process using the following basic construction.

  • A metal frame, referred to as The Basket within which all the speaker components are placed.
  • A Diaphragm that pushes air out through vibration. The vibration patterns is what reproduces the desired sound waves received by your ears. The Diaphragm is often referred to as the Cone. Although a vibrating cone is most commonly used, there are some variations (discussed later in this article).
  • An outer ring of rubber, foam, or other compatible material, referred to as a Surround (not be confused with surround sound or surround speakers) that holds the diaphragm in place, while providing enough flexibility so that it can vibrate. Additional support is provided by an 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 electro-magnet is placed at the back of the diaphragm. The Magnet/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 (which may also be referred to as speaker driver, or driver), once assembled, can now reproduce sound, but the story doesn't end there.

To make sure the speaker performs at its best, and also looks aesthetically pleasing, it needs to placed inside an enclosure. Although most of the time, the enclosure is some type of wood box, other materials, such as plastic and aluminum are sometimes used (I have actually seen speakers with a marble enclosure). In addition to a box appearance, speakers can come in various shapes, such as a flat panel or sphere.

Also, as mentioned above, not all speakers use a cone design to reproduce sound. For example, some speaker makers, such as Klipsch, use Horns in addition to cone speakers, while some speaker makers, most notably, Martin Logan, use Electrostatic technology in speaker construction, and still others, such as Magnepan, utilize Ribbon technology.

The Full Range Speaker

The simplest loudspeaker enclosure is one that contains just one speaker, which is tasked to reproduce all of the frequencies that are sent to it. However, things aren't that simple. If the speaker is too small, it will only reproduce higher frequencies. If it is "medium-sized", it may reproduce the sound of a human voice and similar frequencies well, but fall short in both the high and low frequency range. If the speaker is to large it may do well with lower frequencies and, perhaps, mid-range frequencies, but will not do well with higher frequencies.

The solution, optimize the frequency range that can be reproduced by having speakers of different sizes inside the same enclosure.

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Woofers, Tweeters, and Mid-range Speakers

Paradgim Cinema Tweeter and Mid-range Woofer Examples
Paradgim Cinema Tweeter and Mid-range Woofer Examples. Images provided by Paradigm

Woofers

A woofer is a speaker that is sized and constructed so that it can reproduce low or low and mid-range frequencies well (more on this later). This type of speaker does 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-to-8-inch diameters are common in floor standing speakers, while woofers with diameters in the 4 and 5-inch range are common in bookshelf speakers.

Tweeters

A tweeter is a specially designed speaker that is much not only much smaller than the woofer, but is tasked with only reproducing audio frequencies above a certain point, including, in some cases, sounds that human ear cannot directly hear, but can sense.

Also, another reason that a tweeter is beneficial is that since high-frequencies are highly directional, tweeters are designed to disperse high-frequency sounds into the room so that they are heard accurately. If the dispersion is too narrow, then the listener has a limited amount of listening position options. If the dispersion is too wide, then the sense of direction of where the sound is actually originating is lost.

Tweeters come in several types:

  • Cone - A smaller version of a standard speaker.
  • Dome - The voice coil is attached to a dome that can be made of fabric or a compatible metal (such as the one shown in the photo).
  • Piezo - Instead of a voice coil and cone or dome, an electrical connection is applied to a piezo-electric crystal, which in turn vibrates a diaphragm.
  • Ribbon - Just as the name implies, instead of a traditional diaphragm, magnetic force is applied to a thin ribbon(s) 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 they become out-of-phase, thus alternately attracting and repelling the suspended diaphragm, creating the needed vibration to create sound.

Mid-range Speakers

Although in many cases, a speaker enclosure might incorporate a woofer and tweeter to cover the entire frequency range desired, some speaker makers take it a step further by adding a third speaker that separates the low and mid-range frequencies further. This added speaker is referred as Mid-range.

Speaker enclosures that incorporate just a woofer and a tweeter are referred as a 2-Way Speaker, while a enclosure that houses a woofer, tweeter, and mid-range is referred as a 3-Way speaker.

Now, you might think that you should always opt for the 3-way speaker, but that would be misleading. You can have a well designed 2-way speaker that sounds excellent, or a poorly-designed 3-way speaker that sounds terrible.

It is not just the size and number of speakers that matter, but what materials they are constructed of, the interior design of the enclosure, and the quality of the next needed component—the Crossover.

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Crossovers

Example of a Loudspeaker Crossover Circuit
Example of a Loudspeaker Crossover Circuit. Image provided by SVS Speakers

For a speaker to meet the demands of reproducing sound as accurately as possible, you just don't throw a woofer and tweeter, or woofer, tweeter, and mid-range in a box wire them together, connect it all to your amplifier and receiver and hope it sounds good.

When you have a woofer/tweeter, or woofer/tweeter/mid-range speaker in your cabinet, you also need what is referred as crossover.

A crossover is an electronic circuit that assigns the appropriate frequency range that different speakers reproduce.

For example, in a 2-way speaker, the crossover is set a specific frequency point—any frequencies above that point are sent to the tweeter, while the remainder are sent to the woofer.

By the same token, in a 3-way speaker, a crossover can be designed so that it has two frequency points—one the handles the point between the woofer and mid-range, and the other for the point between the mid-range and tweeter.

The actual frequency points that a crossover is set at varies and is determined by the person that designed the speaker, but a typical 2-way crossover point might be 3kHz (anything above goes to tweeter, anything below goes to the woofer), and typical 3-way crossover points might be 160-200Hz between the woofer and mid-range, and then the 3Hz point between the mid-range and the tweeter.

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Passive Radiators and Ports

A Pair of 3-Way Loudspeakers with a Port
A Pair of 3-Way Loudspeakers with a Port. Matejay - Getty Images

In addition to the speakers and crossover, there are two additional things that are sometimes included in a speaker enclosure that are specifically designed to enhance low frequency response, without having to add another speaker. The added feature can take of form of either a Passive Radiator or a Port.

A Passive Radiator of looks just 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.

The Passive Radiator creates a complementary effect in which the woofer is actually providing the energy to power both 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 aids in producing a more effective bass output. This type of system works well in smaller speaker cabinets, as the main woofer than 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 to use a Port. The port is a tube that is placed on the front or rear of the speaker enclosure so that the air being pumped out by the woofer is sent through the port, creating a similar complementary low frequency enhancement as a passive radiator. However, in order to do its job well, a port has be a specific and diameter, and has to be tuned to the specific characteristics of the enclosure and woofer that is i complementing. Speakers that include a port are referred to as Bass Reflex Speakers.

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The Subwoofer

SVS SB16 Sealed and PB16 Ported Subwoofers
SVS SB16 Sealed and PB16 Ported Subwoofers. Images provided by SVS

There is one more type of loudspeaker to consider - the Subwoofer. A subwoofer is a specialized speaker that is designed to reproduce only very low frequencies and is used mostly in home theater applications.

Examples were a subwoofer is desired would be reproducing specific low frequency effects (LFE), such as earthquakes and explosions in movies, and for music, pipe organ pedal notes, acoustic double bass, or tympani.

Most subwoofers are powered. This means that unlike a traditional speaker, they have there own built-in amplifier. On the other hand, just like some traditional speakers, they may employ a passive radiator or port to enhance low frequency response.

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The Bottom Line

Home Theater Speaker System Example
Home Theater Speaker System Example. N_Design - Digital Vision Vectors - Getty Images

Loudspeakers are primarily designed to reproduce recorded sound so that can hear it in a different time or place. There are several ways to design a loudspeaker, including bookshelf and floorstanding size options.

However, the important thing to remember is to not just note how the speaker is put together, its size, or how much it costs, but how it actually sounds to you.

Before you by a loudspeaker or a loudspeaker system, if possible, do some critical listening with content (CDs, DVDs, Blu-ray/Ultra HD Blu-ray Discs, or even Vinyl Records) that you are familiar with.

If you are ordering speakers online, always check to see if there is 30 or 60-day listening trial available as despite any claims relating to potential performance, you won't know how they will sound in your listening room until you start them up. Listen to your new speakers for several days, as speaker performance benefits from an initial break-in period of between 40-100 hours.

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