Woofers, Tweeters, Crossovers - Understanding Loudspeakers

Dive inside the loudspeaker box

Example of Typical Loudspeaker Driver Construction
Example of Typical Loudspeaker Driver Construction. Klipsch

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

With our technological prowess, humans can also capture sound using 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. Hearing recorded sound requires a playback device, an amplifier, and, most critical of all, a loudspeaker.

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What is a Loudspeaker?

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

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 referred to as The Basket within which all the speaker components are placed.
  • A Diaphragm that pushes air out through vibration. The vibration patterns are 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 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/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.

To make sure the speaker performs well 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. Instead of a box, speakers can also come in other shapes, such as a flat panel or sphere.

Also, as mentioned above, not all speakers use a cone 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. There are even cases where the sound is reproduced by non-traditional methods.

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Full-Range, 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

The Full Range Speaker

The simplest loudspeaker enclosure contains just one speaker, which is tasked to reproduce all of the frequencies that are 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, but fall short in both the high and low-frequency range. If the speaker is too large, it may do well with lower frequencies and, perhaps, mid-range frequencies, but may 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.

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.

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, 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.

Types of Tweeters:

  • 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 piezoelectric crystal, which in turn vibrates a diaphragm.
  • Ribbon - Instead of a traditional diaphragm, a 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 a speaker enclosure might incorporate a woofer and tweeter to cover the entire frequency range, some speaker makers take it a step further by adding a third speaker that separates the low and mid-range frequencies further. This is referred to as a Mid-range speaker.

2-Way vs 3-Way

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

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's 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

You just don't throw a woofer and tweeter, or woofer, tweeter, and mid-range in a box wire them together and hope it sounds good.

When you have a woofer/tweeter, or woofer/tweeter/mid-range 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 a specific frequency point—any frequencies above that point are sent to the tweeter, while the remainder is sent to the woofer.

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 frequency points that a crossover is set at varies. A typical 2-way crossover point might be 3kHz (anything above goes to the 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

A Passive Radiator 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.

This creates a complementary effect in which the woofer is 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 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 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.

In order to do its job well, a port has to be of a specific and diameter and has to be tuned to the specific characteristics of the enclosure and woofer that is 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 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 their 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 designed to reproduce recorded sound so that it can heard in a different time or place. There are several ways to design a loudspeaker, including bookshelf and floor standing size options.

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.

Also, not only take note how the speaker is put together, its size, or how much it costs but how it actually sounds to you.

If you are ordering speakers online, check 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 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.

Bonus Article: How To Clean and Maintain Your Speakers

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