What is a tweeter?

Why tweeters matter

The earliest electromechanical speakers – developed after the recording and reproduction of sound moved on from vibrations in a record groove amplified via a huge horn – used single speaker units driven by an amplifier. These full-speakers worked well enough, but it was soon realised that the demands of audio fidelity required different drivers, each optimised for a different part of the frequency range: large drivers to deliver the long wavelengths of bass by moving more air, and smaller drivers for the fine detail of higher frequencies, which are all about very small movements delivered very quickly.

In fact, while the first electromagnetic speaker was developed by Chester W. Rice and Edward W. Kellogg of General Electric in 1921, it took only ten years for the team at Bell Telephone Laboratories to design what was called the "divided range" loudspeaker, with separate drivers for high and low frequencies and a crossover network to split the music signal between them.

For almost 100 years, most Hi-Fi speakers have used a tweeter to deliver the highest frequencies and a mid/bass driver for the rest of the range. But what’s special about these small, specialist high-frequency drivers, is how they’re designed and the role they play in the way we hear our favourite music. 

The basics of a tweeter

By definition, high frequencies require the drive unit delivering them to move much faster than midrange or bass frequencies. Operating over a typical range from 2 kHz upwards to 20 kHz or even higher, a tweeter needs to move at least 2,000 times a second – or much faster for really high notes – whereas a bass driver may only be moving 100 times a second or less. But fortunately, the wavelength of these higher notes is much shorter than those for bass, so small, precise movements are what’s required. Bass units need big cones to move air, creating that bass you can feel, while tweeters can be small, light, precise and above all capable of moving very fast. 

What does a tweeter do?

While you may not hear much sound coming from the tweeter in your speakers, even with your ear close to the speaker (there won’t be that much apparent output), the role of the tweeter is vital: it gives the sound its natural freshness and sense of air and ambience. In a stereo system, it creates that all-important feeling of three-dimensionality and soundstaging.

The higher a frequency, the more sensitive our ears are to its directionality – maybe it’s to do with our ancient ancestors needing to be aware of the snapping of a twig as something crept up on them! So just as a bass driver can fill a room with low frequencies without us being aware where that element of the sound is originating, so the finest of high frequencies, right up to the highest limits of our hearing, will provide us with directional clues, helping us place performers in the stereo “picture”.

How does a tweeter work?

There are two major requirements in the design of a tweeter: one is the speed and accuracy of the sound it reproduces, and the other is the control of the dispersion of that sound. After all, given the directionality of high frequencies, it’s important that those frequencies are delivered in a focused manner, but also that the nature of the sound doesn’t change significantly when you move off the ideal listening axis of the driver, either in the horizontal or vertical planes.

Due to the nature of high-frequency soundwaves, they will tend to “beam” unless controlled strictly, i.e. the sound will be heard strongly directly in front of the driver but drop off significantly when you listen off axis. That’s one of the reasons why the most popular designs of tweeter use dome diaphragms, with their wide dispersion characteristics. But that’s not the only reason, as domes also has significant advantages when it comes to the balance between lightness and rigidity, both of which are also vital for accurate high frequencies.

A dome can propagate sound over a wide arc, so some control then needs to be exerted to control that dispersion: as we’ll see, different speaker designs use various strategies to achieve focused treble and consistent high frequencies across a wide listening area. This helps create a stable stereo image and allows multiple listeners to enjoy a consistent tonal balance. 

Placing the tweeter in the speaker design

Attention must also be paid to the positioning of the tweeter in the baffle holding the drive units on the front of the speaker cabinet. This ensures integration with the other drivers and that the height of the tweeter matches the position of the listener’s ears. For that reason, the tweeter is usually mounted toward the top of the speaker, above the drivers delivering the midband and bass.

However, in some very large speakers, the tweeter may be mounted centrally in the baffle, with mid/bass drivers above and below it: this D’Appolito array, named after its inventor, is designed to give more consistent dispersion from all the drivers, and bring the sonic effect closer to the theoretical idea of a “point source” of sound, in which the entire frequency range appears to emanate from a single place.

Also important is the need to isolate the delicate output of the tweeter from the more powerful vibrations being developed by mid/bass drivers: to achieve this, many speakers place the high-frequency unit in its own sub-baffle, isolated from the main structure of the speaker using compliant mounting, or even in a completely separate sub-enclosure within the whole design. This also ensures the sound radiating from the rear of the diaphragm doesn’t “leak”, which impinges on the forward radiation and smears the treble image. Some speaker designers even choose to place the tweeter in an entirely separate housing.

The dispersion of a tweeter can also be controlled by placing the drive unit in a dished “horn” within the baffle, or by placing what’s called a waveguide in front of the diaphragm, typically in the form of a meshed grille. This type of grille also provides some protection for the ultra-thin dome itself against small inquisitive fingers. Some older speakers take this to an extreme, placing what looks like a louvred shutter in front of the driver, but this is rare these days except in retro models.

What makes a tweeter?

The key components of a tweeter are the same as those of any speaker driver – a diaphragm, driven by a “motor” comprising a voice-coil in a magnetic field – but here it all needs to be on a miniature scale to meet those requirements of speed and responsiveness. This requires the use of hair-fine windings in the motor system, which can be vulnerable to fusing together if the speaker system is overdriven into amplifier distortion: it’s unusual for the heavier windings in mid/bass drivers to be affected in this way, but “blown” tweeters are a pretty sure sign that the speaker has been abused.

The material used for the dome can vary from the more traditional fabric weave – once silk but now more likely to be a man-made material – which is then doped with chemicals to improve its damping and stiffness, to ultra-thin aluminium or even more exotic materials, all in the quest for ultimate rigidity without the weight.

Some companies favour alloys, others rare metals such as beryllium. It’s also possible to find examples made from ceramics, carbon or in some extreme cases even diamond dust, deposited using evaporative techniques. If there’s a lightweight material known to science, the chances are it’s been tried as a means of making tweeter diaphragms.

A lot of work goes into ensuring the dome keeps its form when being driven: yes, the shape is inherently rigid, but some designs also incorporate a “double dome” in the form of a ring of extra material around the rim of the diaphragm where it meets the “motor”, which gives some additional stiffening without adding significant weight. 

Alternative tweeters

While so much work goes into improving the materials used in dome tweeters, other designs have also been investigated in the quest for lightness while maintaining the output of these high-frequency drivers.

Some speakers use a ribbon tweeter, comprising an ultra-light flat diaphragm, usually of aluminium, driven by an adjacent membrane connected to the motor. This design has the advantage of even sound distribution as well as greater cooling, which sometimes makes it the best choice for high-power speaker systems. However, ribbon tweeters are more expensive to implement than dome designs and may not offer such wide dispersion.

A variation on this theme is the Air Motion Transformer tweeter, in which a pleated ribbon-type diaphragm is driven to act like a bellows, squeezing the air to produce sound. The advantage here is higher power handling and output, plus extended higher-frequency bandwidth.

Another tweeter technology is the planar magnetostatic driver. Known for its precision, speed, and wide dispersion of high-frequency sound, this type of speaker transducer uses a thin, flat diaphragm embedded with conductive traces (usually arranged in bands or strips). This diaphragm is suspended within a strong magnetic field, and when an audio signal passes through the conductive material, it interacts with the magnetic field to produce sound by vibrating the diaphragm. 

Hybrid tweeters

A few speaker manufacturers have embraced the concept of a hybrid tweeter design featuring both a conventional dome tweeter and a ribbon/planar tweeter. Working together, the tweeters combine the best properties of both designs to reproduce a smooth, wide and well-rounded sound that. You can learn more about hybrid tweeter technology here.

Tweeter exotica

The history of speaker design is littered with all kinds of experiments aimed at improving high-frequency response. As far back as the 17th century, the effect of “electric wind” from high-voltage discharges was first noted, while in the 19th century, inventor Nikolai Tesla studied the sound created by an electrical arc. The “musical” qualities of small “singing flames” in tuned glass tubes was also studied, resulting in the novel Pyrophone.

In the 1950s, a work on electroacoustics noted that “the speaking incandescent electric lamp, the singing arc, and the talking flame – measures of desperation these were,” while Edward Kellogg, mentioned earlier as a multi-driver pioneer, was even more forthright: in 1925, he had already concluded that “I tried talking arcs and thermophones, and also a corona discharge device – all of which avoid mechanical moving parts – but none of these appeared promising.”

Not surprisingly, dome tweeters won through on the grounds of practically, supplemented with ribbon-type designs. 

Tweeters in different environments

So far, we’ve covered the use of tweeters in conventional speakers designed for Hi-Fi and home cinema applications, but they can be used in different ways in various applications.

Modern in-car systems have moved on from the early single-driver speakers, sometimes with an unpowered “whizzer cone” mounted in front of the mid/bass driver in an attempt to deliver some treble output. Some speakers designed to be used in the doors of vehicles have a coaxial design featuring a tweeter mounted with the main driver. However, in better factory-fit and aftermarket setups, separate tweeters are mounted higher up – typically in the windscreen pillars or dashboard – to give more precise positional cues, leaving the mid/bass speakers in the door where their output is less readily localised.

In soundbar speakers for use with TVs, multiple tweeters can be used to give the illusion of sound coming from all around the listener: upward-firing tweeters can give the effect of height, while speakers angled to the sides take advantage of wall reflections to create a wrap-around effect. Using digital signal processing to modify the phase, amplitude and time-alignment of these drivers can enhance this impression of space.

Why tweeters are essential for great sound

Enhancing clarity and detail

Accurate high frequencies are vital for the realistic reproduction of voices and instruments: not only do they have harmonics extending up from their fundamental sound, which all play their part in creating the character of the sound, but above the music itself there’s that sense of the space and ambience of the recording. When captured and reproduced accurately, this can make the difference between a “flat” sound and one that really springs to life. 

Realistic sound imaging and spatial accuracy

The way your speakers are set up will have a major effect on the imaging and spatial accuracy of the sound of your system. After all, stereo is all about the illusion of solid, three-dimensional sonic images – it doesn’t mean two channels, but making your speakers disappear to leave a realistic image before you.

Take advice from the set-up instructions provided with your speakers, but don’t be afraid to experiment with positioning to get the best sound. With many speaker designs, just a slight angling of the speakers toward the listening position – and that means a slight change of the alignment of the tweeters – can make all the difference. 

Choosing the right speaker setup

2-way vs. 3-way speakers – what’s best for you?

In smaller rooms, and with smaller speakers, you may find a well-designed two-way speaker more than adequate for your needs. But three-way designs, incorporating a separate midrange driver between the tweeter and woofer(s), can offer a more open and involving sound, simply because the tweeter and bass driver(s) will be doing less work in the “overlap” area where much of the character voices and instruments is being reproduced. Particularly with vocals and classical music, a well-integrated midrange driver can be beneficial.

Look out for a future article on multi-way speaker designs.

Listen before you buy – why tweeter design makes a difference

Don’t get too hung up on the kind of tweeter your speakers use: whether they're soft-dome fabric tweeters, metal domes or some other material, the most important thing is that they sound good to you. As mentioned earlier in the article, we’re more sensitive to the quality of the treble in our systems, even if the tweeter doesn’t appear to be doing as much. So, choose speakers with a treble quality you like as part of the overall sonic balance and, above all, trust your ears… 

Explore DALI loudspeakers featuring class-leading tweeter design

DALI speakers have an extensive range of acclaimed floorstanding speakers, bookshelf speakers, custom install speakers and more. Every DALI speaker is designed with wide dispersion, spreading the treble frequencies over a wider area than traditional speakers to keep the sound balanced and clear, even when you’re not directly in front of the speakers. By doing this, the ideal listening sweet spot is expanded, allowing more listeners to enjoy the best possible sound quality. 

Wide dispersion sound also helps with room integration. While freeing you to place your speakers in what may seem like less than perfect positions, it can also reduce the impact of reflections from walls, which can create unwanted distortion and colourations.

To discover DALI loudspeakers, visit dali-loudspeakers.com.