Exacting high frequency reproduction is critical to creating hi-fidelity sound. Today’s article focuses on the part of the speaker responsible for high frequencies: the tweeter. It includes a brief romp through speaker history and details the types of tweeters you’re likely to find on the speaker market today.
Birth of the Modern Speaker
A century ago, non-electrical horn speakers were used to amplify the entire range of frequencies produced by a variety of sources. While horns were relatively effective, they were limited in their ability to project sound with breadth, clarity, and volume. It wasn’t until the 1920s that consumers were exposed to the first commercially available electrically amplified speaker system called the Radiola Loudspeaker #104. This pricey system (which sold for the equivalent of $3,400) was produced by RCA and its “electromagnetic” driver was created using the same essential design parameters used by the vast majority of speaker manufacturers today. While materials and craftsmanship have changed for the better, the bones of electromagnetic speaker design have proven to be effective and highly reliable.
Arrival of the Tweeter
Let’s take a step back for a moment and consider how the average stereo system creates sound. It all begins with a source paired to an amplifier that sends an alternating current to speakers. The typical driver in a speaker has a coil surrounded by a fixed magnet. As electricity is delivered to the coil, an electromagnetic field is created and the coil moves back-and-forth within the fixed magnetic field. A cone attached to the coil also moves back-and-forth and that’s how sound waves are projected into a room.
While that’s a truncated explanation, it’s essentially how most speakers work. Taking that concept a step forward, most modern hi-fi speakers rely on more than one driver to create sound. This is achieved by using a crossover network that filters frequencies and delivers them to specific drivers within a speaker. The first speaker to do this was developed by Bell Labs in 1931. Using two drivers, the company’s “divided range” design created high frequencies (above 3,000 Hz) using a small horn-loaded driver (horn-loading is akin to cupping your hands around your mouth) and low frequencies with a larger 12-inch driver; thus the job of a tweeter was born. Delegating high frequency reproduction to a smaller driver has its benefits, largely because tweeters require less power and move less air to make sound. Keeping them small allows them to work quickly with precision.
Various forms of the tweeter are found on nearly every kind of hi-fi speaker, save for a small segment of speakers that feature full-range driver implementations. Let’s take a peek at the different types of tweeters used by manufacturers throughout the speaker industry.
Exotica: Plasma and Electrostat
One of the rarest and most exotic tweeters uses ionized plasma to make sound. Plasma tweeters work by creating gas ionization or burning gas at high temperatures in a small sphere. The resulting plasma is nearly weightless, can instantly create oscillating air when charged by electricity, has zero distortion, and is said to deliver near-perfect sound. A major negative to the technology is a byproduct called ozone, which is bad for the environment (not to mention audiophiles). This fact, in addition to cost (which, if you can find a plasma speaker, will be high), makes plasma tweeters rarified air.
A level down in the exotic tweeter chain is the electrostatic design. Electrostats utilize an ultra thin, conductive, film diaphragm that’s stretched between two perforated steel sheets coated with an insulator. These sheets (or stators) are literally sandwiched on either side of the diaphragm. When in use, the diaphragm is charged with a fixed positive voltage while the stators are charged with opposing voltages by a transformer. This fosters an environment that vibrates the diaphragm and thus creates sound waves.
Electrostats are interesting animals simply because their physical presence can be quite big (MartinLogan’s Neolith model features an electrostat driver that measures 48-inches tall and 22-in wide). Despite being large, the diaphragm is extraordinarily light which translates into lightning fast response times, negligible distortion, and mesmerizingly transparent sound. They’re also capable of handling frequencies dipping down to 400Hz, which removes any kind of mid-range frequency crossover. On the flipside, electrostats require extremely high voltage to operate, can be delicate, and project a relatively small listening sweet spot in the high frequency range; electrostats are best known for superior midrange performance.
Other Non-Dome Designs
While plasma and electrostats are certainly unique, there are other semi-rare tweeter designs that make use of ultra-thin diaphragms made from various films or foil. Much like electrostats and plasma, these tweeters do not have a voice coil within a fixed magnet.
Ribbon tweeters typically have a flat, ultra-thin, foil diaphragm that’s folded in a pleated fashion and positioned between two magnetic poles. This diaphragm uniformly vibrates as it’s energized, and moves ultra-fast due to its incredibly light weight. Enthusiasts laud them for their handling of ultra high frequencies and precise sound reproduction capabilities (largely due to their even and linear movement). Manufacturing costs and stingy power requirements make them a more expensive option, and ribbon technology is less effective in mid-range frequency reproduction.
Planar magnetic tweeters are a more accessible form of the ribbon tweeter because they have less restrictive power requirements and better midrange frequency performance. They typically use an imprinted polymer or PET film as a diaphragm positioned between magnetic fields, which eases the cost of manufacturing and improves durability. Their overall performance is considered nearly as good as the transparent capabilities of ribbon and electrostat designs.
Air Motion Transformer
Air motion transformer tweeters (AMT) utilize a thin, folded, film diaphragm that’s lined with aluminum circuit traces and placed between opposing magnetic fields. Unlike ribbon and planar magnetic tweeters, AMT tweeters are folded in an accordion-like fashion. As an electrical signal is applied to the tweeter, the diaphragm’s circuit traces change polarity causing the diaphragm to squeeze in and out, a process that creates sound waves. AMT tweeters are frequently pegged as having a transparent and uncolored sound, but fall on the pricey side of the manufacturing equation.
The vast majority of modern tweeters have the same basic internal design as larger low frequency drivers, but sport a dome-like appearance. These are known as dome tweeters and they’re capable of creating exceedingly exceptional sound.
Dome tweeters represent the tried-and-true category of high frequency drivers and are widely used because of low manufacturing costs, wide dispersion capabilities, and large listening sweet spots. Much like a traditional driver design, most dome tweeters rely on a coil inside of a magnetic field to produce high frequency sound (typically ranging from 2,500 Hz to 20,000 Hz). Of course, every manufacturer employs its own spin on dome tweeter design, but all strive to maintain a stable linear driver motion, achieve quick and rapid cone movement paired with superior damping (the ability to stop moving when a sound ends), and strike a balance between lightness and stiffness to avoid distortion.
The diaphragm of a dome tweeter can be made from any number of materials, including woven fabric, silk, aluminum (and other exotic metals), and synthetics. Each material has its own sonic flavor. Metal tweeters are typically identified as having a crisp or bright sound. Some ears may find aluminum tweeters to lean toward the harsh side, while titanium is known to be a bit tamer. Silk and woven fabric tweeters are typically more laidback than metal variations, and possess mellow sonic characteristics. Synthetics are probably the least desirable of the three because of poor damping and sonic accuracy. These, of course, are generalizations and vary from manufacturer to manufacturer, so you’ll need to let your ears decide which type of dome tweeter you prefer.
Aperion Audio’s New V.2 Silk Dome Tweeter
Aperion Audio recently unveiled a new 1-inch Axially Stabilized Radiator V.2 silk dome tweeter. As previously mentioned, linear driver motion is a key performance goal; if a cone rocks too much it can cause distortion. Aperion’s new V.2 tweeter addresses this issue by applying driver suspension on two different planes: by the voice coil and at the top of the cone. This suspension system minimizes cone rocking and allows the tweeter to reach lower frequencies (approaching 2,000 Hz). These two performance parameters result in better sound dispersion and a larger listening sweet spot, while allowing midrange drivers to focus on a more natural frequency spectrum.
Aperion has also added ferrofluid inside the tweeter (common in high-end drivers) to help dissipate heat build-up and allow the speaker to handle more power, in addition to a newly designed waveguide that enables the signal to propagate with minimal loss of energy. The company has also taken steps to maximize electrical current to the tweeter by using 16-guage internal cabling to the tweeter, itself.
Because Aperion’s dome material is silk, listeners can expect a laidback mellow upper-end that avoids levels of harshness that metal dome tweeters can project. The V.2 tweeter can be found on the company’s new Versus II Grand Bookshelf, Center Channel, and Tower speakers.
Modern tweeter technology is allowing manufacturers to achieve absolutely stunning levels of high frequency performance. It’s important to remember that everyone’s ears will respond differently to the various types of tweeters on the market. Your best bet is to audition a wide variety of tweeters – preferably in your listening space or home theater room – before deciding which one will work best for you.