March 01, 2013
| by Igor Levitsky
The moving-coil loudspeaker driver was patented by Rice and Kellogg in 1924 and since then it remains largely the same in principle. Through the years engineers have worked hard to develop alternative solutions to reproduce sound. The first ribbon transducer was patented in the early 1920s, for example, followed a few years later by a patent for the electrostatic loudspeaker.
In the early years of audio, power amplifiers had tubes with very low output capabilities; usually they produced only several watts. To compensate, speakers had to have high sensitivity to deliver useful sound levels. Woofers required a large cone area with large voice coils to handle power and produce high output. Tweeters however, needed small and light-moving systems to achieve high sensitivity and extended high-frequency range.
All early tweeters were actually compression drivers that had stiff phenolic or aluminum diaphragms loaded on a horn. While such designs possessed very high sensitivity, sound quality was largely compromised. It didn’t truly become the primary goal of speaker design until 1958, when Edgar Villchur developed what was considered the first high-fidelity speaker, the Acoustic Research AR-3.
To get it all right in one speaker design is a huge task. Driver design requires a precise and skillful balancing between components, motor topology and materials. And there has always been the pursuit of clarity and resolution, which has led to expensive and exotic designs.
Typical challenges of tweeter design have been largely solved, and top-notch tweeters are capable of relaxed, airy signal reproduction that transcends listeners and emotionally connects them to music or movie. Here’s a look at some pros and cons of the major tweeter types called on to reach those sonic highs.
Villchur’s invention is still the most common tweeter type; probably about 95 percent of all speaker systems use dome tweeters. This tweeter is very well developed, but there are still some engineers that are working on new shapes, materials, motors and waveguide designs. Overall, dome tweeters are easy to manufacture and they are mass-produced, mostly in China.
The design features a dome with integral suspension and aluminum wire voice coil attached to the rim. This moving piece is then mounted on a plastic frame and positioned in a ring magnet motor system. A faceplate is installed from the top. (A shallow waveguide or small horn can be used instead of a faceplate to control directivity, response and/or increase sensitivity.) Typical dome materials include plastic (polycarbonate, Mylar, PEN), doped fabric, aluminum, titanium and rare beryllium or exotic composite materials.
Dome tweeters have very smooth response, good dispersion, and average sensitivity. Typical operating range is 2.5 kHz to 20 kHz, and sound quality ranges from basic to excellent. Some designs employ ferrofluid (a viscous emulsion of magnetic particles in mineral oil) in the magnetic gap around the voice coil to improve cooling and to boost power handling. While it has clear benefits, it also introduces additional dampening and potential long-term issues related to fluid thickening, and many top designs avoid ferrofluid.
A good dome tweeter can be made for a manufacturer’s of about $5, though some high-performance tweeters from companies like Scan-Speak and Dynaudio can cost as much $300 to $400 on a retail level.
Instead of being driven by electro-dynamic force applied to a voice coil that carries signal current and immersed in a magnetic field, electrostatic speakers rely on an electrically-charged diaphragm.
An electrostatic speaker has a thin (6-12 microns) polyester film diaphragm that’s coated with an electrically conductive layer with high resistance that provides a constant surface charge. The diaphragm is tensioned on a frame positioned between two stators.
The stators are made of a perforated metal coated with an insulating layer, and an extra high voltage (EHT) source creates a bias voltage between diaphragm and stators. When an audio signal is applied to stators through step-up transformer, the bias voltage is modulated and the diaphragm vibrates between stators driven by electrostatic force.