With the growing popularity of high-sounding low-power tube amplifiers, there is increasing interest in high-efficiency full-range drivers. As a result, many speakers have been manufactured based on these types of drivers. Unfortunately, many of them offer questionable sound quality. Designing cabinets for these drivers is challenging unless they are fully understood and correctly implemented. Consequently, long-term satisfaction with such speakers is often elusive—unless, of course, they are exceptionally well-designed. As everyone claims their speakers are the best, this raises the question: how can one navigate this landscape?

This paper offers insights and observations to help you understand why these drivers sound the way they do. With a better grasp of these potentially remarkable components, you’ll be better equipped to choose a good one or enhance the performance of what you may already own.

Voice Coil Design

The efficiency of high-efficiency full-range drivers comes from their voice coil design, stronger magnets, and lighter cones and spiders. This setup creates higher magnetic force while controlling a lighter moving mass.

A visual representation of this can be seen by comparing voice coil designs between high-efficiency and conventional drivers. The high-efficiency driver will feature a thin or flat voice coil wire wound to a width close to the magnetic field surrounding it. This magnetic field exists between a metal pole piece inside the voice coil bobbin and a steel ring surrounding the outside. The magnetic gap (or flux) typically ranges from ¼ to ½ inch in modern drivers. The key difference between high and low-efficiency drivers lies in the width of this gap. Low-efficiency drivers often have “overhung” voice coils with winding widths exceeding the magnetic gap by 400% or more, while high-efficiency drivers typically feature “underhung” coils.

In high-efficiency drivers, the gap between the voice coil and pole is often so tight that a dollar bill barely fits between them. In contrast, a low-efficiency driver might accommodate 1 or 2 business cards. If you build a driver with a high-efficiency approach and measure its frequency response, you might find peaks and dips exceeding 12 dB. This is often managed by weighting the response recorder or reducing the frequency sample rate, which averages out the response on paper but doesn’t alter the actual sound. Reducing the response peaks by widening the gap and lengthening the coil, while making the response smoother on paper, can result in a loss of 6 dB in efficiency, requiring four times the power for the same output. However, the high-efficiency driver will always provide more detail and be faster, making it far more linear and accurate.

Phase Response

The resistance, capacitance, and inductance created by moving the voice coil inside the magnetic gap (known as impedance) introduce phase shifts across the driver’s frequency response. For example, at 100 Hz, and moving up the frequency scale, you’ll find that absolute phase shifts can reach 180 degrees or more by the time you reach 5 or 6 octaves. This phase shift can be a challenge, but it can be managed creatively. Some companies design drivers with nearly flat phase angles, but this results in rising efficiency at higher frequencies, which might not sound desirable or look appealing on paper.

In an ideal scenario, a linear-phase driver would be used in a full-size straight horn, where the horn would exponentially raise efficiency as the music goes down in frequency, offsetting the driver’s response and resulting in both flat frequency response and minimal phase shift. Downsizing or folding the horn introduces phasing problems, a key reason why certain drivers, like those from Lowther, sound the way they do.

Cone Mass

Cone mass is often the most overlooked aspect of high-efficiency full-range drivers. These cones are typically very thin—thinner than a business card. Many cabinet designs fail to address the rearward motion of the cone, which is equally significant as the forward motion. High-efficiency drivers are especially sensitive to the rearward sound; without proper treatment, this sound will reflect back through the cone and into the air, degrading performance. To illustrate this, you can test by placing a battery-operated radio inside the cabinet, reinstalling the driver, and listening to how the radio's sound comes through, highlighting the need for better cabinet design.

Whizzer Cones

Whizzer cones are not perfect, and their flaws are often misunderstood. For instance, the infamous Lowther “Shout” is often blamed on the whizzer cone, when it is usually the issues described above that are to blame. Whizzer cones on 8-inch drivers extend response above 5 or 8 kHz and do not affect the 2 kHz range where the “shout” is perceived. Improving whizzer cone performance often involves the type and application of the phase plug. A bullet-shaped phase plug extending past the voice coil, rather than a dust cap, can improve phase coherence. Small pieces of foam under the whizzer cone can dampen the top-end response of the main cone.

Warm-Up

High-efficiency speakers require a warm-up period due to the light voice coil and tight voice gap. As the voice coil heats up, its diameter increases, reducing the gap between it and the plate. This is significant in high-efficiency drivers, where even small changes in diameter can affect sound quality. Typically, about 30 minutes of playing at a normal listening level is needed for the voice coil to stabilize and sound its best.

Set-Up

Speaker placement in a room greatly affects frequency response. Experimenting with placement can significantly enhance long-term satisfaction. Many full-range drivers perform best between 5 and 25 degrees off-axis due to the whizzer cone’s effects.

Cables

High-efficiency speakers, or high-resolution speakers, demand high-quality cables. Generic cables will not suffice, and investing in high-quality speaker cables and interconnects is essential. Without proper cables, these speakers may not perform to their potential.

Amplifiers

For high-efficiency speakers, high power amps are not suitable. Tube amplifiers often work better with these drivers. Your system’s performance is limited by the quality of your amplifier. If your amp isn’t as good as or better than the speakers and cables, you are wasting your time.

Preamp/Sources

The quality of the preamp and source also affects the performance. For instance, a high-quality DAC might still sound inferior compared to a reference vinyl rig. Even the best source is often the weakest link unless all other components are equally high quality. Compromises are inevitable, but balancing them is crucial for optimal sound.

Conclusion

In general, high-efficiency full-range drivers can offer superb frequency balance and clarity if designed and implemented correctly. They can deliver full bass and extended highs without midrange glare if placed in a well-designed cabinet. Success comes from balancing the pros and cons, as there is no perfect design. If you're satisfied with conventional solutions, high-efficiency full-range drivers might not be for you. However, for those willing to experiment and fine-tune, they offer a rewarding audio experience.