Super-Shallow High-Excursion Speakers Speaking of Something Completely Different...


Guest Editorial
Mike Klasco

Super-Shallow High-Excursion Speakers
Speaking of Something Completely Different...

Last week we took a quick tour of the sophisticated engineering implemented by soundmatters into the UpStage compact high-performance audio system. As discussed, the already available UPstage180 speaker was the launch platform for BDNC's high-power, flat glass woofer that uses a 0.2 mm glass diaphragm. We briefly touched on the application of the BDNC glass diaphragm, as the high-excursion yet ultra-low profile woofer in the UpStage, so this time we will drill deeper. 

From the transducer side, there are three noteworthy pillars. Starting with the ultra-thin tempered glass diaphragm, complemented with the anti-rocking thin yet high-excursion suspension, and coupled with low-end operation extended well below in-box resonance (small box eq'ed for extended bass). In this article, we will expand on those pillars.
Details of BDNC's flat-thin, long glass-composite-diaphragm woofer.

But let's step back and start at the beginning. BDNC, Ltd. began in 1995 as an R&D think tank by a Taiwan semiconductor company. The founder bought out the technology and assets in 2007, and then in 2016 raised funding from Taiwan. The core business was consulting in signal processing, mixed-signal, and electroacoustics.

With expanded resources for product development, the BDNC team was able to pursue their dream, which they call the miniMAX concept. This design philosophy is a ground-up approach for audio system development, where all the speaker design aspects are optimized using signal processing when effective. As BDNC explains, miniMAX is "a system-level technology" and an integrated approach from product design to manufacturing, applying DSP where it makes economic sense. Focusing on drawbacks that can be effectively "corrected" by signal processing while the remaining transducer parameters are concentrated on where the processing cannot mitigate performance.

BDNC considers frequency response - and with more effort, harmonic distortion - to be "DSP-friendly" while "non-DSP friendly" factors are maximum excursion ("Xmax"), specific force factor (BL/√Re), sensitivity, and power handling. If we can disregard the frequency response in the design stage, the speaker topology can be dramatically changed, allowing for thinner, more compact, high aspect-ratio rectangular or even tube-shaped products.

The well-crafted UPstage180 thin speaker design directly benefits from BDNC's flat glass speaker driver.
The smartphone and more specifically LCDs have inspired high-performance glass development. BDNC realized this new generation of high strength, ultra-thin glass from Schott, Corning, and others also has applications for speaker diaphragms. Let's take a look at some of these engineering glass formulas.

Schott AS 87 eco is an aluminosilicate glass suited for chemical strengthening (via an ion exchange treatment) that offers a high level of mechanical impact resistance and bending strength in a thickness range of 0.1 mm to 0.4 mm. Corning's Willow (available from 0.8 mm down to 0.03 mm), is roughly the thickness of a sheet of paper. Fabricated from Corning's amorphous Si O2 fusion process, molten glass is homogenized and conditioned before it is released into a collection trough with a V-shaped bottom (isopipe). Flowing evenly over the top edges, forming two sheet-like streams along the outer surfaces, the sheets meet and fuse into a single sheet which feeds into drawing equipment as it lengthens and cools in midair, as thin as 100 microns.

This process adds to the toughness of Willow (and also Gorilla) Glass and is not prone to breaks and cracks. Post processing for glass speaker diaphragm applications can include attributes such as a constrained layer damping of other materials (e.g., 3M's Viscoelastic Damping Polymer 112 acrylic polymer).

Another sophisticated element is the anti-rocking thin high-excursion suspension. The rocking problem is a known issue in high aspect ratio speakers, as there are billions of these smartphone microspeakers with the omniscient 10 mm x 15 mm x 3 mm format. While these microspeakers utilize a one-point suspension, BDNC realized that a high linearity spider was also needed, but would have to fit into a shallow form factor. The epiphany solution was actually derived from a pre-World War II field coil speaker. Specifically, a phenolic spider in a Jensen field coil speaker found in the designer's personal collection. 

A solid spring spider has better linearity performance than a cloth spider in small signals. Over the years, there have been other attempts with varying results, including Sony's low distortion old-style spider and Doug Button's EVX 1800 high output 18" pro-sound woofer, which featured a glass epoxy spider.

Achieving high excursion in shallow high-aspect-ratio transducers means fighting rocking modes (undesired rotational vibration patterns). In a number of Audio Engineering Society (AES) papers the Klippel team has explored the various rocking mechanisms and the resultant voice coil scrapes with the observation that the suspension is the main cause and cure.
Actual implementation of the glass-composite-diaphragm woofer with details of the patented design.

Rocking modes are caused by the inhomogeneous distribution of mass, stiffness, and force factor shifting the center of gravity, and electro-dynamical excitation away from the pivot point, which is the cross point of the nodal lines of the two rocking modes. Due to the high Q of the rocking resonators, only a very small asymmetrical force is required (which is usually a few percent of the transversal force) to generate a critical rocking behavior having more energy than the desired piston mode. Assessing the relative rocking level (RRL) and identifying the imbalances is a convenient way to keep voice coil rubbing under control and to avoid impulsive distortion from impairing the quality of the reproduced sound.

Woofer diaphragms vibrate through the compliances suspending the diaphragm. Compliance is the inverse of stiffness (stiffness = Young's modulus). Aside from many other considerations, the Young's modulus of the diaphragm should be as high as possible while the surround stiffness should be as low as possible.

A more compliant suspension enables the enclosure to be a smaller volume but also offers a more consistent bass response in mass production - as low-end response is dependent on the consistent back volume air compliance (air suspension) rather than variations in the surround compliance. To restate this, the resonant frequency is dominated by the effect of the back volume. This is defined as acoustic suspension (air suspension or sealed box). The compliance of the membrane must be far higher than the compliance of the back volume. 
Prototype of a new D48 glass dome diaphragm tweeter from BDNC.

Invented in 1954 by Edgar Villchur of AR speaker fame, acoustic suspension systems reduce distortion in the bottom end of the speaker's response that can be caused by stiffer nonlinear suspensions. While Acoustic Research, Inc. and Bose are odd bedfellows in the same sentence, the Bose 901 of the late 1960s used an array of 4" full range cones speakers in the tight volume enclosure and eq'ed the heck out of them below 200 Hz (almost 20 dB at 20 Hz!). So the entire bass range was used below resonance where the frequency (and phase) response was consistent and could be electrically compensated. Same here for the miniMAX approach.

On its product development roadmap, BDNC has three different sizes for its rectangular series of glass woofers. There's the middle size FP-SPK-M (160 mm x 40 mm x 17mm), the large size FP-SPK-L (584 mm x 92 mm x 25mm), and a small size FP-SPK-S (80 mm x 20 mm x 7.2mm). The next steps on the company's speaker road map are glass cone speakers (see image above), audiophile headphone drivers, and another long-term project of very low impedance (VLO) speakers.