论文和技术资料

Introduction

We at Sonos engineer premium audio products where delivering on user experience demands careful navigation of competing engineering and program constraints. Arc Ultra, Sonos' flagship soundbar, is the first to feature Sound Motion technology, a novel transducer architecture that delivers four times the bass output per unit volume relative to its predecessor. This work describes how COMSOL Multiphysics combined with rapid prototyping techniques enabled engineering innovations that made this product possible.

Engineering Challenges of the Sound Motion Architecture

Traditional loudspeaker designs center the voice coil (and motor) on the moving diaphragm, creating inherent structural and dynamic stability. The Sound Motion architecture relocates multiple motors to the diaphragm perimeter to achieve distributed driving force. This in turn enables opposing membranes in a much more compact form factor relative to a single conventional transducer of equal acoustic output.

These two architectural changes lead to the following engineering challenges: 1. Traditional fabric “spider” suspensions cannot be accommodated. 2. Distributed coil masses and driving forces create a diaphragm rigidity challenge, compromising rigid-piston behaviour.

As a result, the spider needs to be completely redesigned and ribs are introduced to the transducer assembly.

Spider Design

The spider, a mechanical suspension element connecting voice coil to frame, faces the following competing objectives: 1. Provide system stiffness contribution to achieve target resonance. 2. Ensure natural modes fall outside the operational bandwidth, or are sufficiently damped. 3. Maintain a linear stiffness characteristic across the full displacement range. 4. Constrain motion to linear axial translation only. 5. Withstand expected stress/strain at peak displacement without plastic deformation or fatigue failure over its lifetime.

These objectives are met through optimal design of the spider leg's path, width, and thickness. Two COMSOL Structural Mechanics studies with Geometric Nonlinearity were used: one to capture large-deformation stress distributions and out-of-axis motion, and a second to predict natural modes.

Rib Topology Optimization

The Rib structure connecting the distributed motors faces the following competing objectives: 1. Provide the structural stiffness needed to keep the diaphragm moving as a rigid surface, suppressing deflection modes. 2. Be as low in mass as possible. Total moving mass has a significant impact on Power Efficiency of a transducer. 3. Withstand expected stress/strain for peak forces without plastic deformation or fatigue failure over its lifetime.

Starting from the "solid" rib (top-left in Fig 4), the Optimization Module was used within a Structural Mechanics model to reduce rib mass by 33%, while maintaining natural mode frequencies and keeping stresses below fatigue strength of the material.

Validation

Key design iterations were prototyped using in-house 3D printing and CNC machined parts. We accept differences in material grade and longevity relative to final production in exchange for increased confidence in the COMSOL models. Measured stiffness and mode shapes were compared directly to simulation predictions, with agreement within ±5%. These validated COMSOL models enabled rapid design space exploration without compromising the program timeline.

Conclusion

COMSOL-driven design, validated through rapid prototyping, enabled confident engineering decisions within the constraints of a mass-production program timeline. The Sound Motion woofer in Arc Ultra delivers twice the bass output of the 8 mid-bass drivers found in the previous Arc, while occupying the space of just 4 drivers. This results in a 4x increase in bass output per unit of volume. The freed volume accommodated additional mid-range and tweeter units, directly improving spatial audio and home theater performance.