Take a Gel Audio transducer, place it on a surface, and get sound?
Yes, but it required an analysis that goes several steps beyond normal FEA.
We think of loudspeakers as separate boxes. But engineers at SFX Technologies Ltd. are using Comsol Multiphysics to design loudspeaker drivers that use nearly any surface — from a tabletop to walls, mirrors, dashboards, billboards or even bus shelters — to produce high-quality sound. When a GA (Gel Audio) transducer from the Dunfermline, Scotland company is placed against a surface, this becomes the loudspeaker. It’s very difficult to model this complex process on paper, which makes it a perfect candidate for numerical simulation.
When placed on a panel such as a wall, mirror or dashboard, a Gel Audio transducer turns that surface into a loudspeaker with good frequency response across the audio range.
Gel as the interface
Gel speaker technology is unique; you mount a GA transducer in permanent contact with a panel without being bonded to it, such as with two-sided adhesive tape. The transducer’s magnet and coil receive analog audio signals from an amplifier, and the gel acts as an intermediary material that transfers the acoustic waves to the panel. The result is a “speaker” with good high-frequency response, very good bass, and no need for a large speaker box.
GA transducers were initially used in applications where speakers were required to be not visible or accessible, such as in a bus stops or public-address systems. Now, though, they are being incorporated into products such as small TVs, thus achieving good bass response without requiring a subwoofer. They are being considered for use in mobile phones for better audio response. In these cases, it is difficult to test prototypes because handset manufacturers have tight time-to-market schedules.
Simulation results can show them what they can expect when drivers are incorporated into their phones so that development can simultaneously proceed on both the phone and the driver.
An isosurface plot generated with Comsol Multiphysics illustrates the sound-pressure levels (SPL) that emit from a typical panel surface when driven with the GA transducer. Here the results are shown for one frequency (1092 Hz), while the graph shows the extent of surface displacement.
Previously, it would take SFX three months to build an initial prototype; now with simulation using Comsol Multiphysics and the Acoustics Module add-on, our design team completes a first prototype within a month. This time difference is vital for this new technology.
Complicated route, simple results
The modeling process takes place in three major stages.
First create a model of the coil and magnet to determine the forces that are generated at all frequencies.
Next, use these results in a simulation of the panel to obtain its deformation and acceleration across the frequency band of interest.
Finally, simulate the acoustic field that the panel will generate. Although we model intricate movement of the loudspeakers, the result we want from the simulation is relatively simple: a plot of the sound pressure level coming from the loudspeaker versus frequency. Obtaining this result, however, requires a complicated process.
GA transducer placed inside a television can produce very good bass response using the TV itself as the speaker surface. The simulation illustrates and measures the SPL created by placing a driver against the case of a TV set and the sound pressure level (SPL) at 1900 Hz. The surface plot on the screen of the TV shows its structural displacement, while the close-up 2D plot shows the SPL around the transducer as a slice plot.
Simulation is first necessary to size the various mechanical components, such as the coil and magnet in the driver, and examine their effects. Further, while those two components produce unidirectional movement, the panel on the other side of the gel can produce a very complicated waveform, especially at high frequencies, consisting of movement that might lead to sound distortion and modal shapes that change with frequency.
So a primary task of the modeling is to find the optimal assembly — the right amount of gel and the best way to attach it to the surface — and doing so by considering structural-acoustic interactions. Too much gel makes the driver inefficient and nonresponsive; too little leads to sound distortions.
Initially our design group employed an FEA package that loosely coupled structural mechanics to acoustics iteratively. But this made modeling the acoustics domain an elaborate operation and didn’t model what we wanted. Now we model this in a straightforward manner using Multiphysics.
Every case unique
This modeling is essential for us at SFX because almost every design we consider is unique; we work with a wide variety of panels and panel materials, and for each, a different driver and mounting is required. For instance, the panel that acts as the loudspeaker might be made of plastics, various rubbers and elastomers, or composite materials such as cardboard. In this case, our modelers improvise by representing the panels with isotropic materials. We are now examining new ways of shaping panels, and are starting to combine traditional loudspeakers and GA transducers in the same system. Modeling is essential in these efforts where the acoustics-structure interaction from both devices, and their effects on each other, must be considered.
Filed Under: FEA software, Simulation, Software