Metamaterial Absorption Technology

In speakers using dynamic drivers, the rear wave of each driver presents a problem: what does the designer do with it? Typically, the objective is to get rid of it, so that only the exterior side of the driver is heard. But, absorbing the rear wave of the speaker is not as easy as it sounds. Because treble doesn’t seem to have as much energy as bass, it is less obvious is that the rear wave of the tweeter tends to face a series of reflection points and when the reflections return to the tweeter dome they can pass through as problematic distortion.

KEF has developed an ingenious solution to absorbing the rear wave of the tweeter. This involves the use of metamaterial (basically, an engineered material not found in nature).

Metamaterial Absorption Technology (MAT) was first introduced with the KEF LS50 Collection (2020). A mazelike disc sits behind the tweeter, and its function is to absorb the rear sound wave radiated by the tweeter dome.

The MAT disc comprises 30 channels of differing lengths, formed into tubes sharing an opening on one end and closed on the other. The tubes act as quarterwave resonators, each tuned to a different frequency with a high Q, which effectively absorb a narrow frequency band and its harmonics. The absorption of these channels is tuned to overlap in frequency, leading to almost 100% absorption across the spectrum above 620Hz – well below the lower threshold of the tweeter’s working bandwidth.

Coupling MAT to the Tweeter Dome

Of equal importance to the application of MAT is how the back wave is shepherded into the disc. This requires a complete redesign of the area behind the tweeter to form a waveguide with particular characteristics. Essentially, the acoustic impedance of the waveguide must match that of the opening of the MAT disc to avoid a reflection of the wave within the waveguide back intothe tweeter dome. This led KEF to designing a tapered duct, which reduces in diameter towards the disc opening with a conical profile. This inverted conical horn eliminates said reflections, but also fulfils the design requirements of allowing for easier accommodation of the absorber into the driver package and reducing the size of the MAT disc itself.

In addition, the sheer increase of acoustic volume behind the tweeter increases the venting and reduces non-linear distortion related to the spring effect of compressing this rear acoustic volume. KEF has done detailed mathematics behind MAT and the tweeter coupling technique.

The metamaterial application to the Reference and Blade series is an example in miniature of the extensive theoretical, computer simulation, design, manufacturing and testing efforts that KEF has made on many performance variables of the new Blade and Reference speakers.

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