How are Tectonic Plates more feedback resistant than traditional speaker systems?

It’s all back to the diffuse characteristics of the DML.

Feedback is reliant on the looping signal encountering itself in a well-defined way. If the waves coming off a speaker (wavefronts) are ‘coherent’ then it’s easy for the signal to encounter itself at just the right time to generate feedback. It’s a bit like watching a kid on a swing. You can quite easily work out the best time to push him because the motion is so clearly defined. This is because the cone moves rigidly, as one.

For a DML the wave fronts are not coherent, as different parts of the DML are moving in different directions, so the wave fronts arriving at the mic are not in a simple order. It’s a bit like trying to push 10 kids all swinging at random—it would be pretty hard to work out how to step in and push them all so that they suddenly all swing together in unison.

Usually feedback occurs at a specific frequency that is often the loudest individual frequency in the speaker’s bandwidth.

For a conventional speaker there are usually two loud frequencies; the first mode, (or more correctly the zero-th mode), which is the fundamental mass-spring resonance of the system, and possibly the first break-up mode at the upper end of the drive unit’s pass band. Whichever frequency the feedback locks to, it can often be one of these.

The response of a DML is intentionally designed to have as many modes (resonances) as possible, such that any energy prone to a feedback loop to lock into a specific mode is greatly diminished because there are so many of them. The energy is spread across these multiple modes and no single mode is strongly excited into feedback.

The Critical Distance to generate feedback is the distance from the source at which the direct sound level equals the reverberant sound level. For more directional speakers this distance is further from the source (speaker). For more Omni-directional speakers this distance is closer to the source. For feedback to occur, the mic should usually be within the Critical Distance to ensure a strongly correlated feedback path.

DMLs have wide directivity (more omni-directional) dispersion so the Critical Distance is closer, therefore the mic can be brought closer to the speaker before feedback ensues. Conventional speakers with narrower directivity push the Critical Distance further away from the speaker, therefore feedback occurs from much further out.

In most cases, this results in a much greater “gain before feedback.” There are other factors that can reduce this that must be considered, such as type of microphone (dynamic or condenser) and pattern (Cardiod, Omni). However, in the worst case scenario, we find that Tectonic loudspeakers are no worse than traditional loudspeakers and in most cases are much better.