Not every driver is suited to horn use. I'm not sure what all the characteristics are, but a rising SPL level on a graph is an example of a "horn friendly" characteristic. I used a PHL driver which PHL suggests for possible horn use.

Dr. Edgar played with a lot of drivers years ago to find the best to use. I'm sure Wayne chose his drivers for specific characteristics as well which the Focal's may or may not satisfy.

Grant.