1977: Forsythe added two way compact systems and vented subwoofer systems with a unique interchangeable tube venting system such the enclosure could be field re-configured for use with various drivers from JBL Professional, Gauss Loudspeakers, Altec and EV.
Subsequent to Keele's work and using his principles, Clifford A. Henricksen and Mark S. Ureda of Altec designed a strikingly different hybrid horn displaying constant directivity traits, the horizontal diffraction or "Mantaray" horn.
The Altec A4 could also be used as an speaker alternative.