THX-type dipole surrounds produced irregular mid-frequency depressions, not neutral nulls

Evidence

The loudspeaker shown in Figure 8.10 was not optimized for the dipole mode as conceived by THX. The direct sound represented by the on-axis frequency response exhibits a large irregular mid-frequency depression instead of a broadband timbrally neutral “null” as might have been expected from a true dipole. — Toole, Floyd E, Sound reproduction- the acoustics and…, 827