Horizontal localization is most acute straight ahead and degrades sharply to the sides.

Evidence

We are able to localize sounds in the horizontal plane with good accuracy because of the powerful interaural time and amplitude differences. With speech localization blur (uncertainty) is about 0.9° to 1.5° in the forward direction. As mentioned earlier, localization to the sides is much less precise, with an angular uncertainty range of about 20° for noise pulses, compared with about 7° in front and 11° in the rear (Blauert, 1996). — Toole, Floyd E, Sound reproduction- the acoustics and…, 747