The condition for the excitation of Alfvén eigenmodes is one of the most basic properties of fast ion driven instabilities that must be understood in order to predict the stability of modes in the burning plasma regime. Until recently the quantitative data available on the excitation condition for Alfvén eigenmodes was limited due to the lack of sensitive core fluctuation diagnostics on the one hand and the use of Ion Cyclotron Resonant Heating (ICRH) excitation on the other where the tail ion temperature is typically not known with high accuracy. However, new data obtained on the JET and DIII-D devices using sensitive core fluctuation diagnostics and precisely calibrated neutral beam injection velocities reveal for the first time (a) a much lower velocity threshold for the excitation of Alfvén eigenmodes than previously observed and (b) high sensitivity of mode excitation to the direction of neutral beam injection. The asymmetry in beam direction can be attributed to differences in the fast ion distribution owing to finite orbit width effects and to a possible intrinsic sensitivity of these modes to the direction of the particle. These observations are particularly relevant to advanced confinement regimes where core localized Alfvénic activity is readily excited in existing experiments.