“…Previously found waves are generally outgoing, i.e., propagating away from the wave source, till recent observation of waves that propagate toward the source, called ingoing waves (also referred to "antiwaves", e.g., antitarget waves and antispiral waves) in experiments [23,24] as well as in numerical simulations [25][26][27]. In terms of physical language, the outgoing (ingoing) waves have positive (negative) phase velocity.…”