Because of the huge tsunami created by the gigantic earthquake of March 11, 2011, Japan's Tohoku District sustained widespread and severe damage. Reportedly, alert speech sounds from open- Air loudspeakers of governmental emergency radio communications systems were often not sufficiently intelligible because of the superposition of long-path echoes. A similarly huge tsunami is expected to occur about once every 1,000 years in this area, necessitating preparation for future tsunamis. Alert sounds are useful to convey specific alert signals using non-verbal sounds. Therefore, conditions to produce an alert sound that is tolerant to long-path echoes were investigated in this study, which examined musical chords for the design of alert sounds. To increase the saliency, we also applied an octave-up sweep from stationary chords. Subsequently, we investigated the influences of harmonic structure and sweeping characteristics so that an alert sound can have optimal alert impressions. The alert impression of the sounds, which consist of five tones and the fifth harmonic, was higher than those of single and second harmonic chords. That result indicates that open- Air loudspeaker systems to convey alert information must have a sufficiently broad frequency range.