Especially during summertime, public squares, stadiums and other kinds of open spaces, are frequently used for live concerts (pop, jazz, classical, etc.) – both with and without amplification. Based on the first author’s PhD thesis, this paper aims at illustrating whether reverberation time (EDT, T30) and other acoustical parameters normally used to test the acoustical quality of closed auditoria, such as concert halls, theatres, opera houses, are suitable and sufficient for testing the acoustical quality of open performance spaces. Simulations as well as measurements were carried out to study the acoustics of open squares surrounded by hard, vertical, reflecting building facades. Especially when concerts are amplified, echoes or flutter echoes are often found to be the most important characteristic – and problem! Therefore, emphasis was given to finding an acoustical parameter – or a set of parameters – which could illuminate this problem. The results indicated that the value of Centre Time (Ts), compared with its expected value based on Reverberation Time, has a potential for echo detection, but also Speech Transmission Index, (STI), Strength (G), and Clarity (C80) – and when computer simulations are carried out – auralisations, are valuable for “Agora acoustics” investigations.