In concert halls it’s desirable that the time between the arrival of the direct sound and the first reflection is not greater that 20ms. Highly reflecting parallel walls may cause undesirable flutter echo. The distance from the source where the intensity levels of the direct and reverberant sound are the same is known as the Room Radius; a typical value for a large hall is 5m. Closer than the room radius you hear mostly direct sound and at greater distances mostly reverberant sound. Room modes are particularly important in small rectangular rooms such as recording studios. Tangential modes are combinations of two axial components and oblique modes are combinations of all three. The vineyard design of concert hall gives an even distribution of sound. Seats should be designed to minimize changes in reverberation between the hall being empty and full.