In this paper, we measured ground vibrations excited by rhythmic actions of large audience at an outdoor concert. This kind of noticeable ground vibration is excited by large audience at least over 1,000. However, our knowledge of this vibration is based on vibration tests excited by several people. So, we carried out this measurement. As a result, the following conclusions are obtained. 1) The vibrational energy can be concentrated in a narrow band even if the vibration is excited by large audience. 2) The effect of audience size reduces the high frequency components of the exciting force.
