The acoustics of the “clapping circle” at Purdue University

The unusual acoustical properties of a particular landscape architecture feature of Academy Park on the Purdue University campus have been the subject of speculation for years. The feature, known informally as the “clapping circle,” consists of sixty-six concentric rings of stone tiles. When someone claps while standing at the middle of the circle, they hear a high-pitched squeak immediately afterwards. Experiments were conducted by the Purdue student chapters of the Acoustical Society of America and the Audio Engineering Society to characterize this effect. The response to a clap played from an omnidirectional speaker placed at the center of the circle was recorded using a microphone positioned above the loudspeaker. Spectrograms of the recorded responses revealed the squeak to consist of a descending tone at around 1500 Hz, and its harmonics. This tone disappeared from the spectrogram when the tile rings were covered with absorbing blankets. A mathematical model based on scattering from the gaps between the tile rings reproduced the descending frequency of the squeak, and reproduced the effect of the source and receiver height on the rate of change of frequency. Thus, it was concluded that the squeak is an example of repetition pitch produced by the tile formation.