In March 2015 a group of students from the Eindhoven University of Technology organized a research expedition to three ancient Greek outdoor theaters to investigate the acoustical properties and the exceptional claims made about them.
The data for approximately 12000 impulse responses were collected, and are still being analyzed. How did the team succeed in performing this many measurements over the course of just three days?
The acoustical measurements were performed using two light-weight B&K 4292-L omnidirectional sound sources and 20 microphones attached to digital (handheld) recorders. Each source was rotated over 5 angular positions to remove any residual source directivity effect. The stimulus was specifically designed for this purpose and based on an e-sweep allowing the retrieval of two impulse responses from a single measurement. Measurements were performed during the day and in the evening, to be able to investigate the effect of environmental conditions on the acoustics.
A close look at the video above reveals the cameo appearance of the Echo speech source for some of the measurements. We are eagerly awaiting the results of the Echo measurements.
The responses to the e-sweep signals were recorded and not processed in real time. This means that the quality of all the resulting impulse responses can only be judged at a later time, when the team has returned to Eindhoven. It requires a thorough preparation of the measurement procedure and testing of the equipment to have enough confidence the results will be useable.
With thousands of measurements, it is essential that the processing of these measurements can be automated to a high degree. The scripting feature in DIRAC has proven to be a real timesaver here.
More information on this fascinating project can be found on the Ancient Acoustics website.