Project Ancient Acoustics Part 1 of 4 : a method for accurate impulse response measurements in large open air theatres

Selecting an appropriate method for measuring ‘normal’ indoor concert hall acoustics is always a trade-off between time, stimulus type, number of measurements and measurement quality. For ISO 3382 room acoustic parameters to be derived accurately from impulse responses, this tradeoffaims at a certain minimum decay range. Previously, there have been many attempts to obtain room acoustical parameter values in ancient amphitheatres, using hand claps, balloons, fire crackers, blank pistols, deterministic signals played by loudspeaker sound sources, etcetera. However, results are presented mostly without describing or discussing the quality of the obtained impulse responses (IR’s). Part of the Ancient Acoustics research project was to find out how to accurately measure many open air impulse responses in accordance with the ISO 3382 standard, under restricted time and resource conditions. Practical ‘measurement training’ in a Dutch open air amphitheatre resulted in a measurement setup and procedure that would meet the predefined requirements for measuring ancient amphitheatres. To reduce measuring time, asynchronousmeasurements (without cables) were carried out using multiple microphones and recording devices, as well as multiple dodecahedron sound sources playing the stimulus signals simultaneously. In addition, speech intelligibility measurements were performed using a separate speech source. This finally led to more than 10,000 accurate impulse responses for three theatres. This part of the study will focus on the method used to perform such a large number of good quality measurements within a day per amphitheatre. We will discuss the effect of deviceclock speed mismatch due to signal asynchronicity, sound source directivity errors, system variance by meteorological influences and the effect of background noise on the impulse response decay range, using the Impulse response to Noise Ratio (INR) and the Just Noticeable Difference (JND) as quality measures