The storage tank measured used to contain 5.6 million gallons of shipping oil. You can find out more about the oil tanks and their acoustics (and why the World record is incorrectly called an echo) in my new book.
The measurements were carried out according to ISO 3382-2 2009 “Acoustics – Measurement of room acoustic parameters Part 2: Reverberation time in ordinary rooms”. The impulse response measurement method was used using a starting pistol firing 9mm revolver blanks. Two ½” inch laboratory quality omnidirectional microphones were used (Bruel and Kjaer 4165 condenser microphone connected to Bruel and Kjaer 2669 preamp). These were connected to a Norsonics 336 Front End power supply and pre-amplifier. The outputs from the Norsonics 336s were recorded on a Roland R44 sound recorder at a sampling frequency of 44.1 kHz with a 24 bit precision uncompressed.
Six source-microphone combinations were measured. The two microphones were placed roughly a third of the way from the rear wall of the room, approximately 5 m apart, roughly 2 m and 3 m from the centre line running along the length of the room. One was approximately 1.5 m above the ground the other approximately 2 m above the ground. Three source positions were measured, starting roughly a third of the way from the entrance and ending about half way into the room. Source positions were away from the centre line of the room. In feeesound the impulses are labelled 1-6. 1&2 were measured at the same time (similar for 3&4 and 5&6).