Design of a New Testing Chamber to Measure the Absorption Coefficient Down to 25 Hz
* Presenting author
This presentation will describe a new 286 m3 testing chamber that will allow the measurement of the random incidence absorption coefficient, according to ISO 354 and the normal incidence low frequency absorption coefficient, according to ISO 10534-2. The rev room’s cuboid dimensions were determined from an optimization procedure that minimizes the standard deviation of the modal response. Both dome-shaped boundary diffusors and hanging panels are used to provide satisfactory diffusivity, which is verified using a 200 mm reference absorber according to ISO 354. The isotropy of the incident sound is measured with a new Sound Field Analysis Recorder (SOFAR) and quantified with an analysis of the wavenumber spectrum. 12 impulse responses are collected from 2 sources and 6 distributed microphones. The MLS impulse response data is processed with a proprietary Matlab program, which automatically removes the rms noise floor, creates an energy decay from the optimal linearity limit, achieving maximum dynamic range, and determines an averaged plot of T60 versus frequency, from all of the microphone-speaker pairs. A steel and concrete lined low frequency impedance tube measuring 600 mm x 600 mm x 5.8 m long and 7 tons is used to measure absorption from 25-250 Hz.