Measuring Reverberation Time and Speech Intelligibility using Balloons and an MP3 Player as External Sound Sources
The non-profit organisation Sarakasi Trust owns a theatre in Nairobi Kenya, inside the former Shan Cinema. In this theatre, youths from the city slums are trained to be acrobats, so that they learn how to make a living without having to resort to crime. Shows and concerts are also organised inside the theatre and in community centres and parks across the city. Following the first concerts inside the theatre, it became clear that the acoustics were insufficient for proper communication between the acrobats and for live music performances.
The founders of Sarakasi Trust, Rudy and Marion van Dijck, got in touch with the Eindhoven University of Technology via Studium Generale, its cultural organisation. Acoustical researchers and consultants Constant Hak and Remy Wenmaekers of the university’s Laboratorium voor Akoestiek (Faculty of Architecture, Building and Planning, Building Physics and Systems Unit) and Level Acoustics were willing to work together to improve the acoustics of the theatre.
Sound Absorption Measurements
In the theatre, 6 mm multiplex panels on a 25mm cavity filled with low-density foam were used as lowfrequency, sound absorption material. The absorption of these types of materials relies on the resonance of the panel at a typical frequency. The absorption coefficients of the 400 m² wooden panels had to be estimated for the ODEON prediction model. The resonance frequency was determined by recording the sound when tapping on the panels could be determined from a clear peak in the FFT spectrum graph. The resonance frequency of the wooden panels seemed to lie in the 250 Hz octave band (theoretically 200 Hz). This explains the shape of the reverberation time curve which has a dip at the 250 Hz octave band.
Promising sound absorbing materials were gathered at the nearby market in the slums. These are the only materials directly available to the theatre. One of these materials is a low-density foam that is used for cushions and mattresses. The cushions could be used as baffles and the mattresses could be put against walls and ceiling.
The sound absorption of the cushions has been tested in an improvised laboratory setup in an empty reverberant room inside the theatre. The baffles proved to be sound absorbing up to 100% for frequencies from 125 to 4000 Hz. The mattresses proved to be sound absorbing comparable to mineral wool of the same thickness.
Room Acoustical Predictions
To be able to predict the effect of adding sound absorbing materials to the theatre, room acoustical modelling software ODEON was used. A geometric model was made from drawings during the flight from Amsterdam to Nairobi. Upon arrival, some dimensions were checked and overseen details were added. The theatre floor plan is symmetrical which makes it possible to only model half of the room and mirror the surfaces along the x-axis.
The same sound source and receiver positions were added to the model as used earlier during the measurements. Also, the estimated material properties of the wooden panels and curtains were added to the sound absorption database and the materials were assigned to the surfaces of the model.