Ensuring Good Acoustics for Youth Theatre in Nairobi Slums

DENMARK - August 2009
The non-profit organisation Sarakasi Trust’s Sarakasi Theatre in Nairobi Kenya is not only used for shows and concerts but also for training youths from the city slums to become acrobats. However, following the first few events, it became clear that the acoustics were just not good enough for proper communication between acrobats and for live music performances. As a result, room acoustical measurements, using Dirac Room Acoustics Software, were performed inside the Theatre using an ultra lightweight travelling measurement set. The predicted effect of adding sound absorbing materials to the theatre's performing spaces was then modelled using the Odeon Room Acoustic Modelling Software.

In the theatre, 6 mm multiplex panels on a 25mm cavity filled with low-density foam were used as low-frequency, 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 using DIRAC external input and a microphone. From the recording, the resonance frequency of the panels could be determined.

Sound Absorbing Materials from the Local Market
Promising sound absorbing materials were gathered at the nearby market. These are the only materials directly available to the theatre. One of these is a low-density foam that is used for cushions and mattresses. The cushions could be placed against walls and ceiling. By using an improvised measurement setup, the sound absorption of materials was measured and used in room acoustical prediction modelling.

To predict the effect of adding sound absorbing materials to the theatre, room acoustical modelling software ODEON was used. The goal for the acoustic design is to achieve at least a reverberation time reduction from 3.5 to 1.5 seconds for an empty theatre, which is acceptable for rehearsing. Because there are no seats in the theatre to compensate for audience sound absorption, the reverberation time will be down to 1.0 seconds in a situation with audience, which is ideal during performances. With the use of the measurement results and the calibrated computer model, available materials were proposed for the theatre.

To effectively absorb mid and high frequencies, a total of 700 m² of cloth was added to the theatre walls and ceiling. 300 m² of heavy curtain was hung in front of the theatre back wall, below and above the balcony. The other 400 m² of cloth was draped in a double-layered curtain on the ceiling like a tent structure. And 50 mm of foam was put in between the double-layered curtain.

Even the acoustics of the bar was treated, probably making it the first bar in Nairobi with excellent acoustics! Wrapped foam bags were put against the bar’s ceiling to reduce the reverberation time from 3.0 to below 1.0 seconds. Unfortunately, the background noise will always be high during opening hours – up to 60 dB (A) – because the front doors to the crowded Ngara Road are always open in the hot climate.

The acoustic treatment was done just in time for the Sawa Sawa festival from 30 May to 1 June 2009, where the new theatre was inaugurated with dance, acrobatics and music.

For more information about ODEON and DIRAC, please visit www.bksv.com or contact your local sales representative.