By: Tony Hoover, FASA, INCE Bd. Cert., MCH Principal
Sound for Bollywood films is rarely recorded live in their sound stages, but is "looped" or added later, primarily because of poor ambient acoustics. Construction had commenced on several new sound stages in Film City, Mumbai, India, but ceased shortly thereafter. Following several years’ hiatus, the project was renewed with the directive that the existing fragmentary construction be used in the new design and as the foundation for subsequent construction, and with the goal of “Hollywood quality” acoustics. The design needed to address existing site conditions, encroaching hutments, concerns for local materials and methods, and uncertainty about how local contractors would actually implement the design.
The plan of the sound stage facility is shown to the right, with three sound stages and ancillary support spaces. Approximate dimensions are included.
Post-construction inspection and testing in January 2011 concentrated on the three primary acoustical issues for any project, which are sound isolation (from outside to inside as well as between adjacent sound stages), mechanical system noise (HVAC background noise), and surface finish treatments (especially to provide sound absorption for minimal reverberation).
Sound Isolation
Sound isolation design was heavily influenced by the pre-existing foundations, using concrete structure infilled with locally-made clay bricks. Primary walls included an inner layer of clay brick and an outer layer of clay brick, separated by a wide airspace and glass fiber insulation; this mass-airspace-mass construction is generally desirable. The image to the left shows the concrete structure and portions of the outer wall infilled with clay brick, as well as the pre-existing construction.
Doors would be the weak links in sound isolation to the exterior, and truck doors needed to be very large to allow loading of large equipment and sets. The design called for heavy inner and outer doors, separated by one-meter airspace. The image to the right shows one of these truck doors, with the outer door closed and the inner door open.
After proper adjustment to provide airtight seal upon door closure, the sound isolation was tested with several powerful loudspeakers generating loud levels of pink noise (sounding like a jet engine) outside of the doors. The doors were able to sufficiently block exterior noises, including diesel generators and ongoing construction of another sound stage on the other side of the access road.
Additionally, a cracker band (named after the firecrackers used along with drums and amplified instruments) from an adjacent hutment was hired to play outside the sound stages, since cracker bands play as often as several nights per week for various celebrations in the hutments. The video below highlights how loud the cracker band can be outside of the sound stages.
The interior walls between sound stages were tested using loud pink noise, and were found to provide high levels of sound isolation.
HVAC Noise
The HVAC system was measured to be extremely quiet, due to slow airflow velocities through very large ducts, which minimizes airflow turbulence, plus sound-absorptive linings inside the ductwork. In fact, the HVAC system is essentially inaudible.
Surface Finish Treatments
The remaining concern was the reverberation inside the sound stages, which is determined by the efficiency and extent of sound-absorptive finishes on the walls and ceiling. The design was to install thick glass fiber insulation on almost all wall and ceiling surfaces; in turn, the glass fiber was to be covered by a sound-transparent fabric to prevent the glass fiber from shedding, and colored black to prevent light reflections. Note that the walls are covered with the black sound-absorptive treatment, shown to the left.
The reverberation times were measured to be short and "dry", so that recorded sounds would not have an audible reverberant character such as might otherwise be expected in enormous rooms like these sound studios. Dry sound is desirable so that editors can add any kind of sound effects of their choosing during post-production. Excessively reverberant recordings are very difficult to correct or modify later, whereas reverb, echoes, or other effects are quite simple and commonly added in post-production.
Takeaway
These sound stages are among the finest in Bollywood, finally allowing for live audio recording. All of the challenges of an international project notwithstanding, both the client and the end users acknowledge and recognize that the acoustics in these sound stages are far superior to others in India, and are on par with similar venues around Hollywood.
Note: This Tech Blog is based on an ASA popular version paper for the 2011 San Diego meeting.
