Compressed fibreboard
Explanation
Reverberation refers to the persistence of sound in a closed enclosure as a result of repeated reflections from walls, ceilings, and floors. To ensure acoustic clarity in auditoriums and cinema halls, excessive reverberation must be minimized. This is achieved by using sound-absorbing materials that trap sound energy rather than reflecting it.
Option Analysis
- Polished marble walls, Glass windows, and Metal sheets are incorrect: Polished marble walls, glass windows, and metal sheets are hard, smooth, and rigid surfaces. These materials are efficient reflectors of sound waves. Using them would increase the number of reflections, thereby enhancing reverberation rather than reducing it.
- Compressed fibreboard is correct: Compressed fibreboard is a porous and soft material. When sound waves strike such materials, a portion of the sound energy is absorbed within the pores and converted into heat or dissipated, reducing the intensity of the reflected sound. Consequently, this reduces the reverberation time in the hall.
Key Takeaway: Hard and smooth surfaces reflect sound, causing reverberation, whereas soft, porous, and rough materials (like fibreboard, heavy curtains, and carpets) absorb sound and are used for acoustic dampening.