Terms
- Reflections (early): Heard within approx. 30 ms
- Reverberation/"slap echoes": Heard after 30 ms; excessive reverb can lessen quality of sound/"stereo image" (if original sound is still being echoed when next sound is made)
- Absorption: Curtains, walls, special tiles, seats, and audience absorb sound; Absorption depends on a combination of material and frequency of sound.
- Critical distance: Distance at which direct sound and reverberant sound energy are equal (radius around sound source - depends on sound frequency); More reverb = CD closer to source, More absorption = CD farther from source
Issues with sound quality
- Reverb remains constant throughout room/theater, unlike direct sound, which decreases with distance from source. This means that as direct sound keeps coming from the source, reverb sound keeps bouncing around and adding up until a steady-state is achieved - where the incoming sound equals the absorbed sound. If the reverb energy is much greater than direct sound energy, there can be significant loss of quality, especially at the back of a theater, where direct sound is weaker than at the front. Having the critical distance of a room being as far as possible from the sound source minimizes excess reverberation and is the ideal acoustic condition (reduces echo so sound is clearer). Still, it's hard to determine the "perfect" conditions of a theater - people have various opinions about what sounds best, certain design methods work well for certain spaces and kind of sound being plated, etc.
- Often in large movie theaters, low frequencies are not absorbed well (sometimes due to poor material choices), leading to low sound quality. Good acoustic design can help to mitigate this problem, especially by using fabric (filled with fiberglass is best) offset from walls (often done by pleating fabric hanging against wall) to have a better chance at absorbing lower frequencies that can be the cause. The distance of offset should be ~1/4 of the wavelength of the lowest frequency that will be in the room (the example I found is 6 feet from the wall).
Why it's important
- Poor sound quality can ruin an experience in a theater. If the point of going to see a movie, for example, is to become immersed in the story (the reason why we leave our homes to visit a theater, the lights are turned off, etc.), having sound quality that is inconsistent with the narrative can be very unfortunate. Take Life of Pi as an example movie. When Pi is on the open sea with Richard Parker, there aren't any walls for sound to bounce off of. Watching the movie in a theater with a lot of reverb would diminish the experience of the movie. You couldn't understand how alone Pi must have felt, not having a single thing other than his raft in sight, since your ears/brain know you are in an enclosed space based on the sound quality.
- Architects can help. Theater design can greatly affect an acoustic experience. One of the most obvious solutions is improving absorption to reduce reverb and sound distortion. Materials, including fabric curtains and absorption tiles (on walls and ceilings) can be very effective in improving quality of a theater.
- The Guangzhou Opera House by Zaha Hadid Architects (2011) was designed with help from Marshall Day Acoustics. Clarity and purity of sound was the main goal of the performance theater space. The asymmetrical, flowing geometry of the performance hall allows for greater early reflections. The scheme was modeled and tested using acoustic performance tools, and modified according to results. Many iterations were designed in the process of determining the best balance between acoustic performance and the designers' architectural goals.
http://lenardaudio.com/education/17_cinema_6.html
http://www.crutchfield.com/S-2B00mT6tjJH/learn/learningcenter/home/speakers_roomacoustics.html
http://www.odeon.dk/pdf/Exton_IOA2011.pdf
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