Long before David Copperfield was pulling rabbits out of a hat, kids in the 1960s were glued to the TV set every Saturday morning to watch the Magic Land of Allakazam, starring Master Magician Mark Wilson, the beautiful Nani Darnell and Rebo the Clown (Bev Bergeron). This show was responsible for inspiring many of these kids to become the magicians of today.
Although I didn’t select magic as a career, the show did inspire me to learn as much about magic as I could. But learning magic is a double-edged sword; mastery of each trick comes at the price of losing the awe for the illusion.
Acoustic clouds work much like magic. Visually, they are awe-inspiring and their mysterious secrets are not discussed in most books about acoustics. But once you know the simple trick behind their design, they lose much of their pizzazz. So this is your last warning: If you don’t want to lose your child-like fascination for acoustic clouds, then DON’T READ THIS ARTICLE!
Sounds like magic
As with the planning of any good magic trick, the design of acoustic clouds starts with an understanding of audience perception. Sound travels from the stage to each individual directly and also by reflecting from the walls and ceilings of the theater. If there are no strong reflections, the audience perceives the theater to be dead, even though the reverberation of the room may be moderate. If there are strong reflections but they arrive at the listener 1/20th of a second or more after the direct sound, then the audience perceives that the room is too live, even though the reverberation may be low.Only if the reflections reach the listeners’ ears between 1/50th and 1/20th of a second after the direct sound does the audience perceive the room to have good acoustics. This is somewhat of a simplification, since everyone perceives good acoustics to be different. The main point to be made here is that it isn’t enough to make sure that the reverberation is appropriate; the strength and timing of the first reflections must also be correct.
In a small room, the first reflections from the walls and ceilings arrive almost simultaneously with the direct sound and timing is usually not an issue for good sound. (The exception to this is the timing of reflections in recording or stereo listening rooms.) As the room size grows to accommodate more people, the walls and ceiling reflecting surfaces necessarily move further away from the audience, with the result that the sound reflected from these surfaces arrives later and later after the direct sound. As the ceiling height approaches 20 feet and the width and length of the room exceed 40 feet, the reflections from them begin to adversely affect the audience’s perception of the room’s acoustics. Reflections arrive too late to support the direct sound and start to produce discrete echoes instead.
Late reflections could be avoided by not letting the ceiling height exceed a calculated maximum but this turns out to be too restrictive in the design of the room’s reverberation and seating capacity. The solution is to hang acoustic clouds from the ceiling in such a way that they provide strong reflections with the right timing for each audience member.
Head in the clouds
For clouds to be effective, the source of sound on stage must be fairly loud. Acoustic clouds won’t be much help to a single orator; he hardly has enough acoustic power to be heard directly in a space large enough to require clouds. Reflected sound is considerably less powerful.The usefulness of clouds is also questionable for electronically amplified bands; the goal for most of their sound engineers is to throw as much sound as far as possible. Any room acoustics (including clouds) just tend to get in the way.
Acoustic clouds are most effective in supporting orchestra and classical stage performances. These sources of sound are loud enough to excite the room acoustics, the first reflections are significant and the acoustics of the space are an integral part of the production. In fact, orchestras and classical stage performers are judged on how well they can “play” a room.
Placement of acoustic clouds starts with the limitations of the room. The lines of sight between the control-room window in the back of the theater and the proscenium opening of the stage must be unobstructed, and so these lines of sight establish the minimum height for the clouds. Also, catwalks and stage lighting positioned above the audience fix the maximum height at various positions throughout the theater.
The goal in designing clouds is to redirect all of the sound that hits the ceiling as evenly and with the right time delay across the entire audience seating area. This is done by dividing the ceiling into sectors (they look like slices of pizza) so that each sector receives the same amount of sound energy, and dividing the audience area into even sections so that each section has the same number of seats. The number of sectors is made to equal the number of sections and so they are paired up. Finally, the shape, size and positioning of each cloud is determined so that the energy in each sector of the ceiling is redirected toward each seating section and arrives there at the correct time. Since the clouds must operate for sources located anywhere on the stage, each cloud is usually designed as a curved surface.
Materials used to manufacture clouds are typically hard, rigid and lightweight. Off-the-shelf clouds look suspiciously like 4-by-8 sheets of 3/4-inch plywood that may include a laminate skin and hardware to bend and hang the cloud. Most clouds have sizes and shapes that are unique to the room they are designed for and must therefore be constructed on site. Gypsum board makes a great cloud surface.
Recent projects have included large clouds constructed of suspended T-grid, acoustic lay-in tile. These projects are typically large and fairly dead auditoriums used for loud rock music. Although they look neat and appear to be strategically angled, these clouds simply act as sound absorbers. Most of the sound in these auditoriums is direct from large speaker arrays and all room acoustics are made subordinate to the electronic sound system.
