Bouncing Off the Walls

Controlling acoustics in a Monolithic Dome

by David B. South, reprinted from the Roundup, Summer 2000

Acoustic Panels

Inexpensive calming of the sound bounce in a Monolithic Dome.

The Monolithic Domes have an interesting technological feature. When they are clear and open they concentrate the sound. Concentrated sound can be dealt with. We have many experts telling us how to do it. Some say this and others say that. But we have learned a few things by just doing. One way to calm the sound is to hang sound absorbing blankets in the center of the dome. It is a very simple and inexpensive cure.

In one of our office domes we had too much sound bounce. The dome was open with a few office cubicles around the edge. We had several people give us advice about calming the sound bounce. One that made the most sense was a salesman selling sound panels -- quilted soft panels. Without further guidance we order 24 panels. We decided to hang them a few at a time and test the results. To our great surprise we achieved good solid calming with just four of the panels located in the center.

Since this worked so well we used 32 larger panels in the Italy Multipurpose Facility (Gym). It has helped immensely. But they were hung around the perimeter. There is no doubt in my mind that a small number of the larger panels hung in the center of a gym would do wonders at calming the sound.

The sound blankets are simple to install. They are virtually indestructible. Balls thrown against them are not a problem. They can be colorful, and present a beautiful design. And most of all they are not expensive.

If you do not have current sound mitigation consider the panels. Call us for names of suppliers and methods of installation.

Sound reverberates in all spaces except outer space. Sound waves travel out from their source forever - unless they impinge on to a surface where they are absorbed, or reflected, or both. Sound tends to travel in a straight line. It will bounce in much the same way as light reflects.

All rooms have their own acoustic behaviors. The sounds will bounce from wall to wall and ceilings until they are absorbed. If a room is empty and the surfaces are hard, sound bounces many more times before it dies. If the room is padded, the sound quickly dies as it is absorbed by the padding.

Monolithic Domes are shaped perfectly to reflect most sound through their focal point - their centers. This means we can capture the sound and absorb it at its center without covering all the other surfaces.

Sound generally follows line of sight. If you see only absorbers you can bet the sound is only seeing absorbers. The absorbers can be people (soft, lumpy, no reflective surfaces, many angles of reflection), carpet, upholstery, acoustic ceiling tile, furniture of all kinds and shapes, plants, flags, etc.

The point is that all extra sound in any room is a bother. It needs to be absorbed. In the dome it concentrates in the center. This concentration makes it more noticeable. In many ways that is good as it provides a much smaller area that must be treated to absorb the extra sound.

The simplest sound treatment is a cloud of sound absorbing material hung in the center (focal point) of the dome. For beauty or effect, the absorbing cloud can be designed as a series of small clouds or as a single hung ceiling. Flagging, vertically suspended, also works well.

With all acoustic treatments it's imperative that the treatment does not compromise the dome's huge heat sink. That heat sink is part of the dome's heating system. It's the passive solar part of the building. Air must be allowed to flow freely through the cloud so the heat can interact with the dome shell. This can be done by properly spacing the clouds or the flagging, or by using power ventilation.

Spraying the dome with fuzzy sound absorbers is a quick fix for short sound waves. It does not help much with long sound waves. And it definitely will foul up the quick heat sinking ability of the concrete shell. The shell stores enormous quantities of heat.

Great care must be taken not to lose this feature, especially in domes where crowds of people will be occupying the building from time to time.

Bouncing Off the Walls

Controlling acoustics in a Monolithic Dome

Acoustic Panels

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