Non-focusing Surfaces

Any time the surfaces of a room focus the sound which is reflected from them, they create spots of high intensity and other spots with low intensity. This is generally undesirable in an auditorium since you want a uniform, evenly dispersed sound to all listeners.

Even large flat reflective surfaces are to be avoided because of the prominant reflection which will be produced. Parallel flat walls can produce a pattern of reflections known as a "flutter echo" as the sound waves travel back and forth between the surfaces. Such flutter echoes are often encountered in high school gymnasiums where there are parallel side walls and also a reflective floor and ceiling.

Even dispersion is such an important contributer to good acoustics that it is sometimes desirable to use anti-focusing surfaces in a music making area. Older architecture often had columns, decorative sculpture and woodwork, and other dispersing surfaces. In modern architecture with its flat expanses, it is necessary to design in some anti-focusing properties.

Focused Reflections

Elliptical Enclosure

An ellipse has two focus points. Sound projected in any direction from one focus point will travel to the other. Sound from any point will tend to be focused toward some point, so ellipses are certainly to be avoided for most acoustical purposes.

Parabolic Surfaces

All rays from the focus of a parabola to its surface will be directed outward as parallel rays. It is useful for projecting sound. Two parabolas as shown below can direct sound from the focus point of one to the focus point of the other with great efficiency. A microphone element can be placed at the focus point of a parabola and then aimed at a distant sound source - parabolic microphones can pick up selected sounds at surprising distances.

Rotunda Effect

A popular practice at the U.S. Capitol building in Washington, D.C. is to send a friend to the far side of the capitol dome. Ordinary conversation can be if the speaker and listener are both close to the wall of the dome. Many buildings with dome-like rotundas exhibit this guided reflection phenomena.

Anti-focusing Surfaces