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.

"When the prelates of the medieval Cathedral of Agrigento in Sicily chose to hear confessions near the great central door, they undoubtedly did so to ensure the privacy of their parishioners' revelations. Then, quite by accident, someone discovered that behind the high altar 250 feet away the murmuring from the confessional could be clearly heard. 'Secrets never intended for the public ear thus became known,' according to one account, 'to the dismay of the confessor and the scandal of the people.'

The architect of the cathedral had inadvertently designed an elliptical gallery . . . The configuration of the walls behind the altar had focused sound waves from a point at the other end of the cathedral, although people in between could hear nothing." (Sound and Hearing, p19)

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.

Locations where the rotunda effect is experienced are sometimes called "whispering galleries". The dome of St. Paul's Cathedral in London is a famous example.

Anti-focusing Surfaces

Since even dispersion of sound is highly desirable in an auditorium, it may be necessary to take steps to overcome any focusing surfaces. If an architect decides that some curved surface is desirable for some reason, then the undesirable focusing effect may be partially overcome by covering the curved surface with anti-focusing surfaces.