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From the April 1988 Edition of The Diapason.

Acoustic in the Worship Space, I, II, III, IV, V, VI, VII, VIII, IX have appeared in The Diapason, May 1983, May 1984, January 1986, May 1987, April 1988, April 1990, July 1991 May 1992, and April 2009 respectively.

"Echo"

Of all acoustical matters, "Echo" seems to be the term most familiar, least understood, and most feared by laymen, musicians, and architects alike. The phenomenon of echo has been known and studied for many years. Even during the age of Greek mythology the story was told of the angry and jealous goddess Hera who limited mountain nymph Echo's speech to repeating only what others said. In acoustical science, echo is a distinct, separate, "repeated" sound, heard when a reflected sound arrives at the listener long after the initial direct sound has arrived. In order to be perceived as an echo, the reflected sound path must be at least 55' longer than the direct sound path from source to listener, and the reflected sound must arrive at the listener at least .04 seconds after the direct sound.

Echo is distinctly different from reverberation. While both are the result of sound reflections, "reverberation" is a set of rather early reflections which reinforce and seem to lengthen the duration of tone, while "echo" is a late arriving reflection; a reflection that is so late it is perceived as a second repeated tone.

Just as the mythological mountain nymph was cursed by the jealous goddess, so is "echo" blamed for many acoustical faults. To be sure, a genuine echo can be considered a severe acoustical fault in a worship space, for it disturbs clarity of speech, and interrupts musical blend and rhythmic accuracy. However, throughout my meetings with various congregations and designers, I have heard such problems as "hot spots," "dead spots," lack of clarity, absence of, or excessive reverberation, the improper selection, or placement of speakers, and even poor diction or articulation falsely referred to as "echo." At the same time, I have encountered few worship rooms with genuine and severe echoes. In one instance, the congregation was largely unaware of the echo until I pointed it out.

The many acoustical requirements and potential faults require special attention in the design of a worship space. Important matters such as reverberation period, proper finish materials and the location of sound sources and listeners are all too often forgotten or deferred to the end of the design stage, while much unwarranted attention is devoted to preventing "echo"—which seldom occurs as a genuine problem, and is relatively easy to repair. Unfortunately, the unnecessary "treatment" of imaginary, feared echo often ruins other important acoustical features of the space.

The prevention or elimination of echo can be accomplished by either absorbing sound energy rather than allowing it to reflect, or by redirecting the reflection to a location where it will not be manifest as an echo. Most often the use of absorptive materials such as carpeting or absorbent wall and ceiling tiles is inappropriate in the worship space where useful sound energy must be maintained for all listeners. The re direction of sound energy by proper orientation of hard and reflective surfaces is the preferable method of repairing echo. This maintains useful sound energy in the space and allows it to be reflected to locations where it is needed. The proper design of the worship space involves careful consideration of all acoustical matters. We must, however, beware of overcompensating for imaginary echoes with absorbants, to the detriment of other necessary acoustical reflections.

Scott R. Riedel is President of Scott R. Riedel & Associates, Ltd., an Acoustical and Organ Consulting firm based in Milwaukee, Wisconsin

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