Sound Diffusion vs. Absorption in Worship Environments

There are many examples of diffusing architectural elements in this synagogue.

Cathedrals, mosques, synagogues and temples are often decorated with an abundance of architectural details (deep coffers, arches, columns, sculptures, intricate engravings etc.). These features are not only beautiful to look at, but also serve the vital acoustic purpose of sound diffusion. Large, uninterrupted spans of hard, flat surfaces reflect sound in a singular, specular wave, which creates discrete echoes and comb filtering (In acoustics, comb filtering is when a delayed reflection interferes with, and distorts the original sound wave). These conditions can contribute to an acoustically uncomfortable environment, in which speech is difficult to understand, and music can be hard to perform and enjoy. Irregular surfaces, on the other hand, scatter these reflections, minimizing comb filtering and distracting echoes. In a “diffused” worship environment, speech is intelligible, music is clear and warm, and there is a sense of envelopment which greatly enhances the congregants’ worship experience.

Absorptive treatment can also be used to control echoes and harmful reflections. Instead of redistributing reflections, these “fluffy” materials (drapery, padded pews, acoustic panels etc.) reduce the overall sound energy from the reflections in the room. However, using to too much sound absorption in a room can often make a space sound ‘dry’ or ‘dead’. Determining whether your space needs absorption, diffusion, or a combination of both is dependent upon the acoustic properties of the space, as well as the type of worship service being conducted.

Acoustic Properties of the Worship Space: Reverberation, a principle acoustic factor, is the sound energy that remains in a listening environment as a result of lingering reflections. The dimensions, construction materials, and furnishings of a given worship space determine its reverberation time (RT or RT60). Large halls with reflective materials (glass, wood, concrete) have longer reverb times, while small rooms with absorptive materials (drop acoustic ceiling, carpet, curtains etc.) will have shorter reverb times. Incorporating sound absorptive materials, Such as fabric wrapped acoustic wall panels, is often the best way to reduce the overall reverberation in a room to a suitable level. However, the target reverb time also depends on the nature of the worship service being conducted in a particular space.

Type of Worship Service: Ideal reverb times for worship environments vary widely.  Non-musical, spoken-word worship requires a very short reverb time (.5-.8s range), ensuring that speech is intelligible. At the other end of the spectrum, cathedrals can tolerate an extremely long reverb time (2s and above) due to the traditional nature of their liturgy. Choir, organ and plainchant worship will actually benefit from longer reverb times that create a sense of ambience and spaciousness by sustaining musical notes. These spaces will often lack a sound system, and instead utilize the hard surfaces to propagate sound throughout the room.

Traditional worship may be enhanced by long reverb times, but contemporary worship requires a significantly shorter reverb time. In these environments, drums, guitars, bass and other amplified instruments are critical to the high intensity worship experience, but have far different acoustical needs compared to the choir and organ in a more traditional service. Contemporary “high impact” churches require a reverb time in the .8-1.3s range to ensure that the music won’t become too “muddy” and indistinct. Contemporary churches must also be more cognizant of late specular reflections (slap echoes) which can inhibit the timing of musicians and contribute to poor music clarity.

Let’s take a look a few common scenarios when it comes to treating traditional and contemporary worship spaces.

While including a mix of absorbing Sonora® Panels and diffusing Double Duty barrels, many people overlook the contribution of the padded chairs and carpet to the sound of this space.

Scenario 1: Conversion from Traditional to Contemporary worship

A growing contemporary church moves into a larger, traditional sanctuary and is confronted by a raucous acoustic environment during their first rehearsal… This “live” space was perfect for traditional music, but is not conducive to a “high impact” contemporary worship service. To reduce the excessive reverberation and distracting echoes, sound absorption should be added to the rear wall (opposite stage) and side walls. Also, if using on-stage monitors, the stage walls should be treated to manage stage volume.  Spot diffusive treatment that provides low-frequency absorption would also be beneficial. A good choice for this would be traditional ‘barrel’ diffusers. These are one of the oldest tried and true solutions for controlling bass issues in a performance space. Also, since these units function as both absorbers and diffusers, you get the benefits of both.

Scenario 2: Mix of Traditional and Contemporary worship services.

A worship facility decides to offer a contemporary service in addition to their traditional services… As more absorption is introduced to cut down on distracting reflections, we want to retain the envelopment and spaciousness which benefits congregational singing and traditional worship music. Sound diffusive treatment would be a great way to control echoes and specular reflections, while keeping the energy in the space. A mix of absorption and diffusion is usually best. Multipurpose spaces can also benefit from variable acoustic treatment which allows the room to “adapt” to each service, but that is a subject for another article.

Scenario 3: Poor Music Clarity in Traditional Worship space

A traditional worship space renovates their facility by adding thicker carpet, padded pews and a drop acoustic ceiling… All of a sudden, their once lively space feels “flat” and dull. Acoustic instruments are more anemic, less distinguishable and choirs have a difficult time blending and tuning.  To “liven up” the space, replace sound absorptive materials with diffusers.  For example, replace 1/3rd of the acoustic ceiling tiles with a combination of gypsum tiles and lay-in diffusers. These days, there are wide variety mid-range quadratic sound diffusers available for drop tile ceiling grids, as well as the more traditional barrel and pyramidal diffusers.  

While this space is more lively, the slap echoes from the back wall are controlled with a mix of diffusers and absorbers.

Sound diffusion can often seem a little mysterious compared to sound absorption. This is at least partly because sound diffusion is a more complex and multi-dimensional phenomenon compared to the more easily quantifiable sound absorption. However, sound diffusion is often times the missing piece of an acoustic puzzle: its benefits can help a bad room to sound good, or a good room to sound great!

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