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A concrete cathedral is designed to achieve acoustic excellence

Steerable speakers and acoustic plaster help overcome sound challenges for one Houston church.

By Brian Dupre

There are many challenges to achieving great acoustics in a sanctuary, and the 1,820-seat, 32,000-square-foot sanctuary of the Co-Cathedral of the Sacred Heart in Houston, TX was certainly no exception. Designers and contractors needed to minimize indoor and outdoor noise ricocheting through the massive concrete and marble structure. At the same time, they had to optimize the liturgical spoken word which is a key feature of Roman Catholic worship.

A few major obstacles stood in the way of the church’s goal of great acoustics. The 8,000-cubic-feet of concrete poured to form the one-foot thick and 72-foot high walls addressed the problem of urban noise in downtown Houston. However, this treatment created the problem of echo from even the smallest sound inside. Of course the marble floors and steel and concrete roof perpendicular to the walls kept the unwanted sounds alive, competing with desired sounds. And the high walls and concave-shaped ceilings — space which could contain a 3-story building — added to the problem by creating a cavernous effect.

Building material doesn’t get much worse at affecting acoustics than when solid thick concrete and marble is used. The Co-Cathedral’s walls were cast-in-place concrete with columns to a height of 80-feet. Concrete is like a buffer, keeping out harsh noise but taking in residual sounds and causing distracting reverberation. Car horns, motorcycles with loud mufflers and emergency sirens could all transform a desired quiet into a cornucopia of sounds.

Minimize unwanted sounds

The team from the Linbeck Group realized early on that they would have to minimize unwanted sound such as outside vehicles and optimize the sound of a priest voicing scripture, the homily and prayer throughout the mass.

The sanctuary required precise planning and the expert help of acousticians as subcontractors. Turning the concrete shell into a place to meet God demanded excellent sound quality. Interior wall surfaces were a big priority. Pyrok Acoustement sound-sensitive plaster was used on the walls and the dome. The 86,000-square-feet of plaster applied to the walls and ceiling dampened sound and reduced echo and vibration. The advanced make-up of the plaster provided an excellent finish with the high quality sound absorption needed for such a large space.

Many churches want stage production-quality AV in their sanctuaries. From tiered seating on the sanctuary floor, worshipers attending contemporary services enjoy congregational singing led by praise bands and musicians. The congregants hear and see all aspects of the service on bigger-than-life images broadcast via closed-circuit videos on big screens on the front walls. The AV systems augment a bold worship experience preferred by many church leaders.

Some churches like the Co-Cathedral cherish an atmosphere of solemn dignity during worship services. Liturgical worship with the spoken word and organ and choral music are standard for traditional Catholic masses following the Latin rite.

Traditional arrangements

The priest, or in the case of this archdiocese, the bishop is the predominant figure leading in spoken greetings, prayers, scripture readings, responses, chants, singing, the communion and the homily. Worshipers in traditional seating arrangements of pews from front to back of a long flat sanctuary observe the holy procession, communion, ceremonial scripture reading and other elements of the mass.

Linbeck and its subcontractors chose AXYS Intellivox from Duran Audio, The Netherlands. Designed specifically for superior voice quality output, the technology is ideal for voice in a flat sanctuary. This cutting-edge technology uses computer-controlled precision to optimize the width and height of the sounds waves.

The real advantage was that the loudspeakers were digitally “steerable.” The speakers could be aimed at the ears of the people seated in the pews, creating superlative acoustics for such a dauntingly wide-opened space. Many months before the Co-Cathedral’s dedication in April 2008, technicians stepped inside the unfinished sanctuary to get the geometrical dimensions of the invisible plane making up the location of the ears of the worshipers. Then after installation they returned to aim the speakers with technical exactness.

At 18-feet tall and four-inches wide, the dimensions of the speakers are also unique. This extreme verticality made aiming at both the front row and the back row possible. A total of six speakers were installed. All were surface-mounted on concrete with custom color-matched panels blending seamlessly into the walls.

Acoustics were planned

Long before the sound system for such a sensitive acoustical challenge was determined, the designers and builders had to take into consideration the AV needs. Typically, building the structure came well before decisions about the audio system. The use of concrete walls and cement plaster application on the concrete meant that chase spaces and conduits for mechanical systems and electrical wiring had to be placed inside the walls before pouring cement. To meet the demand, every single conduit and speaker space was drawn into the plan and elevation views early on.

The Co-Cathedral’s designers and contractors began discussing wiring two years before the technical equipment was installed. This required plan management and collaboration with technical subcontractors and product manufacturers. At various points along the way the acousticians were told to stop holding out for the next new innovation and finalize the decision on what product was going inside.

This advanced planning did aid the inclusion of broadcast links to the walls’ exteriors. Video production capabilities were also included for closed-circuit communication between the organist and choir director.

Despite acoustical threats inside and out, the sacred intimacy felt in the Co-Cathedral by worshipers today is due in no small part to the exceptional acoustics built into the structure.

Brian Dupre is a project engineer with Linbeck, Houston, TX. [linbeck.com]

Photos courtesy of Linbeck Group.
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