Built-In Sound
The role of acoustics in preliminary sanctuary design

By Michael Jarzabkowski

Your church has been discussing the matter for some time. Responsible and valid reasons have been presented and the decision has finally been made--you will build a new worship facility. The congregation is getting excited, committees are forming, budgets are being discussed and a fundraising program is underway. Church members familiar with the building industry are beginning to debate land-use codes, architectural services, design/build options and construction methods. Other members with equally important voices are talking about ideal property locations, ones that enhance their vision of a towering atrium and boldly landscaped gardens. Meanwhile, older churchgoers are thinking of comfortable, fully padded seats with individual armrests and maybe even footrests. Other members have the children in mind and hope their new church will feature bright, open play spaces, proper safety features, planned security measures and a décor the youngest members will like.

All of these are valid and important design considerations that need to be addressed during concept formation of the new facility. However, this article focuses on an even more fundamental--though often overlooked--element: acoustics.

When a building committee begins planning a new facility, the quality of the listening environment should be given high priority. Its scientific investigation should begin during the preliminary design stage before plans are committed to blueprints. At the same time seating capacity is being decided, acoustic design of the auditorium should begin taking a lead position in determining the layout and shape of the building. Granted, churchgoers are most often attuned to the visual aspects of a building's design. Even so, true stewardship dictates that basic functionality takes precedence. After all, church auditoriums are primarily places for people to gather and hear the gospel. If that message cannot be heard clearly by every person in the room, the space could be considered a design failure.

This is not a do-it-yourself article on church acoustics. After all, one would not expect a how-to article for calculating the acceptable deflection of a pre-stressed concrete slab at a particular superimposed loading. Acoustics is an engineering science with results that can only be predicted through vigorous mathematical computation and investigation. As a skill, it requires experience and the development of reliable intuition of one of the most complex natural sciences known to modern physics. Rather, I hope to foster an appreciation and basic understanding of acoustics so that intelligent questions can be raised in the early planning stages of your new sanctuary. These are the questions that ensure the completed project is a success, not a poor compromise characterized by remedial treatment.

Very few architects are equipped with the skills to undertake even basic acoustic design, so it is usually overlooked until late in the project, or not addressed at all. In some cases, the acoustic consultant is asked to look at the completed plans and suggest some remedial modifications. An even worse and more common scenario is to hold off on acoustic consultation until after the project is completed and its design flaws have become all too evident. Ideally, an acoustic consultant is engaged at the same time as the architect, saving time and money. Here are some basic acoustic parameters that need to be discussed during the initial preliminary design.

Determine area

The basic floor area of an auditorium can be calculated by multiplying the intended number of seats by about eight square feet (the space typically required for each seated person), then adding aisle space and stage area. Once the basic required floor area has been calculated, the shape and layout of the auditorium should be determined by acoustic requirements and calculation of sight lines. Of course, there may be some architectural and engineering controls caused by site constraints and proposed construction methods, but these need to be discussed along with the acoustic parameters.

Auditorium shape

Some of the most basic auditorium design parameters have a significant effect on the acoustic environment, which is why they must be discussed in the earliest design stage. The ratio of the length to width for a typical auditorium should be between 1.2 and 1.7. Even more important is the ratio of auditorium height to width, which should be between 0.4 and 0.7. If the ceiling is too low, it restricts stage sound from reaching the people at the rear of the room. If too high, sound reflected from the ceiling arrives much later than the initial direct sound from the stage. Actual ratios should be an acoustically informed decision based on auditorium dimensions, shape, layout and internal angles.

One example of an acoustically sound auditorium is the Concertgebouw in Amsterdam, with a length to width to height ratio of 1.5:1:0.63. Built in 1888, critics still consider it one of the best concert halls in the world. Though not a church auditorium, the acoustic principles still apply. The ceiling in the 2,200-seat auditorium is 58 feet high. This height requirement is often difficult to explain to church building committees because many would rather look at floor area as opposed to interior volume. They often view auditorium volume as unnecessary; plus, it costs money better spent on something people can see--a visual feature. To put this issue into perspective, I often ask committees an anecdotal question: If a blind and deaf person both attended your service on Sunday, which of them would leave with a better understanding of what the church is about?

Auditorium geometry

Apart from dimensional ratios, the actual geometry of the room also needs to be considered in the preliminary design stage. Numerous shapes have been used in auditorium design, from the traditional cruciform to rectangles, squares, circles, fans, pentagons, hexagons and multitudes of other polygons. Of these, the most solid choices are fans, rectangles and modified polygons. Square is acceptable if the auditorium is large enough and cruciform and round shapes are the least effective for transmitting sound. After all, the cruciform is actually four rooms joined together in the form of a cross, so sound from each section affects listeners in other sections. The problem with round or partially round rooms is that the walls reflect the sound waves, focusing them on a particular point. This is similar to the way a semicircular reflector in a flashlight focuses light rays into a narrow beam. At the beginning, the committee must resist the temptation to depart from acoustically tried and tested shapes in search of something unique that runs the risk of favoring form over function.

After dimension ratios and shape, the next most important acoustic parameter to consider is the relationships between the auditorium's internal angles. The angles of the walls, floors, balconies and ceilings greatly affect how sound from the stage is reflected into other areas that may be receiving less direct sound. The acoustically ideal room is one in which all seats receive the same sound level and frequency spectrum. This is never wholly possible due to the attenuation of sound with distance, but by carefully modeling the internal angles of the auditorium, the reflected sound can be "aimed" at those areas that need it and kept away from those that don't.

These calculations used to be done by building a plywood one-tenth scale model of the internal shape of the auditorium and using mirrors and light rays to see where reflected sound would concentrate. Modifications were made by adjusting wall and ceiling profiles and testing again. Today, an accurate 3-D computer model of the auditorium is constructed and ray tracing programs are run, showing the amount of direct and reflected sound for every seat in the auditorium. By adjusting interior angles, wall and ceiling materials, the actual level and concentrations of reflected sound can be determined, as well as the frequency spectrum of that sound.

Reverberation time

The physical characteristics of reflected sound bring us to another important acoustic parameter: reverberation time. Reverberation time in an auditorium is the length of time it takes for the reflections from an impulsive sound--like a sharp handclap--to die away or decay a certain amount from its initial level. The physical expression of this parameter is called RT60. Apart from being a measurable quantity in existing auditoriums, RT60 can also be calculated with relative accuracy with various empirical formulae and computer modeling prior to construction.

RT60 is controlled by the amount of absorptive material in an auditorium. A simple example of a room with lots of absorptive material is a lounge room with plush upholstered seats, thick carpets and heavy curtains. Its opposite would be a large, tiled bathroom or changing room in a sports facility with dressing mirrors and porcelain bathroom fixtures. A loud shout in each of these rooms quickly teaches one about the basic concept of RT60 and the effect (or lack of) absorptive materials. It should be noted that every material and item used in construction has an absorption coefficient. Pews, people and even windows absorb some sound, so they must be taken into account during RT60 computations for an auditorium.

Not only does every material have an absorption coefficient, the amount of absorption varies with the frequency of sound. Carpets, drapes and curtains absorb mostly high frequencies while wood, gypsum panels and thin plaster on furring strips absorbs low frequencies. The amount of absorption throughout the frequency range should be even. In practice, lower frequencies usually require the greatest control because they are less likely to be absorbed by padded seats and carpet. Even concrete blocks absorb some sound, so the precisely calculated use of a variety of general building materials can result in excellent acoustics. Contrary to belief, acoustic design does not mean adding padding on the back wall after a project is completed!

The optimum RT60 for an auditorium is determinable by both the room's volume and its intended use. For example, an auditorium to be used primarily for speech should have a shorter RT60 than an equivalent volume room used mainly for music. Even music style and instrumentation must be taken into account when determining the optimum RT60 for an auditorium. Generally, contemporary music requires a shorter RT60 value than orchestral music; traditional organ with choir requires a longer value; and acappella Gregorian chant requires one of the longest. A lot of research is required to define optimum RT60s for church auditoriums because these spaces have to be useful for a number of different functions including speech, music, drama and audience participation during worship.

When discussing optimum RT60s, most texts on acoustics divide churches into three groups: Roman Catholic, High and Low Church Protestants. This is an antiquated concept carried over from an era when the liturgies of each particular denomination were more uniform and predictable. Such generalizations no longer ring true, which is why I propose that evangelical churches--which often have noticeable acoustic problems due to their lively styles of worship--do not fit into either of these categories.

Regardless of denomination, determining the optimum RT60 for any modern church requires a detailed study of that particular congregation's current and future trends. At this point, the ministerial staff and building committee must sit down and clearly define their ministry style and future objectives. Once this is done, the acoustic consultant can provide design specifications that best meet the church's needs. Areas to be considered include liturgy, forms and varieties of congregational worship, different media used in presentations--choir and orchestra repertoire, drama, plays and musicals--and the use of multimedia technology.

Many other parameters affect the listening environment, like noise intrusion from outside (roadways, airports and railways) and from other rooms within the building as well as noise from the mechanical equipment. Even rain on the roof can be an intrusion, and persistent ventilation system noise is one of the biggest problems in speech intelligibility. Still, surprisingly few churches consult an acoustics expert to help solve their noise problems. An acoustic consultant can devise planning concepts that minimize unwanted noise and also provide design measures to stop it wherever necessary.

Finally, we come to the sound reinforcement system itself. The sound system is so closely linked to the acoustics of the auditorium that its design should could be handled by the acoustic consultant. It should be noted here that the installation of a sound system cannot fix inherent acoustic problems in an auditorium. So often, churches are told that specialized design is unnecessary because the sound system will "fix" any problems with the acoustics. This is inaccurate--some churches go through three or four new sound systems before they realize that acoustics are to blame instead. A clear orator in an acoustically sound auditorium should be able to address one thousand people easily without the aid of a sound system. Yet while a sound system does not make the acoustics of an auditorium better or worse, it can certainly amplify any existing acoustic problems.

Building committees should rely on expert acoustic advice in the beginning to avoid an architectural monstrosity. It is usually during the preliminary design stage when a project runs off-track. The final result: an auditorium in which the congregation has to strain to hear the message--or worse, cover their ears in self-defense. Often the acoustic consultant is contacted when construction is almost or completely finished and then asked to help fix the sound problems. By this stage, many of the controlling parameters are literally set in stone, and even the best treatments will yield mediocre results.

Michael Jarzabkowski is the senior consultant of Michael Jarzabkowski & Associates in New Haven, Conn. He can be reached via e-mail at mjarzo@gte.net or by calling (203) 436-3513. Visit the company's Web site at http://home1.gte.net/mjarzo.