Basics of Sanctuary Acoustics by Tom LeFevre tlefevre@soundandsong.com

 

No matter how great the preacher, how inspiring the message, or how marvelous the music, what the worshiper hears depends on the acoustics of the sanctuary. Many a great message has fallen on ears deprived not by deafness, but poor acoustics and inadequate sound systems. This article is an attempt to communicate a few important principles of how sound behaves in church sanctuaries and other large rooms - and what we might do to improve the results.

Mark 4: 1-2, "... He began to teach by the lakeside. The crowd that gathered round him was so large that he had to get into a boat on the lake, and there he sat with the whole crowd on the beach right down to the water’s edge. And he taught them many things ...." Jesus was not only astute in the behavior of crowds, but he knew how sound traveled better over water than over land, and that the multitude would be able to hear his words. Mr. Blair McNair of Integrated Media Group pointed out to me that sound is refracted, or drawn toward cooler surfaces - skipping like a stone over water in Jesus’ case. Therefore the crowd heard him much better than if he had been on a hillside.

A Few Gentle Principles

of Physics

Without getting too scientific or unnecessarily spooky, it’s useful to consider a few of sound’s physical properties, as they may be applied to the rooms in which we worship. Heavy-duty physics can be like a kidney stone - most of us are content to get through life without it. (Apologies to any offended students of the scientific discipline.)

Radiating

Sound, as we know, is the sensation caused by vibrations, or waves traveling through air. Like the ripples emanating from a pebble dropped in a pool, they radiate outward from their source in a circle, unless channeled in one general direction by a reflective object. In fact, sound is more like light than waves in water, because it radiates uniformly as a sphere in three-dimensional space - not just in a flat surface, as a pool is.

Loudness

We could take a lot of space to try to describe the science of loudness. But each of us knows when something is barely audible, or painfully loud. We must consider that these thresholds can vary considerably from person to person. The worshiper who is deaf or nearly so doesn’t need a louder sound system; they need a person who signs. This is a feature of worship becoming increasingly common.

When discussing loudness, we speak of "decibels" or "tenths of a bel". Decibel is abbreviated as "dB". This is a numerical measure of relative loudness - relative to a very small reference sound level. [It is only fair that we mention for the more technically inclined that this base reference level is a sound intensity of "ten to the minus twelve watts per square meter". This is a very small number: 0.000000000001 watts/m. sq. To put this in perspective, a gentle breeze makes a 10 decibel noise, ten times the reference level; a quiet whisper, 15 dB; an empty church, about 30 dB of ambient noise (1000 times the ref. level); a conversation at 3 foot distance, 65 dB (almost 4 million times the ref. level). The threshold of pain is 130 decibels.] Without a lot of further mumbo-jumbo, decibels are like base-ten logarithms in that a small change in the number of decibels means a large change in the "loudness" a person perceives. This is important to know because all mixing consoles - whether in churches or not - use "faders" or "sliders" graduated in decibels. When one boosts a fader from 0 to +6 decibels, they have increased relative loudness of that channel by four times. An increase from 0 to +10 decibels boosts loudness ten times; 0 to +20 decibels, 100 times; 0 to +30 decibels, 1000 times; and so on. The sound person needs to be gentle accordingly, when raising or lowering sound console faders. The amount of change these sliders cause is exponential, not linear. That is, it happens faster than one would expect from the distance they move. Some mixers help compensate for this, but smoothness and gentleness on the faders is always a virtue.

Dispersion

Sound is like light in that its intensity diminishes by the square of the distance from its source. When one moves twice as far from a loudspeaker, the sound is one-fourth as loud. At three times the distance, loudness is one-eighth. At four times, one sixteenth. This principle alone is crucial to understanding why some people can’t stand the volume in the front pews, and others can scarcely hear in the back rows. It also explains why some seventies-era survivors of rock concerts have lost a lot of their high-end hearing. Rare is the church that doesn’t sometimes lean too far toward the opinion of a "very important member". If that person or persons have a sense of hearing that is not representative of the overall congregation, it can greatly complicate the process of designing and managing sound, as one might imagine.

The principle of "shaping" or directing the pattern of dispersion is the key to speaker placement in sanctuaries and other auditoriums. Just as a megaphone directs a human voice, electronic speakers project a cone-shaped region of sound, which, as mentioned above, diminishes in power by the square of the increase in distance from the speaker. Depending on manufacture, the angle of the cone off center line of aim can vary from about 30 to about 60 degrees. The most common type is 45, therefore projecting a sound cone that is a 90-degree angle. This is an important sound design consideration when placing and aiming speakers so that their "sound dispersion cones" do not overlap more than necessary. Depending on how the loudspeakers are aimed, their overlapping sonic cones produce a "wave-intersecting" effect much like the choppy wake area immediately behind a speedboat - except in three dimensions. To the listener, this "sound chaos region" can have strange and unpredictable spots where some frequencies are too loud, while others are canceled out, and can hardly be heard at all. Some overlap is both inevitable and necessary - especially in "reflected" sound that combines to produce what we call "reverberation", but a goal should be to minimize the originating overlap patterns of "direct" source sound.

The worshiper hears sound that is "direct", and sound that is "reflected". Direct sound comes in a straight line from a loudspeaker or other natural sound source. Reflected sound in a sanctuary typically comes from walls, floor (unless absorbed by carpet and people), ceiling, and "upstage" backdrop. Naturally, the reflected sounds arrive later to the ear, because they travel farther. This delay is about one thousandth of a second per extra foot traveled. A room’s natural reverberation is the net mixed effect of these "later-arriving" reflected sounds which trail off, or "decay" over time. A certain amount of this reverberation effect is necessary to what we think of as a "pleasing" sound. Importantly, different styles of music are associated with different reverberation decay times. We’ll speak more of this shortly.

Frequency and Wavelength

Spoken and musical sound varies from low notes, or pitches, to high ones. This is measured by frequency, or the number of waves or cycles per second. This is also referred to as Hertz (abbreviated "Hz"). The distance between wave peaks is called wavelength. The higher the frequency, the shorter the wavelength; the lower the notes, the longer the wavelength. High notes have short wavelengths (the top note of a piano, 3.1 inches), while very low notes (organ or bass guitar), have wavelengths of many feet. The lowest piano note has a wavelength of 39 feet. Low frequencies with longer wavelengths tend to penetrate longer and farther, are less completely reflected by room surfaces, and are less subject to being absorbed by fabric and other materials. High frequencies, in contrast, do not penetrate as far, are reflected more by room surfaces, and are more completely absorbed by fabric and (especially) people in the room. One of the key sound management devices in most church sound systems is a multi-band frequency equalizer. This device can selectively boost or diminish a range of frequencies to promote better-sounding results in any particular room. Just for reference, the lowest note on a piano is 28 cycles per second (Hz), while the highest is just over 4000 Hz. Human voices range from a low bass of 80 Hz to a high soprano of 1000 Hz. The human ear cannot typically discern frequencies below 20 Hz, or those above 15,000 Hz. Equalizers can be especially useful for "tuning" a room to de-emphasize those frequency bands where troublesome "feedback" tends to occur. Another article will deal with a simple and practical method for adjusting these devices so as to minimize the natural feedback tendency of the sanctuary. This method maximizes the "gain before feedback" of the sound system.

Reflection, Delay, and Reverberation

Key to understanding a room’s natural reverberation are the physics of the speed of sound, and the nature of reflection. Sound travels through air at a speed of about 1100 feet per second. In a small room, it seems instantaneous, but at a distance greater than about forty feet, delay becomes something to consider. Each foot that sound travels from its source represents about one millisecond (thousandth of a second) of delay (actually 0.91ms). Loudspeakers in or under a church balcony fifty feet from a main speaker, for example, should be connected to a digital delay device that can make those faraway speakers produce sound fifty milliseconds later in time than the main sanctuary speakers. This results in sound arriving at the worshipper’s ear at about the same time. Otherwise, a jumbled, echo-type effect occurs. This same technique should be applied to speakers outside the sanctuary - in a narthex, for example.

Sound waves are reflected off surfaces like light from a mirror - at an angle equal to the angle at which it hits. If sound from an overhead speaker hits a sanctuary brick wall at 45 degrees, it is reflected back toward the congregation at a similar angle. This is especially important to consider when designing the overhead space above where a choir sings, for example. A troublesome echo from a balcony front, might be avoided by a slight tilt in the angle of the front balcony surface.

The sum of sound reflections - usually occurring after the listener hears "direct sound" from the source - is the "reverberation" of the room. The natural reverberation time can be approximated by standing close to the center of the room and shouting "Hey" loudly dly (or whatever other monosyllable feels right), and literally timing the decay with a stopwatch. A hand clap or loud burst on the organ also work. Officially, the reverberation threshold is measured as a loudness difference in decibels, at sixty. Practically, it’s the time from when a sharp attack sound occurs, and when its last echoes are no longer audible. A pleasing concert hall might have reverberation that lasts from 1.6 to 1.8 seconds or so (with audience in attendance). A large cathedral, at one extreme, might reverberate from three to four seconds or longer. A "dead" room with lots of carpet, draperies, and porous ceiling tile might have little or no reverberation. Traditional sacred music, with organ, choir, piano, and stringed instruments, benefits from some natural reverberation. Longer held notes and phrases are enriched by the reflections continuing as long as two seconds. Spoken voice and rock music, on the other hand, is not enhanced by long reverberation - since the shorter, staccato notes and spoken syllables tend to overlap and get jumbled in the worshipper’s ear. To lend a little perspective, traditional organ music is enhanced by reverberation times of from two to four seconds. An orchestra, from 1.5 to 2.0 seconds. A choir, from one to two seconds. A pop or rock band, from about one-half to three-quarters of a second. Please note that these times are approximate, and that they will tend to be longer as the volume, or size of the sanctuary increases. Spoken voice is generally best suited by short reverberation situations - as is a rock band. In some sanctuaries that have little or no natural reverberation, it makes sense to install a digital effects unit that can add an effective "hall" synthetic reverb aspect to both music and spoken word for better-sounding sermons and times of Scripture reading. The final measure depends on how the music and spoken word actually sound when the sanctuary is attended. Be sure to get more than one or two opinions.

Regulating Reverberation by Absorption

As most sound system operators have learned, loudness can appear to vary with humidity, air temperature, and attendance in worship. As humidity increases, and air temperature increases, the speed of sound does theoretically increase also. The biggest effect of this is on the tuning of a pipe organ. Practically, temperature and humidity don’t affect the need for more or less amplification as much as does the number of persons attending worship.

Worship attendance is the single biggest factor causing sound system loudness and reverberation to vary. Different materials will absorb sound to a greater or lesser extent. The least absorbing materials (hence the more reverberant spaces) are marble, glass, brick, concrete, stone, and hard lime plaster on lath over wood studs. In such spaces, there tends to be considerable reverberation. Fabric materials such as draperies (cotton or velour), and carpets absorb sound substantially. For this reason, it is not usually desirable to carpet an area upon which a choir performs. Conversely, carpet under an amplified praise band may be very advisable. A seated worshiper wearing a suit absorbs about twice as much sound as an empty upholstered auditorium chair, and about twenty times as much sound as an empty chair without upholstery. So when the sanctuary is packed (as we know around Christmas and Easter), not only is reverberation usually greatly reduced, but significantly more amplification need be employed for sermon and music to be heard by all present. Rehearsals are very different from worship services in this regard. Sound operators need to be aware of this (as church audio consultants are) when diagnosing the acoustics of an empty sanctuary, or estimating amplifier power requirements.

Speaker and Control Booth Placement

The two most common approaches to loudspeaker placement in churches are the "single point source" cluster, and the "symmetrical right and left" main speakers. In the single point source, a cluster of several speakers is hung at or close to a central axis at the front of the sanctuary, usually above the chancel area. Sound radiates from that single source (even though it consists of multiple speakers) throughout the sanctuary. Sometimes there also are additional speakers in and under balconies - often attached to delay devices if the distance is greater than forty feet. Increasingly commonplace are low-frequency boosting "sub-woofers" placed unobtrusively, which can greatly augment the quality of music with low bass parts. Sub-woofers usually boost sound in the 25Hz to 125Hz frequency range. The listener "feels" the sound almost as much as they hear it. The symmetrical approach uses a right and left pair of speaker clusters equally spaced on either side of a center line. This approach is used at Ginghamsburg United Methodist Church near Dayton, Ohio. That installation also uses sub-woofers on both right and left sides, and the resulting sound mix quality is excellent. This symmetrical approach may be more appropriate for a sanctuary space that is wider than it is deep.

Regardless of where speakers may be placed, it is extremely important for the sound control area to be placed in the "mainstream" of sanctuary sound. Since professional mixing consoles and high quality sound reproduction systems are relatively new to many churches, it can be politically difficult to obtain space on the main floor - not under a balcony. The unfortunate result is that the poor person who is faithfully striving to carry out a sound ministry from a corner of the balcony, hears a mix of sound quite different from what most of the church hears. They may alternatively mix using an earphone output of the mixer. But remember that the true test of what the worshiper hears is to listen without earphones. If a church is installing a good sound system, it is extremely important to locate the control console on the main floor, close to the center of the sanctuary, in front of a balcony if one is present. This is not easily achieved, but there are many frustrating stories from those who did not go the distance on this issue.

In summary, sound is a fascinating and critically important aspect of worship in today’s church. It will only grow in importance in the years ahead. Because so many factors are involved, the "acoustic performance" of any given space is very complex. These factors include the size of the space in cubic volume, the shape and configuration of the space, the various absorption characteristics of materials, the source and style of music and speech, the furnishings, the size of congregation, and (not least) the nature and quality of the sound reproduction system. And none of us has yet seen a sound wave. With the help of a good and experienced consultant, and decent quality equipment, a church can move smoothly from the unamplified world into a modern world of greatly improved sound management. Ask around. Take a group to visit churches where you like what you see and hear. Jesus would want us to get His Word and good music out in His name, and have it be pleasing to the ear - and capable of being heard and understood.

Sources:

Buildings for Music: The Architect, the Musician, and the Listener from the Seventeenth Century to the Present Day, Michael Forsyth, The MIT Press, Cambridge, Massachusetts.

The Science of Music, John R. Pierce, Scientific American Books, New York, New York.

Music, Physics, and Engineering, Harry F. Olsen, Dover Publications, Inc., New York, New York.

Conversations with Mr. Blair McNair, Consultant with Integrated Media Group, Byron Center, Michigan.