The reverberation period is the number of seconds that sound will linger in a given space. Specifically, it is the amount of time in seconds that sound energy will take to drop 60 decibels after the source has ceased producing a tone.
The science of sound has been a topic that I’ve been interested in for a while, starting with my work in the 1980’s developing a performance space with Hunger Artists Ensemble Theatre. We ran into many problems with acoustics and sound separation. Eventually, we had to cancel the project because we couldn’t solve the sound problems. Good sound is important, in theatres and churches.
Consultant Scott R. Riedel authored the pamphlet Acoustics in the Worship Space (AITWS), which is part of the Church Music Pamphlet Series edited by Carol Schalk. AITWS is a tight, technical document which addresses many of the acoustical issues which need to be considered in building or renovating a church.
One of those issues is reverberation, or reverb. Riedel says,
In rooms to be used for liturgical worship, where speech, corporate spoken response, corporate singing, and liturgical choral and organ music are the primary sound sources, a reverberation period near two seconds at mid-range frequencies is desired.
My copy of AITWS arrived last week, and I’ve been studying it in hopes of learning more about the science of acoustics. AITWS points out the different effects that different surfaces have on sound. When a sound wave hits a surface, the energy will be either reflected, absorbed or transmitted. The material and geometry of the surface will determine what happens to the sound. The geometry part is complex (especially for a liberal arts major who struggled with trigonometry in college) but the material effects are pretty straightforward. Reidel offers this chart, which is straightforward enough to use without calculus.
Sample List: Percent of Sound Energy Absorbed by Materials (per square foot of material) (@ 500 Hz.)
Unglazed Brick | 3% |
Glazed Brick | 2% |
Unpainted Concrete Block | 31% |
Painted Concrete Block | 6% |
Drywall | 5% |
Marble | 1% |
Plaster | 3% |
Plywood Panel | 17% |
Plate Glass | 4% |
Drapery | 55% |
Acoustical Tile | 70% |
Terrazzo | 1% |
Glazed Tile | 1% |
Carpeting (average) | 60% |
Wood Pew, Occupied | 75% |
Wood Pew, Unoccupied | 39% |
Padded Pew, Occupied | 96% |
Padded Pew, Unoccupied | 88% |
Here’s the same chart sorted from lowest absorbance to highest.
Marble | 1% |
Terrazzo | 1% |
Glazed Tile | 1% |
Glazed Brick | 2% |
Unglazed Brick | 3% |
Plaster | 3% |
Plate Glass | 4% |
Drywall | 5% |
Painted Concrete Block | 6% |
Plywood Panel | 17% |
Unpainted Concrete Block | 31% |
Wood Pew, Unoccupied | 39% |
Drapery | 55% |
Carpeting (average) | 60% |
Acoustical Tile | 70% |
Wood Pew, Occupied | 75% |
Padded Pew, Unoccupied | 88% |
Padded Pew, Occupied | 96% |
What these charts mean is that we’ll need to consider using a variety of materials to improve the sound quality. The numbers point out why the sound in the Overflow (plaster, drywall, glass, painted concrete block) is so bad. They also show that every square foot of fabric we put in the church will require us to add more power to the sound system amplifier. These are all trade-offs that we’ll have to address.
Sound and acoustical issues will impact one of our primary goals – Proper Accommodation of the Music Ministry. Fortunately, we have planned to use the services of an acoustical consultant to help us sort out these issues.
Queen of Peace in Aurora and Our Lady of Loreto both have high ceilings, stone floors and and open feeling (1 has about 1/2 in the round seating and the other has a wide (rather than deep) distribution of pews without being “in the round.” I think the sound quality in Our Lady of Loreto is much better than Queen of Peace. I’m sure our acoustic engineers could give us some insight as to why (I think the committee and architect might want to visit these churches).
I should qualify my comment by stating I do not want an “open feeling” or church in the round. However there are some features present in those newer churches that we are considering such as the stone and marble, that affect the sound quality.