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KnowHOW: Taming the acoustics

KnowHOW: Taming the acoustics
The geometric placement of sound absorption panels on walls

KnowHOW: Taming the acoustics

Gordon Moore reveals how to get acoustics under control on a budget

On our first visit to the church where I currently serve, the first thing my wife and I noticed was the incredibly poor acoustics. The room was reflective with virtually no intelligibility. It was large with a 9m ceiling, 18m across and nearly 36m deep – note that the width was twice the height and the depth was twice the width. This is almost the perfect formula for an echo chamber. If you clapped your hands, the flutter echoes would last over five seconds. I would see couples arguing – she could hear and he could not, even though they were sitting next to each other. He would be sitting where everything phase cancelled, she would be in a hot spot even though they were sitting side by side. It took considerable concentration to hear the message clearly. After six months we formally joined the church and I asked if I could offer some assistance. No good deed goes unpunished and 25 years later I am still involved with our church’s technology and acoustics.

Installing a panel; note the insulation between mounting strips on wall
Installing a panel; note the insulation between mounting strips on wall

The church had large volume and very hard surfaces – drywall and concrete. The only soft surface in the room was the carpet, which was very thin glue down carpet with no padding. The pews were upholstered but added hundreds of square meters of reflective hard oak.

Acoustical problems are the bane of every sound operator. Many churches are designed by architects who are unschooled in acoustics. Some even deliberately follow the old-school theme of making the space reverberant as though they were designing a 12th century cathedral. Last time I checked, most churches do not conduct services with Gregorian chants anymore. So, how do you identify and resolve acoustical issues?

When sound reflects off a surface, the nature of that surface plays a big factor in the behaviour of the reflection. The worst-case scenario is a perfectly smooth, hard, unbending surface such as glass, a polished hardwood wall or smooth drywall with no texturing. The reflection will be very sharp, clear and retain much of the original energy. As the surface becomes less smooth, more flexible and softer, it has a tendency to absorb the energy of the sound and/or scatter it in a less coherent form. So, if you are having acoustical issues the first step is to determine where the problem originates.

Three acoustical panels home made from cloth and insulation. The drapery also helps quell excessive echoes
Three acoustical panels home made from cloth and insulation. The drapery also helps quell excessive echoes

You can do this with sophisticated software such as Smaart Live or you can do it using a simple recording program such as Audacity and an omni-directional microphone. The process is known as taking an impulse measurement. You need a sharp, loud acoustical source. In the past, some acousticians used a starters pistol (not recommended), but any loud sudden noise will work. I like using two pieces of hard wood slapped together. If you are using Smaart, follow the prescribed procedure for a transform function using a known sound source. Smaart will parse out the impulse response on a live basis. If you are using Audacity, simply place a microphone in a listening position, start recording, then make your loud noise once. One very loud clap. Make a note of how far from the microphone you are at the time.

Review the recording by looking at the wave form in your editing program with the timeline. You will see the initial sound from the clap. Then you will see the reflections in the display. Use the timeline to measure how much time elapsed between the first sound and the reflections. You can use these times to calculate the distance the reflections had to travel. Then simply look at the room and see which surfaces are that far from where the microphone was placed. These are your offending surfaces. Once you know which surfaces are problematic, you can treat them to reduce your problems.

In this image, the reflections are about 400ms later, the time it takes for the sound to travel 60m out and then back at the speed of sound. Therefore, the offending wall is about 60m away
In this image, the reflections are about 400ms later, the time it takes for the sound to travel 60m out and then back at the speed of sound. Therefore, the offending wall is about 60m away

Hard surfaces need to be softened so that the sound that hits them is widely dispersed in a non-cohesive waveform or, even better, absorbed. An absorbent surface is a soft surface. There are many commercially available treatments for surfaces sold by manufacturers that can help cover these surfaces with aesthetically attractive solutions. If your church has the budget available, I strongly recommend them. They will meet all fire-retardant standards, they will be proven and designed for years of use. But many churches, such as mine, simply do not have the resources financially for these beautiful solutions.

For my church we used a combination of commercially available solutions and some homemade ones as well.

To tame the large room problems, we used a wooden panel called Tectum which is a wood shaving product sprayed with a stiff mineral silicate covering. Highly fire retardant, Tectum was designed for use in gymnasiums which are typically very poor acoustically. We opted for the 4ft x 8 ft panels. To tame the echoes off our walls we needed to add over 223m2 to the walls. Tectum is good at dispersing the higher frequencies – the straw-like nature of its surface disperses the high frequencies very well. But we wanted a wider range of absorption, so we mounted the panels on the wall on furring strips – 1cm x 1cm wooden strips that elevated the panels away from the wall. Before we screwed the panels in place, we placed batts of fibreglass insulation (without barriers) between the strips, fuzzy side out. (Make certain that you wear dust masks, eye protection and wear protective clothing while hanging insulation.) This insulation absorbs the sound energy extending the low-frequency absorption. When the sound waves hit the insulation, the fibreglass strands convert the sound waves into mechanical kinetic motion.

This treatment worked well for the high areas of the walls, which caused the majority of the reflections. Up high (over 2.5m) and out of reach, the panels look good and we arranged them in an attractive geometric pattern. We also placed a panel behind the sound booth to stop reflections from the back wall. The RT60 (reverberation time for a 60dB drop) for the room dropped from 5.5s down to 1.75s – still lively and sounds great for music.

A different pattern was used for the back wall to help suppress the long axis echoes
A different pattern was used for the back wall to help suppress the long axis echoes

But we also struggled with reflections from the wall behind the stage where the praise team and the choir stood. For that section we need treatment lower down where people would interact with them. The treatment needed to be inconspicuous, providing no visual distraction. Commercially available panels were beyond the budget. So, we placed a wooden frame in each section using 40mm X 90mm framing wood. The frames covered the reflective wall sections. We again placed insulation (R19) – fuzzy side out between the frames – and then covered the entire frame with black cotton-based embroidery cloth. The embroidery cloth is a stiff, open weave, perfect for allowing sound to pass. The cloth is treated with fire retardant to safeguard the congregation. Always make sure your insulative or sound-proofing materials are fire retardant.

These panels effectively eliminated the annoying reflections and made the clarity of sound coming from the stage sharper and better defined.

An alternative design uses a similar framing but covered on top of the fabric with a criss-cross pattern of fine hardwood strips with varying gaps in between. It looks like a wooden fence with random gaps between the boards. By randomising the gaps, the absorption of different frequencies is addressed and the diffusion of any reflected sound more evenly distributed.

How else can you tame unwanted reflections? Remember, any soft absorbent surfaces help. Does your church have a banner or sewing committee? If so, have them make large attractive banners that can be hung on walls or from ceilings. Rugs on the floor can help soak up unwanted sound, carpeting with thick comfortable padding is best. Quilted banners hanging across the ceiling will suppress flutter echoes between parallel walls.

This large wall hanging made by the quilting group suppresses echoes in the narthex area
This large wall hanging made by the quilting group suppresses echoes in the narthex area

Set designs can help as well. If you have a black box stage – a common trend among many churches today – the stage design can help with acoustics. Avoid hard smooth surfaces and materials. A cloth panel has the same visual impact as a smooth, hard Masonite panel but weighs less and can help tamper reflections. If it is quilted with absorbent material, it will be even more effective. Heavy curtains or rough surfaces (such as rough-cut wood) can help as well.

A strong word about safety: Stages are often built on very minimal budgets and without consideration about the flammability of the materials. Sets will incorporate lighting fixtures which can generate a lot of heat. Make absolutely certain that:

  • All stage materials are flame retardant or treated with a flame retardant. Cost effective flame retardants can be purchased in liquid form and sprayed on with simple pump sprayers like those found in garden supply houses. Wool is naturally flame resistant – modern synthetics can be highly flammable. Treat everything. Be safe.
  • All power cable extensions are power strips with built-in safety fuses. Whenever I find a simple extension cord without a safety fuse, I immediately cut the cord into small pieces and throw it away.
  • All set materials are situated far enough from lighting fixtures to avoid contact and the possibility of igniting the material.

Poor acoustics are the sound operator’s enemy in delivering a great worship experience. With some hard work, simple materials and careful analysis, you can tame the echoes and help your congregation hear the Word clearly, joyfully and worshipfully.

Be Blessed and mix well.

Gordon Moore is the director of technical ministries at Rio Rancho United Methodist Church in Rio Rancho, New Mexico and president of Lectrosonics Inc.

This article was first published in the November-December 2019 edition of Worship AVL. Subscribe at www.proavl-central.com/subscribe/worship



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