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72 degree principle OF ACOUSTICS

Churches all over the world have plans for growth, expanded influence, increased connection in ministry, and to make a greater impact in people’s lives. There can be huge obstacles on the journey to achieving these goals that can elude even the best of planners and thinkers.

A major hurdle that churches encounter is how to achieve the desired sound so that the listener can enjoy worship and hear the spoken word. Sometimes this barrier is a combination of room acoustics and the right sound system.

So what is the perfect combination? Often, the main focus is put on the sound system, so that the problems involving room acoustics are forgotten. This isn’t as easy a problem to fix as simply purchasing a replacement sound system. It’s what I like to call the “72 degree” principle of acoustics.

There is no doubt that people may feel differently in varying environments. My wife, for example, is “freezing” almost everywhere we go; at the same time I can be burning up. It is seemingly 40 degrees where she’s standing and 85 degrees where I’m standing just a foot away. Can this be possible? The answer is obviously no. My wife is not really “freezing,” at least not to death. She just feels colder or hotter at that particular moment. The most comfortable temperature for the majority of people is 72 degrees; it seems to be the middle ground where the too hot or too cold people can agree. The key is to find the “72 degrees” for your room acoustically.

Characteristics of Your Room
Everyone has their opinions; it’s too loud, it’s too quiet, it’s too live, it’s too dead, this place has perfect acoustics, what we need is a new system, etc. So what is the best temperature for your room acoustically?

Anything that happens in a service, whether it is the spoken word, special video presentation, high-energy worship or quiet worship, has to operate within the physical parameters of the room. The room will always “color” and have an impact on the overall “feel” of the sound. It’s important to realize that a room’s acoustics are either increasing or decreasing the intelligibility (what is actually heard) of the sound.

There are several factors that cause most of the acoustical problems in a room, but the two most important are echoes/reflections and excessive reverberation.

An echo is a distinct reflection of sound, arriving at the listener after the direct sound. There are several different types of echoes and reflections (e.g. flutter echo between two parallel surfaces, slap echo off a back wall, focused echo from a concave curved wall area). Reflections and echoes can be problematic in smaller rooms, but the larger the room, the longer sound travels, and the more negatively it can impact the listening experience.

Reverberation can easily be defined as the amount of time it takes for sound energy to dissipate (bouncing around a room before being absorbed or running out of steam). Reverberation is measured by the time it takes sound to decay 60dB. This is referred to as the decay time of a room or the RT-60. These tests can be professionally done to measure each frequency from low to high, providing information on how long the room is reverberating at each frequency (in some cases an overall RT average can be very helpful). Reverberation time is a function of the volume of the room and the surface areas in the room. The larger the facility, the longer its starting reverberation time will be.

Sabine There, Done That
W.C. Sabine, physicist and pioneer of architectural acoustics, is noted for discovering the relationship between the volume of a room, the amount of absorptive material within it, and the role the absorptive material played with the remaining energy in the room, which he called the Reverberation Time (RT). The equation that he discovered is still widely used today.

RT60 = (.049V)

V is the volume of the room (cubic feet) and A is the total absorption within the enclosure. A is the sum of the surface area (in ft squared) times the absorption coefficient of each material used within the enclosure.

The absorption coefficient, or Noise Reduction Coefficient (NRC), of any material, as originally defined by Sabine, is the ratio of the sound absorbed by that material to that absorbed by an equivalent area of open window. Thus, a perfectly absorbent material would have an absorption coefficient of 1. An absorption unit of 1 sabine (named after W.C. Sabine) represents a surface capable of absorbing sound at the same rate as 1-ft squared of open window. There are some acoustical products that have been invented since that have achieved over this 1 sabine per square foot standard because of the product’s ability to both absorb and diffuse the sound energy. The product is not only a 100% absorptive “open window”, but may also contain geometric shapes that diffuse and diffract sound waves. With a product that may have an NRC of 1.60, you basically don’t have to use as much material on the walls, thus saving you money.

This RT measurement is very helpful in setting the acoustical temperature of your room, as there are various types of calculations and formulas acousticians use to aid in solutions for the room.

There are even intelligibility rating formulas. These show that the higher the reverb time, the less intelligible the room is, or in other words, if the room has a high RT then you may only be able to understand 65% of the information. This can be a devastating impact on your presentation if attendees can only physically understand 65% of what you are trying to communicate. The answer to this is the lower the reverberation time, the better people can hear and understand.

So what is the desired reverberation time for your room? Consider the room’s uses while taking steps to make sure the room does not have any major echoes or reflections. Most churches need a reverberation time below two seconds (1.5 or lower is ideal). The lower the better! Conference rooms and classrooms need a reverberation time below one second.

It’s important to note that different products reduce reverberation more effectively at different frequencies. Some acoustical absorbers are better at absorbing high frequency energy while others are better at absorbing low frequency energy. Low frequency energy, to a point, is best absorbed by thickness and mass, and it really takes both. Carpet for instance, has a lot of weight or mass, but it is too thin to absorb low frequency well. Carved foam products, on the other hand, can be cut quite thick and still not produce a good amount of low frequency absorption because they have very little mass. Because they provide both thickness and mass, products of rock wool or fiberglass high-binder/high-density design in 3” thickness or greater work best for low frequency absorption.

Using Diffusers
Diffusers are a great solution for a room that has bad echo problems but already has a low reverberation time. Whether it is natural decay or after the proposed amount of absorption treatment, there still could be some open reflective wall areas. A sound diffuser is anything that scatters sound so that it turns echo into reverberation. Large, smooth, flat areas echo with opposite large, smooth, flat areas.

Removing these flat areas in the design process will help eliminate echo. There are an amazing number of diffusers on the market today, but consider this one fact about diffusion of sound; it takes an object roughly the size of a waveform to diffuse that waveform (10,000 Hz is a waveform of about 1 inch, 1000 Hz is a waveform of about 1 foot, and 100 Hz is a waveform of about 10 feet). Placing 4” bumps on the wall will not diffuse all frequencies. It is important to develop diffusers with over-all large bumps with complex hills and valleys of various sizes and shapes to catch everything from the fundamental size or frequency all the way down in size (or up in frequency, which ever way you prefer to look at it) to diffuse all the frequencies in as broad a range, and in as non-selective a way as possible. Modern “diffsorbers” (diffusing and absorbing in the same unit), natural clutter, room furnishings, etc. actually provide an ample amount of diffusion to break up most echoes.

Acoustics as A Priority
Movie theaters where multi-million dollar films are shown, corporate executive conference rooms, studios where superstar artists record, and auditoriums in Las Vegas all share one thing in common, the acoustical feel of the room was one of the top priorities in the facility. The multi-million dollar movie wouldn’t have that great of an impact if the sound of the car blowing up remained for eight seconds into the next quiet dialogue scene. The business deal will not go through if the presentation is misunderstood due to resonance problems. The same fervor needs to be applied to the acoustical make up of churches. These houses of worship are proclaiming the Word of God through speaking and through music and video presentation. The key to making the most out of a room is that it has to be at the correct acoustical temperature. When trying to communicate a message clearly, the room has to be set to a low reverb time and must not have any echoes.

However, the room acoustics, as important as they are, may not be all that needs to be addressed. Get with a good, credible company that will help you through these hurdles. With the right planning and professional advice, you’ll be on your way to reaching that perfect “72 degrees” in your room, acoustically speaking.

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