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)
AV 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), or 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 tie 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
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 bout diffusion of sound; it takes an object roughly he 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 100Hz 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 of 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 a ample amount of diffusion to break up most echoes.
