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Acoustic Ceiling Tiles
Buying a new ceiling tile strictly for its ability to control sound is a little like buying a new car strictly for its CD player. Some people might actually make the choice based on that single feature, but cars were no more invented as transport devices for surround sound systems than ceiling tiles were invented for their acoustics. Can ceiling tiles enhance the acoustical characteristics of a room? Absolutely, just like a Blaupunkt can make a sweet ride even sweeter. But when shopping for a car or a ceiling tile, it is probably best to evaluate the whole package and what you want from it not just how good it "sounds".
Before we get into the specific acoustic values of ceiling tiles and how they are measured and represented, here are a few things to consider:
- The term "acoustical ceiling tile" has as much to do with marketing as it does with making a difference in the "sound environment" within a given room. Conventional drop ceiling tiles came into wide usage because they were easy to install, provided convenient access to the areas above the ceiling, and were cheap. People certainly didn't buy them because they were pretty and they didn't buy them for their acoustics. Yet, acoustically, mineral fiber tiles were significantly better than conventional drywall or the gypsum that was widely used for drop ceiling panels. Marketers seized on this as a way to differentiate these tiles as "new and improved" and began to sell them as "acoustical ceiling tiles". That name added perceived value to the product, and soon "acoustical" is what people thought of first when buying tiles for their ceiling.
A conventional room has, at a minimum, six structural surfaces: four walls, a floor, and a ceiling. Unlike hot air, which rises, sound moves equally in all directions and has the potential to be reflected, to pass through, or to be absorbed by all surfaces - not just the ceiling.
So, why is it that we mostly think of the ceiling when we think of acoustics? Marketing. A carpeted floor can do much more to absorb sound than most ceiling tiles, yet we don't think of buying acoustical carpeting. Wall hangings and drapes can have a significant and easily noticeable effect on the sound signature of a room, yet they are rarely purchased for their acoustic characteristics. And what about furniture? One doesn't usually shop for an "acoustical chair", yet furnishings have a huge impact on the behavior of sound within a room.
Should you consider the acoustical properties of ceiling tiles before making a selection? Certainly. But it is also important to make sure that your expectations are aligned with reality. Given all the factors that contribute to the total sound signature of a room, will you be able to actually hear the difference between a tile that absorbs 50% of the sound that hits it versus one that absorbs 25%? For most people in most situations, probably not.
Think of it this way: Most drivers going 60 MPH down the freeway probably aren't even conscious of the difference between a $300 car stereo and one costing $3,000. Instead, they are appreciating the entire driving experience: the look, the feel, the handling, the convenience, the maintenance, the cost, the miles per gallon of the vehicle - the whole package. Approach your selection of ceiling tiles in a similar fashion and you can't go wrong.
There are three major ratings that relate to ceiling tile acoustics: Noise Reduction Coefficient, Sound Trasmittance Class, and Ceiling Attenuation Class. Only some tiles are rated in all three categories, and the value of each rating is only meaningful in the context of the actual installation where the tile will be used. Before describing each rating, it is important to note that these values are achieved in a laboratory, and the measurements are not considered field data. Said another way: "Your results may vary," depending on where and how the tiles are used and what you expect of them.
Sound Transmission Class (STC) is used to rate several different types of building materials for their ability to block sound and prevent it from passing through them. This is commonly used on walls and partitions, as it is very helpful in deciding which types of partitions will properly isolate different areas of a building. STC ratings are represented as a number, usually between 1 and 100, although not limited to that range. The higher the number, the better the material is at blocking sound. A rating of 10 is considered terrible, allowing talking voices to be heard through the rated system, while a rating of 60 or more makes for an excellent sound blocker.
The STC rating is helpful, but does not tell you what will happen to sound once it is blocked. For example, a thick concrete wall might have an excellent STC rating, but it will send all sound bouncing right back into a room. This will keep the sound from passing through the wall, but will make for a very echo filled room.
Noise Reduction Coefficient (NRC) is a general rating that tells you how much sound a surface (in this case a ceiling tile) will absorb. This rating is represented as either a percentage or a decimal, with 1 (or 100%) being perfect absorption and 0 (or 0%) being perfect reflection. In other words, the higher the decimal or percent, the more sound the tile will absorb. While this does give a very good idea of how a tile will perform acoustically, it does not tell you what happens to the sound the ceiling tile does not absorb.
Ceiling Attenuation Class (CAC) is really a completely separate rating system. While it does deal with sound absorption and attenuation, it is a bit more specific.
In an installation where a drop ceiling has been installed and walls have been installed that only reach up to the drop ceiling level, sound can pass up into the ceiling plenum, over the wall, and then back down into another room. The rate at which ceiling tiles inhibit this passage of sound is called the CAC. This rating is commonly misunderstood as a rating that describes how much sound reflects off of the face of a tile. This is a misconception, and the CAC rating should not be given as a rating to describe any situation other than the aforementioned.
NRC, STC, CAC and YOU
When looking for a ceiling tile, consider this: If a tile completely absorbs about 50% of the sound it contacts (NRC), the other 50% has got to go somewhere. If the tile also has a good STC, then it's a good bet that most of the remaining sound will bounce back into a room. If it has a bad STC, then that sound will most likely travel through to the area above the ceiling. If you do not have full walls in an office building or other partitioned building, make sure to look for a CAC rating in your ceiling tiles, or find another way to block sound above your partitions.
While there is no hard and fast rule to properly combine these ratings, it is most important that you are thinking about it while shopping. Hard surfaces like metal and concrete REFLECT sound, while softer and more plush surfaces ABSORB sound. Regardless of material, thickness of an item will drastically change its ability to affect sound waves. Most importantly, use these ratings to compare tiles, and do not expect a good rating in any of them alone to translate to desirable characteristics in the field.
So how important are these ratings? If you were going to be considering tiles in isolation, comparing one type of tile to another and only looking at acoustic performance, these numbers are all you'd have to go on. Fortunately that isn't the case. You are also going to be considering appearance, ease of installation, price, selection, indoor air quality, availability, and a host of other things. Add these acoustical ratings to that mix, think about which overall qualities are most important in your specific installation, and in no time at all you'll know which tile "sounds" right for you.