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For anyone new to the world of acoustics, there is a multitude of terms, coefficients and numbers that are thrown around. This flood of information can seem intimidating, especially to beginners. In this series, acoustician Cameron Girard of Acoustics First® hopes to help you distinguish between what’s useful and what’s not.
Part 1: Acoustic Terminology – Sound Absorption vs Sound Isolation
In order to make informed decisions about acoustical treatment, it is vital to know the difference between materials that are meant to absorb sound within a room and materials that are meant to block sound from leaving or entering it. In an overly reverberant auditorium, absorptive treatment is needed to reduce echoes and improve speech intelligibility. If the problem is sound passing in between spaces, like offices or apartments, then isolation treatment is required. These are two separate acoustic issues which require separate solutions.
In both scenarios, it is important to know which data is relevant and helpful. Also, given sheer volume of information available on the internet, it is perhaps unavoidable that some info will be incomplete or simply incorrect. It should not be assumed that something which sounds technical is, in fact, backed up by proper testing.
Terms for Sound Absorption
We recently encountered an acoustical ceiling tile which was said to “absorb 50% of sound”. On the surface this sounds like an extremely efficient product. However, let’s delve in closer and decipher what is actually usable information, and what is just marketing.
When sound waves meet a room surface such as a wall, ceiling or floor, some of the sound energy is reflected back into the room and the rest is considered to be “absorbed”. The absorbed sound energy has not vanished, it’s actually been converted into kinetic (vibration of a solid material) and thermal energy (heat due to friction within a porous material) or has simply passed right through the material (transmission). The more surface area a certain material has the better absorber it will likely be. “Soft” materials, like heavy blankets, fabric and fiberglass, have loads of nooks and crannies, which sound tries to “fill”. These porous materials are great for reducing reverberation within a room, but will only marginally reduce the sound that leaves it (but more on that later).
When comparing sound absorbing products, there is a particular set of terms you should look for: The Sound Absorption Coefficient (SAC) and Noise Reduction Coefficient (NRC). These are used to specify the fraction of incident sound that a material absorbs per 1’x1’ area. An NRC of 1.0 indicates perfect absorption (an open 1’x1’ window) and a value of 0.0 represents perfect reflection (polished concrete has an NRC of .02).
To measure sound absorption, a large sample of the material is placed in a reverberation room with all other surfaces being hard and reflective. The time it takes a test sound signal to decay by 60dB (rough point of inaudibility) after the source of sound is stopped is measured first with the sample in the room and again with the room empty. The difference in decay time defines the efficiency of the absorbing material and thus the absorption coefficients. Large spaces with low-NRC materials (tile, drywall, etc.) have longer reverberation times, while small rooms furnished with high-NRC materials sound much more “dead”.
Clearly, a single 2’x2’ ceiling tile is not going to reduce the reverberation in a real-world space by 50%. So is the above claim false? Not exactly… The ceiling panels do have an NRC rating of .50, so the tile does absorb 50% of incident sound. However, one might assume a much more drastic improvement based on the “50%” claim. In reality you’d need a large square footage of these ceiling tiles to cut the amount of total reflected sound in half. Always be sure to check the NRC number!
Terms for Sound Isolation
Our customers often call with issues related to neighbor noise or office-to-office privacy and are looking for “sound proofing” treatment. Unfortunately, many do not realize that simply installing acoustic foam or fiberglass panels will not appreciably reduce the level noise entering and leaving their space. These absorptive materials are great at reducing unwanted reflections within a room because they are porous and air/sound energy can flow through them. That being said, they are generally poor sound barriers for this exact reason. They will help to reduce noise buildup in a room and improve the ‘acoustics’, but will do very little to “block” sound coming in or out.
Sound is like water; it will “flow” into an adjacent space if everything isn’t sealed up. Materials that are air tight and heavy, like our BlockAid® sound barrier, provide the most relief of air-born sound transmission. Continuous coverage of floors/ceilings or walls is necessary to ensure that sound doesn’t ‘flank’ around these barriers. Multiple layers of varying materials, the use of resilient clips or channels, and additional walls will provide even more control. For a demonstration of how different materials affect sound isolation, check out our video http://acousticsfirst.com/educational-videos-the-barrier-and-the-bell.htm
Like NRC for sound absorption, there is also a laboratory tested figure that can be used to compare the sound “blocking” properties of acoustic barriers and wall constructions: Transmission Loss (TL) and Sound Transmission Class (STC). These describe how much air-born sound is attenuated through a given material.
In the lab, the material to be tested is mounted over an opening between two completely separated rooms, one with a speaker (source) and the other with a microphone (receiver). Save for the open “window”, these rooms are completely isolated with thick and massive walls, so virtually all the sound energy transmitted between rooms will be through the test specimen. The difference between sound levels in the source room and the receiving room is the transmission loss (TL). The TL is measured at multiple frequencies, which is fitted to a Sound Transmission Class (STC) “curve” at speech frequencies (125Hz-4kHz). The STC of the material is the TL value of the fitted curve at 500 Hz. For example, a material with an STC of 27 typically “blocks” 27dB of sound. Keep in mind though, the STC’s of materials do not add up linearly; in other words, adding a material with an STC of 27 to an existing wall with an STC 45 will not result in an STC of 72.
As always, Acoustics First is here answer questions and help you find the best solutions.