As was discussed in the first part of this two-part series, sound environments are determined by the production of soundwaves, the movement of the waves through a medium (typically, air), and the reception of the soundwave. This reception may involve the absorption of a soundwave by a living ear, the kinetic energy of the wave received, transformed into reverberations deep inside the ear canal, and then translated into environmental sensory information by the hearer’s auditory processing systems.
However, living beings aren’t the only “receivers” of sound waves. Solid objects also “receive” soundwaves, producing one of three reactions. The surface absorbs the kinetic energy of the wave by converting it into heat which is then diffused by and through the absorbent material.
The receiving surface may also transmit the sound wave through its surface by, once again, transforming the wave’s kinetic energy into a type of heat which sets the molecules of the surface material vibrating. These vibrations reverberate through the material until it is passed through or “transmitted” to the other side of the surface in the form of its own unique soundwaves (often lower in intensity and speed (or loudness and pitch) than the original wave.
Finally, the receiving surface may simply cause the sound wave to bounce back into the environment from which it came, causing echoes and reverberations that can quickly lead to excessive background noise through the concentration of soundwave reverberations (1).
In most cases, however, surfaces do not wholly absorb, transmit, or reflect sound. Rather, they do a combination of all three, though in varying proportions. And this is where construction, design, and materials selection play a key role.
Improving Sound Environments through Acoustic Design
You can modify the proportion of sound wave absorption, transmission, and reverberation in a built environment by using acoustic principles to construct and outfit the space. In this, the conclusion of the series, we describe practical applications of these fundamental principles of sound to help remind designers and architects, how to create an ideal sound environment for varying spaces and purposes.
A Short Sound “Proofing” Dictionary
The term “soundproofing” is pretty ubiquitous in ordinary conversation, but anyone with experience in acoustics can tell you that, like the unicorn, a truly “soundproof” room or product just doesn’t exist. There are, however, a number of highly effective design strategies, sound mitigation materials, and noise reduction products which, particularly when used in combination, can yield some pretty extraordinary results.
To understand how they work, though, there are some important terms you need to know:
Noise reduction coefficient (NRC)
The NRC is a single number rating that indicates how much sound energy can be absorbed by a specific material type. The NRC is determined by the average absorption coefficient a material exhibits at 200, 500, 1000, and 2000 Hz. As a general rule of thumb, the higher the NRC rating, the better the sound absorption.
As the name suggests, the absorption coefficient is similar to the NRC but is more useful for differentiating between materials with similar NRCs. It can be a more nuanced and sensitive measure of a material’s acoustic properties because it is dependent on a test of sound absorption capacities at various sound frequencies. The absorption coefficient is measured by the octave or one-third octave band.
That might sound confusing or inordinately complex, but all it really means is that materials will perform differently at different sound frequencies. And that means that you can use the absorption coefficient to select the best materials for the environment, factoring in not only the intended uses of the space but also the external environmental noises that may leach into the space.
Sound transmission class (STC)
As the name implies, the STC refers to the degree of soundwave transmission a material type exhibits. More precisely, the STC is a single number rating that describes how well a material type or surface partition isolates sound waves, preventing them from bleeding into adjacent rooms or exterior spaces.
Harder and denser materials, such as concrete, brick, and drywall, will generally have a higher STC, which means they are better at preventing sound from traveling to adjacent spaces than softer or more porous materials. (However, these harder surfaces will cause sound to bounce around within the room.)
Sound transmission loss (STL)
Much like the STC, the STL refers to the capacity of a material type or surface constructed to filter sound and prevent its transmission to adjoining spaces. The STL is measured in decibels and is based on levels of decibel reduction at specific octaves or one-third octave band.
The octave-based measurement facilitates the comparison of materials because, as has been seen, in regard to sound mitigation, materials perform differently at different frequencies. For this reason, the STL is highly useful in assessing sound filtering capabilities of different materials when tested in the same octave or one-third octave band.
Decoupling is one of two strategies used in acoustic design to reduce sound transmission. Decoupling involves the physical separation of spaces by a sound transmission-resistant surface, partition, material, or space.
Decoupling can be accomplished in a number of ways. You create air gaps or air spaces between partitions. You can use so-called “resilient channels'' to reduce sound transmission between layers and structural framing elements in floors and ceilings. This might include, for example, “floating” a floor to prevent impact sounds such as footsteps in shared floor/ceiling assemblies. This can be accomplished by using springs, rubber isolators, or other materials to isolate the layers of the shared structure.
In addition to decoupling, adding mass is another way to reduce sound transmission. Adding mass can help to increase sound absorption, preventing the intrusion of unwanted noise from external sources or the leaking of internal sounds into adjacent rooms or other adjoining spaces. Mass that includes softer and more porous materials is usually more effective than rigid mass for this purpose. However, the best strategy for adding mass to isolate the sound environment is to use a combination of soft and rigid materials to keep the wanted sounds inside and unwanted noises outside.
From Terminology to Technique
Now that we have some of the essential lingo in place, it’s time to consider how to translate this knowledge into actionable design strategy.
As suggested above, designing a quality sound environment requires you to control the amount of sound reverberation, transmission, and absorption that occurs in the built space. Doing this means choosing the proper materials, using architectural principles that minimize reverberations, echoes, and similar soundwave concentrations (i.e. using straight rather than curved walls and adding mass to buffer the sound intensifying effects of high ceilings and large, open floor plans), and decoupling spaces with air gaps and other isolation construction principles described above.
When you’re using acoustic design to optimize the sound environment of a built space,
it's not only important to focus on the reduction of background noise or sound transmission. You also need to consider the role that the sound environment plays in the functionality of the space. This is particularly important in school and work environments.
For instance, in acoustic design, the concept of sound intelligibility is used to calculate the ratio of voice volume and frequency to ambient noise in order to determine the threshold at which human speech is no longer intelligible for listeners with normal hearing as well as those with hearing loss or other sensory challenges. In general, the higher the level of background noise, the shorter the distance at which a voice becomes unintelligible when spoken at average volume and intensity. In other words, the higher the level of background noise, the closer you have to be to the speaker to understand what is being said.
Speech intelligibility is a particularly significant concern in academic settings, where students may suffer academically, socially, and emotionally from the inability to hear and understand the teacher accurately. This is especially true for children with attentional deficits, hearing loss, or sensory processing issues, as their capacity for filtering irrelevant sound, such as background noise, is often compromised to some degree.
Another relevant issue in acoustic design is speech privacy. This is an especially significant concern in healthcare facilities and office environments, particularly those with open floor plans. The concept refers to the use of architectural design principles, added mass, and ambient white noise to control sound absorption, reflection and reverberation, and transmission.
In addition to the use of materials with high STC ratings for the construction of walls, ceilings, and partitions, other elements with a high NRC can be added to increase sound absorption, reduce transmission, and even increase sound reverberation in order to reduce speech intelligibility outside of the immediate environment. This might include the installation of baffles, canopies, and clouds to better contain, concentrate, and more effectively direct sound from the source to the intended recipient.
How FSorb Can Help
At FSorb, we offer a wide array of state-of-the-art, environmentally-friendly acoustic products. Our innovative and highly customizable product lines are engineered to provide world-class sound absorption for academic settings, healthcare facilities, government buildings, business offices, industrial manufacturing facilities, and the list goes on. We are not acoustic engineers, but with 35 years of experience building acoustic solutions, we can often help or refer you to a good engineer for your particular situation.
Our 2” acoustic panels, for example, offer an extraordinary NRC rating of 1.05, are durable, made of recycled materials, and have a class A fire rating. They also come in a range of shapes, styles, and colors. What this means is that no matter your needs or requirements, FSorb has the products you need to transform your acoustic design goals into a reality.
Contact your local FSorb representative today to discuss how our expert team and innovative product lines can help you create sound environments that truly support the health, happiness, and overall well-being of all who enter.
At FSorb, we are motivated by improving human health and do so by creating eco-friendly acoustic products. Our mission is to help designers build beautiful spaces that reduce excess ambient noise while calming the human nervous system. With over 25 years in the acoustic business we stand behind FSorb as a durable, environmentally friendly, and low-cost product. If you want an acoustic solution that is safe to human health at an affordable price, then we are your resource.
Vercammen, M. L. (2013). Sound concentration caused by curved surfaces. Proceedings of meetings on acoustics. 19. https://asa.scitation.org/doi/abs/10.1121/1.4800250