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Space Behavior: The Effect of Audio and Acoustics

An essay from an acoustics expert -- with three important considerations for architectural designers.

By Chuck Walthall   •  July 31, 2017 11:15 am EDT

Tags: acoustics, architectural design, audio, design,

A challenge some architects face is implementing acoustical elements into the interior design of a space. Most owners face budget challenges. The acoustical designer faces both. Form or function? Can both be achieved when designing a space and still stay in budget? By all means, yes.

What needs to be considered when designing a space is that the space must acoustically support the programmed use of the room. Anything less renders a dysfunctional space. In other words, spaces should be acoustically well behaved. Too often, performance spaces (worship centers, cathedrals, etc.) are deprived of the proper acoustical tailoring and fall victim to “the audio system will take care of it’” mindset. It is very important to understand that the audio system is at the mercy of the acoustics of the room where it is located. No audio system can compensate for poor room acoustics. The proper mindset should be “acoustics first, then audio.”
 Acoustical design is a science art. Sound behaves according to the laws of physics, yet all spaces are not used for the same function. A classroom should render high speech intelligibility (low reverberation) due to its academic purpose, where a recital hall should be musical in nature but not so reverberant that speech intelligibility suffers. Blending the science of sound with the art of acoustics produces a balance that gives way to a well-behaved space.

1. The importance of absorption vs. reflection

When designing a space, an architect should take into account the basic tenet that a space must work acoustically for its planned use. This is achieved, in part, by ensuring a balance of absorptive and reflective surfaces. Every material has acoustical properties—even people. However, it goes much deeper than that. The acoustical properties of a given material can be qualified somewhere between absorptive and reflective. For example, concrete has absorptive properties, be they minute, which are grossly outweighed by its reflective properties. Thus, concrete is classified as a reflective material. Carpet is more absorptive by nature, yet possesses reflective properties in the low-mid frequency range. Common materials are classified as either absorptive or reflective, yet these properties are frequency dependent. When designing a space, architects should steer away from hard parallel surfaces—and implement a balance of absorptive and reflective finishes.

2. Space experience and how occupants “listen”

How do the acoustical characteristics of a space affect the listener’s experience? We listen with our brain, not our ears. The ears receive the sound pressure wave and transmit the information to the brain for interpretation. Based on copious amounts of information contained in the sound pressure wave, the brain tells the listener where the sound is coming from (directionality), how large or small the room is (reverberation time) and the tonal quality of the sound source. That said, every surface and element in the room has an effect on what the brain interprets.

Too much absorption in a space will reduce the reverberation—and the brain will perceive the space to be small. Too little absorption (increased reverberation), and the brain perceives the space as large. In other words, the room’s cubic volume and the reverberation time should match so the room sounds the same as it looks. A simple test to perform is to close your eyes and listen to the space. Your brain will paint a mental picture of how large or small the room is. Open your eyes and see if they match.

3. Balance is everything

A third tenet an architect should take into account when designing a space is to achieve an acoustical balance—so the space sounds the same size as the room’s physical size. Too often, spaces suffer from an acoustical imbalance; they sound two-dimensional. This is common when the floor is fully carpeted and the walls and ceiling are hard surfaces. The floor is absorptive and the walls and ceiling are reflective. What’s worst is when the floor and ceiling are both absorptive and the walls are flat, reflective and parallel. Almost all reverberation (energy) is in the horizontal plane (side to side) with no auditory clues perceived from the floor or ceiling. The brain does not perceive a floor or ceiling’s presence due to inaudible clues. 

What can be implemented to create a well-behaved space or fix an acoustically challenged space? The short answer is to study the space rather than just throwing treatment in it. Determine the purpose (use) of the space, identify a reverberation target then calculate the room’s reverberation time (RT) based on the finishes. A healthy acoustical balance is the goal. 
Acoustical treatment can be grouped into absorption and diffusion categories. The most common absorption panel (adiabatic) is compressed fiberglass wrapped with porous fabric. These panels are most effective for absorbing mid and high frequency. Adiabatic absorbers are the most common type of acoustical products used to lower a room’s reverberation time and control reflections. 

Diffusion can be defined as the scattering of sound waves by reflecting them in various directions or breaking them into smaller pieces. Common diffusers include barrel, pyramidal and quadratic. Diffusion is commonly used to limit or eliminate specular reflections and echo without adding absorption to the room. Diffusion is also used to direct sound energy to specific areas.

The acoustics in a space are largely responsible for how the audio signal is perceived. Again, poor acoustics cannot be fixed with an audio system. It is important to note that a room’s acoustical characteristics and the audio system [must] work together and should be equally matched. Acoustical properties and characteristics must be considered when designing a critical space such as a sanctuary, an auditorium, or a performing arts space. By way of an experienced acoustical engineer, new spaces can be designed to meet the acoustical requirements of the space and existing acoustical concerns, and conducting a detailed study and applying the appropriate acoustical products can resolve issues.

[Editor's note: This piece was originally published in 2014.]

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