Project Planning and Design: Acoustics

1. What Are Acoustics?

Acoustics are an important and often overlooked aspect of architectural design. While acoustics in architecture doesn't usually make the cover of architectural publications, the design of the acoustic environment is important in the creation of interior spaces which are functional and satisfactory to occupants. Aside from considerations of comfort and functionality, sound in architecture can also be understood at a qualitative level. That is to say - each architectural space has its particular acoustical character. The same orchestra will sound different playing in one concert hall compared to another. Sound also shapes our experience and enjoyment of a space. The chatter of conversation in a cafe or the sound of a fountain in a piazza are examples of sound contributing to the ambience or experience of a place. For the purposes of the ARE exam, the more measurable aspects of sound and how they relate to space planning or building assemblies and details are what I will cover.

2. Acoustic Design Aspects

Among aspects of acoustic design to be considered are acoustic comfort, speech intelligibility, and privacy. The acoustic needs for a project will vary for different types of spaces and the programming which is envisioned for them. In planning for the design of a contemporary office plan with open floor plan, for example, acoustics are often poorly considered so that workers by turns experience uncomfortable level of noise from other workers and a sense of a lack of privacy in their own conversations. A high level of noise, whether from equipment, human speech, or sounds from the exterior environment can cause worker distraction, discomfort, stress, and even health issues. In multi-unit residential projects, as well as in hospitality projects, adequate sound attenuation between units can mean the difference between a successful and profitable project and a failed one. As suggested, each project will have its own set of requirements and challenges. Before discussing specific architectural issues and solutions, it is worth briefly examining the basic physics of sound.

3. Basic Physics of Sound

Sound travels in waves and is transmitted from a source through air as well as other mediums. The basic parameters of sound waves include frequency, amplitude, and wavelength. The human ear has a particular range of frequencies that are perceivable. Frequency is measured in Hertz or Hz. It should be noted that different material assemblies may be more successful at blocking particular frequencies than others, and consequently, the types of sound being mitigated may need to be considered when looking at different products or assemblies. Amplitude may be understood as pressure or loudness. Loudness is measured in decibels, dB, with louder sounds being at larger decibel levels. It should be noted that sound levels double with every increase of 10 dB. For example, 40 dB is twice as loud as 30 dB. The architectural designer should be generally familiar with the ranges of sound level which are preferable for different types of spaces.

4. How Are Acoustic Properties Measured?

There are several commonly used measures of the acoustic properties of materials and assemblies which an architect should understand. The Noise Reduction Coefficient (NRC) is a measure of a product's ability to absorb sound. The higher the rating and the more of the product utilized in a space, the more sound will be absorbed. Sound Transmission Class (STC) is a measure of the ability of a partition system to reduce sound transmission from one side of the partition to the other. The higher the rating, the greater the reduction in sound transmission. Noise Isolation Class (NIC) is similar to STC, but NIC is the actual measure reduction in sound taken in the field, while the STC is a rating given to a product based on lab tests. Impact Isolation Class (IIC) is a measure of how much sound is attenuated as it carries through an assembly due to impacts to it.

4.1. Sound Mapping

It is important to consider acoustics from the earliest stages of design since the careful planning of spaces, in terms of adjacencies and separations, can significantly reduce the potential of unwanted noise or other acoustical issues. One method is to do what is called a sound mapping exercise, wherein each space of the building program is identified with particular goals in terms of acceptable noise levels or other criteria. Once these requirements are understood, initial floor plan options can be evaluated based on the compatibility or incompatibility of the proposed spatial adjacencies. For example, it would be poor planning to locate a study room or a conference room where important meetings are held immediately next to a noisy space like a cafeteria, fitness room, or heavily used lounge space.

4.2. Reverberation

The reflection of sound within a space produces what is known as reverberation. Although some level of reverberation is tolerable and even desirable, too much reverberation causes problems with speech intelligibility within a space. The acoustic environment can be improved in this regard by adding finish materials with absorptive capacity to the space. Utilizing products with a high NRC value can improve the acoustics of a space in this regard. Products that can be utilized include acoustic tile ceiling panels, acoustic baffles, and wall panels. Additionally, the use of carpeting and even fabric upholstered furniture can help mitigate reverberation in a room.

4.3. Sound Masking

Another technique for reducing unwanted noise within a space is to utilize sound masking. This can also be effective in increasing the level of speech privacy and is sometimes utilized in open offices or in healthcare settings.

4.4. Noise Source Isolation

When a noise source is located in a separate space, it is desirable to isolate it and reduce its transmission into any adjacent spaces where a lower level of noise is necessary. The assemblies of the walls separating the spaces should utilize appropriate STC ratings. Underwriters Laboratory publishes details of wall assemblies and the STC ratings which they achieve. Some of the methods for increasing the STC rating of a wall assembly include the use of sound batt insulation within the wall cavity, such as between metal studs, in addition to increasing the stud spacing; adding resilient channels on one side of the wall; adding additional drywall layers; and utilizing a double stud wall with air space in between. Properly sealing edges and areas of penetrations is also critical. Doors should be solid core rather than hollow core, with seals and gaskets utilized at edges. For acoustic separation between floors, a floor-ceiling assembly with an appropriate STC rating should be selected, but the IIC should also be considered in order to ensure an attenuation of potential impact sounds such as footsteps. Utilizing floor underlayment beneath the finish flooring is one method of reducing impact sound transmission.

4.5. Noise from Mechanical Systems

Another area where potential noise is to be considered is from mechanical systems. Equipment selection may sometimes depend on how quiet a particular space may need to be. Sound attenuation is also possible with products which help to reduce sound traveling through ductwork, as well as from the duct to the surrounding space, such as with duct lining. Along with providing equipment rooms with walls and floor-ceiling assemblies of adequate STC ratings, the mechanical equipment can also be placed on isolation mounts or pads to reduce noise-causing vibrations.

5. Consider Hiring an Acoustical Consultant

On projects where potential sound issues are of significant concern, the inclusion of an acoustical consultant or specialist on the project team is best practice. On all projects, however, the practicing architect would do well to have a firm grasp of the principles of acoustics in construction since acoustical outcomes are impacted by design decisions ranging from initial project planning and preliminary layout of spaces to product selections and technical detailing.

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