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Acoustics

Acoustics affect critical aspects of a building’s function, from productivity in the office and communication in the classroom, to the price an apartment, condominium or single-family home can command. Understanding how to select a combination of building materials, system designs and construction technologies that will provide the most appropriate sound control is key to creating a successful acoustical design.

What is Sound?

Sound is transmitted as a wave in an elastic medium, such as air or steel, when that medium is set into motion by some disturbance.  In the case of airborne sound, the disturbance in the medium is usually caused by a vibrating sound source, such as a loudspeaker or human vocal cords.  But sound can also travel through solid structures when airborne sound waves are powerful enough to impart energy into the structure, or when the structure is energized directly by an impact with another object, such as foot steps. 

Energy from sound waves interacts with every object and surface in a room – some energy may be absorbed or scattered by room furnishings and some energy may reflect off or transmit through partitions.  Sound can bend around barriers and squeeze through small openings, all of which can allow noise to reach surprisingly far beyond its point of origin.  As a result, designers must consider the dynamics of sound when determining how they will control noise within a building.

The basic mechanisms involved in noise control for buildings are sound absorption and sound transmission. Sound absorption is necessary to reduce the reverberation, in a space to help improve clarity of speech and reduce excessive noise within a room. Sound transmission, either airborne or structural, must be reduced to prevent sound from travelling between two spaces.

Sound Absorption

Sound Absorption

Sound is absorbed by a material when a portion of the sound wave incident upon that material is converted to heat. Porous materials, such as glass fiber or mineral fiber insulation, generally make for very efficient sound absorbers, while non-porous materials, such as stone or gypsum board, tend to reflect sound.

Common Measures of Sound Absorption:

  • sound absorption coefficients
  • noise reduction coefficient (NRC)
  • sound absorption average (SAA)
  • sabins

Sound Transmission

Sound Transmission

Sound transmission is the phenomenon of sound traveling in air or structure and transmitting to adjacent spaces through partitions and openings. There are many factors that determine the sound transmission properties of a material or system, including surface density, stiffness, internal damping, and mass. 

Common Measures of Sound Transmission:

  • sound transmission loss
  • sound transmission class (STC)
  • ceiling attenuation class (CAC)
  • impact insulation class (IIC)

Key Concepts and Definitions

NOISE REDUCTION COEFFICIENT (NRC) – A single-number rating for comparing sound absorption of building materials, tested per ASTM C423.  Products with a high NRC value absorb sound and help reduce reverberation.

CEILING ATTENUATION CLASS (CAC) – A single-number rating for comparing sound attenuation of ceiling systems, tested per ASTM E1414.  Products with a high CAC value help contain or block sound transmission between rooms with a common ceiling plenum. They inhibit unwanted noise from disrupting activities.

SOUND TRANSMISSION CLASS (STC) – A single-number rating for comparing sound attenuation of partitions, such as walls or floor-ceiling assemblies, tested per ASTM E90.  The higher the STC value, the greater the attenuation of airborne sound provided by the partition.

IMPACT INSULATION CLASS (IIC) - A single-number rating for comparing attenuation of impact sounds, such as foot steps, in floor-ceiling assemblies, tested per ASTM E492.  The higher the IIC value, the greater the attenuation of impact sound provided by the assembly.

SABINS – A measure of the total sound absorption provided by an object or surface.  Sound absorption performance of non-continuous sound absorbers, such as baffles, canopies, and clouds, are typically expressed in terms of sabins, tested per ASTM C423.

REVERBERATION – The persistence of sound in an enclosed space due to repeated reflections of sound waves.  A common measure of reverberation is reverberation time, which is the time it takes, in seconds, for a sound to decay by 60dB in a space. Excessive reverberation can result in noisy environments with poor speech intelligibility.

SPEECH INTELLIGIBILITY – A measure of comprehensibility of speech in a given set of room conditions.  Speech intelligibility is influenced by a number of factors, including reverberation and background noise.  Speech Transmission Index (STI) and Articulation Index (AI) are common measures of speech intelligibility.

SPEECH PRIVACY – The opposite of speech intelligibility, speech privacy is the inability to understand conversations by an outside listener.  Speech privacy in enclosed spaces is primarily influenced by the sound transmission properties of the walls, floor, and ceiling construction, as well as background noise.  Common measures of speech privacy include Speech Privacy Potential (SPP), Speech Privacy Class (SPC), and Privacy Index (PI).

BACKGROUND NOISE – Also known as ambient noise, background noise is any sound other than the primary sound being conveyed, such as speech.  Background noise can reduce speech intelligibility or enhance speech privacy.  Steady state background noise levels are often governed by HVAC systems and/or supplemented by sound masking systems.  Background noise is often measured in A-weighted decibels (dBA), or stated in terms of Noise Criteria (NC) or Room Criteria (RC) values.

Acoustic Measurement Standards

ASTM C423
Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method

ASTM E90
Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements

ASTM E1414
Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum

ASTM E413
Classification for Rating Sound Insulation

ASTM E492
Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine

ASTM E989
Standard Classification for Determination of Single-Number Metrics for Impact Noise

ASTM E336
Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings