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Guideline I.6: Effective Acoustics

Intent

To promote interior conditions that avoid harmful noise effects and produce a basis for a positive soundscape acceptable to occupants and appropriate to their tasks. The benefits are avoiding exposure to unhealthy noise levels, the elevated stress which accompanies higher background noise levels, and noise distraction impacts on mental work. Effective acoustics enable effective speech communications at normal speaking voice while providing for local speech privacy.

Required Performance Criteria

  1. Recurrent background noise from external and internal sources cannot not exceed 70 A-weighted decibels (dBA).
  2. All continuously occupied office space must meet a Noise Curve (NC) of no greater than NC-50. (See recommended levels below.)
  3. All classroom space must meet an NC of no greater than NC-45.(See recommended levels below.)
  4. Reverberation time for all continuously occupied space must be no less than 0.2 seconds and no greater than 0.8 seconds. Reverberation time must be evaluated on the 500 Hz octave band, and must be appropriate to the uses of the space. (See recommended levels below.) Note that this requirement would not apply to concert halls or other music performance auditoria.

Recommended Performance Criteria

  1. Articulation Index must be less than 0.20 for open offices, where a low level of speech intelligibility is required (speech privacy is desired), and greater than 0.70 for enclosed offices where a high level of speech intelligibility is required.

Promote Positive Acoustics Appropriate to Tasks:

  1. Reduce NC criterion to NC 45 or lower for continuously occupied spaces.
  2. NC can be no greater than NC 40 for intermittently occupied meeting spaces like conference rooms and classrooms. (Note that this is less stringent than the ANSI S12.60 standard for classrooms.)
  3. Provide reverberation times optimal for space use based on professional acoustic judgment. General Guidelines are as follows:
    1. Open Office: 0.2–0.5 seconds
    2. Enclosed offices: 0.2–0.4 seconds
    3. Classrooms 0.2–0.7 seconds
    4. For other space types, such as gymnasiums and auditoriums, use acoustic professional judgment and advice.

 

There are several US university programs focused on architectural acoustics and many sources of room acoustics modeling software that are commercially available. There are also free acoustics modeling and analysis software available from some universities and companies. These programs provide calculated estimates of quantities like reverberation time and sound pressure levels given certain parameters that account for room size, shape, surface absorption, and activity types.

Implementation in the Design Process:

Include performance criteria in programming document. Develop any additional special acoustical performance requirements to support functional programming of building (e.g., sources of recurrent noise that need to be controlled, special user populations which may have distinct auditory performance limitations, multiple uses of building spaces which may have different acoustic criteria. Investigate and choose appropriate acoustics modeling software for the project. (See Additional Resources below.)

Consider performance in building layout and form. Ensure that there are no inherent acoustic conflicts or limits to meeting performance criteria at schematic design. Perform preliminary software simulations to ensure that general acoustics parameters are met. Through Design Development demonstrate complete compliance with acoustical performance criteria via updated calculations or more detailed simulation modeling.

Address explicit performance criteria in design and materials selection and specification. Check to ensure materials selection meets necessary criteria for acoustical controls.

Final Design:

  • I.6A: Verification that expected background noise is within guideline limits.
  • I.6B: Verification that expected office background noise NC is within guideline limits.
  • I.6C: Verification that expected office background noise NC is within guideline limits.
  • I.6D: Verification that expected reverberation time is within guideline limits.
  • I.6E: Verification that expected articulation index is within guideline limits.
  • I.6F: Verification that continually occupied space expected background noise NC within guideline limits.
  • I.6G: Verification that intermittently occupied space expected background noise NC within guideline limits.
  • I.6H: Verification that expected reverberation time is within limits specified in guideline.

Closeout:

  • I.6A: Verification that expected background noise of installed condition is within guideline limits.
  • I.6B: Verification that office expected background noise of installed condition NC is within guideline limits.
  • I.6C: Verification that office expected background noise of installed condition NC is within guideline limits.
  • I.6D: Verification that expected reverberation time of installed condition is within guideline limits.
  • I.6E: Verification that expected articulation index of installed condition is within guideline limits.
  • I.6F: Verification that occupied space expected background noise NC of installed condition is within guideline limits.
  • I.6G: Verification that intermittently occupied space expected background noise NC of installed condition is within guideline limits.
  • I.6H: Verification that expected reverberation time of installed condition is within guideline limits.

 

The Engineering Toolbox architectural acoustic calculator: http://www.engineeringtoolbox.com/room-absorption-sound-d_69.html

IRCAM (a French research project with many useful publications and free software): www.ircam.fr

ODEON distributes room acoustics modeling software: www.odeon.dk

SARA – A Spatial Audio & Room Acoustics Project from the Academy of Finland: www.acoustics.hut.fi/~vpv/projects/sara.htm

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