Room acoustics
The term ‘room acoustics’ has slightly different meanings to different people, but a simplified version might be the behaviour and perception of sound within closed spaces. The paragraphs that follow will review reverberation, sound absorption, and steady-state background noise—all of which play leading roles in the overall room acoustic environment.
Sound absorption co-efficient
Sound absorption co-efficients (a) are a measurement of a material’s ability to absorb sound. Defined as the fraction of the randomly incident sound power absorbed by a surface or material, they are typically measured and reported in one-third octave bands on a scale from 0.0 (perfectly sound-reflective) to 1.0 (perfectly sound-absorptive.)
Noise reduction co-efficient
A single-number rating describing a material’s ability to absorb sound, the noise reduction co-efficient (NRC) is the arithmetic average of the sound absorption co-efficients at 250Hz, 500Hz, 1000Hz, and 2000Hz octave bands.
Reverberation time
The reverberation time (RT) is the period it takes, in seconds, for a sound within a room to decay by 60dB from its initial sound level. In its simplest form, reverberation time is a function of the physical volume of the room, the total surface area of the room envelope and any interior elements, and the sound absorption co-efficients of those surfaces.
A-weighted and C-weighted sound pressure levels
Decibels are the overall sound pressure level (single-number) calculated from one-third octave or octave band sound pressure levels, then weighted according to predetermined spectrum curves. A-weighting (dBA) is the most common and is intended to imitate the response of the human ear at normal sound levels, giving less weight to sound at very low and very high frequencies. The C-weighting (dBC) curve is meant to imitate the response of the human ear at very high sound levels (over 100 dB).
Noise criteria
Noise criteria (NC) is a single-number value describing the spectrum of background noise typically associated with steady-state noise sources, such as air circulation systems. Noise criteria is often assigned in preferred ranges for different types of spaces and is used as a guideline for designing HVAC systems. For example, concert halls should be designed for NC-20 or lower (very low background noise), while open plan office spaces may be designed for NC-35 to NC-40 (moderate background noise.)
Room criteria
Similar to NC, the room criteria (RC) is a single-number value for the background noise within a space with additional descriptors for the quality of noise, based on the background noise spectrum. These descriptors are neutral, rumble, hiss, and perceptible vibration. A variant on RC, referred to as RC Mark II, includes even more descriptors for the quality of background noise.
Room noise criteria
While NC and RC are most applicable to steady-state background noise conditions, room noise criteria (RNC) is a single-number value for background noise with sensitivity to low-frequency time-varying fluctuations or surging that can be produced by HVAC systems.
Room acoustics summary
Similar to the aforementioned sound transmission loss metrics, room acoustic criteria are single-number ratings (with the exception of a) that do not describe full-spectrum effects of the behaviour and perception of sound within closed spaces. For high-performance buildings and critical use spaces, it is important to consider a broader spectrum of frequencies with respect to background noise criteria, sound-absorptive materials, and reverberation times.
Background noise criteria (e.g. NC and RC) are often referenced in mechanical systems’ standards and specification documentation.
Speech intelligibility and privacy
Indices for speech intelligibility and privacy combine various elements of sound transmission and room acoustics to paint a more complete picture of the behaviour of sound in spaces and the way humans perceive it.
Articulation index
Articulation index (AI) is a single-number rating, on a scale from 0.0 to 1.0, intended to define speech privacy in open-office environments with partial-height walls. Low AI values have better speech privacy than high AI values. Calculation of AI takes into account speech spectrum level, background noise level, the time-varying quality of background noise, and reverberation.
