Terms & Definitions

Reverberation Time (RT60) (T30, T20): The time in seconds for sound in a room to decay by 60 dB after the source stops. Long RT60 blurs speech; short RT60 makes a music room feel dead. It is set by room volume and the total absorption of its surfaces.
Sabine and Eyring equations: The two classical formulas for predicting RT60 from volume and absorption. Sabine is accurate in fairly live rooms; Eyring is the better estimate once average absorption is high, because Sabine over-predicts there.
Absorption coefficient (α, NRC, αw): The fraction of incident sound energy a surface absorbs rather than reflects, from 0 to 1, quoted per octave band. NRC and αw compress that curve into one number for quick comparison.
Sound Transmission Class (STC): A single-number rating of how much airborne sound a partition blocks, from ASTM E413. Higher is better. It weights speech frequencies, so it can flatter a wall that leaks bass.
Weighted sound reduction index (Rw, DnT,w): The ISO 717-1 counterpart to STC. Rw is the laboratory element rating; DnT,w is the rating actually achieved between two rooms on site, which is always the number that matters to an occupant.
Spectrum adaptation terms (C, Ctr): Corrections added to Rw for the real noise you are blocking. C suits speech and office noise; Ctr suits traffic, music and anything bass-heavy, and is usually the harsher of the two.
Impact sound rating (Ln,w, IIC): How much footfall and dropped-object noise a floor transmits to the room below. For Ln,w lower is better; for IIC higher is better, the two run in opposite directions, which is a common source of specification errors.
Flanking transmission: Sound that bypasses the partition entirely: through a continuous floor slab, a shared ceiling void, or a duct. It is why a wall tested at Rw 55 in a laboratory can deliver DnT,w 42 on site.
Noise Criteria / Noise Rating (NC, NR): Single-number descriptions of acceptable background noise in a room, compared against a family of octave-band curves. An NC 30 office is quiet; an NC 45 one is not suitable for concentrated work.
Equivalent continuous sound level (LAeq): The steady A-weighted level carrying the same energy as the fluctuating noise measured over a period. It is the basis of almost every environmental noise limit, including Indonesia's.
Room mode (axial, tangential, oblique): A standing wave between a room's parallel surfaces. Modes make some bass notes boom and others disappear depending on where you stand, and they are fixed by the room's dimensions, not its finishes.
Schroeder frequency: The frequency above which a room behaves statistically and below which individual modes dominate. It marks the point where RT60 stops being a useful description on its own.
Speech Transmission Index (STI, STIPA): A 0–1 measure of how intelligible speech is in a space, from IEC 60268-16. Voice alarm systems typically have to demonstrate STI at or above 0.50 at every measurement position.
Potential / Needed Acoustic Gain (PAG, NAG): The classic feedback-stability check for a live sound system. If potential gain is lower than needed gain, the system will ring before it is loud enough, a geometry problem no equaliser fixes.
Illuminance (E, lux): Light arriving on a surface, in lumens per square metre. It is what a lux meter reads and what almost every lighting criterion is written in.
Luminance (L, cd/m²): Light leaving a surface toward the eye, the brightness you actually perceive. Glare, contrast and screen legibility are luminance problems, not illuminance ones.
Maintained illuminance (Ēm): The average illuminance a design must still deliver at the end of the maintenance cycle, not on day one. Designing to the day-one figure is the most common way a compliant scheme fails two years later.
Maintenance factor (MF, LLF): The multiplier accounting for lamp lumen depreciation, dirt on luminaires and room surfaces, and survival rate. Typically 0.7–0.8 indoors; assuming 1.0 quietly under-designs the installation.
Unified Glare Rating (UGR): A calculated index of discomfort glare from luminaires in the field of view, per CIE 117. Offices and classrooms are usually held to UGR ≤ 19.
Uniformity (U0): Minimum illuminance divided by average illuminance across the task area. A high average with poor uniformity still reads as a patchy, uncomfortable room.
Colour rendering and colour temperature (CRI/Ra, CCT, Rf/Rg): Ra describes how faithfully a source renders colour; CCT in kelvin describes whether the light looks warm or cool. TM-30's Rf and Rg give a fuller picture than Ra where colour actually sells something.
Lighting power density (LPD, W/m²): Installed lighting load per unit floor area. SNI 6197 caps it by room function, and it is usually the first number an energy reviewer checks.
Lumen method: A whole-room average calculation: how many luminaires are needed for a target average illuminance, given room geometry, surface reflectances and a maintenance factor. Fast, and correct only for regular spaces with regular grids.
Point-by-point method (E = I·cos³θ / h²): Calculates the direct illuminance one luminaire delivers to one specific point from its intensity distribution. The right tool for accent, façade and sports lighting, where an average tells you nothing.
Room index (RI, k): A shape number combining room length, width and mounting height above the working plane. It is what a utilisation factor table is looked up against.
Utilisation factor (UF): The proportion of a luminaire's bare lumens that actually reaches the working plane, after the room's geometry and surface reflectances take their share.
Photometric data file (IES, LDT): The measured intensity distribution of a real luminaire, from an LM-79 test. Without one, a lighting calculation is an estimate of an imaginary product.
Daylight factor (DF): Indoor illuminance from daylight as a percentage of the unobstructed outdoor illuminance under an overcast sky. The basis of SNI 03-2396 and a quick screen before full simulation.
Display Image Size for 2D Content (DISCAS): The AVIXA method that derives minimum display height from the farthest viewer distance and the smallest element that must be read. It replaces rules of thumb with an auditable number.
Image System Contrast Ratio (ISCR): Contrast measured on the screen with the room's real ambient light on. It is the metric that exposes a bright meeting room specified with a projector that cannot win against it.
Overall Thermal Transfer Value (OTTV, RTTV): Area-weighted heat gain through the building envelope in W/m², combining conduction through walls and glass with solar gain. SNI 6389 sets a 35 W/m² ceiling; RTTV is the same idea for the roof.
Window-to-wall ratio (WWR): Glazed area as a fraction of the façade. Together with shading and glass selection it is the main lever for bringing an OTTV back under the limit.
Solar heat gain coefficient (SHGC, SC): The fraction of solar radiation admitted through glazing. Low SHGC with high visible transmittance is the combination that keeps a façade both cool and daylit.
Thermal transmittance (U-value): Heat flow through a construction per unit area per degree of temperature difference, in W/m²K. Lower is better insulated; it is the reciprocal of R-value.