Acoustics and lighting

In short

Acoustics and lighting are the two "environmental" services of building technology that decide whether a space can actually be heard in and seen in. Acoustics covers room acoustics (reverberation, absorption, defects in halls) and building acoustics (insulating a space from outside and adjacent noise); lighting covers daylighting and artificial illumination, measured in lux and designed by the lumen method. In India they are governed by NBC 2016 Part 8 (Section 1 Lighting and Ventilation, Section 4 Acoustics, Sound Insulation and Noise Control), the IS 3646 and IS 2526 series, the Noise Pollution (Regulation and Control) Rules, 2000, and the energy codes ECBC 2017 (succeeded by ECSBC 2024 where a state adopts it) / Eco-Niwas Samhita. The topic carries steady marks in B.Arch building-science papers, the GATE AR environmental-design section and the Council of Architecture's NATA/ARE-type questions, and is asked at interviews by architectural practices, acoustic consultants (auditoria, studios, multiplexes), lighting-design firms and green-building (GRIHA/IGBC/LEED) consultants.

Explained

Sound: quantities, units and the decibel

• Sound is a longitudinal pressure wave in an elastic medium. Speed in air ≈ 343 m/s at 20 °C (rises ~0.6 m/s per °C); wavelength λ = c/f, so 100 Hz ≈ 3.43 m and 1,000 Hz ≈ 0.34 m — wavelength decides whether a surface absorbs, reflects or diffracts.
• Audible range 20 Hz–20 kHz; speech intelligibility lives mainly in 500–4,000 Hz; octave-band analysis is done at 125, 250, 500, 1k, 2k, 4k Hz.
• Decibel (dB) is a logarithmic ratio. Reference pressure p₀ = 20 µPa (2 × 10⁻⁵ N/m²) = 0 dB = threshold of hearing; 120–130 dB is the threshold of pain.
• Weighting: dB(A) approximates the ear's reduced sensitivity to low frequencies and is the legal/measurement unit for environmental noise; dB(C) for peaks.
• Loudness is subjective: phon (loudness level, equal-loudness contours) and sone (loudness). A 10 dB rise ≈ doubling of perceived loudness; 3 dB is the smallest change normally noticed in the field.
• Intensity falls as 1/r² from a point source in free field (6 dB per doubling of distance); a line source (traffic) falls 3 dB per doubling.
• Behaviour at a surface: incident energy splits into reflected + absorbed + transmitted. Other effects: diffraction (bending around barriers/openings — strong at low frequency, which is why noise barriers work poorly for lorry rumble), diffusion (scattering by irregular/convex surfaces), refraction (wind and temperature gradients outdoors).

Room acoustics: reverberation and absorption

• Reverberation is the persistence of sound by repeated reflections after the source stops. Reverberation time (RT or T60) = time for the sound-pressure level to fall by 60 dB (to one-millionth of energy).
• Sabine's formula (statistical, for reasonably diffuse rooms with α up to ~0.3): RT = 0.161 V/A. Eyring–Norris is used for highly absorptive rooms, where Sabine over-predicts.
• Absorption coefficient α = absorbed energy / incident energy, 0 (perfect reflector) to 1 (open window). Absorption unit = 1 m² sabin (metric) = 1 m² of open window; total absorption A = Σ(Sα) + Σ(objects, persons, seats in sabins).
• Types of absorbers:
  1. Porous / fibrous — mineral wool, glass wool, coir/jute felt, acoustic plaster, carpets, perforated boards over wool. Absorb high and mid frequencies; effectiveness rises with thickness and with an air gap behind.
  2. Panel / membrane (diaphragmatic) — thin plywood, hardboard, gypsum board on battens with a cavity; a mass-spring system absorbing low frequencies at resonance f = 60/√(m·d) approximately.
  3. Cavity / Helmholtz resonators — perforated or slotted blocks and bottle-type cavities, absorbing a narrow band tuned by neck and cavity volume; broadened by lining the cavity.
  4. Variable absorbers — rotating prisms, drapes, retractable banners for multipurpose halls.
• Audience is a major absorber (≈0.4–0.5 sabin per person seated on upholstered seats); hence seats are upholstered so RT changes little between a full and empty hall.
• Optimum RT rises with volume and with the type of use: speech needs short RT for intelligibility; music needs longer RT for blend and fullness.
• Volume per person (guide): speech 3.5–4.5 m³, cinema 3.5–4.5 m³, multipurpose 5–6 m³, concert/opera 7–9 m³. Excess volume is the commonest cause of excess reverberation.

Acoustic defects and the design of halls

• Echo — a distinct repetition; occurs when the reflected path exceeds the direct path by more than about 17 m (delay > ~50 ms). Cure: absorb or re-shape the offending rear wall/ceiling.
• Flutter echo — rapid multiple reflections between parallel hard surfaces; cure by splaying walls (3–5°) or adding diffusers/absorbers on one face.
• Sound focusing / hot spots — concave domes, barrel vaults and curved rear walls concentrate sound; cure by convex or faceted diffusing geometry, or by absorptive treatment.
• Dead spots — areas starved of reflected energy, the complement of focusing; cure with distributed reflectors.
• Sound shadow — under deep balconies and behind columns; limit balcony depth to about twice the opening height.
• Resonance / standing waves (room modes) — boom at particular low frequencies in small rooms with simple proportions; avoid cubic and 1:1:1 / simple integer proportions; use panel absorbers and bass traps.
• Insufficient / excessive loudness — fix by ceiling and splayed side-wall reflectors near the stage, raked floor (rise ~10–15 cm per row) for direct sound and sight lines, reduced source-to-last-row distance (speech limit ~25 m unassisted), and electro-acoustic reinforcement where unavoidable.
• Noise and HVAC intrusion — sound lock lobbies with two doors, no openable windows, lined ducts, low duct velocities, vibration isolators, plant rooms remote from the hall.
• Design sequence for a hall: fix use and audience → volume from m³/person → plan shape (fan, rectangular, horseshoe, reverse-fan for studios) → sight lines and rake → reflective zone (stage, front ceiling, front side walls) and absorptive zone (rear wall, rear ceiling, back of hall) → compute RT octave-band and tune absorption → noise criterion and insulation → services noise control.
• IS 2526 gives the Indian code of practice for acoustical design of auditoriums and conference halls; NBC 2016 Part 8 Section 4 gives recommended noise levels, RT ranges and insulation guidance.

Noise control, sound insulation and Indian limits

• Airborne sound insulation is measured by transmission loss (TL) of a partition in dB; a weighted single figure is STC (ASTM) or Rw / DnT,w (ISO, used in IS 9901 series measurement methods).
• Mass law: TL rises ~6 dB per doubling of surface mass and ~6 dB per doubling of frequency; above the critical frequency there is a dip from coincidence (resonance of bending waves).
• Better than mass alone: cavity (double-leaf) construction — two independent leaves, wide air gap, cavity lined with wool, no rigid bridges; staggered studs or resilient channels; double glazing with unequal pane thickness and a 50–100 mm gap.
• Flanking transmission — sound bypassing the partition via floors, ceiling voids, back-to-back sockets, continuous ducts and door undercuts. One 1% unsealed gap can cost 10–20 dB; sealing is cheaper than mass.
• Impact / structure-borne sound — footfall, machinery. Controlled by floating floors, resilient underlays, carpets, suspended ceilings and anti-vibration mounts; rated by IIC / Ln,w.
• Site and planning measures (IS 4954, noise abatement in town planning): distance and the inverse-square law, buffer green belts, earth berms and barriers (effective when they break line of sight), orientation of noisy roads, planning bedrooms away from the source, service cores as buffers.
• Noise Pollution (Regulation and Control) Rules, 2000, under the Environment (Protection) Act, 1986, enforced by CPCB/SPCBs — ambient limits in dB(A) Leq, day 6 a.m.–10 p.m. / night 10 p.m.–6 a.m.: industrial 75/70, commercial 65/55, residential 55/45, silence zone 50/40 (zone of 100 m around hospitals, educational institutions and courts). Loudspeakers are restricted at night (10 p.m.–6 a.m.), with limited State-notified exemptions for festivals.
• Occupational noise: 90 dB(A) for 8 hours is the long-standing Indian industrial limit, with halving of exposure time per 5 dB rise; the occupational safety provisions now sit under the Occupational Safety, Health and Working Conditions Code (one of the four Labour Codes in force from 21 November 2025, replacing the Factories Act).
• Noise criteria (NC) curves specify permissible octave-band background noise: auditoria/studios NC 20–25, classrooms and hospital wards NC 25–30, private offices NC 30–35, open offices NC 35–40.

Light, vision and lighting quantities

• Luminous flux (Φ) — lumen (lm), total visible light emitted. Luminous intensity (I) — candela (cd), flux per unit solid angle (lm/sr). Illuminance (E) — lux = lm/m², light falling on a surface. Luminance (L) — cd/m², brightness of a surface as seen. Luminous efficacy — lm/W, the efficiency measure.
• Daylight outdoors: clear-sky horizontal illuminance can exceed 100,000 lux; Indian daylight design uses a design sky illuminance commonly taken as 8,000 lux on the horizontal (NBC/IS 2440 practice) for computing daylight factor.
• Quality factors besides quantity: uniformity (min/avg illuminance, usually ≥ 0.4 on the task area), glare — disability glare (veiling) and discomfort glare, rated by UGR (≤19 for offices and classrooms); contrast, modelling/shadow, colour temperature (CCT, K) — 2,700–3,000 K warm, 4,000 K neutral, 5,700–6,500 K cool/daylight; colour rendering index (CRI, Ra) — ≥80 for interiors, ≥90 for art, retail textiles and operation theatres; flicker and stroboscopic effect.

Daylighting

• Daylight factor (DF) = indoor illuminance at a point / simultaneous outdoor unobstructed horizontal illuminance × 100, expressed as a percentage — independent of absolute sky brightness, which is why it is used for design.
• Three components: SC sky component (direct sky seen through the opening), ERC externally reflected component (from facing buildings and ground), IRC internally reflected component (from room surfaces). DF = SC + ERC + IRC, each reduced by glass transmission, framing and dirt factors.
• Indicative DF targets (IS 2440 / IS 7942 practice): dwellings and general living 1–2%, classrooms and offices 2–4% (with higher values at the task), drawing offices and laboratories 4–6%, art studios 6% and above. Values differ between codes and climates — check the current edition before quoting.
• Devices and strategies: orientation (north light gives steady diffuse light in India; south and west need shading), window-to-floor-area ratio (NBC requires minimum openings for light and ventilation — commonly 10% of floor area for habitable rooms, subject to the local building bye-laws), bilateral and clerestory windows, light shelves, light pipes/tubular skylights, skylights and monitors (give 2–3 times the daylight of an equal vertical window but also heat gain), courtyards and atria, jaalis, chajjas, louvres, verandahs, light-coloured ceilings (reflectance 0.7–0.8) and walls (0.5–0.6).
• Shading design uses sun-path charts, solar-geometry (altitude and azimuth), shadow angles (HSA/VSA) and the Mani–Rangarajan Indian solar tables; horizontal devices suit south, vertical/egg-crate suit east and west.
• Deep rooms are daylit effectively only to about 1.5–2 times the window head height; beyond that supplementary electric light is needed — hence daylight-linked dimming and task-ambient design.
• Green rating systems (GRIHA, IGBC) award credits for daylight availability in a defined percentage of the regularly occupied floor area, and ECBC 2017 mandates daylight-responsive controls for perimeter zones in large buildings.

Artificial lighting: sources, luminaires and design

• Sources and typical efficacy: incandescent (GLS) 8–15 lm/W and effectively phased out; tungsten halogen 15–25; CFL 50–70; linear fluorescent T8 ~70–85 and T5 ~85–105 (with electronic ballast); metal halide 70–100; high-pressure sodium 80–130 (poor CRI, yellow — street lighting); LED 100–160 lm/W and rising, with life 25,000–50,000 h and instant restrike. India's UJALA and Street Lighting National Programme (EESL) moved domestic and street lighting largely to LED.
• Luminaire classification by distribution: direct, semi-direct, general diffusing, semi-indirect, indirect; and by light control — specular reflectors, prismatic/opal diffusers, louvres, batwing optics for offices, asymmetric for washing walls and roads.
• Lighting systems: general (uniform), localised, local/task, accent and decorative; emergency and escape-route lighting is mandatory under NBC 2016 Part 4 (Fire and Life Safety).
• Lumen (flux) method gives the number of luminaires for a required average horizontal illuminance; the point-by-point (inverse-square) method gives illuminance at a specific point from a known intensity distribution.
• Utilisation factor (UF / CU) depends on the room index, luminaire type and surface reflectances (tables in IS 3646 Part 3). Maintenance factor (MF / LLF) combines lamp lumen depreciation, luminaire dirt and room-surface dirt; typically 0.7–0.8 for clean interiors.
• Spacing-to-mounting-height ratio (SHR) of about 1:1 to 1.5:1 above the work plane gives acceptable uniformity; the perimeter row sits at roughly half the spacing from the wall.
• Recommended illuminance (IS 3646 Part 2 practice, lux on the task plane): circulation and corridors 100–150; general classrooms and lecture halls 300; offices 300–500; drawing offices, laboratories and detailed drafting 500–750; fine inspection and fine assembly 1,000–1,500; operation theatre task lighting 10,000–100,000 (local); hospital wards 100 general with higher bed-head task light.
• Energy codes: ECBC 2017 — and its successor, the Energy Conservation and Sustainable Building Code (ECSBC) 2024, binding once a state adopts it — sets maximum lighting power density (LPD, W/m²) by space type for commercial buildings with ≥100 kW connected load, plus mandatory controls (occupancy sensors, daylight dimming, time controls); Eco-Niwas Samhita (residential code; Part II, 2021, covers electro-mechanical systems) and the BEE star labelling of lamps and luminaires apply to housing and products. Quote the limit from the current ECBC edition rather than from memory, as the tables are revised.

Key points

• RT (T60) - time for sound to decay 60 dB; computed by Sabine, RT = 0.161 V/A in SI units.
• Absorption coefficient α - fraction of incident energy absorbed; 1 m² sabin = 1 m² of open window.
• Porous absorbers - treat mid/high frequencies; panel absorbers and cavity resonators treat low and narrow-band frequencies.
• Optimum RT - roughly 0.8–1.2 s for speech, 1.2–1.4 s multipurpose, 1.6–2.1 s concert music; rises with hall volume.
• Volume per person - 3.5–4.5 m³ for speech, 7–9 m³ for orchestral music; excess volume causes excess reverberation.
• Echo limit - reflected path more than ~17 m longer than the direct path (≈50 ms delay) is heard as a separate echo.
• Concave surfaces focus, convex surfaces diffuse - domes and curved rear walls are the classic acoustic defect.
• Mass law - partition TL gains about 6 dB per doubling of surface mass or of frequency.
• Cavity construction with no rigid bridges beats added mass for a given weight; any gap destroys insulation.
• Flanking paths - ducts, ceiling voids, back-to-back sockets and door gaps carry sound around a good wall.
• Noise Rules, 2000 - day/night limits 75/70, 65/55, 55/45 and 50/40 dB(A) for industrial, commercial, residential and silence zones.
• NC curves - octave-band background noise limits; auditoria NC 20–25, classrooms NC 25–30, offices NC 30–40.
• Lux = lumen/m² (illuminance received); candela is intensity; cd/m² is luminance (perceived brightness).
• Daylight factor = indoor/outdoor illuminance × 100 = SC + ERC + IRC; design sky commonly taken as 8,000 lux in India.
• Lumen method - N = E·A/(Φ·UF·MF); UF from room index and reflectances, MF for depreciation and dirt.
• Efficacy ranking - LED > sodium/metal halide > fluorescent > CFL > halogen > incandescent.
• Glare and uniformity matter as much as lux - UGR ≤19 and uniformity ≥0.4 for offices and classrooms.
• Codes - NBC 2016 Part 8 Sections 1 and 4, IS 2526 (auditoria), IS 1950 (sound insulation), IS 3483 (industrial noise), IS 4954 (town planning), IS 3646 (interior illumination), IS 2440 / IS 7942 (daylighting), ECBC 2017.

Formulas and examples

Example 1 — RT of a lecture hall and the treatment needed. Hall 20 m × 12 m × 6 m, 200 seated students.

• V = 20 × 12 × 6 = 1,440 m³; volume per person = 1,440/200 = 7.2 m³ — high for speech.
• Absorption: floor (concrete/tiles) 240 m² × 0.05 = 12.0; ceiling (plaster) 240 m² × 0.10 = 24.0; walls 2(20 + 12) × 6 = 384 m² × 0.05 = 19.2; audience 200 × 0.45 = 90.0 → A = 145.2 m² sabin.
• RT = 0.161 × 1,440 / 145.2 = 231.84/145.2 = 1.60 s — too reverberant for speech.
• Target RT = 1.0 s → A required = 231.84/1.0 = 231.8 m² sabin → extra 86.6 sabins.
• Using 25 mm mineral-wool panels, α = 0.75, replacing wall α = 0.05 → net gain 0.70 per m² → area = 86.6/0.70 = 124 m² of panelling (about one-third of the wall area) — place on the rear wall and rear side walls, keeping the front walls and front ceiling reflective.

Example 2 — decibel addition and distance.

• Four identical AHUs, each 85 dB(A) at 1 m: L = 85 + 10 log 4 = 85 + 6.02 = 91.0 dB(A).
• A machine at 86 dB plus a fan at 80 dB: 10 log(10^8.6 + 10^8.0) = 10 log(4.981 × 10⁸) = 87.0 dB — the quieter source adds only 1 dB.
• Free-field level 90 dB at 2 m → at 8 m: 90 − 20 log(8/2) = 90 − 12.0 = 78 dB. Halving noise perception needs 10 dB, i.e. a little under two doublings of distance (6 dB each, so ~1.7 doublings, r × 3.16).

Example 3 — partition insulation. A 115 mm brick wall plastered 12 mm both sides: m = (0.115 × 1,920) + (2 × 0.012 × 1,800) = 220.8 + 43.2 = 264 kg/m².

• At 500 Hz: TL ≈ 20 log(264 × 500) − 48 = 20 log(132,000) − 48 = 20 × 5.1206 − 48 = 102.4 − 48 = 54 dB (theoretical). Site performance is typically 8–12 dB lower (42–46 dB) because of flanking, workmanship and the coincidence dip — never quote the mass-law figure as the field value.
• Taking a realistic TL = 45 dB, partition area S = 15 m², receiving room absorption A₂ = 30 sabins: NR = 45 + 10 log(30/15) = 45 + 3 = 48 dB. A source room at 85 dB therefore leaves 37 dB in the next room — acceptable for an office, marginal for a bedroom at night.

Example 4 — lumen method for a classroom, with LPD check. Classroom 9 m × 7 m, required 300 lux on desks, LED luminaires 40 W / 4,000 lm, UF = 0.60, MF = 0.80, mounting height above the work plane 2.4 m.

• A = 63 m²; N = (300 × 63)/(4,000 × 0.60 × 0.80) = 18,900/1,920 = 9.84 → 10 luminaires, laid out 2 rows × 5, spacing ≈ 3.5 m between rows (9/5 = 1.8 m along each row), i.e. SHR ≈ 1.5 across and 0.75 along — within the acceptable range.
• Achieved average E = (10 × 4,000 × 0.60 × 0.80)/63 = 19,200/63 = 305 lux.
• RI = (9 × 7)/[(9 + 7) × 2.4] = 63/38.4 = 1.64 (use the UF column for RI ≈ 1.5).
• Installed load = 10 × 40 = 400 W → LPD = 400/63 = 6.35 W/m², comfortably within typical ECBC classroom allowances; the same job with 36 W fluorescent battens would need roughly twice the wattage.

Example 5 — point illuminance and daylight factor.

• A downlight of 1,200 cd mounted 3 m above the floor: directly below, E = 1,200/3² = 133 lux; at a point 4 m away horizontally, d² = 3² + 4² = 25 and cos θ = 3/5 = 0.6 → E = (1,200/25) × 0.6 = 28.8 lux — showing why one luminaire cannot light a room uniformly.
• Measured 150 lux indoors when the unobstructed outdoor horizontal illuminance is 8,000 lux → DF = (150/8,000) × 100 = 1.88% — adequate for a living room, short of the 2–4% wanted in a classroom; to reach 300 lux at DF-design conditions one needs DF = 300/8,000 × 100 = 3.75%.

Where it is used

• Auditoria, multiplexes, studios and places of worship: acoustic consultancy for RT tuning, shaping and insulation — e.g. multiplex chains, NCPA-type halls, TV and film studios, convention centres.
• Architectural practice and interior design: daylight and shading design, window sizing to NBC and bye-laws, lighting layouts and specification of LED luminaires; drawings and lux/RT calculations in the services set.
• Green-building and energy consultancy: GRIHA, IGBC and LEED daylight and glare credits, ECBC 2017 LPD and control compliance, Eco-Niwas Samhita for housing; BEE-labelled products.
• Environmental and infrastructure projects: noise mapping and barrier design for metros, highways and airports in EIA reports; compliance with the Noise Pollution Rules, 2000 for diesel-generator and loudspeaker enforcement.
• Industrial and healthcare buildings: machine-noise reduction (IS 3483 practice), enclosures and plant-room isolation; speech privacy in consulting rooms, NICU and operation-theatre lighting.
• Job roles: architect / project architect, acoustic consultant, lighting designer, building-physics or building-performance analyst (Ecotect/Radiance/DIALux/ODEON/Insul workflows), green-building facilitator, government PWD and CPWD design offices.

Interview questions

• Q: What is reverberation time and how is it calculated? A: The time for sound-pressure level to fall 60 dB after the source stops; Sabine's formula gives RT = 0.161 V/A, with V in m³ and A the total absorption in m² sabin.
• Q: Why is the optimum reverberation time shorter for speech than for music? A: Long decay smears successive syllables and lowers intelligibility, whereas music needs overlap of sound for blend and fullness — so speech halls aim near 0.8–1.0 s and concert halls 1.6–2.1 s.
• Q: How does a porous absorber differ from a panel absorber? A: A porous absorber converts the air-particle motion of mid and high frequencies into heat by friction; a panel absorber is a mass-spring system that flexes and absorbs low frequencies near its resonance.
• Q: Why are domes and curved rear walls avoided in auditoria? A: Concave surfaces focus reflected sound into hot spots and leave dead spots elsewhere, and a curved rear wall can send a strong delayed reflection back to the stage as an echo.
• Q: When does a reflection become an echo? A: When the reflected path exceeds the direct path by more than about 17 m, i.e. a delay over roughly 50 ms, so the ear hears it as a separate sound.
• Q: State the mass law and its practical limit. A: Transmission loss increases about 6 dB for each doubling of surface mass or frequency; doubling mass is costly and ineffective near the coincidence dip, so a lined double-leaf cavity wall is preferred.
• Q: Why does a well-built partition still fail on site? A: Flanking transmission and gaps — ceiling voids, continuous ducts, back-to-back electrical boxes, door undercuts and unsealed junctions; a small opening can lose 10–20 dB.
• Q: What are the Noise Pollution Rules limits for a residential zone? A: 55 dB(A) Leq by day (6 a.m.–10 p.m.) and 45 dB(A) at night, under the Noise Pollution (Regulation and Control) Rules, 2000; a silence zone is 50/40 dB(A) within 100 m of hospitals, schools and courts.
• Q: Differentiate lux, lumen, candela and cd/m². A: Lumen is total luminous flux emitted, candela is intensity per unit solid angle, lux is lumens per square metre falling on a surface, and cd/m² is luminance — the brightness of a surface as seen by the eye.
• Q: Why is daylight factor used instead of absolute lux in daylight design? A: It is a ratio of indoor to simultaneous outdoor illuminance, so it stays constant as the sky brightness changes and lets a design be judged independently of the moment of measurement.
• Q: Write the lumen-method formula and explain UF and MF. A: N = E·A/(Φ·UF·MF); the utilisation factor is the fraction of lamp flux reaching the work plane, set by room index and surface reflectances, and the maintenance factor allows for lamp lumen depreciation and dirt.
• Q: A room has 500 lux but users complain of discomfort — why? A: Quantity is not quality: likely discomfort glare from bare high-luminance sources or a bright window (high UGR), poor uniformity, veiling reflections on screens, or unsuitable colour temperature and CRI.
• Q: Which orientation is preferred for studio daylighting in India and why? A: North, because it gives steady diffuse sky light with no direct sun for most of the year, avoiding glare, contrast swings and heat gain.

Common mistakes

• Using Sabine's formula in highly absorptive or very irregular spaces where Eyring applies, or forgetting to include audience and seat sabins — the single largest absorption term in a hall.
• Mixing up the unit of absorption: α is dimensionless, the absorption *A* is in m² sabin, and only Σ(S·α) goes into the formula.
• Treating sound absorption and sound insulation as the same thing — acoustic foam on a wall barely improves transmission loss; insulation needs mass, cavity and airtightness.
• Adding decibels arithmetically (85 + 85 = 170) instead of logarithmically (85 + 10 log 2 = 88 dB).
• Quoting theoretical mass-law TL as the achieved field value and ignoring flanking and the coincidence dip.
• In lighting, omitting UF or MF, taking lumens per *luminaire* when the table gives lumens per *lamp*, or reporting average illuminance as if it were the value on every desk.
• Over-glazing for daylight without shading and glare control, which adds heat gain and fails both comfort and ECBC compliance.
• Quoting withdrawn or superseded code numbers and editions; check the current IS/NBC edition and the local building bye-law before stating a limit.

Related topics

Building services: plumbing, electrical and HVAC · Climatology and passive solar design · Building materials and finishes · Landscape and site planning · National Building Code and building bye-laws · Green buildings, GRIHA/IGBC and ECBC compliance