# Architectural Acoustics Fundamentals Architectural acoustics is the science of controlling sound within and between building spaces. For the architect, acoustic design determines speech intelligibility in auditoria, privacy between dwellings, noise control in hospitals, and the perceived quality of every interior environment. Good acoustic design is inseparable from spatial planning, material selection, and building services coordination. --- ## Table of Contents - [Sound Fundamentals](#sound-fundamentals) - [Room Acoustics Theory](#room-acoustics-theory) - [Reverberation Time](#reverberation-time) - [Absorption Coefficients](#absorption-coefficients) - [Speech Intelligibility](#speech-intelligibility) - [Acoustic Metrics](#acoustic-metrics) - [Room Types and Target Criteria](#room-types-and-target-criteria) - [Acoustic Modeling](#acoustic-modeling) - [Material Selection for Acoustics](#material-selection-for-acoustics) - [See Also](#see-also) --- ## Sound Fundamentals | Property | Description | Unit | |----------|-------------|------| | **Frequency** | Number of oscillations per second | Hz | | **Wavelength** | Physical length of one sound wave cycle | m | | **Amplitude** | Magnitude of pressure variation | Pa | | **Sound Pressure Level (SPL)** | Logarithmic measure of sound pressure relative to threshold | dB | | **Sound Power Level** | Total acoustic energy emitted by a source | dB (re 10⁻¹² W) | | **Speed of Sound** | ~343 m/s in air at 20°C | m/s | ### Frequency Ranges | Range | Frequency | Architectural Relevance | |-------|-----------|------------------------| | Low (bass) | 20–250 Hz | Structure-borne sound, HVAC rumble, music | | Mid | 250–2000 Hz | Speech range, most building noise | | High (treble) | 2000–20,000 Hz | Sibilance, electronic noise, birdsong | Human hearing is most sensitive at 1–4 kHz; sound insulation and absorption data must be considered across the full frequency spectrum, not just as single-number ratings. --- ## Room Acoustics Theory Sound behaviour in rooms involves: | Phenomenon | Description | |-----------|-------------| | **Direct Sound** | First-arrival sound travelling straight from source to listener | | **Early Reflections** | Sound reflected once or twice from room surfaces (arriving <80 ms after direct) | | **Late Reverberation** | Diffuse sound field from multiple reflections | | **Flutter Echo** | Rapid repetitive reflection between parallel surfaces | | **Standing Waves** | Resonance at frequencies related to room dimensions | | **Diffusion** | Scattering of sound energy in multiple directions | | **Focusing** | Concentration of sound by concave surfaces (usually undesirable) | ### Room Modes In rectangular rooms, standing waves occur at frequencies determined by room dimensions: f = (c/2) × √((n/L)² + (m/W)² + (p/H)²) Where L, W, H are room dimensions and n, m, p are mode integers. Problematic in small rooms (studios, practice rooms) where modes fall within the audible speech/music range. --- ## Reverberation Time Reverberation time (RT60 or T60) is the time for sound to decay by 60 dB after the source stops. It is the most fundamental acoustic metric for room design. ### Sabine Equation T = 0.161 × V / A Where: - T = reverberation time (seconds) - V = room volume (m³) - A = total absorption (m² Sabins) = Σ(αᵢ × Sᵢ) ### Eyring Equation (for highly absorptive rooms) T = 0.161 × V / (-S × ln(1 - ᾱ)) Where ᾱ is the average absorption coefficient and S is total surface area. ### Target Reverberation Times | Space Type | RT60 Target (mid-frequency) | |-----------|---------------------------| | Recording studio | 0.2–0.4 s | | Classroom | 0.4–0.7 s | | Office (open plan) | 0.5–0.8 s | | Conference room | 0.5–0.8 s | | Lecture hall | 0.7–1.0 s | | Theatre (drama) | 0.8–1.2 s | | Concert hall (orchestral) | 1.8–2.2 s | | Cathedral/church | 2.0–5.0+ s | --- ## Absorption Coefficients The absorption coefficient (α) measures the fraction of incident sound energy absorbed by a material, ranging from 0 (fully reflective) to 1.0 (fully absorptive). | Material | 125 Hz | 250 Hz | 500 Hz | 1000 Hz | 2000 Hz | 4000 Hz | |----------|--------|--------|--------|---------|---------|---------| | Concrete (painted) | 0.01 | 0.01 | 0.02 | 0.02 | 0.02 | 0.03 | | Plasterboard on studs | 0.29 | 0.10 | 0.06 | 0.05 | 0.04 | 0.04 | | Carpet (heavy on concrete) | 0.02 | 0.06 | 0.14 | 0.37 | 0.60 | 0.65 | | Mineral wool (50mm) | 0.15 | 0.45 | 0.70 | 0.80 | 0.85 | 0.80 | | Acoustic ceiling tile | 0.25 | 0.45 | 0.65 | 0.75 | 0.80 | 0.70 | | Audience (per person) | 0.25 | 0.35 | 0.42 | 0.46 | 0.50 | 0.50 | | Heavy curtain | 0.07 | 0.31 | 0.49 | 0.75 | 0.70 | 0.60 | --- ## Speech Intelligibility | Metric | Description | Target | |--------|-------------|--------| | **STI** | Speech Transmission Index (0–1) | >0.60 good, >0.75 excellent | | **RASTI** | Rapid STI (simplified) | Equivalent to STI | | **C50** | Clarity — ratio of early (0–50ms) to late sound energy | >0 dB for speech | | **D50** | Definition — fraction of energy arriving within 50ms | >0.50 for speech | | **SNR** | Signal-to-Noise Ratio | >15 dB for clear speech | Factors degrading intelligibility: excessive reverberation, high background noise, flutter echoes, poor loudspeaker placement, unfavourable room geometry. --- ## Acoustic Metrics | Metric | Application | Description | |--------|-------------|-------------| | **RT60** | All rooms | Reverberation time | | **EDT** | Performance spaces | Early Decay Time (more perceptually relevant than RT60) | | **C80** | Music venues | Clarity for music (early-to-late energy ratio, 80ms) | | **G (Strength)** | Concert halls | Sound level relative to free field | | **LF (Lateral Fraction)** | Concert halls | Proportion of lateral early reflections | | **IACC** | Concert halls | Interaural Cross-Correlation (spatial impression) | | **Lnw / L'nw** | Dwellings | Impact sound insulation rating | | **Rw / STC** | Partitions | Airborne sound insulation rating | --- ## Room Types and Target Criteria | Room Type | Key Acoustic Goal | Critical Metric | |-----------|-------------------|-----------------| | Classroom | Clear speech for all students | STI > 0.60, RT < 0.6s, BNL < 35 dB | | Open office | Speech privacy, reduced distraction | RT 0.5–0.8s, D₂,S > 7 dB | | Hospital ward | Patient rest, speech privacy | BNL < 40 dB, RT < 0.8s | | Concert hall | Rich, enveloping musical sound | RT 1.8–2.2s, C80 -1 to +3 dB | | Recording studio | Controlled, neutral environment | RT 0.2–0.4s, NC-15 to NC-25 | | Restaurant | Ambient buzz without shouting | RT 0.6–1.0s | --- ## Acoustic Modeling | Tool | Type | Application | |------|------|-------------| | **ODEON** | Ray-tracing/image-source | Room acoustics simulation | | **CATT-Acoustic** | Ray-tracing | Auditoria, concert halls | | **EASE** | Ray-tracing | Electroacoustic system design | | **Pachyderm** | Grasshopper plugin | Acoustic analysis in parametric environment | | **I-Simpa** | Open-source | Sound propagation research | | **Insul** | Prediction software | Partition sound insulation calculation | --- ## Material Selection for Acoustics | Strategy | Materials | Effect | |----------|-----------|--------| | **Absorption** | Mineral wool, foam, fabric panels, perforated boards | Reduces reverberation, controls flutter | | **Diffusion** | Schroeder diffusers, QRD panels, profiled timber | Scatters sound, improves spatial quality | | **Reflection** | Concrete, glass, stone, plaster | Directs sound, maintains energy | | **Mass** | Dense concrete, brick, multilayer drywall | [[Sound Insulation Design]] between spaces | | **Resilience** | Resilient channels, floating floors, neoprene mounts | Decouples structure, reduces flanking | --- ## See Also - [[Sound Insulation Design]] - [[Room Acoustics Design]] - [[Noise Control in Buildings]] - [[Acoustic Materials and Systems]] - [[Vibration Isolation]] - [[Thermal Comfort Fundamentals]] --- #acoustics #room-acoustics #reverberation #sound #building-performance #speech-intelligibility