Building Acoustics Glossary

Building acoustics uses standardized engineering metrics to evaluate sound insulation, transmission loss, and environmental noise exposure. This reference glossary defines key technical terms, laboratory test metrics, and physical mechanisms used across building specifications.

1. Acoustic Measurement Metrics and Units

Decibel (dB)

A logarithmic unit used to express the ratio of a measured sound pressure level relative to a standardized reference level (20 µPa, representing the threshold of human hearing). Because the decibel scale is logarithmic, a 3 dB increase represents a doubling of sound energy, whereas a 10 dB increase is perceived subjectively as approximately twice as loud.

dB(A) (A-Weighted Decibels)

A frequency-weighting filter applied to measured sound pressure levels to approximate the non-linear sensitivity of the human ear across audible frequencies. A-weighting reduces the emphasis on very low and very high frequencies, reflecting human loudness perception.

R_w (Weighted Sound Reduction Index)

A single-number laboratory rating defined under BS EN ISO 717-1 expressing the airborne sound insulation of a building element tested in isolation under BS EN ISO 10140 laboratory standards. Higher R_w values indicate greater airborne sound attenuation across standard test frequencies (100 Hz to 3150 Hz).

C (Spectrum Adaptation Term)

An adjustment factor applied to R_w accounting for noise sources dominated by medium and high frequencies, such as fast highway traffic, close-range jet aircraft, or high-pitched human speech.

C_tr (Urban Traffic Spectrum Adaptation Term)

An adjustment factor applied to R_w accounting for noise sources containing significant low-frequency energy, such as slow-moving urban road traffic, diesel engines, heavy rail, or low-frequency machinery.

D_nT,w (Weighted Standardized Level Difference)

An in-situ field measurement metric describing airborne sound insulation between two rooms in a completed building. Unlike laboratory single-element ratings, D_nT,w evaluates the complete installed assembly, including reverberation time and flanking transmission paths.

D_n,e,w (Weighted Element Normalized Level Difference)

A specialized laboratory rating expressing the airborne sound insulation of small building components, such as trickle ventilators or air bricks, tested under BS EN ISO 10140 standards.

L_n,w / L_nT,w (Weighted Impact Sound Pressure Level)

Laboratory (L_n,w) and in-situ field (L_nT,w) metrics describing impact sound transmission through floors, evaluated using a standardized tapping machine. Lower impact sound values indicate superior impact isolation.

2. Sound Transmission Mechanisms

Airborne Noise

Sound originating in the air (such as speech, television sound, or passing road traffic) that generates sound pressure waves penetrating building elements.

Impact Noise

Sound generated by direct physical impact on a building element (such as footsteps, dropped objects, or moving furniture on a floor) vibrating the structure and radiating audible sound into adjoining spaces.

Structure-Borne Noise

Vibrational energy generated by mechanical plant, machinery, or heavy ground impacts traveling through solid building materials before radiating as audible sound into connected rooms.

Flanking Transmission

The indirect transmission of sound energy between adjoining spaces along paths other than the primary separating element. Common flanking routes include unisolated side walls, continuous floor voids, unsealed window reveals, roof spaces, and structural junctions.

3. Physical and Acoustic Principles

Coincidence Dip (Critical Frequency, f_c)

A distinct reduction in sound insulation occurring at a specific critical frequency (f_c) where the speed of bending waves in a stiff panel matches the speed of sound in air. At coincidence, sound energy passes through the panel with minimal resistance.

Mass-Air-Mass Resonance (f_m)

A resonant frequency occurring in double-leaf structures (such as double glazing or cavity walls) where the air trapped between two leaves acts as a spring connecting two masses. Sound insulation near or below f_m is temporarily reduced compared to a single leaf of equivalent total mass.

Decoupling

Mechanically isolating building layers (using double stud frames, resilient channels, or independent secondary framing) to break direct structural pathways and prevent vibrational energy from transferring across the assembly.

Damping

The dissipation of vibrational energy within a material, preventing continuous panel resonance. In acoustic glazing, viscoelastic interlayers (such as acoustic PVB) introduce internal shear damping to flatten the coincidence dip.

4. Regulatory Standards and Testing Frameworks

Approved Document E

The UK Building Regulation specifying mandatory performance requirements for resistance to sound passage across separating walls, floors, and internal spaces in residential dwellings, flats, and school buildings.

Approved Document F

The UK Building Regulation specifying minimum ventilation requirements for healthy indoor air quality. Acoustic window and door specifications must balance sound insulation targets against mandatory background airflow requirements.

BS EN ISO 10140

The multi-part international laboratory standard governing procedures for measuring the airborne and impact sound insulation of building components under controlled test conditions.

BS EN ISO 717-1

The international standard defining calculation methods to convert frequency-dependent sound insulation laboratory data into standardized single-number ratings (R_w, C, C_tr).

Reviewed by Sebastian Paszek, CCENM
Technical Director & Lead Acoustic Reviewer

Sebastian Paszek holds the Institute of Acoustics Certificate of Competence in Environmental Noise Measurement (CCENM) awarded in 2018. All terminology, testing standards, and measurement definitions in this glossary are verified against official UK and ISO acoustic frameworks.