Glossary

Acoustic Panels – Surface-mounted elements designed to control sound within a space by absorbing or diffusing acoustic energy. They help reduce reverberation, improve speech clarity, and balance the overall sound environment in rooms such as offices, studios, and residential or commercial spaces. Learn more

Acoustic Treatment – The process of modifying a room’s acoustic response using absorption, diffusion, and/or bass control elements to improve sound quality, clarity, and balance. Learn more

Bass Traps – Acoustic absorbers specifically designed to reduce low-frequency resonances (typically below 250 Hz), where conventional absorbers are less effective. Commonly placed in room corners where low-frequency pressure accumulates. Learn more

Comb-Filter Effect – Characteristic null and peaks that occur in the sound frequency response due to combination of the direct sound with its reflection coming from untreated surfaces.

Decibel (dB) - The scale on which sound pressure level is expressed. It is a logarithmic unit defined as 20 times the logarithm of the ratio between the root-mean-square sound pressure and a reference sound pressure of 20 x 10^-6 Pa in air. Because it is logarithmic, the decibel scale compresses the wide range of human hearing into a more practical numerical range, making it easier to represent very small and very large sound pressure variations.

Direct Sound - Sound that arrives at the listener’s position directly from the sound source, i.e. without being reflected from any objects or surface.

Early Decay Time (EDT) – A measure of how quickly sound energy decays in a room during the initial 10 dB drop, extrapolated to estimate reverberation behavior. EDT is closely related to perceived clarity and is often more representative of subjective acoustic perception than RT60.

First Reflections - Normally defined as the sound reflections that reach the listening position up to approximately 20 ms after the direct sound.

Flutter Echoes - Repeated sound reflections caused by sound waves travelling between parallel reflective surfaces such as walls.

Late Reflections - Sound reflections that arrive at the listening position after the early reflection window, typically after 50–80 ms depending on room size and acoustic criteria. These reflections form part of the reverberant sound field and contribute to the perception of spaciousness, reverberance, and warmth. However, excessive or poorly controlled late reflections can reduce clarity and speech intelligibility, resulting in a blurred or overly reverberant sound.

Lombard Effect – A psychoacoustic effect in which people involuntarily raise their voice when speaking in noisy environments. It is named after the French otolaryngologist Étienne Lombard, who first described the phenomenon.

Noise Rating Curves (NR) – NR curves are used to describe as a single value noise from mechanical sources. These curves are based in human audition and are used to target background noise targets for different room types according to their noise sensitivity.

Noise Reduction Coefficient (NRC) – A single-number rating that represents the average sound absorption performance of a material across the mid-frequency range (typically 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz). NRC values range from 0 (no absorption) to 1 (perfect absorption), although values above 1 can occur for some highly absorptive materials. NRC is widely used for quick comparison of acoustic materials, but does not describe low-frequency performance or frequency-specific behavior.

Reverberation - An acoustical phenomenon that occurs in enclosed spaces, when sound persists in that space as a result of repeated reflection or scattering from surfaces enclosing the space or objects within it.

Reverberation Time (RT) - A measure of the degree of reverberation in a space. It is equal to the time required for the level of a steady sound to decay by 60 dB after it has been turned off.

Room Modes - At specific frequencies, called room resonance frequencies, standing waves are created within rooms. These frequencies depend on the dimensions and shape of the room. This group of resonance frequencies are normally referred to as room modes. When a sound source generates sound with frequencies equal or close to the room resonance frequencies, the room response will be enhanced and patterns of maximum pressure levels and minimum pressure levels will be produced. The shape of these patterns differs with the room resonance frequency.

Scattering Coefficient – A measure of how evenly a surface disperses reflected sound energy in different directions, distinguishing diffusion quality beyond simple absorption metrics.

Sound Absorption - The portion of the sound energy that is absorbed and not returned when a sound wave hits a surface. Learn more

Sound Diffusion - Sound diffusion occurs when a sound wave hits a complex surface such as a diffuser and its energy is distributed in many directions. Learn more

Sound Insulation - The process of reducing the transmission of airborne sound between enclosed spaces to prevent noise from entering or escaping a room. Sound insulation improves acoustic privacy and comfort by limiting the transfer of sound through walls, floors, ceilings, doors, and other building elements.

Sound Masking - Usually white noise that is used to originate a constant background noise within open plan areas to mask other unwanted sounds such as speech from other people – it is used in open plan offices to improve speech privacy.

Sound Reflection - The portion of the sound energy that is returned when a sound wave hits a surface.

Speaker Boundary Interference Response (SBIR) – Acoustic phenomenon caused by the interaction between direct sound from a loudspeaker and reflections from nearby boundaries (such as walls, floor, or desk surfaces). These interactions create frequency-dependent cancellations and reinforcements, typically resulting in comb-filter-like effects, especially in low frequencies. SBIR is strongly influenced by speaker placement relative to room boundaries and is a key factor in studio monitoring accuracy.

Speech Privacy – It is the inability of an unintentional listener to understand another person’s conversation. Lack of Speech privacy is frequently related with acoustic dissatisfaction within offices – e.g. overhearing unwanted conversations, or feeling overheard.

Standing Waves - A standing wave is originated from the interaction of two sound waves with equal frequency and amplitude but travelling in opposite directions. Unlike the travelling waves, the standing waves do not cause a net transport of energy, since the two waves that form it are carrying equal energy in opposite directions. The resulting standing wave alternates between maximum and zero amplitude.

Weighted Sound Absorption Coefficient (αw) - A single-number rating defined in ISO 11654 that expresses the sound absorption performance of a material based on a standardized weighting procedure across octave bands. Unlike NRC, αw is derived from measured absorption coefficients and fitted to a reference curve, providing a more standardized and internationally comparable value. It is commonly used in European acoustic classification systems (Classes A-E).