Acoustic Glossary

Please see common acoustic definitions below:

General acoustic terms

• Sound: The audible transmission of vibrations through air or water.
• Noise: Unwanted sound. Sound that causes disturbance.
• Ground-borne Vibration: Vibration transmitted through the ground. Has the potential cause disturbance, even damage at sufficient levels. Typically measured as Vibration Dose Values (VDVs).
• Re-radiated Noise (or 'Ground-borne Noise'): Ground-borne vibration can cause walls, floors and ceilings to radiate noise. This is often referred to as ground-borne noise. Mechanical plant may also generate noise by similar means.
• Cross-talk: Sound transmission between rooms via ventilation ducting.
• Decibel (dB): The standard unit for defining sound pressure levels. The range of normal hearing is between 0 dB and 130 dB Where 130 dB is the upper threshold of pain. A change of 1dB in sound pressure levels is barely perceptible and 3dB is normally the minimum audible difference. A change of 5dB is clearly audible. A change of 10dB roughly corresponds to a halving or doubling of perceived loudness.
• dBA (A-weighted decibel): A-weighted decibels use a frequency weighting to correspond to how the human ear hears sound.

BS 4142 terminology

• BS 4142:2014+A1: A British Standard that provides guidance on assessing the effect of noise from industrial and commercial operations on residential dwellings.
• Ambient sound level (La): The sound that comprises the total sound for a specific situation and time (e.g. distant road traffic plus wildlife plus an air conditioning unit or other commercial noise source).
• Specific sound level (Ls): The sound arising from the source being assessed (e.g. an air conditioning unit or other commercial noise source).
• Residual sound level (Lr): The sound remaining when the specific sound is inaudible (e.g. distant road traffic plus wildlife)
• Rating level (LArTr): The specific sound level with penalties for characteristic features of the noise (i.e. tonality, intermittency or impulsivity)
• Background sound level (LA90,T): The sound level that is exceeded for 90% of the time.
• Ambient sound level (La): This is equivalent average noise level (dB LAeq) at a given location.

Control of noise at work terminology

• LEP,d: The estimated daily noise exposure (used in the Control of Noise at Work Regulations 2005).
• LC,peak: The C-weighted peak noise level over an 8-hour workday (used in the Control of Noise at Work Regulations 2005).Environmental noise terminology
• LAeq,T: (equivalent continuous noise level)**: The A-weighted equivalent average noise level (LAeq) is commonly used to describe the average noise level in a given environment over the measurement period.
• LA10,T: The A-weighted level of noise exceeded for 10% of the specified measurement period (T). It gives an indication of the upper limit of fluctuating noise and is commonly used in traffic noise measurements.
• LA90,T: The A-weighted level of noise exceeded for 90% of the specified period (T). It is commonly used to define background noise level; the underlying level in the absence of intermittent noise.
• LAmax (maximum noise level)**: The highest A-weighted noise level recorded during the measurement period. It is measured using the fast sound level meter response.
• Hz (Hertz): Hz is the unit of frequency, equal to one pressure fluctuation cycle per second. Frequency is related to the pitch of a sound.
• Free-field: A sound measurement taking in the absence of any reflecting objects. Generally measured outside and away from buildings.
• Façade-level: A measurement taken in close proximity (e.g. 1 m) to a reflective surface other than ground, such as a building façade. This typically increases the measured level by around 3 dB.
• NSL: Noise-sensitive location. This is typically a dwelling, church, meditation space or other location likely to be significantly affected by noise.

Music noise terminology

• MNL (Music noise level): The A-weighted equivalent average noise level averaged over a 15 minute period (LAeq,15min) . This noise level does not include noise produced by patrons attending the event or other associated noise source.

Room acoustic terminology

• Reverberation: The reverberation time (RT60) is the time required for the sound pressure level in an enclosed space to decay by 60 dB.
• Tmf: The mid-frequency reverberation time. This is the average RT60 of the 500 Hz, 1 kHz and 2 kHz octave bands.
• Sound absorption: Directly related to reverberation. More sound absorption results in a shorter reverberation time.
• Sound insulation: A general term describing how much the sound level is reduced by a partition or facade.
• Dw (Weighted level difference): A single-number quantity describing airborne sound insulation between rooms. This values is not normalised to reference conditions.
• DnT,w (Weighted standardised level difference): A single-number quantity describing airborne sound insulation between rooms. The values is corrected for a standard reverberation time of 0.5 seconds.
• Dne,w: A single-number quantity that describes the sound insulation of ventilators.
• L'nT,w (Weighted standardised impact sound pressure level): A single-number quantity that describes the impact sound insulation of floors. Impact sound is caused by physical impacts on the floor and is quantified by placing a tapping machine on the floor to measure its response. LnTw is the sound level in the room below that has been corrected to a reference reverberation time (typically 0.5 seconds unless stated otherwise).
• Rw (Weighted sound reduction index): The weighted sound reduction index is a single number laboratory-measured rating used to describe the sound insulation performance of building elements.
• R'w (Apparent weighted sound reduction index): Similar to Rw but measured onsite. R'w normally tends to be lower than the laboratory rated Rw level due to onsite conditions.
• C: A spectral correction applied to Rw values to account for the type of noise made by activities such as talking, music, radio, TV, children playing, railway traffic at medium and high speeds, highway road traffic > 80 km/h, jet aircraft at short distances, and factories emitting medium and high frequency noise.
• Ctr: A spectral correction applied to Rw values to account for noise from urban road traffic, railway traffic at low speeds, propeller driven aircraft, jet aircraft at large distances and factories emitting low and medium frequency noise.
• NR (Noise Rating): Noise rating is a method for assigning a single number level to a noise spectrum. It is typically used to specify allowable noise emissions from mechanical ventilation systems in buildings. For a typical noise spectrum the NR level is approximately equivalent to the dBA level minus 6.