June 4, 2015
|Sound||The transmission of vibrations through a medium such as air or water.|
|Noise||Sound that causes disturbance or annoyance. Noise is assessed in terms of sound pressure levels.|
|Ground-borne Vibration||Vibration transmitted through the ground. Can cause disturbance or annoyance if at sufficient levels. Usually measured as Vibration Dose Values (VDVs).|
|Re-radiated Noise (or ‘Ground-borne Noise’)||Ground-borne vibration can cause room walls, floors and ceilings to radiate noise in the audible frequencies. This type of noise is often referred to as ground-borne noise. Mechanical plant may also generate structure-borne vibration that is reradiated as noise.|
|Decibel (dB)||The standard unit for defining sound pressure levels. The decibel is a logarithmic quantity reflecting the human perception of changes in sound pressure levels. The lower threshold of normal hearing is around 0dB and 130dB 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 to the average listener. A change of 10dB roughly corresponds to a subjective ‘halving’ or ‘doubling’ of loudness.|
|dBA (A-weighted decibel)||A-weighted decibels have been corrected using a frequency weighting that corresponds to the frequency sensitivity of the human auditory system. Measurements of noise in dBA are generally used for assessing human perception of noise.
The table on the right shows indicative A-weighted noise levels (sound pressure levels over the range of human perception) for a variety of sources.
|LAeq,T (equivalent continuous noise level)||The sound level of a notionally steady sound having the same energy as the A-weighted fluctuating sound over the measurement period (T). The A-weighted equivalent continuous 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 to describe road traffic noise levels.|
|LA90,T||The A weighted noise level exceeded for 90% of the specified measurement period (T). It is commonly used to define “background” noise level, i.e. the underlying noise level in the absence of intermittent or short-duration noises.|
|LAmax (maximum noise level)||The highest A-weighted noise level recorded during the measurement period. Unless described otherwise, it is generally measured using the ‘fast’ sound level meter response.|
|Hz (Hertz)||Hz is the unit of frequency, equal to one cycle per second. Frequency of sound waves refers to the number of pressure fluctuations per second. Frequency is related to the pitch of a sound.|
|Octave-band or third-octave band (1/1 or 1/3)||The spectrum of audible frequencies can be separated into proportional ‘bands’ to facilitate ease of computation and analysis (the bandwidth being proportional to the band centre-frequency). Third-octave bands (1/3-octave) split each octave into 3 bands, which allows greater accuracy.|
|Free-field||A sound field determined at a point away from reflective surfaces other than the ground with no significant contributions due to sound from other reflective surfaces. Generally as measured outside and away from buildings.|
|Facade-level||A measurement taken in close proximity (e.g. 1m) to a reflective surface (other than ground) such as a building facade. It is generally accepted that reflections from the surface will increase the measured noise levels by around 3 dB compared to free-field levels, i.e. free-field levels = facade levels – 3 dB.|
|Reverberation||The sound in a room or enclosed space that results from reflections from the room boundaries. The reverberation time (RT) is the time required for the steady sound pressure level in an enclosed space to decay by 60dB.|
|Sound absorption||The process whereby sound energy is converted into heat, leading to a reduction in the sound pressure level. Directly related to reverberation in an enclosed space, i.e. more absorption = lower reverberant level and shorter reverberation time.|
|Sound insulation||A general term referring to the degree by which a material or separating construction reduces sound energy passing through it.|
|Dw (Weighted level difference)||A single-number quantity that describes airborne sound insulation between rooms, but which is not adjusted to reference conditions.|
|DnT,w (Weighted standardized level difference)||A single-number quantity that describes airborne sound insulation between rooms, “corrected” to a reference reverberation time of 0.5 s for dwellings.|
|Dn,e,w||A single-number quantity that describes the sound insulation of ventilators.|
|L′nT,w (Weighted standardized impact sound pressure level)||A single-number quantity that describes the impact sound insulation of floors over a range of frequencies. Impact sound is caused by physical impacts (such as footfall) on the floor. Impact sound insulation is quantified by placing a ‘tapping machine’ on top of the floor and measuring the resultant noise levels in rooms below. LnTw is the sound level difference “corrected” to a reference reverberation time (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 specific noise sources.
A “C” correction is used for living activities (talking, music, radio, tv), children playing, railway traffic at medium and high speeds, highway road traffic > 80km/h, jet aircraft at short distances and factories emitting mainly medium and high frequency noise.
|Ctr||A spectral correction applied to Rw values to account for specific noise sources.
A “Ctr” correction is used for urban road traffic, railway traffic at low speeds, propeller driven aircraft, jet aircraft at large distances and factories emitting mainly 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, i.e. NR ≈ dBA – 6.|
|BS4142:2014||A British Standard that provides guidance when assessing sound of an industrial or commercial nature.|
|Ambient Sound (as used in BS4142:2014)||The sound that comprises the total sound for a specific situation and time (e.g. distant road traffic + wildlife + air conditioning unit)|
|Specific Sound (as used in BS4142:2014)||The sound arising from the source being assessed (e.g. an air conditioning unit).|
|Residual Sound (as used in BS4142:2014)||The sound remaining when the specific sound is inaudible (e.g. distant road traffic + wildlife)|
|Rated Sound LAr,Tr (as used in BS4142:2014)||The specific noise level with adjustment for characteristic features of the noise (i.e. tonality, intermittency or impulsivity)|
|Background Sound (as used in BS4142:2014)||The sound level that is exceeded by the residual sound for 90% of the time.|
|BS8233:2014||A British Standard that provides guidance for the control of noise in and around buildings. This guide suggests criteria such as suitable sleeping/resting conditions, and proposes noise levels that normally satisfy these criteria for most people.|