Glossary Of Terms

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ΔL_w

The measured improvement of impact sound insulation as result of installing a floor covering or floating floor. Result is based on installing on a laboratory test floor to BS EN ISO 717-2:1997

A

A-Weighting

Frequency dependent weighting based on the human hearing range.

Absorption

Conversion of sound energy to heat, usually through the use of an acoustic panel or material

Airborne Noise

Refers to a sound that radiates through the air and, if not treated correctly, can pass through partitions. Referred to in documents such as Approved Document E. 

Anechoic

Defined as ‘free from echo’ meaning that sound cannot reflect from any surfaces creating reverberation. Normally associated with anechoic chambers where a full anechoic chamber will be treated on all elevations including the floor, whilst a Hemi-Anechoic Chamber may have the floor left untreated.

Anti-Vibration (AV) Mounts

A material or other system used to isolate machinery or moving parts from another surface and therefore reducing vibration (and noise) through decoupling the various elements from each other.

Attenuation

Common term used to describe the reduction of noise (and vibration), generally shown in a dB (decibel) level.

Attenuator

A system placed in line that reduces sound as air or gas flows through the attenuator. Generally consists of absorptive materials designed to control set frequency levels.

B

Background Noise

The noise normally present for most of the time and measured statistically to a L_A90.

C

C_tr

Is a correction value used to adjust measurements associated with airborne noise for site testing, to take in to account low frequency noise when used in conjunction with D_nT_w. Standard defining this is BS EN ISO 717.
May also be identified by C (A-weighted pink noise spectrum) or C_tr (A-weighted urban traffic noise spectrum

C-Weighting

A frequency weighting defined by BS EN ISO61672-1. Normally used for Peak measurements and in some entertainment noise measurement standards were low frequency (bass) noise can be an issue.

CRN

Is the Calculation of Railway Noise method used in the UK for predicting rail noise. Usually measured in terms of L_Aeq or L_den

CRTN

Is the Calculation of Road Traffic Noise method used in the UK for predicting road traffic noise. Usually measured in terms of L_A10,18hour

D

Decibel (dB)

More commonly shown as dB, the unit for decibel based on the logarithmic scale for sound intensity.

Decibel  (A-Weighted) (dBA)

More commonly identified as dB(A) or dBA, show an expression of relative loudness of sounds in air as perceived by the human ear. A Weighted values are adjusted to show a reduction in low frequency sounds, compared with unweighted decibels, in which no adjustments are made for audio frequency.

Density

Mass per unit volume. kg/m³ for example (kilogram = unit)

D_n

Is the normalized level difference of airborne sound insulation.

D_nT

Is the standardized level difference of airborne sound insulation.

D_nTw

Refers to the on-site testing for sound insulation between areas and derived as the sum of all paths, for airborne noise. The main path is normally the separating partition however this measurement also takes into account flanking transmission, widows, ventilation and other junctions within the building fabric. D_nT_w may need to be corrected for low frequency sound (see +C_tr) D_w relates to the onsite sound insulation and the _nT element allows for the normalization of reverberation (allows all sound insulation results to be compared equally regardless of reverberation differences)

Damping

Process of converting vibration energy in to heat to decrease vibration (and noise) levels.

Diffuser

A specialized panel or material designed to scatter sound waves in a predetermined fashion instead of absorbing them. These can be designed to diffuse sounds on set frequencies or in a set direction.

Doppler effect

The change in observed frequency and sound level caused by motion between the source and receiver.

E

Ear Defenders/ Protection

Equipment or products designed to protect the human hearing through blocking sound levels/ reducing sound levels to acceptable limits.

Equivalent Continuous Noise Level

See L_Aeq.

Environmental Noise

The European Environmental Noise Directive 2002/49/EC defines this as ‘unwanted or harmful outdoor sound created by human activities, including noise emitted by means of transport, road traffic, rail traffic, air traffic and from sites of industrial activities.

Eyring’s Formula

A modified version of Sabine’s formula for calculating reverberation time.

Exposure Limit Value

A daily (or weekly) limit that an employee can be exposed to and as defined in the Control of Noise at Work Regulations 2005.

F

F (fast) Time Weighting

An averaging time sound used on sound level meters.

Façade Noise Level

Noise level measured close to a façade of a building to determine the affect it may have on reflected sound levels. Measurements are usually taken at a distance of less than 3 metres from the façade.

Field Measurements

Measurements taken from on-site locations as opposed to within the controlled environment of a laboratory.

Flutter Echo

Repeating reverberation in a room caused by sound reflecting off of parallel walls.

Flanking Transmission

Refers to a sound that is transmitted around a partition due to a weakness in how the partitions interact at the junction between one or more of them and includes the effectiveness of how acoustic materials are installed. Flanking noise is included as this can be a potentially major and costly issue to resolve. 

Frequency

The number of pressure variations (or cycles) per second that gives a sound its distinctive tone. the unit of frequency is the Hertz (Hz)

H

Hermholtz Resonator

Consists of an air filled enclosure (acts as a spring) connected to the open air by a narrow neck which acts as mass.

Hertz (Hz)

Represented by Hz and is the unit representing sound frequency (sound wave oscillations per second). One Hertz (1Hz) equals one vibration per second with audible sources to the human ear ranging from 20Hertz to 20,000 Hertz (20Hz – 20,000Hz)

I

Impact Noise

Refers to a sound that is transmitted through a partition (relating to floors only) as a result of an impact directly on to the structure and is connected to sound insulation. Impact has been included as a separate entity to Sound Insulation above given that it is referred to in acoustics when looking at floor partitions in all architectural applications. 

Intelligibility

Refers to how we perceive speech and the degree of how each syllable of speech is understood.

Intensity

The power level by which sound travels through a unit of area in a given direction. Measured in the units W/m².

ISO

International Organization for Standardization.

Isolation

The absence of rigid connections between two or more parts of a structure.

L

L

A-weighted sound pressure Level (SPL) shown in decibels (dB)

L_A

Sound pressure Level (SPL) shown in decibels (dB)

L_A10

Percentile sound pressure level exceeded for 10% of the measuring process, A weighted through statistical analysis

L_A90

Percentile sound pressure level exceeded for 90% of the measuring process, A weighted through statistical analysis

L_AeqT

The average of the total sound energy with A frequency weighting over a specified time period. Commonly used as a descriptor of human perception of sound over time.

L_den

The L_Aeq levels measured over a 24 hour period with different weighting values given to the daytime, evening and night time periods.

L_EPd

Daily personal noise exposure dose

L_EPw

Measured exposure to noise over a working week (40 hours, 5 Days)

L_max

Maximum sound pressure levels recorded during the measuring process

L_min

Minimum sound pressure levels recorded during the measuring process

L’_nT,w

Refers to the on-site testing for weighted and standardized impact sound pressure levels between the floor/ceiling construction of a dwelling. See BS EN ISO 717-2:1997

M

Mass Law

Used as an indicator in predicting the sound reduction index of panels, partitions and other sound reducing materials.

Music Noise Level (MNL)

Defined in the Noise Council Code of Practice on Environmental Noise Council at Concerts, 1995, measures the A-weighted continuous equivalent sound level of music noise.

N

NCR (Noise Reduction Coefficient)

An average of the sound coefficients across the 250, 500, 1000 and 2000Hz frequency bands. Gives a single figure measurement for how effective a product absorbs sound.

Noise

Noise is unwanted sound

Noise Dose

Combines exposure levels that a person is exposed to in respect of sound levela nd time.

Noise Mapping

Noise mapping is an modelling technique used to predict noise levels over a wide area. Given the scale of such areas this is undertaken using computer software.

O

Octave Bands

Show the audio spectrum divided in to 10 equal parts and defined in the Iso standards as 31.5Hz, 63Hz, 125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz, 8kHz and 16kHz.
This spectrum can be refined further by sub dividing the audio spectrum in to 33 bands to form third octave bands.

One-third octave bands

A frequency band in which the upper limit of the band is 2.33 times the frequency of the lower limit.

P

Perceived Noise Level

A reference sound pressure level that observers classify as being equally noisy.

Percentile Level

The sound level, in dBA which is exceed for N% of the time interval T, expressed as L_AN,T

Personal Daily Noise Exposure Level

Expressed as L_EP,d is the sound that a person is exposed to over a 8 hour period and contains the same amount of A-weighted sound energy as is received during the working day.

Pink Noise

A random broadband signal which has equal power across all frequencies.

Plenum

A chamber used to store air prior to it’s distribution.

R

Ray Tracing

A method of modelling room acoustics using computer software.

Reflection

The way sound waves ‘bounce’ off of a surface and continue to move within a space.

Resilient Layer

A layer that isolates one element from another.

Reverberant Area

An area that produces diffuse sound. This includes specially designed rooms that are solely meant for test purposes.

Reverberation

Reverberation (Reverb) is defined as a persistence of sound, that continues after the sound is produced and is often mistakenly described as an echo (which occurs when a pulse of sound can be heard twice).  Reverb is created when sound reflects off of hard surfaces causing numerous reflections that build up and decay as the sound is absorbed by the surfaces of objects within the space.

Reverberation Time

Reverb is measured using a sound meter and predefined noise source to a level where the sound ecays to acceptable levels.
RT60 – How long a sound takes to decay to 60dB (Rt10, RT15, RT20 & RT30 are also options if required)
T_{mf –} Is the mid-frequency reverberation time produced by taking an average of the 500Hz, 1kHz and 2kHz frequency bands. 

Robust Details

A set of tested build details for constructing new dwellings. Using this system guarantees that the new dwelling will meet the requirements of approved Document E.

Room Mode

a 3 dimensional standing wave sound pressure pattern.

R_w

Used in identify the sound reduction value of as single entity when tested under laboratory condditons. For example 50dB Rw represents a sound reduction of 50dB

S

S (slow) time weighting

Standard averaging times for sound level meters.

Sabine’s Formula

A method for calculating reverberation time.

Silencer

A device for reducing noise in air and gas flow systems and commonly found on power generation enclosures that are powered by diesel engines. Often referred to as attenuators.

Sound Absorption

The process by which sound energy is converted to heat, through fiction, leading to a reduction in the sound pressure level.

Sound Absorption Coefficients

Are measured at set frequencies and represent by a value between 0 and 1 (0 = 0% absorption and 1 = 100%) and is represented by the symbol α.

Sound Classification

Is a designation given to how well a material or product absorbs sound energy with A being the best and E being the worst.

Sound Insulation

Sound insulation refers to the practice of preventing noise being transmitted from one space to another, ensuring that sound levels in the receiving space are at acceptable levels. Basic sound insulation is the noise level within one space minus the noise level in the receiving room.

Sound Insulating Material

A material that has sufficient mass, either as a single or multiple layer construction, that minimizes sound transmission between two areas.

Sound Intensity

Sound power level that flows through an unit of area in a given direction. Measured in the unit W/m²

Sound Level

a frequency weighted sound pressure level.

Sound Level Meter

A device used to measure sound pressure levels and with the capabilities to allow for A,C,Z, fast and slow weightings.

Sound Power level SWL, L_w

Sound Power Level (measure of the total power radiated by a source) and measured on the decibel scale

Sound Pressure level SPL, L_p

Sound Pressure level (expressed in decibels as measured by a sound level meter)

Sound Propogation

Transmission of sound energy from one area to another.

Sound Transmission

Transfer of sound energy across a boundary from one medium to another.

Speech Intelligibility

Being able to understand speech without excess effort. To be able to speak clearly and define all syllables from speech clearly and without effort.

Speech Transmission Index (STI)

A measurement for measuring speech intelligibility in a room.

Structure Borne Sound

Sound that is borne through a building in such a way that it reaches the receiver from a source via a building or machine structure. Structure borne sounds transmits very easily and is more difficult to identify than airborne noise.

Tangential Mode

A room mode which involves reflections from two parallel walls.

Traffic Noise Index

An index used in the 1960’s to assess environmnetal noise.

Z-Weighting

A zero frequency weighting.