Explanation of Terminology by jackshepherd

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									                    ACOUSTIC TERMINOLOGY EXPLAINED


        Explanation of Terminology                                      Page 2

              Simple Technology                                         Page 2

             Further Explanations                                       Page 4

              Acoustic Definitions                                      Page 7

      Average sound pressure levels
                                                                        Page 9
         in Common Environments




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     Explanation of Terminology

     When the recent amendments to Building Regulations were introduced a, whole new list of
     Terms and Abbreviations came into being that are different from the terminology that we have
     been used to.
     The following jargon buster may be useful to decipher some of the terms and abbreviations that
     we will come across on a day-to-day basis, in literature relating to acoustics, and in particular Part
     E.



     Simple Technology
           Absorptive Material                                 Material that absorbs sound

                                                   Sound propagating through the air. Often linked to
             Airborne Sound
                                                              speech, media equipment, etc

            Building Element                                    Walls, floors and roofs etc

                                                 The spectrum adaption term to take account of specific
                       Ctr                       sound spectra, which are predominantly low frequency.
                                                  Only used as a correction to airborne measurements

                Decibel (dB)                        The most commonly used unit to measure sound

                     DB(A)                              Unit of sound weighted to the human ear

                                                 The measurement used to measure the airborne sound
                     D’nT,w
                                                         insulation between two rooms (on site)
                                                 See above, but with the low frequency correction factor
                 D’nT,w + C
                                                                         included

            Final Floor Finish                        Carpet, vinyl, laminate or other top floor finish

                                                Sound transmitted between two rooms using all indirect
          Flanking Transmission
                                                     path i.e. the top or bottom of a separating wall
                                                  Often referred to as “FFT”- An FFT may use battens,
      Floating Floor Treatment                     cradles or platform base all of which use a resilient
                                                      layer to provide isolation from the base floor

              Floating Layer                          A surface layer that rests on a resilient layer.

                                                   The number of pressure variations per second that
                 Frequency
                                                             gives a sound its distinctive tone




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                                               The unit of the frequency of the sound – also known as
                 Hertz (Hz)
                                                                 “Cycles Per Second”
                                                  Sound resulting from direct impact on a building
              Impact Sound
                                                                        element
                                                   Any floor that is not a separating floor between
              Internal Floor
                                                                       dwellings
                                                 Any wall that does not have a separation function
               Internal Wall
                                                                  between dwellings
                                               The measurement used to measure the impact sound
                     L’n,w
                                                insulation of floors (on site) L’n.w = laboratory testing

                    L’nT,w                                   As above but tested on site

                    Noise                                          Unwanted sound

                                                   A new requirement to Part E where structures
       Pre-Completion Testing
                                                  not conforming to the RSD will be tested prior to
                     (PCT)                     completion to check they reach the required standards
                                                 A layer of resilient material (ISO-MAT 45, 60) that
        Resilient Layer Screed                  isolates an element (e.g. floating floor) from another
                                                                 element (base floor)
                                               A collection of pre-approved constructions that, if used
           Robust Detail (RD)
                                                               negate the need for PCT
                                               The measurement used to rate the airborne sound of a
                      R,w
                                                     material or building element in a laboratory

            Seperating Floor                           Floor that separates adjoining dwellings

        Structure Borne Sound                  Sound which is carried by the structure of the building




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     Further Explanations
        Transmission of sound                 Sound is transmitted between different parts of
                                              buildings and between adjacent buildings in two ways.
                                              When sound generated within a room Document E
                                              requirement for floors reaches the bounding surfaces
                                              some of the energy is reflected back into the room and
                                              some generates vibrations will produce sound on the
                                              other side of the structural element. Music and loud
                                              speech are examples of airborne sound.

                                              R,w (weighted sound reduction index) – a laboratory
                                              measurement of the airborne sound reduction
                                              performance of a building element – higher values
             Airborne sound                   denote better performance.

                                              D’nT,w (weighted standardised sound level
                                              difference) – a single number which characterises the
                                              airborne sound between two rooms: A field/site
                                              measurement – higher values denote better
                                              performance.

                                              D’nT,w + Ctr (weighted standarised sound level
                                              difference) – as above with the low frequency
                                              correction included in the figure shown. Document E
                                              requirement for floors – 45dB minimum value.
                                              Direct impacts upon the ‘building structure – such as
                                              footsteps, hammering or door slamming – cause the
                                              structure to vibrate and produce sound waves on both
                                              sides of the surface impacted.

                                              L’nw (weighted normalised impact sound pressure
                                              level) – a laboratory measurement of impact sound
                                              transmission.

              Impact Sound                    L’nTw (weighted normalised impact sound
                                              pressure level) – a field/site measurement of impact
                                              sound transmission between rooms – lower values
                                              denote better performance. Document E requirement
                                              for floors 62dB maximum value.

                                                Lw (Delta Lw) Nett improvement provided by the
                                              material not including the contribution of the floor.
                                              Document E requirement for floors 17dB minimum
                                              value.
                                              Sound id a form of energy, produced when a source
                                              generates waves (rapid variations in pressure) within a
                                              medium. The sound is heard when the waves reach a
                    Sound                     receiver (ear, microphone etc)

                                              The number of waves radiated every second is
                                              described as the frequency of the sound (measured in




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                                              Hertz, Hz), which determines the pitch doubling of the
                                              frequency of a sound increases the pitch by one
                                              octave.

                                              The loudness of a sound depends upon the magnitude
                                              of the pressure changes in the medium. The basic
                                              measure of loudness is the sound pressure level, which
                                              is recorded in decibels (dB). The sound pressure level
                                              is logarithmic, so a doubling of the actual sound
                                              pressure produces a 6dB increase in sound pressure
                                              level.

                                              The human perception of the loudness of sound
                                              depends upon it’s pitch; the ear is less responsive to
                                              very high and very low pitched sounds compared to
                                              those in the middle range (500 – 4000 Hz)
                                              Specialist reports indicate that when a noise level is
                                              reduced by 10 dB, the human ear perceives it to be half
                                              as loud.

                                              Recording the sound level as a single value is good
                                              enough for most everyday uses, but it is not sufficiently
                                              accurate for use in acoustics because the single value
                                              can mask substantial variations in the sound levels at
                                              different frequencies. For these purposes sound levels
                                              are measured across a range of 6 or 8 octaves
                                              (centred on 63, 125, 250, 500, 1000, 2000, 8000 Hz) in
                                              bands 1/3 of an octave wide.
                                              The measures required to reduce the transmission of
                                              sound within and between buildings will vary according
                                              to the type of structure and its reaction to sound
                                              energy. Key factors which affect the behaviour of
                                              structures receiving sound include.

                                              Mass: a greater amount of energy is required to set up
                                              vibrations in a dense structure than in light one, making
                                              a massive structure less likely to transmit sound. That
                                              is why in timber floors a traditional method of reducing
                                              sound transmission has been to increase the mass by
                                              the use of plugging (filling the spaces between joists
                                              with dense material such as sand, ash or plasterboard).
              Sound control
                                              Stiffness: when a building element consists of several
                                              different parts the amount of sound transmitted will be
                                              affected by the stiffness of the connections between
                                              them, such as wall tie or a resilient layer in a floating
                                              floor. The stiffer the connection the greater the sound
                                              transmission between the 4 parts of the structure.

                                              Absorption: components which absorb sound energy
                                              by friction (converting it to heat) will reduce sound
                                              transmission through the building structure.

                                              Cavity: cavities aid the reduction of sound transmission
                                              by isolating parts of the building elements. The width of
                                              the cavity and the number of connections, which cross



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                                              it, will affect the amount of reduction.


      Floating / Isolation Layers             Paths of impact sound can be reduced by a floating or
                                              isolation layer of resilient material

                                              Flanking transmission occurs when sound is
                                              transmitted from one space to another indirectly,
                                              through adjoining parts of the structure.
                                              For example, impact sound may be transmitted from
                                              one room to another through a timber floor, but also
        Flanking Transmission                 through the supporting wall. Other common
                                              mechanisms for flanking transmission include internal
                                              leaves of external cavity walls, which adjoin separating
                                              walls; and also pipework and ducting. Successful noise
                                              solutions must address the possibilities of flanking
                                              transmission.




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     Acoustic Definitions

                                              The Noise Reduction Coefficient, defines how much
                                              sound specific materials absorb. It is the average
                                              sound absorption between 250 Hz – 2kHz rounded to
                                              the nearest 0.05.

                     NRC                      This is analogous to a room’s finishes. Just as various
                                              colours of paint, or textiles, visually alter a room,
                                              various materials – with different NRC ratings, such as
                                              carpet or tile, audibly alter a room. A material with a low
                                              NRC rating (tile) absorbs little sound and a material
                                              with a higher NRC rating (carpet) absorbs more sound
                                              The human ear does not hear all the frequencies with
                                              the same intensity. It is most sensitive to sounds in the
                                              500Hz – 8kHz range. Above and below this range the
                                              ear becomes progressively less sensitive. To
                                              compensate for this, sound level meters incorporate
                    dB(A)                     electronic, filtering to correspond with the varying
                                              sensitivity of the ear.

                                              This filtering is called A-weighting and Sound Pressure
                                              Levels obtained with this weighting are referred to as A-
                                              weighted and signified as dB(A).
                                              Transmission Loss (TL) is a figure which rates the
                                              ability of a material to block sound. It is usually
                                              measured in 1/3 octave band intervals. Mathematically
                                              it is defined as the ratio of the sound energy transmitted
                                              through a material to the sound energy incident on the
                                              material.

                                              The Transmission Loss of a material is measured by
                       TL                     mounting a sample of the material in an opening of a
                                              wall separating two reverberant test rooms. Broadband
                                              noise is played in one room (source). The difference
                                              between the sound levels in the source room the other
                                              (receiving) room is defined as Noise Reduction (NR).
                                              As the frequency and/or density increases the
                                              Transmission Loss also increases. The density of the
                                              material directly related to Transmission Loss.

                                              The ASTM (American) 1/3rd octave Sound
                                              Transmission Losses measured above are referred to
                                              by the European ISO standards as Sound Reduction
                                              Indices (R).
                      SRI
                                              ISO 717/1 defines a standard contour and a procedure
                                              for fitting the contour to the measured sound reduction
                                              indices to determine a single-number rating of a sound
                                              transmission loss spectrum. This rating is called the
                                              Weighted Sound Reduction Index (Rw).




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                                              Unlike The STC contour, the Rw contour is defined
                                              over a slightly lower frequency range of 100 Hz to 3.15
                                              kHz. The contour fitting procedure requires that:
                                              The sound reduction values be determined to one
                                              decimal place.

                                              The contour be raised in 1dB increments to a point
                                              where the average deficiency over the contour
                                              frequency range is as close to. But not exceeding, 2.0
                                              dB.

                                              If an 8 dB or larger deficiency exists in the sound
                                              reduction index data, then the deficiency amount in dB
                                              and the frequencies at which they occur must be
                                              reported.

                                              The average deficiency is the sum of all efficiencies in
                                              all frequency bands divided by 16, the number of 1/3rd
                                              octave frequency bands spanned by the contour.
                                              The actual Rw value is equal to the fitted contour value
                                              at 500 Hz.

                                              It should be noted that the SRI value has been
                                              developed to approximate the performance of a
                                              material in reducing the transmission of speech. The
                                              SRI value obtained from the TL data is useful for a
                                              quick comparison of materials but does not give a true
                                              idea with respect to non-speech sounds such as music,
                                              traffic, trains, aircraft etc.
                                              STC is the American ASTM, standard E413, equivalent
                                              of SRI and is based on the averaged sound insulation
                                              achieved between 125 Hz and 4 kHz. As before. The
                                              standard defines a procedure for determining the STC
                                              rating for a T Loss spectrum by fitting a contour to the
                                              1/3rd octave data. This procedure involves raising or
                                              lowering the contour following these rules.
                                              The contour may not be raised above the point at which
                                              the T loss in any 1/3rd octave band falls more than 8
                                              dB below the contour.

                                              The contour may not be raised above the point at which
                      STC                     the total number of deficiencies is greater than 32.
                                              A deficiency occurs when the TL data in any 1/3rd
                                              octave band falls below the contour by 1 dB.

                                              The STC rating resulting from the contour fitting
                                              procedure is the TL value of the contour at 500 Hz.
                                              It should be noted that the STC value has been
                                              developed to approximate the performance of a
                                              material in reducing the transmission of speech. The
                                              STC value obtained from the TL data is useful for a
                                              quick comparison of materials but does not give a true
                                              idea with respect to non-speech sounds such as music,
                                              traffic, trains, aircraft etc.




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     Average Sound Pressure Levels in Common
     Environments
                                                                         Average Subjective
           (SPL) dB                 Typical Environment
                                                                            Description

                                 30m from military aircraft at
              140                                                              Intolerable
                                          take off

                                   Pneumatic chipping and
              150                                                              Intolerable
                                 riveting, operator’s position

                               Boiler shop (max levels), ships
              120                                                              Intolerable
                                        engine room

                                 Auto punch press operators
              110                position, sheet metal shop,                   Very Noisy
                                    textile weaving room

                                Automatic lathe / milling shop.
              100              Platform of underground station                 Very Noisy
                                         (max level)

                                     Heavy lorries at 6m.
               90                    Construction site with                    Very Noisy
                                       pneumatic drills
                                 Kerb side of a busy street.
               80                                                                 Noisy
                               Office with tabulating machines

               70                Loud radio in domestic room                      Noisy

               60               Restaurant, department store                      Noisy

               50               Conversational speech at 1m                       Noisy

                                  Average suburban area,
               40                                                                 Quiet
                                whispered conversation at 2m

               30                  Residential area at night                      Quiet

                                    Background in TV and
               20                                                              Very Quiet
                                      recording studios

              0-10               Normal threshold of hearing                   Very Quiet




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