Coincidence Effect : Mass Law provides a good working rule to predict the airborne Sound Insulation of a partition up to the region of the
Critical Frequency and the coincidence effect. The coincidence effect occurs when the
Wavelength of the sound in air is the same as the bending waves in the partition.
At a certain frequency and angle of incidence, the bending oscillation of the partition will be amplified and the Acoustic Energy will be transmitted through the partition almost without attenuation.
Critical Frequency : the lowest frequency at which the
Coincidence Effect occurs in a partition is obtained when the incident sound waves graze the partition (parallel with it). This frequency is called the critical frequency in building acoustics.
Flanking Transmission : flanking is the transmission of sound from a source room to a receiving room by paths other than through the separating partition. For example, impact sound may be transmitted from one room to another through a common timber floor. Other common mechanisms for flanking transmission include suspended ceilings, pipework, ducting, etc.
Flanking sound is always present, except in the 'ideal' acoustics laboratory. In practice the sound insulation is often limited by the flanking transmission.
In all cases where it is uncertain whether results are obtained without flanking transmission the normalized impact sound pressure level should be denoted by L'n.
Normalized Impact Sound Pressure Level : L'n : field measurement.
The Normalisation formulae for Ln directly above also applies for L'n.
Normalized Level Difference : Dn : airborne sound transmission. The sound insulation index measured under field conditions, between 'real' rooms and therefore includes effects due to
Flanking, different room sizes and other on-site considerations.
Sound Transmission Class : STC : American single number rating of a partition's isolation value based on laboratory measurement. Results may not be compatible with Rw as a different range of frequencies is used.
Sound Transmission Loss : STL : a measure of sound insulation provided by a structural configuration. Expressed in decibels, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient of the configuration.
Spatial Averaging : taking measurements at various positions and averaging the results. Mandatory in sound insulation measurements and recommended anywhere multiple reflections are present. See other types of Averaging.
Spectrum Adaptation Terms : C and Ctr : The single number rating method defined in BS EN ISO 717 uses a standard reference curve to determine the weighted value of airborne sound insulation.
The spectrum adaptation terms C and Ctr may be used to take into account different source spectra as indicated in the standard.
Standardized Impact Sound Pressure Level : L'nT : the impact Sound Pressure Level in a stated frequency band, corrected for the standardized reverberation time of 0.5 seconds. Field measurement, written L'nT to differentiate between LnT
Standardized Level Difference : DnT : airborne sound transmission. Similar to the Dn, but this index corrects the measured difference to a standardized reverberation time of 0.5 seconds. This
RT value is often cited as approximately average for a medium sized, carpeted and furnished living room. It does not require detailed and accurate knowledge of the dimensions of the test rooms.
DnT = D + 10 lg T/T0
D = level difference
T = reverberation time in the receiving room
T0 = reference Reverberation Time, 0.5 seconds for dwellings.
Transmission Loss : TL : alternative name for sound reduction index, see R and R'
Weighted Apparent Sound Reduction Index : R'w : a single number rating of airborne sound insulation between rooms, over a range of frequencies - field measurement.
Weighted Level Difference : Dw :
single Integer number found by comparing the measured spectrum with the 'standard' curves for airborne and impact insulation.
The Dw value is where the curve meets the 500 Hz curve and the unfavourable deviation is 32 dB. Dw will be identical to DnT,w when T = 0.5 seconds.
Weighted Normalized Impact Sound Pressure Level : Ln,w : Laboratory measurement of sound performance of a building element floor).
There is no flanking (indirect) transmission loss, so only the element under test needs to be considered.
Weighted Sound Reduction Index : Rw : a single-number quantity which characterises the airborne sound insulation of a material or building element over a range of frequencies.
Laboratory measurements - so Rw may be used to compare building elements.
Weighted Standardized Impact Sound Pressure Level : LnT,w :
single number quantity used to characterize the
Impact Sound Insulation of floors, based on laboratory measurement of LnT.
Weighted Standardized Impact Sound Pressure Level : L'nT,w :
single number quantity used to characterize the Impact Sound Insulation of floors, based on field measurement of L'nT.
Weighted Standardized Level Difference : DnT,w : field measurement of airborne sound transmission. Standardized to a standard 0.5 seconds reverberation time and weighted to provide a single figure value.
The results will include flanking transmission so the test is for the total transmission between the rooms, not just the partition.
Weighted Standardized Level Difference with Spectrum Adaption Term : DnT,w + Ctr : a single quantity which characterises the airborne sound insulation between rooms using the Ctr Spectrum Adaption Term defined in BS EN ISO 717.
Definitions Specific to Sound Insulation Measurements
Normalized : The measured sound level difference and impact sound levels in similar dwellings may vary if the sound absorbing materials (soft furnishings) are different.
If the receiving room levels are normalized by adding 10 lg S/A - the Sabine Equation, then room to room variations in the 'field' will not influence the results.
Standardized : If the reverberation times differ between similar rooms in the field (actual dwellings), then there will be a related change in the sound levels measured in the receiving rooms.
Standardizing the impact and airborne sound pressure levels to a reverberation time of T = 0.5 s is equivalent to standardising the results to an equivalent absorption area of A0 = 0.32 V
A0 is the equivalent absorption area in square metres
V is the volume of the receiving room in cubic metres.
Weighted : to establish a Single Figure Rating descriptor the Normalized or Standardized levels are compared to the Reference Curves published in BS EN ISO 717 for airborne or impact noise.
To evaluate the levels in one-third octave bands the reference curve is moved in 1 dB steps towards the measured curve, until the average unfavourable deviation is not more than 2.0 dB. **
An unfavourable deviation at any frequency occurs when the measurement value exceeds the reference value in the case if Impact levels, or is less than the reference value in the case or Airborne measurements. Only the unfavourable deviations are taken into account.
The Single Rating Number, in decibels, is now the reference curve value at 500 Hz.
** 32 dB is often quoted as the unfavourable deviation. This is the total deviation across the 16 one-third octave bands ... 32/16 = 2.0 dB average deviation