Acoustic measurements

Acoustic measurements and nomenclature

When you see a figure in decibels it must always have one of these potentially confusing initials after it to tell you what is being measured and how. Without them, the number is ambiguous at best or even meaningless. All claims for performance should include these and an description of the construction in question.

Rw(dB) – Weighted Sound Reduction Index, A laboratory measurement of the airborne sound insulation performance of an individual material or a particular construction.  Because this measurement is made under ideal conditions, it is typically around 5 to 8dB more optimistic than the equivalent site measurement (DnT,w).

DnT,w;(dB) – An on-site assessment of airborne sound insulation of a complete floor or wall that quantifies it’s performance. Due to the inevitable variations that will occur on site this measurement will always be around 5 to 8 dB lower than the Rw of a similar construction.

Ctr – a correction made to on-site airborne measurements of sound insulation to add extra significance to low frequency noise, primarily because of the popularity of subwoofers.

L'nT,w (dB) – an on-site measurement of impact noise insulation, assessing the amount of noise heard below from a tapping machine placed on the floor above. Confusingly, a lower figure indicates better performance.

ΔLw (dB) – a measurement used to compare the improvement in impact noise due to the inclusion of a soft, resilient underlay.  The larger the number, the more substantial the improvement. Care should be taken with this metric, it involves a concrete subfloor and (crucially) no hard floor finish such as a laminate or timber floor.

Dnc,w (dB)– a measurement of airborne sound insulation performance where the most significant transmission path is from the source room through a suspended ceiling to an adjacent receiving room. SoundBlocker tiles can be used on suspended ceilings  to improve this figure, or you could use either Soundbar 53 or Soundstop to take the partition line up to full height.

LA,eq(dB) – Equivalent continuous sound level, an assessment of the overall noise level measured over a period of time. Typically this measurement is used to assess fluctuating noise levels such as noise at work or to gauge a noise climate.

αw– Alpha, or sound absorption coefficient, this quantifies the efficiency of an absorber at different frequency, 0 is a completely reflective material such glass, 1 is a completely absorptive layer. SRS Sonata acoustic panels have excellent absorptive performance so that fewer panels are needed for a given improvement.