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Measuring sound

Understanding sound can be complex, so an introduction to the decibel scale is a good place to start. This then provides a basis for where on that scale your glass partition specification should be. The decibel (abbreviated dB) is the unit used to measure the intensity of a sound.

On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB, 100 times more powerful is 20 dB and 1,000 times more powerful is 30 dB. Here are some common sounds and their decibel ratings:

Acoustic performance

Understanding sound

Partitions are acoustically rated according to their performance in laboratory acoustic tests. The resulting figures give an index of performance, which enables us to compare one form of a partition to another and assess a particular type of partition for a specific application.

ISO140 defines the acoustic test procedure used as a means of rating the partitions and ISO717 defines the rating procedure. The procedures enable us to classify building elements according to a single figure, the weighted sound reduction index, or Rw, for ease of comparison and selection.

We construct the partition in a test aperture between two chambers, each isolated from the other. A sound source in one chamber produces a wide spectrum of sound (from 100 – 4000 Hz), we then plot the received sound in the reception chamber on a curve of frequency against the sound reduction in decibels (dB). The value of the selected curve, at a nominal 500 Hz, is deemed to be the ‘weighted rating’ or ‘Rw’ which is used as an international and universally applied index of partition performance.

Understanding sound on-site

We carry out laboratory tests under acoustically optimised conditions. However, the conditions onsite are very different, and most buildings possess a number of acoustic pathways, or flanking transmissions, which will greatly reduce the sound insulation. Typical sources of sound leakage are:

When installed, the nominal acoustic performance of a partition can reduce by 12-15%, even though optimisation of the installation detailing can improve losses. For example, acoustic mastic to abutments, acoustic foam to tracks, thresholds to doors.

The following values provide some general guidance on acoustic privacy in relation to sound reduction:

RwdB effect
25 Normal speech easily overheard
30 Loud speech clearly overheard
35 Loud speech can be distinguished
40 Loud speech heard but not intelligible
45 Loud speech heard faintly
50 Loud speech and shouting can be heard with difficulty

British Standard 5234: 1992: Partitions suggests that the sound insulation performance levels of a general office should be Rw38dB and a private office should be Rw44dB.

 

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