Undestanding The Acoustic Performance Of Glass Partitions
In order to understand how best to specify glass partitions, it’s good to start with an introduction to the decibel scale. This then provides a basis for where on that scale your 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. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB. Here are some common sounds and their decibel ratings:
- Near total silence – 0 dB
- A whisper – 15 dB
- Normal conversation – 60 dB
- A lawnmower – 90 dB
- A car horn – 110 dB
- A jet engine – 120 dB
- A gunshot – 140 dB
Partitions are acoustically rated according to their performance in laboratory acoustic tests. The resulting figures give an index of performance, which enables one form of a partition to be compared with another, and a particular type of partition to be assessed for a specific application.
The acoustic test procedure used as a means of rating the partitions is defined in ISO140 and the rating procedure in ISO 717. The procedures enable building elements to be classified according to a single figure the weighted sound reduction index, or Rw, for ease of comparison and selection.
The partition is constructed 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. – and the received sound in the reception chamber is plotted in a curve of frequency against the sound reduction in decibels (dB). This response curve is plotted superimposed on a family of standard performance curves.
The rating, which is biased towards the human ear response, to provide a subjective weighting to the values, is determined by the highest insulation rating curve to which the measured results conform – when the sum of the unfavourable deviations at the sixteen frequencies (between 125 – 3150 Hz.) is as near as possible to, but not greater than 32dB. The value of the selected curve at a nominal 500 Hz. is deemed to be the rating: the ‘weighted rating’ or ‘Rw’ which is used as an international and universally applied index of partition performance.
Laboratory tests are carried out under acoustically optimised conditions. 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. Areas of glazing and the insertion of door openings will also reduce the overall performance of the partitions. Typical sources of sound leakage are:
- Trunking passing through walls, floors, and ceilings
- Flanking transmission above the ceiling line, and along the line of the suspended ceiling
- Floor and wall ductwork passing through the partition wall line
- Acoustic leakage at the junctions of partitions with facades, walls, ceilings, and floors
- Transmission of sound along the façade of the building: this is particularly likely where the façade is glazed
When installed, the nominal acoustic performance of a partition can reduce by 12-15%, even though the installation detailing can be optimised to reduce losses to a minimum. Example: acoustic mastic to abutments, acoustic foam to tracks, thresholds to doors.
There are generally accepted acoustic insulation values which when achieved in laboratory test conditions will provide, with appropriate care in installation, acceptable values on site.
The following table provides some general guidance on acoustic privacy in relation to sound reduction:
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
BS 5234: 1992: Partitions suggests sound insulation performance levels for privacy in some occupational conditions. These are given in the following table:-
General offices Rw38dB
Private offices Rw44dB