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BS EN ISO 15186 โ Acoustics: Sound Insulation Measured by Sound Intensity (Full Series)
Measurement of Sound Insulation in Buildings and of Building Elements Using Sound Intensity โ Parts 1, 2 and 3
๐ BS EN ISO 15186 is the British Standard series that specifies methods for measuring the sound insulation of buildings and building elements using the sound intensity technique. Unlike the conventional reverberation-room / sound-pressure-level methods (e.g., the former ISO 140 series), sound intensity measurement directly senses the vector of acoustic energy flow rather than a scalar pressure level. This makes it possible to separate direct transmitted sound from flanking transmission and background noise, delivering more reliable insulation values for both laboratory and field applications. The series comprises three parts: Part 1 covers laboratory measurements, Part 2 covers field measurements, and Part 3 covers laboratory measurements at low frequencies.
๐ก The key advantage of the sound intensity technique is directional awareness: when the measured intensity is negative (energy flowing back towards the source), the contribution does not come from direct transmission through the element under test and can be excluded, making the measured insulation closer to the true behaviour of the element.
1. Series Overview and Scope ๐
BS EN ISO 15186 applies to the measurement of airborne sound insulation of building elements (walls, floors, doors, windows) and between rooms by means of sound intensity. The series was implemented in the UK by BSI as an identical adoption of the ISO 15186 International Standards, issued in the EN ISO series. In building acoustics practice, the intensity method is used alongside the reverberation-room rating methods: the latter provide standardised ratings for product classification, while the former excel where background noise is high or transmission paths are complex.
| Part | Standard | Subject | Typical Use |
|---|---|---|---|
| Part 1 | BS EN ISO 15186-1:2003 | Laboratory measurements | Element sound reduction index under controlled conditions; product comparison and grading |
| Part 2 | BS EN ISO 15186-2:2010 | Field measurements | On-site acceptance testing of floors, walls and partitions in completed buildings |
| Part 3 | BS EN ISO 15186-3:2010 | Low-frequency laboratory measurements | Frequency range about 25 Hz to 160 Hz; lightweight structures and mechanical noise |
โ ๏ธ The three parts differ in frequency range, probe placement and data processing. In particular, Part 3 requires a denser measurement grid, longer averaging and near-field corrections; applying Part 1 practices to the low-frequency band would introduce significant error.
2. Measurement Principle and Key Parameters ๐ฌ
In the intensity method, two closely spaced microphones form a probe; the pressure gradient is approximated by finite differencing to yield the intensity vector. Combining the normal intensity component In with the pressure level Lp allows the apparent transmission loss (or associated insulation descriptor) to be computed. The standard governs the following critical factors:
- Microphone spacing: determined by the upper frequency limit; too large a spacing adds high-frequency error, too small a spacing reduces low-frequency signal-to-noise ratio.
- Sound field index: quantifies the ratio of active intensity to total pressure energy at a point, used to flag points dominated by reactive flow or near-field effects.
- Background-noise correction: in field work the background level must be measured and corrected according to the standard formulas so that a satisfactory signal-to-noise ratio is achieved.
- Spatial averaging: the probe is traversed along prescribed lines or over a grid in the receiving room; the space-averaged intensity is used for the insulation calculation.
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When direct transmission dominates and background noise / flanking transmission is weak, intensity results agree well with reverberation-room data; when flanking or background noise is significant, the intensity method generally yields more trustworthy insulation values.
3. Key Technical Requirements ๐ ๏ธ
The standard specifies the measurement environment, probe placement and data processing in detail. For field measurements, a source room and a receiving room are set up on the two sides of the element under test; the source is typically white or pink noise with a well-diffused field. In the receiving room the probe is moved along lines parallel to the element; the traversal time, number of lines and distance from the element surface must comply with the standard.
For the low-frequency range (Part 3), room modes are sparse and the field is highly non-uniform. The standard therefore requires a denser grid, longer averaging times, and special treatment of near-field intensity to suppress the uncertainty caused by low-frequency modes. Results are reported in 1/3-octave or 1/1-octave bands and can be converted to single-number quantities (e.g., weighted sound reduction index Rw, weighted standardized level difference DnT,w) by the appropriate rating standards.
4. Engineering Practice Applications ๐๏ธ
- ๐ Product development and grading: manufacturers use the Part 1 method to determine the insulation of new elements and to compare products.
- ๐ Field acceptance testing: the Part 2 method evaluates the actual insulation of built floors and partitions, especially where background noise cannot easily be eliminated.
- ๐ง Low-frequency noise control: the Part 3 method supports evaluation of lightweight floors, floating screeds and structures exposed to HVAC, lifts and machinery noise.
- ๐ Acoustic design optimisation: the directional information from intensity measurements localises weak points (gaps, penetrations, back boxes), guiding remedial work.
๐จ The intensity technique is demanding: probe calibration, traversing paths, grid spacing and frequency range must follow the standard strictly or results are not comparable. Always calibrate the probe and check phase matching of the microphones before the measurement campaign.
5. Frequently Asked Questions โ
โ Q: How does the intensity method differ from the reverberation-room pressure method (ISO 140)?
A: The pressure method measures only scalar pressure and cannot distinguish direct from flanking transmission; intensity measures the energy-flow vector and can identify the direction of sound propagation, which is a decisive advantage under high background noise. In return, it demands more rigorous probe calibration and operator skill.
A: The pressure method measures only scalar pressure and cannot distinguish direct from flanking transmission; intensity measures the energy-flow vector and can identify the direction of sound propagation, which is a decisive advantage under high background noise. In return, it demands more rigorous probe calibration and operator skill.
โ Q: What is the difference between Part 1 and Part 3?
A: Part 1 covers the conventional frequency range (typically 100 Hz to 5 kHz) in the laboratory; Part 3 is specialised for low frequencies (about 25 Hz to 160 Hz), requiring a denser grid, longer averaging and near-field corrections for lightweight structures and low-frequency vibration noise.
A: Part 1 covers the conventional frequency range (typically 100 Hz to 5 kHz) in the laboratory; Part 3 is specialised for low frequencies (about 25 Hz to 160 Hz), requiring a denser grid, longer averaging and near-field corrections for lightweight structures and low-frequency vibration noise.
๐ Q: Can the intensity method replace ISO 140 for product certification?
A: Not in general. Product classification normally still relies on reverberation-room methods (e.g., the ISO 10140 series) as the reference; intensity methods serve for on-site diagnosis, separation of complex transmission paths and laboratory cross-checks.
A: Not in general. Product classification normally still relies on reverberation-room methods (e.g., the ISO 10140 series) as the reference; intensity methods serve for on-site diagnosis, separation of complex transmission paths and laboratory cross-checks.
โ Q: What if the background noise at the site is too high?
A: Reduce the background as far as possible (switch off machinery, choose quiet periods) and apply the prescribed background correction; if the signal-to-noise ratio is still inadequate the result is not reliable โ raise the source power or change the measurement strategy.
A: Reduce the background as far as possible (switch off machinery, choose quiet periods) and apply the prescribed background correction; if the signal-to-noise ratio is still inadequate the result is not reliable โ raise the source power or change the measurement strategy.
๐ง Q: How are results used for performance evaluation?
A: Compute the insulation or standardised level difference in the prescribed bands, convert to the relevant single-number quantities with standards such as ISO 717 (Rw, DnT,w), and compare with design targets or regulatory limits.
A: Compute the insulation or standardised level difference in the prescribed bands, convert to the relevant single-number quantities with standards such as ISO 717 (Rw, DnT,w), and compare with design targets or regulatory limits.