Acoustic Underlay: How it factors into Value Engineering (VE)?

1 November 2021
Being acoustic consultants in the Middle East, we are often asked this question. In this article, we will try to answer the pertinent questions – why is this topic relevant in our climate today, and why should people care? 
 

Contact

Harout Taghilian, Senior Acoustic Engineer

Harout Taghilian

Principal Consultant - Acoustics Lead
T: +971 4 3343616

Simply put, people greatly value the benefits of quiet environments, where good acoustic enhances the quality of life, improves mental health, provides better working and sleeping conditions – in general adding to the wellbeing of an individual. 

A person’s reaction to noise differs from day-to-day depending on our mood, state of mind, stress levels, health conditions, and time of day. 

Studies have shown that intermittent, irregular, impulsive or impact noises translate to be more annoying, as compared to a steady-state noise, with stress levels on the rise the more people are exposed to it. It has also shown that people are generally more tolerable to external noise sources (i.e., noise sources outside the building) as compared to noise from their neighbours.

With the COVID-19 pandemic, more people are now working from home, children with their online lectures, and the family tightly-knitted under one roof – indicating that any noise be it from external sources or internal sources, are placed to become more evident, disturbing, and disruptive.

Acoustic Regulations 

Building regulations in different countries have specific acoustic requirements for different types of buildings. However, these regulations don’t always guarantee that occupants are satisfied with the acoustic conditions. 

For this reason, ISO/TS 19488:2021 Acoustics – Acoustic classification of dwellings which was published in April 2021 has classification for six different classes A, B, C, D, E and F for dwellings, with Class A being the highest class and Class F the lowest class. The purpose of ISO/TS 19488:2021 is to make it easier for developers to specify a classified level of acoustic quality of a dwelling and help users and builders to be informed about the acoustic conditions and define increased acoustic quality.

The classification of ISO/TS 19488:2021 includes criteria for 5 acoustic aspects:

  • Airborne sound insulation;
  • Impact sound insulation;
  • Airborne sound insulation of building envelopes against outdoor noise from traffic, industry or other sources;
  • Sound pressure levels in the dwellings from service equipment; and
  • Reverberation time. 

In Dubai, all new buildings shall be designed in accordance with Al Sa’fat Dubai Green Building System Version 2.0-2020. Section 403 of this regulation, Chapter 3: Acoustic Comfort, states that all new Villas / Residential Buildings are to be designed in accordance with Building Regulations Approved Document E (latest version) (UK).

Approved Document E covers 4 parts:

  • E1 Protection against sound from other parts of the building and adjoining buildings;
  • E2 Protection against sound within a dwelling-house;
  • E3 Reverberation in the common internal parts of buildings containing flats or rooms for residential purposes; and
  • E4 Acoustic conditions in schools.

When it comes to controlling noise from adjacent dwelling units, Approved Document E has requirements for airborne and impact sound insulation which are summarized in the table below

 Purpose Built Dwelling-Houses And Apartments
Airborne Sound Insulation (Walls & Floors), 
DnT,w + Ctr (dB)
45 (minimum value) 
Impact Sound Insulation (Floors),
L’nT,w (dB)
 62 (minimum value)

 

It should be noted that unlike airborne sound insulation parameters where a higher performance corresponds to a higher rating, a higher level of impact sound insulation is represented by a lower L’nT,w value and for this reason impact sound insulation is specified in terms of maximum value. In other words, the lower the value of LnT,w, the more onerous it is to achieve.

This article will focus on impact sound insulation only. 

The table below is an extract from ISO/TS 19488:2021 regarding the maximum impact sound pressure level between habitable rooms in dwellings from other dwellings in all directions for the six different classes.

Class A Class B
Class C
Class D Class E
Class F
L’nT,w≤46 dB
and
L’nT,50≤50 dB
L’nT,w≤50 dB
and
L’nT,50≤54 dB

L’nT,w ≤54 dB

L’nT,w≤58 dB
L’nT,w≤62 dB
L’nT,w≤66 dB

 

Differences in Design Requirements: Airborne vs Impact Sound Insulation

It is important to understand the difference between airborne and impact sound insulation, as the method of control differs.

In short, airborne sound refers to noise produced by sources that directly set the air around them into vibration. Impact noise refers to noise caused by sources which produce impulsive mechanical excitation of a part of a building (example footsteps, electric light switches, slamming doors)

A misconception by architects, developers and contractors is that having a thick concrete floor will reduce impact noise. Generally speaking, for any given type of construction, the heavier the wall/floor construction, the better its airborne sound insulation (mass law formula). It should be noted however, heavy slabs do little or none to impact noise. 

Unlike airborne sound transmission where the sound insulation is related to the mass of the specimen (in case of single leaf elements e.g., concrete slab, masonry partition), the control of footfall noise from upper floors is dependent on the appropriate design and specification of structural slabs and suitable floor and ceiling constructions.

To control impact noise, soft floor covering can be used, or acoustic underlay can be specified above the structural floor which can be installed either under the screed or under the floor tile to create a structural decoupling between the spaces.

Based on Approved Document E, the soft floor covering should meet the following specification:

  • Any resilient material, or material with a resilient base, with an overall uncompressed thickness of at least 4.5 mm; or
  • Any floor covering with a weighted reduction in impact sound pressure level (∆Lw) of not less than 17 dB.

Based on the above, where there is no suspended ceiling on the level below and where there is hard floor finish on the level above, an acoustic underlay is required to meet the impact sound insulation criteria.

Thin layers of acoustic underlays are generally stiffer than thicker ones and thus there is a risk of acoustic bridging (i.e., between the floating screed and the structural slab) because in such cases due to unevenness or general building debris, the acoustic underlay might be punched through which eventually reduces the effectiveness of the impact sound insulation.

Attention shall also be paid by the contractor to ensure that there are continuous strips of acoustic underlay alongside the walls, pipe penetrations and door frames. The acoustic underlay shall also not be continuous between adjacent apartments and each apartment shall have its own acoustic underlay. This is shown in the figure below.

For a city like Dubai which is famous for its iconic towers, specifying acoustic underlay as a requirement can have a significant cost impact taking into consideration the material cost and the time required for the proper installation of the material.

The question most architects and developers ask is whether the acoustic underlay is required, or it can be removed from the specifications. 

When it comes to impact sound insulation, most of the laboratory test reports available are for acoustic underlays being tested with 150 mm concrete (as opposed to 230 mm PT/RRC slabs here in Dubai). In addition, the tests are conducted either with the acoustic underlay installed directly below the floor tile (without screed) or installed under the screed but without ceramic tiles. Both scenarios differ from what is seen on-site where we usually have 230 mm slab + screed + floor tile. In addition, the requirements in the above tables are in terms of L’nT,w which are on-site performance and not laboratory performance.

A Case Study: Residential Units

In one of the projects, the same question was raised by the Architect to the appointed acoustic consultant. The Client then approached Ramboll as a third party to carry out on-site testing for a building that was already built and ready for tenants to move in.

Upon Client’s request, Ramboll carried out on-site testing in accordance with the procedures of ISO 16283-2:2020 Acoustics - Field measurement of sound insulation in buildings and of building elements - Part 2: Impact sound insulation and the single number ratings were calculated in accordance with the procedures of ISO 717-2:2020 Acoustics – Rating of sound insulation in building and of building elements Part 2: Impact sound insulation.

The measurements were carried out between 2 residential units (empty living rooms). During the measurement the background noise was low, and no external sources affected on the measurement results.

The build-up of the floor-ceiling assembly tested was:

  • 230 mm RC slab;
  • 60 mm screed; and
  • Ceramic tiles.

The results of the measurement showed 66 dB L’nT,w as opposed to the requirement of 62 dB L’nT,w (maximum value).

It can be seen from the test result, that by omitting the acoustic underlay, the impact sound insulation requirement of Al Sa’fat Dubai Green Building System Version 2.0-2020 has not been met and the building is falling under Class F.

Adding an acoustic underlay at this stage of the project was also not possible and the only way to improve impact sound insulation was to either add suspended acoustic ceiling on the level below which would have a significant time and cost impact or to add soft surface finish (example carpet) which was also not possible/desirable.

ISO/TS 19488 also shows that even if the building complies with the local regulation of L’nT,w ≤ 62 dB, the building will be classified as Class E which is described as: ‘A low protection is offered against intruding sounds’ (Annex B of ISO/TS 19488:2021).

In Conclusion

With the studies wrapped up, we have determined that there is still a great need for residential units to include an acoustic underlay between units (unless soft floor finishing is provided), ensuring that the development complies with local regulations, and the project receiving a better Classification.

Even if soft floor finishing is provided – such as carpet - it is still recommended to have acoustic underlay, as future occupants/tenants may remove these soft finishing during their renovations, leaving a higher-than-expected impact noise on the levels below.

Considering the competitive markets, clients are always on the lookout to make their money’s worth, with a growing concern of the acoustic quality of their homes due to previous bad experiences. Our word of advice? Developers should always have a qualified acoustic consultant onboard during the early stages of the project, to guarantee a successful project which not only meets, but exceeds the clients’ and future tenants’ acoustic expectations.

Referenced Documents

  • Al Sa’fat Dubai Green Building System Version 2.0-2020;
  • The building regulations 2010 Approved Document E Resistance to the passage of sound; and
  • ISO/TS 19488:2021 Acoustics – Acoustic classification of dwellings BS 8233 Guidance on sound insulation and noise reduction for buildings.
  • SO 16283-2:2020 Acoustics - Field measurement of sound insulation in buildings and of building elements - Part 2: Impact sound insulation
  • ISO 717-2:2020 Acoustics – Rating of sound insulation in building and of building elements Part 2: Impact sound insulation

Note: The results of this test are only valid for that particular rooms and floor-ceiling assembly, but it gives an idea of what performance to expect.

Ramboll

Ramboll
First Floor, Emerald Building
Oud Metha Road
PO Box 116921, Dubai, UAE
Tel:+971 4 334 3616
Fax:+971 4 334 3617

Mail: info@ramboll.ae

Other sites

Other sites