Mathias Barbagallo, PhD and senior consultant, Brekke & Strand Akustik AB,
Acoustic solutions to improve impact sound insulation should be evaluated on a project-basis in order to identify the optimal ones in respect to the project’s requirements, both structural (e.g. maximum mass per unit area of the floor construction) and esthetical (e.g. keep the CLT structure as visible as possible).
Besides that, sound classification plays an important role in determining the acoustics of a building. By choosing the sound class for each of the relevant acoustic parameters – typically, airborne and structure-borne insulation, sound from service equipment, sound from outdoor noise sources, reverberation time – the acoustic comfort of a building is determined. The ISO standards to measure acoustic parameters are common worldwide, whereas each country has its own sound classification.
Descriptors for impact sound insulation are defined in ISO 717-2. These descriptors are single-number quantities that summarise the acoustic properties of a building element or system. For instance, one can write that the weighted, standardized impact sound pressure level of a floor is L’nT,w 52 dB, where the prime indicates field measurements, i.e. not in an acoustic laboratory.
Behind single-number descriptors there is a frequency spectrum – i.e. the piece of information showing that e.g. a floor has poor sound insulation at low frequency but good at high frequency. Once the single-number descriptor is computed, the information on acoustic performance as a function of frequency is lost, see Figure 8.
ISO 717-2 descriptors are computed between 100 Hz and 3150 Hz, thus excluding the low frequency range, where the acoustic properties of timber construction are critical. ISO 717-2 presents spectrum adaptation terms (CI,50-2500), which are also single-numbers and which give a hint of the low-frequency performance by extending the frequency range down to 50 Hz. There are very few countries (6 in Europe according to B. Rasmussen 2009 and 2019) that consider these lower frequencies in their sound classification. Sweden is one of them, having basic requirement for impact sound in dwellings L’nT,w + CI,50-2500 < 56 dB. For higher sound classes there is even encouragement to evaluate impact sound as down as 20 Hz, see Table 1. In fact, even lower frequencies than 50 Hz should be considered to properly assess impact sound insulation of timber.
Two questions arise. First, if the minimum sound class is chosen, will dwellers be generally satisfied? A quick answer is that dwellers in concrete buildings will generally be satisfied if minimum requirements on impact sound are met, whereas dwellers in timber buildings not. This may be expected since sound classes do not consider whether a building is in timber or concrete and basic descriptors without CI,50-2500 poorly represent acoustic properties of timber building.
Second, impact sound is measured in a specific way, but how well will this measurement – and accordingly ISO 717-2 descriptors such as L’nT,w – represent the acoustic comfort of a timber building? A quick answer is: not so well to date. Dwellers are not interested whether a certain floor behaves well with the ISO tapping machine, but rather whether they will be disturbed by children running or by chairs being moved during their neighbour’s dinner party. There are descriptors, such as those extending the frequency range down to 20 Hz, that may better correlate to acoustic comfort, but which are to date limited by insufficient standardisation and by lack of support both in measuring and calculation standards as well as in datasheets from producers of building elements.
There is work to be done with timber buildings, both in terms of low-frequency measurements techniques, prediction tools, standardization and local regulations.
A summary to conclude. A) Qualitative energy models help to quickly brainstorm for design solutions; talk to an acoustician to get help at early design stage. B) Choose a smart room layout for your project so that it will be easier to meet project’s requirements and achieve good acoustic comfort; again, talk to an acoustician at early design stage. C) With timber constructions there is a gap between fulfilling local regulations and having most dwellers satisfied with their acoustic comfort; promise to deliver a certain sound class but aim during design at a higher one. D) Even if acoustic regulations in your country do not include them, investigate the acoustics of your building between 50 Hz and 100 Hz; the frequency range 50 Hz – 3150 Hz is typically covered both by commercial software and by building acoustics standards as well as by available material input data, and should not require extra work.
Brekke & Strand Akustik is an independent specialist company with acoustics, noise and vibrations as its field of expertise. Our continued focus on technical expertise, independence, cost-effectiveness and good customer service ensures that we can contribute to the largest and most prestigious acoustic tasks in Sweden and Norway where we operate. We are more than 100 specialists within acoustics, with a master’s degree in engineering, a doctoral degree or equivalent. Together we have accumulated more than 1000 years of experience in the field. If you would like to get in touch, just contact Mathias Barbagallo for acoustics of timber constructions () or Johanna Carpelan for other inquiries ().