The increasing use of cross-laminated timber (CLT) in construction has encouraged research on moisture, fire, acoustics, and energy performance. These areas are crucial for CLT buildings’ functionality and durability and are typically addressed together during design. This study reviewed current knowledge, identifies gaps and explores synergies and conflicts among these areas, while briefly addressing the environmental impact of CLT buildings. Key findings include that airtightness is critical but often overlooked, making field and lab measurements essential. Junctions between CLT elements pose challenges in modeling sound and vibration transfer, and water absorption and removal, necessitating further research on junction robustness. More attention should be given to CLT's limited thermal mass and its impact on overheating risks, cooling demands, and fire development. Enhanced ventilation has a limited impact on off-gassing of volatile organic compounds. Covering CLT panels may be beneficial for energy efficiency, fire safety, and acoustics. Environmental impact assessments of buildings are complex and often neglect operational technical aspects. The aspects identified here are crucial for extending the service life of CLT buildings and enabling the reuse of CLT elements in circular economy value chains. Methodological innovations are needed to enhance evaluation flexibility across entire value chains.

Building with cross laminated timber (CLT) has gain increased interest over the last years, but in common to other wood-based building systems, inadequate low-frequency sound insulation is seen as a problem. This paper deals with two methods to improve the sound insulation of CLT panels, normally made from spruce: 1) heavy CLT, introducing compressed, i.e. densified, spruce as well as alternative wood species, and 2) elastic layer based upon shear motion. In addition to a series of laboratory measurements, a full-scale CLT floor made of two 60 mm birch panels with a 12 mm elastic layer in between was tested in a two-room test mock-up. The results from the acoustical measurements showed that the floor has about 7 dB greater airborne and impact sound insulation for one-third octave bands, 50–3150 Hz, compared to a standard CLT floor of the same total height.

In this paper, the acoustical quality in apartment housing is studied. The purpose is to find out to what extent occupants are annoyed by indoor noise and to compare the annoyance with measured airborne and impact sound insulation. The occupants in 38 building cases in Sweden, grouped into different construction categories, were asked in a questionnaire to rate their annoyance for a variety of potentially disturbing sound sources. In total, 1230 individual responses were used for the statistical analyses. The result shows that on average, the occupants are quite satisfied and reported low annoyance. This is taken as an indication that the present National legislation for sound insulation, airborne sound insulation included, works well. However, annoyance from footstep of walking neighbours is an exception, causing significantly greater annoyance compared to any other source, especially among occupants in lightweight buildings. The commonly used impact sound insulation descriptors are unable to match subjective experience. In combination with the sensitivity of lightweight floors to low-frequency sounds, improper building designs are likely to result in poor noise protection for the occupants. To overcome this issue, a new single number quantity taking frequencies as low as 25 Hz into account is suggested.

Through a series of Swedish research projects carried out over twelve years, the relation between the measured impact sound insulation and the corresponding subjective rating given by residents of multi-storey apartment buildings has been studied. Previous results, based on a limited number of building cases, suggested the need to include frequencies below 50 Hz in a frequency weighted single number quantity to get a reasonable correlation with the subjective ratings.

The purpose of this paper is to include an extended amount of data compared to the previous studies to secure more substantiated results. Data has been examined for 38 building cases, including a variety of lightweight and heavy constructions. More than 1200 questionnaire responses form the basis for comprehensive statistical analyses.

The results confirm the conclusions of the previous parts (I, II), stating that frequencies below 50 Hz are of importance when evaluating impact sound insulation, primarily in lightweight buildings. The correlation between measured weighted single number quantities and annoyance ratings increases significantly when the frequency range is extended down to 25 Hz. The strongest correlation was obtained when evaluating the impact sound insulation, again from 25 Hz, when using the weighted single number quantity L_nT,w + C_I,25€-2500 where the C_I-term is evaluated according to ISO 717-2 amended to the extended frequency range 25-€“2500 Hz. It is also suggested that in the lowest one-third octave bands, 25-40 Hz, there is no need to normalize the impact sound pressure levels with respect to the reverberation time.

Wood-based multi-family houses continue to gain popularity. Related to acoustics, low-frequency sound insulation as well as appropriate single number quantities for the evaluation of sound insulation have been in focus for a long time. In a series of Swedish research projects running for 12 years, the correlation between rated annoyance from residents and measured airborne and impact sound insulation, with alternative frequency ranges and weightings, have been studied. In total, 38 building cases of various constructions were involved and more than 1200 questionnaire responses were collected. While the building code’s present evaluation parameter for airborne sound insulation, D'_nT,w + C_50–3150, seems to be working well, the situation is different with respect to impact sound insulation. L'_nT,w as well as L'_nT,w + C_I,_50–2500 show weak correlation with the rated annoyance from the residents. The reason is that frequencies below 50 Hz are overlooked, although they dominate the response from walking in many common, particularly lightweight, floor constructions. The strongest correlation with the rated annoyance from impact sound, including both lightweight and heavyweight constructions, was found when the measured frequency range was extended down to 25 Hz, using L'_nT,w + C_I,_25–2500. Because footstep noise rendered the highest degree of annoyance in the survey, a somewhat more restricted requirement than what it used today is suggested to offer a higher degree of protection against unwanted impact sounds. It is a delicate challenge to design wood-based floor constructions with great sound insulation at low frequencies to meet the higher requirement. A tested innovative floor design based upon two high-density cross-laminated timber plates with an intermediate damping layer may serve as the basis for future constructions. 

When evaluating impact sound insulation of dwellings, including frequencies below 50 Hz has been reported to improve the correlation between the measurements and the occupants’ rating of annoyance from footstep noise. To determine the impact sound insulation, the reverberation time of the receiving room must be considered, a procedure that may introduce large errors at low frequencies. If it can be shown that normal furniture does not affect the absorption (and thereby neither the reverberation time), the reverberation time below 50 Hz could be omitted in the evaluation of impact sound insulation. This would improve the measurement accuracy of the impact sound insulation and simplify the measurement procedure. The purpose of this paper is to investigate to what extent fully furnished rooms for residential purposes affect the reverberation time. Measurements are conducted using the integrated impulse response method in two empty and furnished bedrooms of different construction. Due to the potential errors in the reverberation time measurement, sound pressure level was measured for comparison. No statistically significant absorption difference due to furniture could be found at frequencies below 50 Hz, neither for the measured reverberation times nor the difference in sound pressure level. As a consequence, impact sound insulation may be evaluated without any reverberation measurement below 50 Hz.

An important part of evaluating impact sound insulation is to measure the reverberation time in the receiving room. The increasing trend of using wooden structures for residential buildings has made it increasingly important to have control of the measurement procedure at low frequencies. The present ISO standards do not give any specific guidelines for how to deal with frequencies below 50 Hz. It cannot be taken for granted that the present guidelines developed for higher frequencies are also appropriate for lower frequencies. This paper includes an empirical study of the spatial variation of reverberation time, measured from 20 Hz in two different rooms, with about 100 microphone positions in each room. The results from two alternative methods are compared, based upon the impulse response and the interrupted noise respectively. The accuracy of an intended normal measurement procedure is estimated with respect to the number of microphone positions.

AkuTimber is the joint name for two research projects carried out at Luleå University of Technology 2019-2021. The projects focus upon low frequency sound insulation, which has been pointed out to suffer from high annoyance among the residents of timber houses. One of the main objectives is to find a single number quantity that in a better way, compared to established quantities of today, describes the relation between impact sound measurement and perception. The goal of the study is to include up to 40 building objects in total, including field measurements from 20 Hz as well as questionnaires, although this interim report will not be able to cover them all. The correlation between the residents reported annoyance from impact sound and various number single number quantities of different frequency range is presented for as many building objects as are ready up to bow.

Rapporterat projekt fokuserar på akustiskt prestanda hos KL-träbaserade konstruktioner och behandlar huvudsakligen två arbetsområden; 1) Uppskattning av steg- och luftljudsisolering av olika konfigurationer i en av Martinson utvecklad bjälklagskonstruktion med övergolv samt 2) Modellering av ljudisolering för dubbelväggar.Totalt uppskattades ljudisoleringen för 54 olika bjälklagskonfigurationer. Bäst ljudisolering, Rw,50 = 58 dB och LnT,w,50 = 51 dB, motsvarande ljudklass B för bostäder, uppnåddes med 240 mm KL-stomme kombinerat med ett 220 mm högt övergolv samt av-vibrerat flanktransmissionsbeslag av gångjärnstyp. En viss förbättring förväntas om väggarna i byggnadskonstruktionen utförs med regelstomme i stället för att bestå av KL-skivor.Ljudisoleringen har uppskattats för 32 varianter av dubbelväggar genom analytisk modellering. Bäst ljudisolering, Rw,50 = 62, motsvarande ljudklass A för bostäder, uppnåddes med gipsbeklädda 120 mm KL-skivor med 170 mm mineralfyllt mellanrum. God ljudisolering, motsvarande ljudklass B indikerades för ett flertal varianter, t.ex. gipsbeklädda 120 mm KL-skivor med 120 mm mineralfyllt mellanrum alternativt gipsbeklädda 160 mm KL-skivor med 95 mm mineralfyllt mellanrum.

During the work with the draft standard ISO/DIS 19488 for the sound classification of dwellings, several studies were presented with recommendations on whether 50 or 100 Hz would be the most appropriate lower frequency limit when evaluating single numbers of airborne sound insulation between dwellings. It was observed that 100-3150 Hz is the range used in almost all national building regulations, but including lower frequencies has been considered in some countries and are recommended for higher sound classes in some standards. The Swedish regulations began including evaluation from 50 Hz in 1999, which means there is a long-term experience from the field to support this discussion. In this paper, several single numbers based on field measurements are compared to subjective ratings given by the residents in a variety of building types. The questionnaire surveys were distributed in total 46 building objects with light-weight or heavy walls and floors. Several single number quantities according to ISO 717-1 were correlated against the ratings given by the residents. The statistical evidence for a 50 Hz limit was found to be small on the average, but analysing light-weight buildings separately showed an importance of including sound reduction indices from 50 Hz. Currently, we consider measurements from 50 Hz the best choice when good sound protection against music at low frequencies may occur, since it will also protect from noise at mid and high frequencies. If protection against disturbing speech is sufficient, measuring from 100 Hz is enough. In 2019-2021, about 15 surveys will be made in buildings with timber joist, CLT- or concrete floors.