Acoustics Research Unit
Research on sound insulation is concerned with all aspects of measurement, prediction and subjective response. An important impact of ARU research has been on the development of International and European Standards in building acoustics.
For cavity wall constructions there are two parameters that can be difficult to model accurately, these are the cavity damping and the foundation coupling. In order to investigate dominant sound transmission paths across masonry cavity wall constructions, measured cavity damping and foundation coupling data have been incorporated in a Statistical Energy Analysis (SEA) model for a test construction in a flanking laboratory . The research presents measured data to validate the SEA predictions for the direct and flanking transmission paths that were dominant. For the masonry cavity wall construction that was tested, structural coupling due to the separating wall foundations was found to be a dominant path for transmission across the separating cavity wall.
Low frequency noise, generated by outdoor and indoor sources, is an increasing problem and there is a need for methods of measurement and prediction of low frequency sound transmission, particularly between dwellings. Experimentally validated models using Finite Element Methods (FEM) have been used to model the sound level in rooms at low frequencies . The sound transmission between adjacent rooms has been modelled using a finite-element method. Predicted sound-level difference gave good agreement with experimental data using a full-scale and a quarter-scale model. Results show that the sound insulation characteristics of a party wall at low frequencies strongly depend on the modal characteristics of the sound field of both rooms and of the partition. The effect of three edge conditions of the separating wall on the sound-level difference at low frequencies was examined: simply supported, clamped, and a combination of clamped and simply supported. It is demonstrated that a clamped partition provides greater sound pressure level difference at low frequencies than a simply supported. It also is confirmed that the sound pressure level difference is lower with equal room volumes than in unequal room configurations. A summary of work on the topic of low-frequency sound transmission between adjacent dwellings is given in .
Artificial Neural Networks (ANNs) are information processing systems that can store knowledge from their environment through a learning process and thus can be used in developing models for prediction. ANNs were applied to existing field data, in the standard frequency range, to confirm the classical mechanisms of sound transmission between dwellings . They then were applied to data from finite element models for the frequency range 40Hz to 100Hz. Results show the influence of parameters in sound transmission at low frequency that are not important at higher frequencies. Of particular significance is the main room dimension, normal to the plane of the party wall, and the edge conditions of the party wall. The results of applying ANN to sound transmission at low frequencies are promising but there remains a need for larger data sets, particularly to improve the prediction of equal room configurations where maximum acoustic coupling takes place.
Beam and block floors are commonly used as separating floors between dwellings. Measurements of the dynamic properties of these floors indicate that beam and block floors are highly orthotropic, and do not act as infinite plates in terms of their driving-point mobility . In addition, transmission suite measurements of beam and block floors with or without a screed show that these floors do not achieve the same level of airborne sound insulation as a homogeneous isotropic concrete slab floor. Significant differences were found in the flanking transmission between masonry walls when coupled to a beam and block floor at a T-junction in comparison with homogeneous concrete slab floors. Vibration measurements indicate that at frequencies below 500Hz, flanking transmission along masonry flanking walls past beam and block floors is significantly higher than with homogeneous concrete slab floors of similar surface density.
Natural ventilation using open windows is an energy efficient and effective method of providing a good indoor air climate in schools. However, open windows not only let fresh air into the building but also external noise. The availability of automatically controlled windows enables precise control over the window opening distance in comparison with manually operable windows. Research has considered the benefits of using small opening distances to provide sufficient ventilation for a good indoor climate as well as attenuation of external noise . Airborne sound insulation tests on a window with different opening distances show that the sound insulation varies significantly depending upon the opening distance; the smaller the opening distance, the higher the sound insulation. In many cases, an opening distance as small as 1cm to 2cm can not only provide sufficient ventilation for a good indoor climate, but also adequate sound insulation. The results of sound attenuation tests are presented. In addition, empirical results from three different naturally ventilated schools, including two in Denmark and one in the UK are described. These results indicate that often only small opening distances are required to obtain a comfortable thermal and atmospheric indoor climate based on measurements during the year.
An investigation was carried out on the performance that can be achieved using timber platform floors on concrete floor bases in order to design a platform floor that could provide at least 29dB ΔLw (loaded and unloaded) for use in floor constructions described in Approved Document E (2003 Edition) . Reconstituted open cell foams were identified as resilient materials that could provide suitable static and dynamic stiffness under lightweight timber floating floors. A platform floor with tongue and grooved chipboard and two layers of reconstituted foam achieved 29dB ΔLw when loaded, and 30dB ΔLw unloaded. Indicative tests on double platform floating floors indicated that they may be beneficial in floor designs where negative values of ΔL cannot be tolerated due to critical impact sound insulation and structural stability requirements. A double floating floor could use dynamically stiffer resilient materials than would be needed in a single floating floor and still provide the required impact sound insulation and structural stability.
Measurements of the sound intensity levels caused by artificial heavy rainfall have been carried out on roof glazing, polycarbonate roofing and ETFE roofing, the latter with and without rain suppressors . The measured sound intensity data allow comparison of products and estimation of the reverberant sound pressure level in a room due to rain noise. The intention is to help designers to assess the likely effect of rain noise from lightweight roofs and roof elements on the indoor ambient noise levels in rooms.
The monograph ‘Sound insulation’ is intended for students, engineers, consultants, building designers, researchers and those involved in the manufacture and design of building products . It uses theory and measurements to explain concepts that are important for the application, interpretation and understanding of guidance documents, test reports, product data sheets, published papers, regulations and Standards. Chapters 1 and 2 deal with theoretical aspects of sound fields in spaces and vibration fields on structures as sound transmission in buildings is fundamentally concerned with the coupling between these fields. Chapter 3 looks at sound and vibration measurements relating to sound insulation and material properties. It deals with the underlying theory behind the measurements, and the reasons for adopting different measurement methods. It also forms a bridge between the sound and vibration theory in Chapters 1 and 2 and the prediction of sound insulation in Chapters 4 and 5. Chapter 4 looks at direct sound transmission across individual building elements. Sound and vibration theory from Chapters 1 and 2 is combined with material property measurements from Chapter 3 to look at prediction models for different sound transmission mechanisms. Chapter 5 concerns sound insulation in situ where there is both direct and flanking transmission. Prediction of vibration transmission across idealized junctions is used to illustrate issues that are relevant to measurement and prediction with other types of walls/floors and junction connection.
Two important variables that affect the airborne sound insulation of cavity masonry separating walls in the field are the foundation detail and the soil type upon which the foundations are built. Vibration transmission has been measured between cavity wall leaves on three different types of foundation: concrete deep trench fill, a strip footing and a strip footing with concrete infill . The results indicate that where a strip footing is used, higher sound insulation can be achieved without the concrete infill. Measurements of the dynamic properties of soils indicate significant differences between the compression stiffness per unit area and the loss factor of different soils. These different soil properties were seen to affect the airborne sound insulation of cavity walls. This explains some of the variation in airborne sound insulation between nominally identical masonry cavity separating walls in the field.
To select an appropriate standard floor impact source to simulate real floor impacts, objective and subjective evaluations of the floor impact sounds were conducted in a box-frame-type structure with reinforced concrete slab floors . The sounds simulated in the test were an adult walking barefoot, children running and jumping represented by a heavyweight impact source, such as a bang machine or an ISO rubber ball, as well as those of a person walking in high-heels or a lightweight object being dropped represented by a tapping machine. Similarity tests between human-made impact sounds and standard heavy-weight impact sounds were performed. Sound Quality (SQ) metrics were used to predict the results of the similarity tests. These results showed that the impact sound of the ISO rubber ball is more similar to a human-made impact sound than the sound of a bang machine. A multiple regression analysis showed that loudness and roughness are significant factors describing the results of similarity judgment among SQ metrics. Much of the data from the standard impact sources, measured in reinforced concrete floors with rigid floor coverings, have been collected. An empirical relationship was established to convert the impact pressure sound level from the bang machine or tapping machine to that from the ISO rubber ball. This study indicates that the use of an ISO rubber ball is reliable for simulating human impact sounds.
An experimentally validated analytical model has been developed in order to investigate the effect on impact sound transmission at low frequencies of location of the impact, type of floor, edge conditions, floor and room dimensions, position of the receiver and room absorption . The model uses normal mode analysis to predict the sound field generated in rectangular rooms due to point excitation of homogeneous rectangular plates with different edge conditions. Laboratory and in situ measurements confirm that the models can be used to estimate impact sound transmission at low frequencies.
Classifications of heavy-weight floor impact sounds based on subjective responses were suggested by conducting two experiments . Five categories for heavy-weight floor impact sounds were suggested based on the annoyance responses. The extent of acoustic comfort in each class was clearly captured by the statements related to the range of LAFmax for each class. Effects of temporal decay rates (DR) and noise sensitivity on the classifications were significant. Differences between the class criteria for impact sounds with DR 60 dB/s and 30 dB/s were approximately 5 dBA. In addition, significant differences were found between the high and low noise sensitivity classification groups for impact sounds in the 50–60 dBA range.
Transient Statistical Energy Analysis (TSEA) has been used to predict impact sounds in heavyweight buildings in terms of the maximum Fast time-weighted sound pressure level using transient sources of mechanical excitation that have complex force time-histories . The sources considered were the rubber ball that is used to measure heavy/soft impacts in buildings, and human footsteps with three different kinds of footwear. A force plate was used to measure the blocked force from these sources in order to calculate a hybrid transient power for input into the TSEA model. TSEA predictions were validated against measurements in a heavyweight building where each of the sources in turn were used to excite a 140 mm concrete floor. Close agreement was observed between measurements and TSEA predictions of maximum Fast time-weighted velocity levels on the concrete floor and a connected masonry flanking wall, as well as the maximum Fast time-weighted sound pressure level in the room below the floor. This confirmed the following: (a) correct implementation of transient power from the measured force time-history in the TSEA model, (b) correct modelling of structure-borne sound transmission between the concrete floor and the masonry wall which confirms that the TSEA model has the potential to include flanking transmission and (c) correct radiation coupling between the concrete floor and the room.
Prediction models have been developed using Statistical Energy Analysis (SEA) to calculate the airborne sound insulation of a timber–concrete composite floor . The complexity in modelling this floor is due to it having (1) a multilayer upper plate formed from concrete and Oriented Strand Board (OSB), (2) multiple types of rigid connector between the upper plate and the timber joists and (3) a resiliently suspended ceiling. A six-subsystem model treats the concrete–OSB plate as a single subsystem and three different five-subsystem models treat the combination of concrete, OSB and timber joists as a single orthotropic plate subsystem. For the orthotropic plate it is suggested that bending stiffnesses predicted using the theories of Huffington and Troitsky provide a more suitable and flexible approach than that of Kimura and Inoue. All SEA models are able to predict the weighted sound reduction index to within 2 dB of the measurement. The average difference (magnitude) between measurements and predictions in one-third octave bands is up to 4 dB. These results confirm that SEA can be used to model direct transmission across relatively complex floor constructions. However, this requires the inclusion of measured data in the SEA model, namely the dynamic stiffness of the resilient isolators and the cavity reverberation time.
Sound insulation prediction models in European and International Standards use the vibration reduction index to calculate flanking transmission across junctions of walls and floors. These standards contain empirical relationships between the ratio of mass per unit areas for the walls/floors that form the junction and a frequency-independent vibration reduction index. However, calculations using wave theory show that there is a stronger relationship between the ratio of characteristic moment impedances and the transmission loss from which the vibration reduction index can subsequently be calculated. In addition, the assumption of frequency-independent vibration reduction indices has been shown to be incorrect due to in-plane wave generation at the junction. Therefore numerical experiments with FEM, SFEM and wave theory have been used to develop new regression curves between these variables for the low-, mid- and high-frequency ranges . The junctions considered were L-, T- and X-junctions formed from heavyweight walls and floors. These new relationships have been implemented in the prediction models and they tend to improve the agreement between the measured and predicted airborne and impact sound insulation.
Research has been carried out to develop and assess protocols for the measurement of transmission functions in lightweight buildings . A transmission function is defined that relates the spatial-average sound pressure level in a room to the structure-borne sound power injected into a wall or floor. The intention is to facilitate the prediction of structure-borne sound transmission from machinery to receiving rooms. Errors in the measurement of the power input can be reduced by using a pair of accelerometers on either side of the excitation point rather than a single accelerometer on one side. Laboratory measurements on a timber-frame wall indicate that steady-state excitation using an electrodynamic shaker and transient excitation with a force hammer can be considered as equivalent. Measured transmission functions from a laboratory test construction below 500 Hz are found not to be significantly affected by the choice of excitation position being directly above a stud or in a bay. Laboratory and field results on different timber-frame walls indicate that with transient excitation using a force hammer, the transmission function is measurable in vertically-, horizontally- and diagonally-adjacent receiving rooms over the frequency range from 20 to 1 kHz. The approach has been applied in field measurements which indicate that there is potential to create databases of average transmission functions as a simplified prediction tool for sound pressure levels from service equipment in buildings.
The effects of floor impact noise on humans were investigated using both psychological and physiological methods . During the laboratory experiments, two factors that impact psychophysiological responses were considered: (1) types of impact sources (standard or real sources) and (2) the levels of floor impact noise ranging from 31.5 to 63 dBA in terms of A-weighted maximum sound pressure level (LAFmax). Twenty-one normal-hearing subjects were then asked to judge the noticeability and annoyance caused by the floor impact noises. Meanwhile, the subjects’ physiological responses (heart rate: HR, electrodermal activity: EDA, and respiration rate: RR) were monitored throughout the experiments. Noise annoyance and noticeability increased with increases in noise levels, the impact ball resulted in higher noticeability and annoyance ratings than real sources. All physiological measures varied significantly with noise exposure; HR decreased, whereas EDA and RR increased.
Field measurements were performed in 26 residential apartments in Korea to investigate levels and types of noise from neighbours . Noise recordings were carried out at each residence in unoccupied conditions. The recordings were analysed at 1 minute intervals in terms of the A-weighted equivalent (LAeq) and maximum sound pressure levels (LAFmax) for three different time periods during the day. It was found that 20 apartments met the recommended WHO guidelines during the daytime (07:00–23:00). However, at night (23:00–07:00), eight apartments were in excess of the WHO guideline value in terms of LAeq while LAFmax exceeded the WHO limit level in 21 apartments during the night. Human footsteps, movement of furniture, and dropping of small items were found to be major sources accounting for approximately 80% of all the noise events. LAFmax of children’s jumping and dropping small items were greater than others. Adults’ walking showed larger variation of noise levels than other sources.
 Hopkins C (1997) Sound transmission across a separating and flanking cavity wall construction. Applied Acoustics vol 52 issue 3/4 pp 259-272.
 Maluski S and Gibbs BM (2000) Application of a finite element model to low frequency sound insulation in buildings. Journal of the Acoustical Society of America vol 108 (4) pp 1741-1751.
 Fora-Moncada A and Gibbs BM (2002) Prediction of sound insulation at low frequencies using artificial neural networks. Building Acoustics vol 9 issue 1 pp 49-71.
 Hopkins C (2004) Airborne sound insulation of beam and block floors: Direct and flanking transmission. Building Acoustics vol 11 issue 1 pp 1-25.
 Andersen A and Hopkins C (2005) Sound measurements and natural ventilation in schools. The International Journal of Ventilation vol 4 issue 1 pp 57-69.
 Hopkins C and Hall R (2006) Impact sound insulation using timber platform floating floors on a concrete floor base. Building Acoustics vol 13 issue 4 pp 273-284.
 Hopkins C (2006) Rain noise from glazed and lightweight roofing IP 2/06. BRE Information Paper.
 Hopkins C (2007) Sound Insulation, Butterworth-Heinemann, Imprint of Elsevier, Oxford, 2007 ISBN: 978-0-7506-6526-1.
 Gibbs BM and Maluski S (2007) Low frequency sound transmission between adjacent dwellings. In Crocker MJ ed(s). Handbook of Noise and Vibration Control. Hoboken, NJ, Wiley and Sons.
 Hopkins C (2008) The effect of foundation details and soil types on the airborne sound insulation of masonry cavity walls. Building Acoustics vol 15 issue 1 pp 1-20.
 Jeon JY, Lee PJ, Sato, S (2009) Use of the standard rubber ball as an impact source with heavyweight concrete. Journal of the Acoustical Society of America vol 126 pp 167-178.
 Neves e Sousa A and Gibbs BM (2011) Low frequency impact sound transmission in dwellings through homogeneous concrete floors and floating floors. Applied Acoustics vol 72 pp 177-189.
 Jeon JY, Hong JY, Kim SM, Lee PJ (2015) Classification of heavyweight floor impact sounds in multi-dwelling houses using an equal-appearing interval scale, Building and Environment vol 94 pp 821-828.
 Robinson M and Hopkins C (2015). Prediction of maximum fast time-weighted sound pressure levels due to transient excitation from the rubber ball and human footsteps. Building and Environment, 94, 810-820.
 Churchill C and Hopkins C (2016). Prediction of airborne sound transmission across a timber-concrete composite floor using Statistical Energy Analysis. Applied Acoustics, 110, 145-159.
 Hopkins C, Crispin C, Poblet-Puig J and Guigou-Carter C. (2016). Regression curves for vibration transmission across junctions of heavyweight walls and floors based on finite element methods and wave theory. Applied Acoustics, 113, 7-21.
 Schoepfer, F, Hopkins, C, Mayr, AR, Schanda, U (2017). Measurement of transmission functions in lightweight buildings for the prediction of structure-borne sound transmission from machinery. Acta Acustica united with Acustica, 103(3), 451-464.
 Park SH, Lee PJ (2017) Effects of floor impact noise on psychophysiological responses, Building and Environment vol 116 pp 173-181.
 Park SH, Lee PJ, Lee BK (2017) Levels and sources of neighbour noise in residential buildings, Applied Acoustics vol 120 pp 148-157.