Acoustic Evaluation of Floating Floors with Housekeeping Pads

Mechanical and electrical equipment rooms are one of the main sources of noise and vibration in buildings. In high-rise buildings, it is usually inevitable to locate equipment rooms in mid-floors rather than placing them far from noise sensitive areas such as basements or separate structures. The noise from the mechanical equipment such as chillers, circulation pumps, and air handling units in these spaces can travel via and through structure to adjacent occupant spaces. Structure-born noise from the machinery excitation transmitted as impact sound and vibration can be isolated by choosing proper vibration isolators. Yet, air-borne and flanking noise transmission from the flooring should still be carefully treated. Installing a floating floor provides high levels of air-borne and flanking sound reduction in such cases. A floating floor is either constructed by using an air gap or a resilient layer. Spring or rubber type mounts are utilized to provide an air gap. A composite sound transmission loss value for such types of floating floor applications are calculated and presented in this paper.

Keywords: sound transmission, housekeeping pads, floating floors, sound insulation, equipment noise.

Introduction

High rise multistory buildings involving concrete and steel frames are embarked in many countries. Increase in sound insulation performance requirements result in cost oriented and technically practical solutions [1]. The most effective noise control measure is to locate indoor technical rooms as far away as possible from noise-sensitive areas. However, mechanical equipment rooms in high-rise multistory buildings are typically located on intermediate floors, close to the occupied areas they serve. In such cases, appropriate constructive layers should be selected for walls, ceilings, and floors once the amount of noise is assessed within the mechanical equipment rooms. For floorings, floating concrete floors are usually required to separate mechanical spaces from noise-sensitive spaces that are below the mechanical room [2].

Floating floor is a technical term which implies that the flooring is separated from the structure so that it has no rigid connection with surrounding building elements such as walls, floors and columns. This is achieved by using various insulation materials such as rubber mount isolators, resilient layers, flanking bands and strips. As a term, floating floor may refer to various floor isolation methodologies that can be adopted by using these products. Floorings raised on steel constructions in data centers and laminate parquet floorings installed on resilient layers are also called floating floors, but in our case we will be mostly dealing with concrete slabs raised on rubber mounts or springs as used in most mechanical rooms.

Floating Floor Applications in Mechanical Spaces

Insulation for the flooring in mechanical spaces should be chosen according to the equipment type, equipment weight, noise level and adjacent spaces intended purpose of use. Unnecessary and overqualified insulation may result in excess amounts of investment costs. If the main purpose of floor insulation is to overcome impact noise caused by the machinery, then using vibration isolators, resilient layers or rubber pads is probably a better choice since primary objective to install a floating floor with resilient mounts and air cavity is to prevent airborne sound transmission.

Impact noise insulation performance

Floating floors in mechanical rooms are generally not designed as part of a vibration isolation scheme for plant equipment. Floating floors with resilient mounts consist of concrete slab which is completely disconnected from surrounding building elements by vertical flanking strips to separate it from walls and columns, and resilient mounts to support it above the structural floor. The resilient mounts chosen mostly determine the overall impact noise isolation performance of the floating floor application. Assuming an ideal condition in which flanking transmission are neglected and there are no sound bridges, impact sound insulation improvement can be calculated from equation (1).

           (1)

where ρs1 is the surface weight, η1 is the internal loss factor, cL1 is the longitudinal wave velocity, h1 is the thickness of the floating slab, n is the number of resilient mounts per unit area, and s is the stiffness of the mounts used [3]. It is possible to achieve the same or even better impact sound insulation performance with a similar floating floor construction by using a resilient layer instead of rubber mounts. We can consider this case as a locally reacting floating floor. Thus, we can use the following equations for the calculation of improvement in impact noise insulation performance of a floating floor with a resilient layer under ideal circumstances [3].

   (2)

where f is the frequency. The natural frequency fo of the system is

    (3)

where s is the dynamic stiffness of the resilient layer and ρs1 is the surface weight of the floating slab [3].

Comparing resilient mounts and resilient layers impact noise performance from properties of the available products in the market, it is clear that we can achieve similar impact noise performances by choosing appropriate products according to their mechanical properties (Figure 1). Also, vibration isolation products can be used when the foundation or base of a vibrating machine is to be protected against large unbalanced forces or impulsive forces [4]. However, resilient mounts and elastic underlays performance vary a lot when we are dealing with airborne sound insulation. Even when the so called impact noise, structural vibrations and flanking transmissions are damped by vibration isolation, airborne noise transmission can still be a problem.

Figure 1. Comparison of improvement in impact noise performances of floating floor systems constructed with a resilient layer and rubber mounts.

Airborne noise insulation performance

The heavy equipment should be properly supported to account for additional loads such as seismic loads [5]. Therefore, heavy equipment such as a chiller is usually fastened to a housekeeping pad which is anchored to the structural load bearing slab in floating floor applications (Figure 2). It is possible to overcome impact noise and vibration transmission caused by the machine by using an elastic or resilient member between the machinery and the foundation. The problem is, it is usually questioned whether the floating floor that surrounding the housekeeping pad is doing any good in terms of acoustic insulation since its plinth base already creates a short cut for airborne noise transmission through the cross section of the plinth itself.

Figure 2. ASHRAE compatible floating floor design for heavy equipment.

Floating floors and housekeeping pads have different sound transmission losses. For the ease of our calculations we adopt Goesel’s empirical method of double partitions to predict the floating floors sound transmission loss [6]. Calculating the transmission loss of two constituent single partitions RI and RII assuming that there are no structure-borne connections, and the gap is filled with porous sound-absorbing material, the airborne sound transmission through the floating floor can be calculated from equation (4).

  (4)

where ρo is the density and co is the speed of sound in air trapped in between the gap, s is the dynamic stiffness per unit area of the gap, d is the gap thickness, and RFF is the overall sound reduction performance of the floating floor system.

To calculate isotropic single layered structures sound reduction performances, calculation method described in EN 12354: Annex B is adopted [7]. Assuming that floating floor and plinth structure are exposed to the same average sound intensity on the source side, we can calculate the composite transmission loss from equation (5).

          (5)

where SFF is the surface area of floating floor system, SP is the surface area of the plinth structure, and RP is the sound transmission loss of plinth base structure.

Contribution of Housekeeping Pad to Sound Transmission

We evaluate a mechanical room with the equipment described above installed within. We consider a single rigid base of plinth structure made of concrete with a height of 400 mm which is surrounded by a floating concrete slab of 100 mm. Load-bearing concrete slab has a 200 mm thickness as usual in most mechanical spaces and the air gap between the floating slab and the load-bearing concrete slab is considered to be 50 mm. Composite transmission loss is calculated according to the method described for different surface area of housekeeping pad for a fixed area of 200 m² mechanical space.

Figure 3. Change in composite transmission loss for varying surface area of housekeeping pad to a fixed 200 m² surface area of a floating floor system.

Figure 4. Sound power levels of cooling equipment in a mechanical room.

Figure 5. Insulation performance comparison between various floating floor systems.

As it appears, there is a considerable difference between the insulation performance of a whole floating floor and a floating floor that encloses a housekeeping pad (Figure 3). However, once a rigid base of plinth structure is built within a mechanical space, increasing the surface area of the plinth base does not affect insulation performance in a significant way. At this point, the question is whether the performance of floating floors that enclose a plinth base structure is efficient under real working conditions.

Most equipment manufacturers give single value representations of their products noise levels. Unfortunately, we have to work on broad band – or at least one-third octave band – responses of the relevant machinery to design a working isolation system. Therefore, if it is not possible to make measurements on site, having an archive of measurement results of spectral noise levels of common machinery can be an advantage to start with a reasonable design. As an example, we consider a cooling room with a cold water pump, a chiller and an air handling unit (Figure 4). Even though spectral noise characteristics of these three units vary, their combination gives us a flatter response.

Assuming that the total noise within the mechanical space is transmitted through the flooring to an adjacent space, the difference between the total sound power level (SWL) of the equipment and the composite transmission loss of flooring gives us an idea of sound insulation performance of various floating floor systems with and without housekeeping pads. As expected, having a monolithic floating floor is advantageous. Existence of a housekeeping pad causes an increase in noise between 500 Hz and 1 kHz. However, the change of housekeeping pad surface area does not affect noise transmission dramatically (Figure 5).

Conclusion

The plinth structures contribution to airborne sound insulation is investigated and with some simple calculations available in literature it has been found that - for a realistic case - the contribution of housekeeping pads to noise transmission is mostly in between 500 Hz and 1 kHz. Presence of a housekeeping pad causes a conspicuous increase in noise transmission compared to a monolithic floating floor design. However, if a floating floor design is made considering the equipment noise levels from the beginning and appropriate slab thicknesses and insulation materials are chosen, it is expected that the transmission loss should not vary much according to the changing plinth base surface area. For future work, further analysis and a more detailed model should be developed to investigate floating floors with plinth base structures. It is recommended to investigate more about such composite structures contribution to airborne and impact noise transmission especially in mechanical spaces.

Acknowledgement

This project has been funded by Ulus Yapi Tesisat Malzemeleri A.Ş.

References

[1]  S. Smith: Chapter 1: Profiling Existing and New Build Housing Stock, Building Acoustics Throughout in Europe Volume 1: Towards a Common Framework in Building Acoustics Throughout Europe, COST Action TU0901, 2014.

[2]  J. Lilly, A. Mitchell, B. Rockwood, S. Wise, ASHRAE Technical Commitees: 2011 ASHRAE Handbook - Heating, Ventilating, and Air-Conditioning Applications, SI Edition, Chapter 48 – Noise and Vibration Control, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., USA, 2011.

[3]  I. L. Ver, L. B. Beranek: Noise and Vibration Control Engineering. Wiley, USA, 2006.

[4]  S. S. Rao: Mechanical Vibrations. Pearson Prentice Hall, USA, 2004.

[5]  J.R. Tauby, R. J. Lloyd, T. Noce, J. T. F. M. Tunissen: ASHRAE: Practical Guide to Seismic Restraint, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., USA, 1999.

[6]  K. Goesele: Prediction of the Sound Transmission Loss of Double Partitions (without Structureborne Connections), Acoustica, 45, 218-227, 1980.

[7]  EN 12354-1:2000, Building Acoustics – Estimation of Acoustic Performance of Buildings form the Performance of Elements: Part 1: Airborne Sound Insulation Between Rooms, 2000.