Design, construction and commissioning of a reverberation room John Pearse 1 University of Canterbury Department of Mechanical Engineering Private Bag 4800 Christchurch New Zealand Aaron Healy University of Canterbury Department of Mechanical Engineering Private Bag 4800 Christchurch New Zealand

ABSTRACT In this paper we discuss the design, construction and commissioning of a reverberation room that meets the requirements of ISO 354:2003. The new room was built using lightweight double walls rather than the more usual monolithic construction. We present an analysis of measurements made during the commissioning of the room and discuss these results; these measurements include the effect of adding various diffusing elements, lighting and heating. We made measurements of the sound absorption coefficient of different materials in the commissioned room and these are compared with measurements of the same materials in two other reverberation rooms that also comply with the requirements of ISO 354:2003.

1. INTRODUCTION

In this paper we discuss the design, construction and recent commissioning of a reverberation room that meets the requirements of ISO 354:2003. Product testing, product development work, teaching and research will be carried out using the new room.

2. DESIGN

ISO 354:2003 describes the volume and shape requirements for a reverberation room. A design with parallel walls and with the roof parallel to the floor was chosen in order to provide known modes and ease of construction – contrary to the guidelines in the committee draft version of ISO 354:2019(E). A room volume of 221 m3 was selected with a length of 7.7 m, a width of 6.1 m and a height of 4.7 m.

1 john.pearse@canterbury.ac.nz

3. CONSTRUCTION

The room was constructed within an existing building that had a concrete floor. The concrete floor was cleaned and a 75mm thick concrete slab was poured over it and finished with an epoxy paint to provide a hard surface. A double wall construction was selected to achieve the required sound insulation (STC greater than 60). The wall construction consisted of gypsum plasterboard screwed to 92 mm x 1.15 mm steel studs separated by a 24 mm air gap. The inside walls of the reverberation room were lined with 2 mm thick sheets of aluminium and the outside walls of the room were protected with 18 mm thick plywood. The wall assembly is depicted in Figure 1. The internal wall aluminium facing sheets were bonded to the underlying plasterboard using an adhesive while all other linings were screwed at 300 mm centres.

Figure 1: Wall construction. The roof matched the wall construction with minor modifications to accommodate the structural requirements. In particular the roof structure employed I-beams in place of studs with the interior ceiling being bolted and glued to the lower side of the I-beams and the exterior surface glued and bolted to the top of the I-beams. ISO 354:2003 requires a diffuse sound field and this was achieved by the use of stationary diffusers. The diffusers were fabricated from 1200 mm by 2400 mm galvanised steel sheets of thickness 0.95 mm and surface density 7.7 kg/m2. The surface area of the diffusers corresponded to 16% of the internal surface area of the room. The diffusers were suspended from the ceiling by steel cables through holes cut in the diffusers. The cables were attached to eyebolts in the ceiling and secured in place. This enabled the height and angle of each diffuser to be fixed in such a way as to promote a diffuse sound field. 4. COMMISSIONING

A sample of highly absorbent acoustic media was placed in the room, and the sound absorption measured while adding diffusers. The results are shown in Figure 2 below.

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0.00 5.76 11.52 17.28 23.04 28.80 34.56 38.64 42.72 46.80

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Figure 2: Initial diffuser performance The absorption in the 100 Hz one-third octave band exceeded allowable levels. The final diffuser arrangement consists of five metal diffuser and three boundary diffusers. The boundary diffusers are two triangular surfaces with a base length of 0.5 m and a height of 1.1 m in opposing corners and a larger polyhedron shape with a square base of sides 2.08 m and perpendicular height 0.6 m corresponding to a volume of 0.87 m3 mounted just off the centre of the ceiling. This arrangement improved the empty room absorption curve, see Figure 3 below.

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Figure 3: Final diffuser performance

Room absorption ISO 354:2003 specifies a maximum empty room sound absorption area. Measured values are presented in Figure 4 below.

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Figure 4: Empty room sound absorption area The maximum allowable equivalent absorption area is clearly exceeded in the 100 Hz one-third octave band. The metal diffusers were consequently replaced with 12 mm thick plywood diffusers sealed with an epoxy paint and hung from the ceiling in a random fashion. The diffuser dimensions were three 1.4 m by 1.2 m; three 1.0 m by 1.2 m and one 1.2 m by 1.2 m. The pyramid shaped volume diffuser and the two corner diffusers remained in place. The empty room sound absorption area was recalculated following further measurements and the measured values are presented in Figure 5 below.

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Figure 5: Empty room retest – no diffusers

Frequency uniformity ISO 354:2003 requires that the sound absorption coefficient in each of the one-third octave band must be within 15% of the mean of the sound absorption in the two adjacent one-third octave bands. With the exception of the 100 Hz band this requirement was met, see Figure 4. Ancillaries Operation of the lights was shown to increase the measured absorption in the 100 Hz one-third octave band and this was attributed to the electrical transformers associated with the power supply for the LED luminaires. The empty room absorption area with the lights on and off are presented in Figure 6 below.

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Figure 6: Absorption in the room with and without the lights operating Figure 7 shows the effect on the absorption in the room with and without the heaters. Removing the heaters resulted in a measureable decrease in absorption. The heaters were subsequently moved outside the room before measurements were made.

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Heaters present Heaters removed

Figure 7: Effect of heaters 5. PERFORMANCE COMPARISON

The absorption coefficients of two different fibrous acoustic materials were measured in the reverberation rooms at the University of Auckland and in a now demolished reverberation room at the University of Canterbury. These materials were tested in the new room by placing each in turn directly on the floor of the reverberation room and then testing again mounted with a 400 mm air gap. These mounting systems are type A-mounting and type E-400 mounting respectively as described in ISO 354:2003. The measured results are presented in Figure 8 and Figure 9 below

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Figure 8: Comparison of absorption coefficients for the same material using an A-mounting system and measured in two different reverberation rooms

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Figure 9: Comparison of absorption coefficients for the same material using an E-400 mounting system and measured in two different reverberation rooms

6. CONCLUSIONS

A reverberation room constructed of lightweight framing can meet the requirements of ISO 354:2003. This method of construction can result in significantly reduced construction costs as well as being easier to construct. The success of the full-scale room demonstrated that lightweight construction is a viable alternative to the more common monolithic construction Measurements of sound absorption between three different reverberation rooms, each complying with ISO 354 2003 show very similar results for the same material. 7. ACKNOWLEDGEMENTS

The work reported here was carried out in collaboration with JSK Acoustics and their support is gratefully acknowledged. 8. REFERENCES

1. ISO 354:2003, Acoustics - Measurement of sound absorption in a reverberation room. (n.d.).

Geneva: International Organization for Standardization. 2. ISO 354:2019(E), Acoustics - Measurement of sound absorption in a reverberation room. (n.d.).

Committee Draft Version, Geneva: International Organization for Standardization.