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The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors ♦ PDF Cover Page ♦ ♦ Verso Filler Page ♦ RESEARCH DEPARTMENT The design. of a new free-field sound measurement room: The selection of sound absorbent material RESEARCH REPORT No. L-055 1964/42 THE B R I TIS H B R 0 A 0 CAS TIN G COR P 0 R A TI 0 N ENG I NEE R I N G 0 I VI S ION This Report is the propertf or the British Broadcastin& Corporation and aa1 not be reproduced in anf tOrm without the written pera188ion ot the Corporatlon. li Research Report No. L-055 (1904/42 ) THE DESIGN OF A NEW FREE- FIELD SOUND MEASUREMENT ROOM: THE SELECTION OF SOUND ABSORBENT MATERIAL Section Title Page SUMMARY ..' . 1 1. I NTRODUCTI ON. 1 2. GENERAL . · . 1 3. FIBROUS MATERIALS 2 4. FOAMED PLASTIC MATERIALS. 4 5. CONCLUSIONS . · . 6 6. ACKNOWLEDGEMENTS. · . 8 7. REFERENCE . · . 8 iv July 1964 Research Report No. L-055 (1964/42) THE DE SIGN OFANEW FREE-FIELD SOUND MEASUREMENT ROOM: THE SELECTION OF SOUND ABSORBENT MATERIAL SUMMARY In designing wedge-type sound absorbers for use in a new free-field room, various kinds of plastic foam were considered as possible alternatives to glass wool or glass fibre. The reflexion coefficient of the plastic materials was found to be influenced by mechanical resonances in the material, which in turn were markedly affected by the method of �ounting the wedge. The material finally adopted was a polyurethane ether foam having sufficient mechanical hysteresis to damp the internal reso nances. The introduction of additional mass in the form of a steel rod at the centre of each wedge was found to extend the low-frequency range. 1. INTRODUCTION In the new sound measurement room built in 1963 at Kingswood Warren, it was desired to ensure substantially free-field conditions at frequencies down to 50 cis. The acoustic treatment in the existing sound measurement room consists of glass fibre wedges 8 in. x 8 in. (20 cm x 20 cm) at the base, and occupies a depth of 3 ft 6 in. (101 m); from experience with this room, it was estimated that the required performance in the new room could be obtained by similar means by allowing a total of 5 ft (105 m) for the length of the wedge plus the air space, if any, between it and the wall. However, as an increase in length of wedge would in theory alter the optimum value of flow resistance, it was necessary to experiment with materials other than the original grade of glass fibre. 2. GENERAL Measurements of reflexion coefficient for normal incidence were made by means of a travelling-wave duct having an internal cross-section 16 in. x 16 in. (40 cm x 40 cm) thus accommodating four wedges. Although the base cross-section of the wedge was kept constant throughout the experiments, the remaining dimensions were varied; for converiience, the length of the tapering p�rtion of the wedge, the length of the parallel portion, the air gap between the base of the wedge and the wall and the overall depth of the acoustic treatment are designated, as shown . in Fig. 1, as A, B, C and D respectively. 1 The lower limit of the useful D frequency range of an absorbent material c.,;-.------- for a free-field room is usually taken as the point at which the reflexion co­ efficient rises to 10%. To facilitate c omparison between different types of s ound absorber occupying different 8 A -------I l- -.......i.----- amounts of space, some workers have taken as a figure of merit the value of DIA Dimensions of wedge and position o Fig. 1 - where is the wavelength, in air, of relative to wall. Ao sound at that frequency for which the reflexion coeffici ent is 10%. This pro ­ cedure, however, produces a figure which decreases when the working frequency range of the absorber increases; to avoid this incongruity, the inverse quantity AolD will be used in the present report as a measure of the efficacy of various types of acoustic treatment. For the sake of brevity and to preserve anonymity , commercial brands of absorbent material have been designated throughout by Roman numerals. 3. FIBROUS MATERIALS The first group of materials to be investigated were of the fibrous type, in which the absorption of sound depends principally on the flow resistance. Accord­ ing to theory, the curve of reflexion coefficient as a function of frequency should exhibit signs of interference between the wave reflected from the front of the wedges and that reflected after passing through the wedges and reaching the rigid wall behind them. Fig. 2(a) shows the theoretical form of reflexion characteristic given in a paper by Walther l in 1960 and Fig. 2(b) the reflexion characteristic of a sample of low-density glass fibre, in which the effects of interference are likewise apparent. 50 50 a} Theoretical curve from Walther 1960 b) Experimental data - glass fibre 40 40 -! \V -! ..:0 ..:0 / c , c � � 30 30 V .....� .....� ..... ..... \ 8 If" 8 u I u c: 20 r\ c: 20 \ �r-. Q Q )( )( \ V ......!! \ I � ......!! � � � L L \ ,; 10 \ II 10 \ \ / \/\ �I\. V '" o o 1"\ 30 50 100 500 30 50 100 500 freauenc V. cis frequency, cIs Fig. 2 - Reflexion coefficient of low density porous wedges. 2 Although the reflexion characteristics of absorbent wedges do not always follow the theoretical fonn as closely as in the examples just given, the same general principles appear to hold. The experiments on fibrous materials yielded a number of conclusions, applicable to a wide range of materials, regarding the effect of varylng the dimensions of the absorbent wedges and their spacing from the wall. Fig. 3, for example, shows the effect of varying length of taper A on the reflexion characteristic of mineral wool wedges and Fig. 4 the effect of varying the air gap C for low-density glass-fibre wedges. Fig. 5 shows, for mineral wool wedges, the effect of varying the ratio C/B (air gap to parallel portion), keeping the rate of taper and overall depth of treatment constant. Reflexion Coefficients 5 0 I taper length A (25cm) air C o�� r...... JOin gap .... " �V 16in (41cm) Oin (Oem) � \100.. \ \ \ \ � '. -, 1';b)4in(61cm) 1--4in (1Qcm) ,.-......1-- ... - '\ 1\ 1\ \ .....8 in ��2jn (B1cm) ..: (20cm) '\ \ .( � r 1-12in (31cm) r... 40in (102cm) \ \ \- ...,... /1I1/\� ,...�4in(61cm) \ \ " l� 36ln(92em) u � � 1\1 , II � \' '\. \ \ � Ij �,j ,� r'�, " i 0 �.� \ 1\ '" 1'0.' '\ , �::"".. '\ " ).�� � , �-r\. � \." l...!!::: �-lIII �.... ..... '''1'-.' � ... ..... \. r� 0 o 30 100 300 1000 30 100 300 1000 • frequ�ncy, cIs frequency, cIs Fig. 3 - Variation with length of taper 'A' for wedges Fig. 4 - Variation with depth of airgap 'C' behind of mineral wool. wedges of low density glass fibre. ,.., 5 5 0 0 0 a) - Glass fibre = 60" (150)cm t-. I, '0' 1\' parallel 01\ '1\ air gop C portion B £. ..0 \ ./ Oln <Oem) 16in (41em) 8 , 0 41n (1Ocm) 12in (31cm) i ..i 0 ' \ ,/ 8in (20cm) 81n (2Ocm) j = 1 " b) - Glass fibre II, '0' = 60" (150)cm " , , , 0 n �, C\. � �,.", '- I'�r- -� "'" -- ,,�1-h ...... � ��� 'I-.l )O..:; Q 0 30 100 300 1000 30 100 300 1000 0'=4-3A A ,,3·3 frecJJency, cIs D frequency. cIs Fig. 5 - Variation of ratio of air gap 'C' to parallel Fig.