BS 848-2:1985

Foreword
Committees responsible
Methods
1. Scope
2. Definitions
3. Symbols and units
4. Test methods, general
5. Instrumentation
6. In-duct test methods
7. Reverberant field test method
8. Free field and semi-reverberant test method
9. Conversion rules for sound power levels
10. Information to be recorded
11. Information to be reported
Appendices
A. Precision of the in-duct method of measurement
B. Determination of the signal-to-noise ratio of
     sound to turbulence noise in the test duct
C. Examples of suitable instrumentation systems
D. Anechoic termination
E. Reverberation time qualification test and
     evaluation of environmental correction factor K
F. Absolute comparison qualification test for
     evaluation of environmental correction K
G. Generalized guide for sound power level prediction
H. Characteristics and calibration of reference
     sound source
Tables
1. Uncertainty in determination of the broad band
     sound power levels
2. Example of instrument response correction to
     measured sound pressure levels
3(a) Flow velocity correction C3 for sampling tube in
     outlet side duct
3(b) Flow velocity correction C3 for sampling tube in
     inlet side duct
4(a) Modal correction C4 for a sampling tube
4(b) Modal correction C4 for omnidirectional micro-
     phones
5(a) Maximum standard deviation
5(b) Corresponding maximum range of sound pressure
     levels
6. Coordinates of microphone locations for
     hemispherical measurement surface for type A
     installations and corresponding representative
     surface areas
7. Coordinates of microphone locations for
     hemispherical measurement surface for type B and C
     installations and corresponding representative
     surface areas
8. Coordinates of microphone locations for spherical
     measurement surface and corresponding represent-
     ative surface areas
9. Effect of end reflections
10. Effect of transitions
11. Computation of power from pressure measurements
12. Maximum differences between the sound pressure
     level readings of a microphone fitted with a nose
     cone and of a microphone fitted with a sampling
     tube as a function of the turbulence noise
     suppression triangle Lt of the sampling tube, for
     a minimum signal-to-noise ratio of sound to
     turbulence noise of 5 dB
13. Turbulence noise suppression triangle Lt of
     typical sampling tube as compared with that of a
     13 mm nose cone
14. Maximum pressure reflection coefficient
15. Calibration accuracy for reference sound source
Figures
1. Example of frequency response of microphone fitted
     with protection grid as supplied by a manufacturer
2. Schematic drawing of a typical sampling tube
3. Limiting dimensions of test ducts, intermediate
     ducts and transitions for type B installations
4. Limiting dimensions of test ducts, intermediate
     ducts and transitions for type C installations
5. Limiting dimensions of test ducts, intermediate
     ducts and transitions for type D installations
6. Test room with whole air circuit within walls
7. Test room with type A installation
8. Test room with types B and C installations
9. End reflection correction
10. Microphone positions for a hemispherical measure-
     ment surface for type A installation
11. Microphone positions for a hemispherical measure-
     ment surface for types B and C installations
12. Microphone positions for spherical measurement
     surface
13. Examples of anechoic termination
14. Examples of three catenoidal designs of anechoic
     termination
15. Examples of inlet anechoic termination (catenoidal)
16. Example of stepped inlet anechoic termination
17. Environmental correction K, in dB
18. Example of determination of a and b for typical
     fan series

Determination of the acoustic performance of fans operating against difference of pressure. Four methods are described: in-duct, reverberant field, free field and semi-reverberant. Illustrations are given of suitable test ducting and anechoic terminations.