By Duncan Templeton and David Saunders (Auth.)
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Extra resources for Acoustic Design
F The air absorption has been estimated assuming a temperature of 20°C and a humidity of 60 per cent. 15. Using the Norris-Eyring equation To use the Norris-Eyring equation the mean absorption coefficient of all the absorbing surfaces has to be calculated which means that the effective surface area of the audience must be known. As this quantity is not easily defined use of this equation cannot be guaranteed to increase the accuracy of the estimates of reverberation time. 75 m 2 has been assigned to each occupied seat.
A relatively heavy mass rests on the discs and is preloaded by means of a stiffspring, the whole assembly being contained within a metal housing with a thick base. When the surface to which the accelerometer is attached vibrates, the mass exerts a variable force on the discs which, because they are piezo-electric, develop a variable charge proportional to the force and therefore to the acceleration of the mass. 14. 14 Piezo-electric accelerometer. D> tv 4 3 i 0] 5 ^ Vibration measurement Detail 6 -7 8 9 1 Accelerometer 2 Magnetic attachment to vibrating surface 3 Voltage or charge preamplifier 4 Frequency analyser 5 Read out or recorder 6 Piezo-electric element 7 Mass 8 Spring 9 Base Selecting an accelerometer An accelerometer has to be chosen with sufficient sensitivity and an adequate frequency range and, in addition, it should be of such a mass that it does not load the surface to which it is attached.
It was originally developed in the United States specifically for application in commercial buildings. 7. The noise criterion is obtained from the lowest curve which is nowhere exceeded by the octave band sound levels of the noise. In 1971 the preferred noise criteria ( P N C ) curves were introduced but as yet they are not widely accepted. 8. 9. 10 Noise criteria curves. 11 Preferred noise criteria curves. 9 Octave band centre frequency (Hz) Recommended noise criteria for different environments Environment Noise criterion (NC) Concert halls, opera houses, large auditoria, large theatres ( > 500 seats), large churches, recording studios 10-20 Broadcasting studios, small auditoria, small theatres, small churches, music rehearsal rooms, large meeting and conference rooms, executive offices, hospital wards, operating theatres 20-25 Bedrooms (private to motels), courtrooms, cinemas 25-40 Private offices, small conference rooms, classrooms, libraries, living rooms 30-40 Large offices, reception areas, retail shops, cafeterias, restaurants, laboratories 35-45 Lobbies, drawing offices, general typing areas, swimming pools, gymnasia 40-50 Rooms with computer and office equipment, kitchens and laundries, shops 45-55 33 Properties of sound Speech interference level, SIL The speech frequencies that contribute to intelligibility lie between 200 and 7000Hz and masking of these frequencies by noise will obviously give reduced understanding.
Acoustic Design by Duncan Templeton and David Saunders (Auth.)