In the construction of auditorium, classroom, office buildings, residential and com­mercial buildings architects pay attention to the acoustic quality of such spaces. The acoustic qualities of such spaces are evaluated by few important measured / design parameters like noise Criterion (NC) rating, reverberation time and speech interfer­ence level (SIL). While designing such spaces the architects and engineers aim to obtain desirable value of the above parameters. The acoustical property of the build­ing materials plays a significant role to obtain the target parameters. In this section the use of jute and its derivatives for improvement of the acoustical quality of the architectural space is presented.

The reverberation time of an architectural space is defined as the time taken for a sound level in a room to decrease by 60 dB1. The reverberation time is given by Eq. (15),

where RT is the reverberation time defined as the time taken for a sound to decay by 60 dB after the sound source is suddenly switched off.

V is the volume of the auditorium in m3.

A is the total absorption of the auditorium in m2-sabins.

The absorption unit of 1 m2-sabins represents a surface capable of absorbing sound at the same rate as 1 m2 of a perfectly absorbing surface, for example an open window.

However for practical purposes people use generally the Eyrings’s formula for reverberation time is used which is given in Eq. (16) below

al, a2,•••,an is the respective absorption coefficient

Figure 6Л2 gives the typical variation of overall reverberation time as a func­tion of room volume^ By an international standard the reverberation time of an architectural space can be measured and improvement of the reverberation can be

made by having the appropriate amount of absorption on the wall surface57. The speech interference level (SIL) is the arithmetic average of the sound pressure level in octave band centered at 500, 1000, 2000 and 4000 Hz and the preferred Speech Interference Level (PSIL) is the arithmetic average of the SPL in octave bands of 500 Hz, 1 kHz and 2 kHz. The PSIL as a function of distance from the receiver in presence of background noise is shown in Fig. 6.132. Apart from using reverbera­tion time and speech interference level to characterize the acoustics of a room the noise criterion (NC) curve is widely used in engineering. The NC curves are a set of sound pressure level in the various octave bands. To have a particular NC rating of the room the measures SPL in octave band should be less than or equal to the SPL given for the particular NC rating. A typical NC Curve is shown in Fig. 6.14 and the recommended NC values for various environments are given in Table 6.12.

R VL SH

50 60 70 80 90 100 110 125

13 Communication limits in the presence of background noise (with permission