acoustics,   architectural      The  branch  of  acoustics concerned with the design of rooms and auditoria to produce good  listening.  Acoustical  control  of architecture  is achieved   by arrangement  of  shape;  by  reflection  and absorption of sound; by noise insulation; and by artificial amplification  and  distribution of sound. Design parameter varies  with function. For example, design of a concert hall, a  court  room,  or  a  broadcasting studio will have to be different

Effect of room size and shape : A listener receives sound  directly   from the source and by reflection and diffraction  from  walls.  Good  listening  requires that sound pressure  remains uniform   throughout  the  seating area. In a rectangular room with hard walls, standing waves can be set   up  across  parallel  pair  of walls. Consequently the room  will resonate to certain frequencies which will die out at a slower rate. Effects such as focusing from curved walls or ceiling, and echoes occurring in very large room also interfere with good listening.

Reverberation:  Reverberation is the persistence of sound  in  a  room  after  the  source  has stopped producing the sound.  The  time taken for the sound pressure level at the listener  to  fall  60  decibels from the original level is called  reverberation  time.  The  reverberation time, T in seconds of a room is given by,

       T = 0.049 V/A (Sabine's Law ) (a9)

where  V is the volume of the room in cubic feet, and A is the  total  absorption  of  the  surface  of  the room. The quantity A is found by totaling the product of square foot  area and its absorption coefficient. Absorption coefficient  is the fraction of incident energy absorbed per reflection.  In  Table aI absorption coefficients for some surfaces are  listed.

Reverberation  time  is  the  most important parameter which  governs  the  acoustic characteristic of a room. Too long reverberation time reduces intelligibility of speech.

The optimum value for speech ranges between 0.7 seconds for a  small  room,  to  1  second for a large room. For music however  somewhat  larger  reverberation time is desired. A typical concert hall has reverberation time ~ 1.5 seconds.

Table aIAverage  absorption coefficient for ordinary vocal frequencies