THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

258 11. Acoustics of Enclosed Spaces: Architectural Acoustics11.10 Early Decay time (EDT10)A modification of the reverberation time T 60 is the early decay time, or EDT10,which represents the time interval required for the first 10 dB of decay to occur,multiplied by 6 to produce an extrapolation to 60 dB decay Originally proposed byJordan (1974), EDT10 is based on early psychoacoustical research, and accordingto Cremer and Muller (1982), “the latter part of a reverberant decay excited bya specific impulse in running speech or music is already masked by subsequentsignals once it has dropped by about 60 dB.”11.11 Acoustic Energy Density and DirectivityIn order to account for uneven distribution of sound in some sources, we expressthe sound intensity I (W/m 2 ) due to a point source of power W (W) in the directfield (i.e., reflections are not considered) asI = WQ(θ,φ)(11.19)4π r 2where r is the distance (m) from the source and Q(θ,φ)isthedirectivity factor. Thedirectivity factor Q(θ,φ) equals unity for an ideal point source that emits soundevenly in full space. For an ideal point source above an acoustically reflectivesurface, in an otherwise free half-space, Q(θ,φ) equals 2. The sound or acousticenergy density is the sound energy contained per m 3 at any instant. In the directfield in full space, the direct sound energy density D D (W s/m 3 )isgivenbyDD = I c = WQ(θ,φ)(11.20)4π r 2 cwhere c is the speed of sound in m/s.11.12 Sound Absorption in Reverberant Field:The Room ConstantThe product IS gives the rate of acoustic energy striking a surface area S; and IScos θ gives that rate for the incidence angle θ. In an ideal reverberant field, withequal probability for all angles of incidence, the average rate of acoustic energystriking one side of the surface is given by IS/4. The power absorbed by the surfacehaving an absorption coefficient α isPower absorbed = αIS4 = αcD R S4where D R denotes the reverberant sound field density. In a fairly steady statecondition the power absorbed is balanced by the power supplied by the source tothe reverberant field. This is the portion of the input power W that remains after