Characterization of closed spaces, ISO 3382-3

R.Narasimha SwamySenior consultant

narasimhaswamy@yahoo.com

Introduction• RT60 - just a single number and is a classical measure

of acoustics characteristics of the closed spaces.

• However, RT60 is not sufficient to completely characterize the enclosed spaces, as it does not consider several important characteristics like relative SPL, energy fraction of the early/late reflections, lateral reflections, background noise etc.

• These factors are very important measure for spatial impression of auditoriums, speech intelligibility in class rooms, offices, multi purpose halls etc.

ISO-3382-3 (1997-E)

• ISO-3382 is a method suggested by ISO organization for acoustic measurement of the enclosed spaces.

• All the important characteristics like energy fraction of the early/late reflections, lateral reflections, background noise are derived from the Sabin’s classical sound decay characteristics.

ISO 3382 parametersDescription of

acoustic parameter

Symbol Definition or expression Proposed

by Attributes

Reverberation RT60 (sec)

Time taken for the sound in a room to decay to 60 dB below the source level, after the source is muted.

Sabine 1923

Characterizes the enclosed space

Early decay time

EDT (S)

Early Decay Time is the reverberation time, measured over the first 10 dB of the decay.

Jordan, 1975

Provides subjective evaluation of the RT

Clarity C50Ratio Early to late arriving sound energy

ratio

Reichardt1975

Acoustic clarity with reference to room size perception

Definition D50 (%)

The D50 parameter is the early to total sound energy ratio. It is defined as Thiele

1953Speech intelligibility and sound definition

Signal to noise ratio

SNR (dB)

Lchner e Burger, 1964

Speech intelligibility

Rapid speech transmission

indexRASTI (ratio) [S/N media +15]/30

Steeneken e

Houtgast 1980

Speech intelligibility

5

ISO 3382 Reverberation Time(s)

Early Decay Time (EDT): extrapolated from 0 to -10 dB

Reverberation Time T10: extrapolated from -5 to -15 dB

Reverberation Time T20: extrapolated from -5 to -25 dB

Reverberation Time T30: extrapolated from -5 to -35 dB

Reverberation time T20

Lp (dB)

Time (s)

45 dB

-5 dB

-25 dB

T20

7

Early – Late energy evaluationUseful Energy Detrimental Energy

Early-Late parameters

ms

ms

dττp

dττpC

80

2

80

0

2

80 lg10

Clarity Index C80 (symphonic music):

Clarity Index C50 (speech):

Optimal Value = +/- 1 dB

Optimal Value = +/- 1 dB

ms

ms

dττp

dττpC

80

2

50

0

2

50 lg10

Early-Late parameters

Center Time tS:

0

2

0

2

dp

dpts

100

dp

dp

D

0

2

ms50

0

2

Definition Index D:

Other acoustical parameters

• Strength: dB31LSPLG w

dthdh

dthh

t2s

2d

sd

IACC:

SPL at 10 m

IACC: Inter Aural Cross Correlation

Room impulse response and Schroeder decay function determined from an experimentallymeasured room impulse response in two coupled scaled-down model rooms.

(a) Room impulse responseoctave band-pass filtered at 1kHz with peak-to-noise ratio equal to 53 dB.

(b) Schroeder decay function.

Case studies of reverb and anechoic chamber

Impulse response of reverberant room

Decay curve of reverberant room

Characteristics of reverberant room as per ISO-4482

Impulse response of anechoic chamber

Decay curve of anechoic chamber

Characteristics of anechoic chamber as per ISO-4482

Importance of acoustics in learning

Thanks