SYN-AUD-CON Acoustic Test and Measurement Seminar The Early Sound Field: Properties, Perceptual Attributes, and Measurement Methods NEIL THOMPSON SHADE President/Principal Consultant Acoustical Design Collaborative, Ltd Ruxton, MD Director, M.A. Acoustics Program Peabody Institute of Johns Hopkins University Baltimore, MD 28 – 29 August 2002

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Overview of Talk For the Early Sound Field I Will Discuss –History –Simple Definition –Differences and Acoustical Attributes –Psycho-Acoustic and Perceptual Factors –Related Metrics Used for Evaluation –Doing Measurements and Interpretation

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Some History on Early Reflected Sound Field H. Matthews, London, 1826 book: “Observations on Sound: Shewing the Causes of Its Indistinctness in Churches, Chapels, Halls of Justice &c. with a System for Their Construction” “That part of the sound which proceeded in a direct line would arrive first; and that reflection from an oblique wall, having to perform a longer journey, would arrive later…. Echo does not politely wait until the speaker is done; but the moment he beings and before he has finished a word, she mocks him as with ten thousand tongues.” From Buildings for Music, Forsyth, M. (1985) Cambridge, MA: MIT Press

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Some History on Early Reflected Sound Field Joseph Henry studied audibility of reflections at Smithsonian Lecture Hall (around 1850) CONCLUSION: Echoes not perceptible if listener within about 50 ft of reflecting surface. Helmut Haas studied audibility of secondary sources (1949–1951) CONCLUSION: Level of secondary source can be up to 10 dB higher than primary source and not be perceived if within 30 to 35 ms of secondary source. Schultz and Beranek studied late-to-early sound index (1963–1965) CONCLUSION: Sound arriving within 50 ms of direct sound is “useful” for determining sense of reverberance with actual program sources. Called it “running reverberance.”

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Some History on Early Reflected Sound Field Riechardt studied speech clarity (C 50 ) and musical clarity (C 80 ) (1973– 1975) CONCLUSION: Speech clarity important for sound within 50 ms and music clarity important for sounds arriving within 80 ms. Harold Marshall studied lateral reflections (1967–1968) CONCLUSION: Reflections within 50 to 80 ms after direct sound from side walls increase perceived “envelopment” and spatial impression. Michael Barron studied lateral reflections based on Marshall’s work (1971–1972) CONCLUSION: Delay of 20 to 80 ms has slight influence on spatial impression, but level differences between primary and secondary sources more important.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Importance of Early Sound Field Early Sound Field Provides the Room “Sound” Rooms with Same Reverberation Time Will Sound Different –Due to Early Reflections Often Desirable to Have Strong Early Reflections –Speech — Improves Clarity and Level –Music — Helps With Intimacy, Definition, Spaciousness, and Musical Ensemble Strong Early Reflections Directed at Audience Can Alter the Late Reflected Sound Field Both in Duration (Time) and Strength (Level) Early Reflections Not Desirable in “Studio” Rooms –Effects Sense of Localization and Timbre of Sound

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Importance of Early Sound Field Haas/Precedence Effect Data

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Importance of Early Sound Field Barron Spatial Perception Data

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Importance of Early Sound Field Toole/Olive Spatial Perception Data

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Importance of Early Sound Field Toole/Olive Audibility of Single Reflection Data

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Description of Early Sound Field Early Sound Field Is Room and Position Dependent –Based on Room Size, Geometry, and Location of Sound Reflecting Surfaces Approximate Guidelines for Early Sound Limits –Large Rooms (Concert Halls, Large Churches, Arenas) 0 to 150–250 ms –Medium Rooms (Recital Halls, Most Churches, Large Lecture Rooms) 0 to 50–100 ms –Small Rooms (Studios, Screening Rooms, Classrooms) 0 to 10–30 ms Idealized Room Response 1. Direct Sound (L D ) 2. Initial Time Delay Gap 3. Early Reflections (L RE ) 4. Build-up of Reverberant Field (L R ) 5. Steady State Energy Balance (L R ) 6. Decay of Reverberant Field (L R ) 7. Ambient Noise (L AMB )

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Description of Early Sound Field

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Psycho-Acoustic Aspects Perceptual Attribute Reverberance Speech Clarity Music Clarity Spaciousness Envelopment Intimacy Stage Support Physical Parameter Early Decay Time (EDT) C 50 C 80 Interaural Cross Correlation Coefficient (IACC EARLY ) Lateral Fraction (LF EARLY ) Interaural Cross Correlation Coefficient (IACC LATE ) Lateral Fraction (LF LATE ) Initial Time Delay Gap (ITDG) ST1 Early Sound and Perception Factors

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field Early Decay Time (EDT) Units: sec –Measures sound decay from 0 to -10 dB down and extrapolates decay to full 60 dB decay limit.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field Clarity C 50 and C 80 Units: dB –Measures energy ratio of early time (0 to 50 ms or 0 to 80 ms) to late time (50 ms to infinity or 80 ms to infinity).

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field Inter-Aural Cross Correlation Coefficient (IACC) –Measures difference in sounds arriving at left and right ears. IACC Early (IACC E3 ) Units: none –Measures sound over 0 to 80 ms time period at 500, 1,000, and 2,000 Hz bands and averaged. –Evaluates spaciousness (apparent source width).

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field IACC Late (IACC L3 ) Units: none –Measures sound over 80 to 750 ms time period at 500, 1,000, and 2,000 Hz bands and averaged. –Evaluates sense of envelopment.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field Lateral Fraction (LF) –Measures energy ratio of sound arriving from side walls to total sound. Lateral Fraction Early (LF E4 ) Units: none –Measures sound over 5 to 80 ms time period at 125, 250, 500, and 1,000 Hz bands and averaged. –Evaluates sense of spaciousness.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field Lateral Fraction Late (LF L4 ) Units: none –Measures sound over 80 to infinity ms at 125, 250, 500, and 1,000 Hz bands and averaged. –Evaluates sense of envelopment.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field Initial Time Delay Gap (ITDG) Units: ms –Time difference between arrival of direct sound and first significant reflection. –Evaluates sense of intimacy.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Definition of Metrics to Assess Early Sound Field Stage Support (ST1) Units: dB –Measures energy ratio of sound arriving from 0 to 10 ms to sound arriving from 20 to 100 ms. –Evaluates reflections that provide useful support to musicians to hear each other.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation General –Recommended Equipment Omni-directional sound source Omni-directional microphone (pressure or random incidence) –Generate impulse response or Energy Time Curve (ETC) Pink noise, MLS signal, or swept sine signal –Data collection procedures in ISO 3382, “Measurements of Reverberation Time of Rooms with Reference to Other Acoustical Parameters”

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation Measurement Methods –Measurement in octave bands 125 to 4,000 Hz –Minimum distance mic-to-source given by DMIN = 2 {(V/cT)} V = room volume, m 3 C = 343 m/s T = mid-frequency reverberation time, sec

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation Measurement Methods –Excitation time of signal must exceed room reverberation time. –Minimum number of source positions: 3 Stage Left Stage Center Stage Right

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation Measurement Methods –Minimum number of receive (mic) positions based on room size (number of seats) Number of Seats 500 1,000 2,000 Minimum Number of Mic Positions Total number of measurements: need to multiply the number of source positions (3) by the minimum number of mic positions.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation EDT –Measure in octave bands 125 to 4,000 Hz. –Average all measurement values in each octave band. –Characterize EDT at 500 and 1,000 Hz average or 250 to 2,000 Hz average values. Shaded = Positional dependent range of values

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation Clarity –Measure in octave bands typically limited to 500, 1,000, and 2,000 Hz. –Limit of direct plus early sound set to 50 ms for speech clarity and 80 ms for music clarity. –Values averaged to obtain a single value for 3 frequencies and all measurement positions. Shaded = Positional dependent range of values

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation IACC –IACC mics at right and left ears in dummy head to evaluate differences in sound levels arriving at right and left ears. IACC Early (IACC E3 ) –Measure in 500, 1,000, and 2,000 Hz octave bands over 0 to 80 ms time period. –Values averaged to obtain a single value for 3 frequencies and all measurement positions. Shaded = Positional dependent range of values

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation IACC Late (IACC L3 ) –Measure in 500, 1,000, and 2,000 Hz octave bands over 80 to 750 ms time period. –Values averaged to obtain a single value for 3 frequencies and all measurement positions. Shaded = Positional dependent range of values

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation Lateral Fraction (LF) –LF energy ratio of output of figure-of-8 mic with lobes pointed towards side walls (CH 1) to omni-directional mic (CH 2). Lateral Fraction Early (LF E4 ) –Measure in 125, 250, 500, and 1,000 Hz bands over 5 to 80 ms time period. –Values averaged to obtain a single value for 4 frequencies and all measurement positions. Shaded = Positional dependent range of values

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation Lateral Fraction Late (LF L4 ) –Measure in 125, 250, 500, and 1,000 Hz bands over 80 ms to infinity time period. –Values averaged to obtain a single value for 4 frequencies and all measurement positions. ITDG –Measure in center of seating area at 500 or 1,000 Hz. –Time difference between direct sound and first reflection.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Measurements and Data Interpretation ST1 –Measure with music stands, chairs, and percussion on stage at 250, 500, 1,000, and 2,000 Hz. –Values averaged to obtain a single value for 4 frequencies and all measurement positions.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Summary and Conclusions Studies about early sound field nearly 150 years old. –Studies continue to evolve relating perception to measurement parameters. Early sound field is a major factor in room “sound.” –May be more important than reverberation time. Early sound field measurements different than standard acoustical measurements. –Special equipment or software needed to properly evaluate. –Measurement data very positional dependent — requires numerous measurement locations to quantify.

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Summary and Conclusions Important early sound metrics include –Early Decay Time (EDT) –Speech Clarity (C 50 ) –Music Clarity (C 80 ) –Interaural Cross Correlation Coefficient (IACC EARLY ) –Interaural Cross Correlation Coefficient (IACC LATE ) –Lateral Fraction (LF EARLY ) –Lateral Fraction (LF LATE ) –Initial Time Delay Gap (ITDG) –Stage Support (ST1)

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Summary and Conclusions Appropriate values of early sound metrics dependent on type of music (organ, orchestral, chamber, soloist, pop/jazz) or speech programs (talk, drama, opera). Typical ranges –C 50 Opera > 0 dB Talks and Drama > +2 dB –C 80 Organ +2 dB –EDTOrgan > 2.5 sec Symphony 1.8 to 2.2 sec Chamber 1.3 to 1.6 sec Opera 1.4 to 1.6 sec Pop/Rock < 0.8 sec

© ACOUSTICAL DESIGN COLLABORATIVE, LTD Summary and Conclusions Typical ranges –IACC EARLY < 0.50 –IACC LATE < 0.50 –LF EARLY < 0.40 –LF LATE < 0.40 –Initial Time Delay Gap (ITDG) < 25 ms –Stage Support (ST1) -13 to -11 dB