Title: SYNAUDCON Acoustic Test and Measurement Seminar The Early Sound Field: Properties, Perceptual Attrib
1SYN-AUD-CONAcoustic Test and Measurement
SeminarThe Early Sound Field Properties,
Perceptual Attributes, and Measurement
Methods NEIL THOMPSON SHADE President/Principa
l ConsultantAcoustical Design Collaborative,
LtdRuxton, MD Director, M.A. Acoustics
ProgramPeabody Institute of Johns Hopkins
UniversityBaltimore, MD 28 29 August 2002
2Overview 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
3Some 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
4Some 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 (19491951) - 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 (19631965) - CONCLUSION Sound arriving within 50 ms of
direct sound is useful for determining sense of
reverberance with actual program sources. Called
it running reverberance.
5Some History on Early Reflected Sound Field
- Riechardt studied speech clarity (C50) and
musical clarity (C80) (19731975) - CONCLUSION Speech clarity important for sound
within 50 ms and music clarity important for
sounds arriving within 80 ms. - Harold Marshall studied lateral reflections
(19671968) - 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 Marshalls work (19711972) - CONCLUSION Delay of 20 to 80 ms has slight
influence on spatial impression, but level
differences between primary and secondary sources
more important.
6Importance 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
7Importance of Early Sound Field
Haas/Precedence Effect Data
8Importance of Early Sound Field
Barron Spatial Perception Data
9Importance of Early Sound Field
Toole/Olive Spatial Perception Data
10Importance of Early Sound Field
Toole/Olive Audibility of Single Reflection Data
11Description 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 150250 ms - Medium Rooms (Recital Halls, Most Churches, Large
Lecture Rooms) 0 to 50100 ms - Small Rooms (Studios, Screening Rooms,
Classrooms) 0 to 1030 ms - Idealized Room Response
- 1. Direct Sound (LD)
- 2. Initial Time Delay Gap
- 3. Early Reflections (LRE)
- 4. Build-up of Reverberant Field (LR)
- 5. Steady State Energy Balance (LR)
- 6. Decay of Reverberant Field (LR)
- 7. Ambient Noise (LAMB)
12Description of Early Sound Field
13Psycho-Acoustic Aspects
Early Sound and Perception Factors
- Perceptual Attribute
- Reverberance
- Speech ClarityMusic Clarity
- Spaciousness
- Envelopment
- Intimacy
- Stage Support
- Physical Parameter
- Early Decay Time (EDT)
- C50C80
- Interaural Cross CorrelationCoefficient
(IACCEARLY)Lateral Fraction (LFEARLY) - Interaural Cross CorrelationCoefficient
(IACCLATE)Lateral Fraction (LFLATE) - Initial Time Delay Gap (ITDG)
- ST1
14Definition of Metricsto 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.
15Definition of Metricsto Assess Early Sound Field
- Clarity C50 and C80 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).
16Definition of Metricsto Assess Early Sound Field
- Inter-Aural Cross Correlation Coefficient (IACC)
- Measures difference in sounds arriving at left
and right ears. - IACC Early (IACCE3) 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).
17Definition of Metricsto Assess Early Sound Field
- IACC Late (IACCL3) 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.
18Definition of Metricsto Assess Early Sound Field
- Lateral Fraction (LF)
- Measures energy ratio of sound arriving from side
walls to total sound. - Lateral Fraction Early (LFE4) 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.
19Definition of Metricsto Assess Early Sound Field
- Lateral Fraction Late (LFL4) Units none
- Measures sound over 80 to infinity ms at 125,
250, 500, and 1,000 Hz bands and averaged. - Evaluates sense of envelopment.
20Definition of Metricsto 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.
21Definition of Metricsto 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.
22Measurements 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
23Measurements and Data Interpretation
- Measurement Methods
- Measurement in octave bands 125 to 4,000 Hz
- Minimum distance mic-to-source given by
- DMIN 2Ö (V/cT)
- V room volume, m3
- C 343 m/s
- T mid-frequency reverberation time, sec
24Measurements 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
25Measurements and Data Interpretation
- Measurement Methods
- Minimum number of receive (mic) positions based
on room size (number of seats)
Minimum Number of Mic Positions 6 8 10
Number of Seats 500 1,000 2,000
Total number of measurements need to multiply
the number of source positions (3) by the minimum
number of mic positions.
26Measurements 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
27Measurements 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
28Measurements 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 (IACCE3)
- 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
29Measurements and Data Interpretation
- IACC Late (IACCL3)
- 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
30Measurements 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 (LFE4)
- 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
31Measurements and Data Interpretation
- Lateral Fraction Late (LFL4)
- 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.
32Measurements 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.
33Summary 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.
34Summary and Conclusions
- Important early sound metrics include
- Early Decay Time (EDT)
- Speech Clarity (C50)
- Music Clarity (C80)
- Interaural Cross Correlation Coefficient
(IACCEARLY) - Interaural Cross Correlation Coefficient
(IACCLATE) - Lateral Fraction (LFEARLY)
- Lateral Fraction (LFLATE)
- Initial Time Delay Gap (ITDG)
- Stage Support (ST1)
35Summary 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
- C50 Opera gt 0 dB Talks and Drama gt 2 dB
- C80 Organ lt -4 dB Symphony -2 to 2
dB Chamber 0 to 2 dB Pop/Rock gt 2 dB - EDT Organ gt 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 lt 0.8 sec
36Summary and Conclusions
- Typical ranges
- IACCEARLY lt 0.50
- IACCLATE lt 0.50
- LFEARLY lt 0.40
- LFLATE lt 0.40
- Initial Time Delay Gap (ITDG) lt 25 ms
- Stage Support (ST1) -13 to -11 dB