1. ROOM ACOUSTICS
MEASUREMENT
AND
ASSESSMENT
METHODS

2. ASSESSMENT OF ROOM ACOUSTICS CAN BE DIVIDED INTO:
OBJECTIVE MEASUREMENT METHODS
SUBJECTIVE ASSESSMENTS

3. OBJECTIVE METHODS Signal to Noise Ratio Loudness
Decay curves --Level.v.Time
Reverberation Time
T30, T20, EDT
RT ratios
Initial Time Delay Gap, ITDG
4. Energy Distribution in the Decay
Lateral Energy Fraction, LF
Clarity -- C80, etc.
4. Room Resonance (modes)
6. Transfer Functions

4. 1. Signal to Noise
Background noise must be low enough so as not to affect the listener.
Simple method
“A” Weighted Level
No information on failing
frequencies if too high
Better methods
Rating Methods (NR, RC)
Speech Interference Level, SIL

5. NR (Noise Rating), NC (Noise Criterion), RC (Room Criterion)
plot measured octave band sound pressure levels
RECOMMENDED
Offices NR25-55
Hospitals
Wards NR30
Domestic
Bedroom NR25
Lounge NR30
Concert Hall
NR20
Recording Studios
NR15
This would be ≈NR46
Determined by the highest curve that is “cut”

6. SIL (Speech Interference Level)
Based on average level in the 500Hz, 1, 2, 4 kHz octave bands.
Determines the possibility of face to face communication so is a measure of privacy
How far apart do I need to position desks in an open plan office?
ISO , Ergonomic Assessment of

7. 2. LOUDNESS
Loudness gives a measure of whether or not the audience will be able to hear the performance at the correct loudness level.
For an orchestra the sound power of the instruments determines the maximum size of the auditorium
For electronic music the level in all parts of the auditorium can be measured and adjusted to give an even and sufficient level.
Loudness is given by the strength factor, G which can be determined from the reverberation time, RT.
RTmid is the average of 500 Hz & 1000 Hz
The best concert halls have a strength factor G between 4.0 and 5.5 dB

8. 3. DECAY CURVES
Measured with interrupted sound source and a plot of the decay through time. RT and EDT calculated from the average of many decays.
Compare with recommended RT for type of space and performance --- found from published data (next slide)
Faults such as flutter echoes can be detected from the decay curve (and by listening)

9. Recommended RT’s

10. WARMTH AND BRILLIANCE Warmth --- smoothness to the music.
If a sound field is too warm, the hall can be undesirably "dark."
Brilliance --- bright, clear, ringing sound.
With too much brilliance, the sound is harsh, brittle, and metallic sounding.
Balancing warmth and brilliance is achieved by balancing the ratio of low frequency Reverberation Time (RT) to high frequency RT. Thus, the Bass Ratio (BR) has been suggested as an objective measure of Warmth:
A bass ratio between 1.1 and 1.25 is desirable in halls with a high RT,
A bass ratio between 1.1 and 1.45 is recommended for any hall with an RT of 1.8 sec or less.

11. INITIAL TIME DELAY GAP (ITDG)
The ITDG is the time delay between the direct sound and the first reflection shown on the idealised reflectogram below.
ITDG affects our perception of the spaciousness of the hall short gap – intimacy long gap -- remoteness
ITDG

12. ACTUAL REFLECTOGRAM
ITDG = 5 ms
ITDG = 47ms

13. Measured ITDG for Various Halls (rated as good
Concert Hall ITDG (ms)
Amsterdam, Concertgebouw 21
Boston, Symphony Hall 15
Vienna, Gr. Musikvereinssaal 12
Basel, Stadt-Casino 16
Berlin, Konzerthaus 25
Cardiff, Wales, St. David’s Hall 25
New York, Carnegie Hall 23
Tokyo, Hamarikyu Asahi 16
Zurich, Großer Tonhallesaal 14
Note:- 25ms = 8.5m path difference (direct-reflected)

14. 4. ENERGY DISTRIBUTION in the Decay

15. CLARITY
Clarity refers to how clear the sound quality is. Can you hear every separate note distinctly, or do the notes tend to blur into one another?
Some blending is often desired for music, but for speech and opera, greater clarity leads to better speech intelligibility.
C80, the Clarity Index, is the ratio of energy <80ms to >80ms used for music. (C50 for speech)
C80 = +5dB, C80 = - 13 dB

16. Spatial Impression
Reflections from side walls affects the spatial impression.
Lateral reflections can be measured using directional figure-of-eight microphones.
Lateral / total energy
fraction, LF determined
early LF affects auditory
source width (ASW)
Late LF (>80ms) affects
listener envelopment (LE)
Popular concert halls have early LF = 0.2 (mid frequencies)

17. Recommendations for room acoustic criteria (Gade et al [2007])

18. 5. ROOM RESONANCES
Measured using a swept sine signal and plotting the results or by white noise and FFT
Evenly spaced modes are OK

19. 6. Transfer Functions There are various methods:-
Speech Transmission Index (STI)
Uses complex sound over a 7 octave frequency range to simulate speech
Rapid Speech Transmission Index (RASTI)
Simplified method uses only 2 octave bands
Impulse methods
Dirac Delta (Measures the RTF)
This can be used to predict STI / RASTI & most other acoustical parameters

20. SPEECH TRANSMISSION INDEX (STI)
Developed in the early 1970’s, the STI is a machine measure of intelligibility.
A signal (80-12kHz) modulated by low frequency sine waves and designed to approximate long-term spectral characteristics of speech is generated.
When transmitted through the air the signal becomes corrupted by both background noise and reverberation reducing the effective modulation of the original signal.
The STI is a measure of the amount of information in the original signal that is transmitted to the listener.
If STI = 0 (completely unintelligible)
If STI = 1 (perfect intelligibility).

21. Methodology
The Modulation Transfer Function (MTF) is measured

22. Instrumentation

23. Results
If the MTF is measured over a wide range of frequencies it is possible to identify any acoustical problems
A constant MTF at all frequencies indicates the cause as background noise
A constantly decreasing MTF with frequency indicates excessive reverberation
An MTF which first decreases and then increases with frequency indicates an echo.
If measured in only two octave bands 500Hz and 2kHz (RASTI)
Only the index is available

24. SUBJECTIVE METHODS Musicians Criteria Listeners Assessments Speech

25. 1. Musicians’ Criteria
Room should respond to their instruments (should have reflections back from the listening area to the stage)
Musicians should be able to hear each other (reflecting surfaces around the performing area)

26. 2. LISTENERS -- SPEECH PERCEIVED ARTICULATION INDEX, AI
Traditionally the method used to assess speech intelligibility is for different speakers to read out a series of phonetically balanced words or phrases.
Listeners in different locations write down what they hear the words to be.
Normally if a word is misunderstood it is because the consonants are not heard properly.
The number of words understood out of the total is expressed as a percentage as the phonetically balanced (PB) word score or perceived %AI

27. Method
Listeners write down what they think they heard and then check this against the original.
95% accuracy best possible
80% accuracy audience will understand every sentence
75% accuracy listener has to concentrate
65% accuracy intelligibility is poor
Note that AI can also be predicted from measurements of the room transfer function (which can be predicted using modeling)

28. Typical Word List
are bad bar bask box cane cleanse clove crash creed death deed dike dish end feast fern folk ford fraud fuss grove heap hid hive hunt is mange no nook not pan pants pest pile plush rag rat ride rise rub slip smile strife such then there toe use wheat

29. Typical Phrase List Oak is strong and also gives shade.
Cats and dogs each hate the other.
The pipe began to rust while new.
Open the crate but don't break the glass.
Add the sum to the product of these three.
Thieves who rob friends deserve jail.
The ripe taste of cheese improves with age.
Act on these orders with great speed.
The hog crawled under the high fence.
Move the vat over the hot fire

30. COMPARISON OF PHONETICALLY BALANCED WORD SCORE AND STI
Measured speech transmission index, STI %

31. 2. LISTENERS –MUSIC
EVALUATION GUIDE TO BE COMPLETED BY EXPERIENCED LISTENER
CLEAR SOUND BLURRED SOUND
(varies from clear or distinct to blurred or muddy)
LIVE REVERBERANCE . .DEAD REVERBERANCE
(liveness or persistence of mid-frequency sounds)
WARM BASS COLD BASS
(relative liveness of bass or longer duration of reverberance at bass compared with mid and treble frequencies)
INTIMATE SOUND REMOTE SOUND
(auditory impression of apparent closeness of orchestra)
SATISFACTORY LOUDNESS UNSATISFACTORY LOUDNESS
(indicate early or direct sound (symbol D) and reverberant sound (symbol R) on scale
RICH DIFFUSION POOR DIFFUSION
(envelopment of sound which surrounds the listener from many directions)
GOOD BALANCE POOR BALANCE
(observe between musicians and soloists or choir, and amongst sections of the orchestra)
SATISFACTORY BACKGROUND NOISE UNSATISFACTORY BACKGROUND NOISE
(from HVAC equipment, or intruding from ancillary spaces or outdoors)
ECHOES No Yes Direction.
(long delayed reflections that are clearly heard)
OVERALL IMPRESSION…………………………………………………….

32. Links
Paper giving the assessment of 58 concert halls (Leo Beranek )
Concert Hall Acoustics
History of acoustical parameters