1. MODELLING ROOM ACOUSTICS

2. Points covered History of physical models
Development of acoustic scale models
Difficulties and limitations
Case studies.
Computer (mathematical) models
Approaches
Data requirements (inputs)
Outputs
Problems.

3. HISTORY OF PHYSICAL MODELS
Simple 2-D models
1900 to 1930
Photographic (light and smoke)
Water ripples
Beams of light (reintroduced in the 1980 using lasers)
Light intensity distribution.
Acoustic 3-D models
1930 (1/5 scale)
1970 (1/50 scale).

4. Physical Model - Ultrasonic
Uses a technique called Schlieren photography
2D section of a scale model of an auditorium
Smoke filled air is lit from behind
Spark source produces high frequency sound waves
Photograph of wave fronts and reflections is taken
Research published by WC Sabine in 1913.

5. Physical Model - Ripple Tank
Mechanical vibrator creates ripples in a shallow water tank shaped like the cross section of the auditorium
1/50 scale, 2-dimensional
Vertical or horizontal profile
Frequency of vibration can be altered to produce
wavelengths representative of the full scale to
investigate diffraction and interference effects
Ripple tank video.

6. Physical Model – Light Beams
Light source in a slotted cylinder showing reflections
from the ceiling in a scale model (Circa 1929)
Reintroduced into 3-D modelling using laser light
sources in the 1980’s.

7. Cylinder Light Source

8. Physical Model – Intensity Distribution
Early models used a translucent glass plate in place of the audience
Distribution of light observed by eye
Light Box - 3D model
Small lamp and photocell used to investigate reflections.
model of a school gymnasium (circa 1970).

9. EARLY ACOUSTIC MODELS
Based on 1/10 and then later 1/20 and 1/50 scale physical models made from plywood.
Miniature sound sources and microphones
Scaled frequency range (4kHz 40kHz for 1/10)
Internal surfaces selected to match absorption at these higher frequencies (thin felt used for carpets, etc.)
Internal air dried to very low %RH to reduce the effect of air absorption at these higher frequencies.

10. Various Levels of Accuracy
Technicolor model
All frequencies and absorption coefficients modelled
Subjective – play and record music
Record at x10 speed & playback at x1 speed
Half-tone model
Spark impulse
Only audience absorption modelled accurately
Measure binaural room impulse response (Auralisation)
Black and white model
Absorption either 1 or 0.

11. Example 1 Sydney Opera House (1966) 1/10 scale

12. Example 1 Sydney Opera House (1966) 1/10 scale

13. Example 2 Royal Festival Hall, London 1/20 Scale model
The model was built in 1999 to help with the redesign and refurbishment of the hall which was originally built in 1951.

14. Plywood surfaces sealed with multiple layers of varnish
People modelled with corks and bits of carpet.

15. Fine details (lights, handrails, surface texture, etc
Fine details (lights, handrails, surface texture, etc.) not required since
objects must be larger than a few wavelengths in size to have a significant
effect of scattering and reflection.

16. Royal Festival Hall

17. Example 3 Walt Disney Concert Hall, 2003
Built in at the Los Angeles Music Centre. The architect was Frank Gehry, and the acoustician,Yasuhisa Toyota (Nagata Acoustics (Tokyo, Japan))
Aim was to design a concert hall with a sculpted shape, warm acoustics and exceptional clarity
Designed using a mix of scale models 1/50 up to 1/10 (large) -- over 30 models were constructed
Lasers used in the models to map the reflections from the complex ceiling design
Sound impulses used to verify wall and ceiling curves and fine tune the reverberation time.

18. Model Showing External View

19. Model Showing Interior Detailing

20. Full Scale Hall Nearing Completion

21. Recent Developments
1/100 scale models have recently been use with smaller microphones and a spark noise source
Air absorption is now removed using computer techniques
Used for other types of buildings
Industrial buildings
Tunnels and underground railway concourses

22. COMPUTER MODELLING These models can be based on:
Image Sources
Ray Tracing
Beam Tracing
All need precise information:-
CAD drawing of the room
Information on surface properties
Absorption coefficients (as a function of angle)
Scattering coefficients.

23. IMAGE SOURCE METHOD Uses law of reflection
Limits the number of reflections
Strengths of image sources
from the final reflections
are added to find the
level at M

24. THE RAY TRACING METHOD (RTM)
Uses a large number of particles, which are emitted in various directions from a source point.
The particles are traced around the room losing energy at each reflection according to the absorption coefficient of the surface.
The reception point is defined as a volume and all particles “caught” by this volume are used to find the sound level at this point.
Originally used for sound distribution only
Particles must be very closely spaced otherwise some surfaces are missed.

25. Ray Tracing (Particles)

26. Problems Rays need to be very close together otherwise parts
of the room surfaces are not correctly represented

27. Conical Beam Tracing

28. PROBLEM OF OVERLAPPING CONES Gaps between cones or Overlapping Cones
Solution --- Triangles

29. Triangular Beam Tracing

30. Surface Effects The model must also include:- Reflection -
Absorption -
Diffusion (Scattering)-
Edges (Diffraction)-

31. Diffusion (Scattering)
Ray & beam tracing can model reflections and absorption but not scattering by surfaces or diffraction around edges.
Scattering and diffraction can be modelled by RADIOSITY where each reflection generates new secondary sources on the surface. These are determined by the scatter coefficient

32. Scattering Coefficient

33. HYBRID MODELS
Current models are combinations of beam tracing and radiosity modelling
The inclusion of scattering effects and angle dependent reflection with phase shifts makes it possible to calculate the room impulse responses with a high degree of realism.
The model gives the Room Impulse Response which can be used with the Head Related Transfer Functions (HRTF) to get the Binaural Room Impulse Response (BRIR)
Combining the BRIR with anechoic sound recordings produces Auralisation of high quality.

34. Auralisation process

35. Binaural Room Impulse Response

36. Binaural Room Impulse Response

37. Head Related Transfer Function (HRTF)
The transfer function
between sound pressure
at the entrance of the
ear canal and sound
pressure in the middle of
the head when
listener is absent.
Varies with frequency
and angle – found from measurements in an anechoic room

38. Head Related Transfer Function

39. Typical Model Outputs

40. Detecting Problems

41. 3D Billiard Ball Effect (shows scattering, focusing, etc)

42. Complex Models Surfaces can be modelled in detail

43. Auralisation Listen to a concert before the hall is built
(head related transfer function)

45. Links Salford University Kirkegaard Acoustics Arup’s ‘SoundLab’
Concert Hall Acoustics
Kirkegaard Acoustics
Typical Software -
examples of auralisation and a downloadable demo of software limited to models provided
Arup’s ‘SoundLab’
auralisation consultancy service
OpenAir Library of Auralisation examples