MODELLING ROOM ACOUSTICS

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.

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).

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.

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.

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.

Cylinder Light Source

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).

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.

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.

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

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

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.

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

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.

Royal Festival Hall

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.

Model Showing External View

Model Showing Interior Detailing

Full Scale Hall Nearing Completion

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

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.

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

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.

Ray Tracing (Particles)

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

Conical Beam Tracing

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

Triangular Beam Tracing

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

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

Scattering Coefficient

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.

Auralisation process

Binaural Room Impulse Response

Binaural Room Impulse Response http://www.ee.usyd.edu.au/carlab/UserFiles/SOH/index.html

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

Head Related Transfer Function

Typical Model Outputs

Detecting Problems

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

Complex Models Surfaces can be modelled in detail

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

Links Salford University Kirkegaard Acoustics Arup’s ‘SoundLab’ Concert Hall Acoustics Kirkegaard Acoustics http://www.kirkegaard.com/ Typical Software - www.odeon.dk/ examples of auralisation and a downloadable demo of software limited to models provided Arup’s ‘SoundLab’ auralisation consultancy service OpenAir Library of Auralisation examples