Course:PHYS341/2018/Calendar/Lecture 32

Phys341 Lecture 32: Summary and web references

2018.03.28

Textbook Ch.26, 27.1, 28.1-5.

Architecture and Music

1. Outdoor spaces
2. Room modes
   • Standing waves happen in 3-D spaces like rooms: room modes.
   • At particular frequencies there will be pressure nodes and different points in 3-D around the room.
   • There will always be pressure antinodes at the walls.
3. Surfaces (Figs. 26.4-5)
   • When sound meets a surface, three things can happen (all frequency dependent):
   • Reflection (specular and diffusive) – often hard.
   • Absorption – often soft.
   • Transmission – thin (relative to the wavelength).
4. Reverberation (Figs. 26.11-12)
   • Single percussive note in concert hall:
     • Individual audience members perceive:
     • Direct sound
     • Individual reflections off near surfaces for tens of milliseconds
     • Then multiple reflections mush together into a broad envelope of sound
     • Sound level decays away
     • Reverberation time is that required for sound to decay to a millionth of its initial level (60 dB).
5. Desired reverberation times: (Fig.27.1)
   • Depends on room size
   • ~ 1 for speech
   • ~ 1.5 s for an orchestra
   • ~ 10 s in San Marco, Venice (Canzones of Giovanni Gabrieli)
   • Controlled by:
     • Size
     • Shape
     • Surfaces (reflecting/absorbing)
     • Audience (we absorb!)
6. Shapes (Fig. 28.3)
   • Try to avoid focussing.
   • Confuses the listener as to where the sound is coming from.
   • Diffusers http://acoustics.phas.ubc.ca/room-acoustics/
7. Architectural acoustics - Wallace Sabine (Fig. 28.6)
8. Tweaking Concert Halls (Fig. 28.9)
   • Added reflectors improve the acoustics.
   • https://en.wikipedia.org/wiki/Orpheum_(Vancouver)
9. Modelling music spaces
   • Tapio Lokki’s Virtual Acoustics project: http://research.cs.aalto.fi/acoustics/