Room Acoustics. Bouncing Around. Experiment. Listen to a tone and move your head from side to side. What do you hear? Why do you hear it??. Move yer head. WALL. Different Distances. Schematic. Head Movement. DIRECT SOUND REFLECTED SOUND. Consider this Table. Two surfaces.
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Consider a Wall • How smooth is it? • Smooth is in the feel of the feeler! • Smooth or Rough are Relative terms. • We define: • SMOOTH – Variations occur on a scale much smaller than a wavelength of the sound we are considering. • ROUGH – The variations in the surface are comparable to the size of the wavelength.
SOFT Walls • A soft wall (like rubber or cork) will yield when you push on it. • Sound (music) pressure pushes on the wall. • IF the wall deforms, than a force (pA) times a distance (the deformation), means that the wave does WORK. • The sound therefore loses some energy when it hits such a wall. • The reflection isn’t as strong as one from an “un-yielding” wall.
Consider an outdoor concert • Musicians on stage • People in the audience • No Walls or Ceilings • Only reflections possible are from structures in back of the musicians.
Care in a band-shell • The focus can’t be too good because then all of the performers need to be at the same place. • Since they can’t be, a vertical wall might be better. • Real Band shells look right but really do NOT properly focus. ON PURPOSE!
What does “focus” mean • Sound waves hit a surface which can be called a mirror. • The mirror surface can be curved so that rays of sound from different directions can be made to come together at the same place. • Like a lens • In a concert hall, too much focusing can also mean that there is only ONE good seat in the house!
Whispering Gallery Note – This Wren design was actually a spherical surface that doesn’t really focus that well. It probably comes close to a portion of an ellipse.
An interesting application With this device, you can magnify faint or distant sounds with a clarity you never thought possible. You can listen to bird calls in the forest! (Advertising Pitch).
Diffraction • Sound can “bend” around objects. • Sound can change its properties depending upon the size of the wavelength compared to objects. • The Diffraction effect can be understood via one of the early theories of waves.
Huygen's Principle • Every point on the front of a wave (wave front) acts as a source of spherical waves. • The next position of the wave front will be the surface that is tangent to all of the other parts of the surface created in the same way. • The spherical wave travels at the speed of sound. vt
Sound • Travels in straight Lines. • Travels in crooked lines. • Can be focused. • Can be absorbed by a surface • Can be diffracted • Can interfere “with itself” • Is dependent on the properties of the room.
What else? • Small objects will scatter or diffract sound so it can be heard in non-straight lines. • Around edges, etc. • Small objects do very little to long wavelength sounds (low tones). They are like the eEverready Battery … they keep going and going and going ….. • Higher frequency sounds will be deflected or absorbed more than low frequency sounds.
