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room acoustics: reverberation

12-Nov-01. Intro to Acoustics: Reverberation. 2. Learning Outcomes. Explain reflection, absorption, energy density, sound decay and reverberation.Understand the energy balance equation in roomsExplain and use Sabine's reverberation time equation. 12-Nov-01. Intro to Acoustics: Reverberation. 3. Sound in Rooms.

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room acoustics: reverberation

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    1. Room Acoustics: Reverberation University of Salford Acoustics Audio and Video Group

    2. 12-Nov-01 Intro to Acoustics: Reverberation 2

    3. 12-Nov-01 Intro to Acoustics: Reverberation 3 Sound in Rooms reflection absorption rays or waves?

    4. 12-Nov-01 Intro to Acoustics: Reverberation 4 The Sound Field Imagine a large number of rays emitted from a sound source Growth of sound field described by energy density, D D = acoustic energy (in a sound field) per unit volume Draw 2D room with number of rays increasing till huge.Draw 2D room with number of rays increasing till huge.

    5. 12-Nov-01 Intro to Acoustics: Reverberation 5 Energy balance (in) Energy in room = DV Energy is supplied by sound source of power W Watts Energy per second supplied = W

    6. 12-Nov-01 Intro to Acoustics: Reverberation 6 Energy balance (out) At each reflection, let the proportion of energy absorbed be called a ( it can be shown that) the rate at which energy falls on wall area S is

    7. 12-Nov-01 Intro to Acoustics: Reverberation 7 Energy Balance Rate of change of energy in room = power input - power lost

    8. 12-Nov-01 Intro to Acoustics: Reverberation 8 Reverberant Sound Decay If source switched off at time t = 0 Starting from: Leave the source on for a while, so that the energy balance reaches a steady state. Switch off the source, so W = 0: Just before source switched off, we have steady state with rate of change = 0 and D=D0 where We can say that, at t = 0, D = D0. So, k = ln(D0)Starting from: Leave the source on for a while, so that the energy balance reaches a steady state. Switch off the source, so W = 0: Just before source switched off, we have steady state with rate of change = 0 and D=D0 where We can say that, at t = 0, D = D0. So, k = ln(D0)

    9. 12-Nov-01 Intro to Acoustics: Reverberation 9 Example

    10. 12-Nov-01 Intro to Acoustics: Reverberation 10 Reverberation Time Definition: Reverberation time T is the time taken for D to fall to D0 / 106 Sabines equation: A = total absorption in room (m2) Exercise: Use Sabines eqn to calculate the RT of the lecture theatre described previously. Exercise: Use Sabines eqn to calculate the RT of the lecture theatre described previously.

    11. 12-Nov-01 Intro to Acoustics: Reverberation 11 Conclusions Sound field in a room is controlled by an energy balance Decaying sound field is described by reverberation time Sabines equation can be used to predict reverberation time - important for good room acoustics

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