Sound in Rooms

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Sound in RoomsReverberationDirect and reflected soundRT60Digital reverberationUsually modeled using feedback delay lin ...Comb filters and allpass filters can be ...Schroeder proposed two modelsGardner reverbRoom modelingSchroeder later proposed modeling reverb ...Griesinger proposed modulating the filte ...Dattorro proposed a filter network that ...Feedback delay networksWaveguide based reverbsImpulse response and convolution
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Sound in Rooms
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Reverberation
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Direct and reflected sound
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Direct sound arrives first and is generally louder than later reflections
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Reflections travel longer and are absorbed when they bounce off surfaces
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When rooms are very reflective (i.e. they absorb little) the reverb is longer
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When they are not, reverb is shorter
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The duration of reverberation is not necessarily related to room size
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A tiled bathroom might have a longer reverb than a large carpeted room.
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The auditory system will fuse reverberation to the direct signal
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So reverberation becomes an "attribute" of the sound event rather than a separate auditory event
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Early reflections
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Right after the direct sound some clearly separated reflections might arrive
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The ear is sensitive to small variations in reflection times and the perception of reverb is rich, complex and nuanced. A lot of aesthetic and cognitive information is provided by minute differences in reverb.
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RT60
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RT60 is a measure of reverb time
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It is defined as the time it takes for reverberation to decay 60 dB from its peak amplitude
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Digital reverberation
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Usually modeled using feedback delay lines
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Comb filters and allpass filters can be designed from feedback delay lines
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Feedback Comb filter
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Allpass Filter:
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Nested allpass filters
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All pass filters have the interesting property that they can be nested
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https://ccrma.stanford.edu/~jos/pasp/Nested_Allpass_Filters.html User Link
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Schroeder proposed two models
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Five allpass in series
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4 comb filters in parallel followed by two allpasses
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Comb filter delay times (ms): 29.7, 37.1, 41.1,43.7
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Allpass times: 5 sm and 1.7ms
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Gardner reverb
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General algorithm to simulate small, medium and large rooms
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Room modeling
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Ray tracing/Beam tracing
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Phantom source
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Although this appears to be very accurate, the produced reverberations don't sound like the modeled rooms!
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"This somewhat startling fact means that one or more of our assumptions does not hold water. [...] we have been ignoring [...] the effect of diffusion."
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"The rough surfaces completely wash out all but the first few images, making any use of geometric acoustical modeling ,either with phantom sources or ray models, useless for determining the actual sound of a concert hall"
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James A. Moorer. *About This Reverberation Business*. Computer Music Journal , Vol. 3, No. 2 (Jun., 1979), pp. 13-28
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Schroeder later proposed modeling reverb by generating the early reflections separately from the reverb tail
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Griesinger proposed modulating the filters
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This can model the variances of sound in the air due to turbulance and temperature variations
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Dattorro proposed a filter network that allowed parametric control
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6 parameters in total
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Decay
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Decay diffusion
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Damping
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and others
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See article by Beltrán and Beltrán for details
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Fernando A. Beltrán † , José R. Beltrán † , Nicolas Holzem †† , Adrian Gogu , Matlab Implementation of reverberation algorithms. Proceedings of DAFx '99.
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Feedback delay networks
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Proposed by Stautner and Puckette
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Generic feedback delay network
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Waveguide based reverbs
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Waveguides can be thought of as a generalization of the feedback allpass filter design
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For details see Valimaki et al. "Fifty years of artificial reverberation."
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Impulse response and convolution
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In a future lecture