4.6. Implementation: Room Acoustic Simulation
To create a realistic 3D acoustic simulation in software, we must synthesize not only the direct sound and HRTF cues, but also the reflective acoustic characteristics of an enclosed room.
The Image Source Method
A standard method for simulating room acoustics is Allen & Berkley’s Image Source Model.
- Each boundary wall acts as an acoustic mirror.
- For each physical reflection, a “virtual mirror source” is placed behind the reflecting wall at an equal distance.
- The impulse response of the room is constructed by summing the direct sound and each mirror image source, attenuated by the wall absorption coefficients and delayed by the acoustic propagation time:
$$t_i = \frac{d_i}{c}$$
where $d_i$ is the distance to image source $i$ and $c$ is the speed of sound (~343 m/s).
Room Acoustics Breakdown:
- Direct Sound: Travels directly from source to listener; localized via direct-path HRIRs.
- Early Reflections (10–80 ms): Low reflection density; each individual reflection is spatially positioned using the HRIR corresponding to its angle of arrival.
- Late Reverberation (>80 ms): High reflection density; modeled efficiently using feedback delay networks (FDNs) or diffuse artificial reverberators.
Implementation Considerations in Csound
In Csound, early reflections can be synthesized using tapped delay lines (delay, deltapi) routed into spatial panning or HRTF opcodes (hrtfmove, hrtfstat, spat3d), while late reverberation is generated via comb and allpass filter networks (reverbsc, freeverb).