Binaural Room Reverb (BiRR) 2D and 3D

A realistic room simulator based on a combination of a convolution processor and a virtual room impulse response calculator.

BiRR3D

BiRR2D

Info

BiRR2D and BiRR3D are room simulators that accurately emulate the sound produced by a monophonic and omnidirectionnal source perceived by a listener in a rectangular (2D) or parallelepipedic (3D) room.

User can control $(x,y,z)$ dimensions of the room, and position of source and listener in the room. The orientation of the listener can also be adjusted. Wall properties are adjusted with only two parameters:

• Wall absorbtion : a coefficient between 0 an 1, representing a reflection coefficient. The acoustic wave amplitude is multiplied by this coefficient at each wall rebound. Higher values produce a shorter reverb.

• Wall high frequency absobtion : this parameter controls the high frequency absorbtion for each wall rebound. Higher values produce darker reverb sound.

The input is stereophonic and for now, left and right signal are mixed to one mono signal sent to the reverb. It is the same as if both input signals were placed at the same position in the virtual space. The output sound is a stereo reverberberated sound with one of the following configurations:

• Two cardioid microphones in XY configuration

• One cardioid and one eight-pattern microphones, in MS configuration

• One omni and one eight-pattern microphone, in MS configuration

• Binaural receptor based on the HRTF provided by MIT medialab (https://sound.media.mit.edu/resources/KEMAR.html).

Direct path and reflections paths

when a parameter is changed, two stereo impluse responses are calculted and sent to convolution processors:

• The direct path impluse response.

• The impulse response due to multiple reflections on walls.

The effect level of direct path and reflections can be adjusted separately. These can be sought as dry and wet parameters of the reverb, although the produced sound is physically acurate when both parameters are equal.

Stereo width

In the XY configuration, this parameter specifies the angle between the two cardioid microphones. At its minimum the angle is zero and the produced sound is mono. At its maximum value, the angle equals 180 degrees, the microphones being perpendicular to the head axis. In the standard XY configuration, the angle equals 90 degrees, which occurs when the width parameter equals 0.5.

In the MS configurations the stereo with quantifies the amplitude of the figure eight microphone signal (between 0 and 1).

The stereo width parameter has no effect in the binaural configuration.

Parameters updating

At each parameter update (except reverb levels), the impulse response is calculated. As each acoustic path has to be calculated, the computation time can be high, in particular if the damping is small.

A number of threads equal to the number of CPUs is employed for the impulse response calculation, which allows reasonable computation times (At most a few seconds for largest reverberation times on recent CPUs).

Important note: the fact that some calculation time is necessary after each parameter change prevents parameter automation, as they cannot be updated in real time. Consequently, this plugin cannot be used to move sounds in the virtual space.

Future improvements

In priority order:

• Ambisonic output (FOA)

• Absorption parameters for each wall

• Multiple inputs (and ability to place multiple virtual sources in the room in the same UI)

• Faster convolution (?)

• Rooms not necessarly parallelepipedic

Contact

olivier.doare@ensta-paris.fr