Binaural Room Impulse Responses

Binaural room impulse responses (BRIRs) are measurements that capture the spectral filtering properties of the head and ears, as well as the loudspeakers and any room reverberation present. Measurements are typically made in reverberant rooms using dummy heads that are rotated above their torso to capture multiple head orientations for a number of source locations within the room, as shown in Fig. 1 below.

Figure 1: BRIR measurements taken at the University of Salford’s ITU-R BS.1116-1 compliant listening room [1]

One key application of BRIRs is the synthesis of spatial audio over headphones. Convolution of an audio signal with a BRIR converts the audio to that which would be heard by the listener if it had been played at the source location. This process can be repeated for all channels in the audio signal and their respective source locations in the room to create spatial surround sound on headphones.

Audio Spatialisation for Headphones

This dataset includes a set of BRIRs that were derived from a range of publicly available binaural datasets. The BRIRs were measured using dummy heads and Head and Torso Simulators (HATS) in a variety of reverberant rooms, each containing unique acoustical properties. A comprehensive set of rooms have been compiled including control rooms, listening rooms, offices, and studios, among other types of rooms. For each room, a set of BRIRs have been provided for a range of source directions around the head on the horizontal plane. Improvements have been made to the frequency responses of the BRIRs in an effort to improve the quality of the binaural simulations on headphones.

Binaural Room Response Improvements

A key feature of this dataset is the high quality binaural simulations provided by the BRIRs. This is largely a result of the improvements made to the frequency responses of the BRIRs. The BRIRs have been equalised with compensation filters to compensate for undesired spectral colouration in the binaural signal transmission chain and to improve the externalisation and localisation of sounds within the acoustic environments reconstructed by the BRIRs. The main sources of colouration in the binaural signal chain include the loudspeakers, microphones, dummy heads, and room acoustics during binaural measurement, and the listener's headphones and ears during binaural reproduction. Each of these sources of colouration were taken into consideration during equalisation. Limitations of the measurement equipment and quality of the acoustic spaces measured were also taken into consideration for further enhancements. A summary of this process is provided below.

Frequency Response Compensation

• Spectral colouration introduced by the loudspeakers, microphones, and dummy heads during binaural measurement were compensated. The dummy heads were equalised to a flat diffuse-field frequency response to provide compatibility with diffuse-field equalised headphones.

• Spectral colouration introduced by the pinna during binaural reproduction on headphones was compensated. The types of headphones used for binaural reproduction were taken into consideration to ensure compatibility with different headphone types. Over-ear (circumaural), on-ear (supra-aural), and in-ear headphones were all considered. As over-ear and on-ear headphones were expected to be used more frequently for binaural reproduction, the BRIRs were equalised for compatibility with over-ear and on-ear headphones. An approximation of the typical acoustic interactions between headphones and the pinna is shown in Fig. 2 below. A separate equalisation filter is required to provide compatibility with in-ear headphones due to differences in pinna interaction. The required filter can be found in the Compatibility_Filters folder.

• Room correction was applied to compensate for frequency response problems caused by the room's acoustics. The rooms were equalised to a smooth room target curve resulting in a downward slope of approximately 8dB from 20Hz-20kHz. The applied room target curve is shown in Fig. 3 below.

Low Frequency Extension

• Due to the limited frequency ranges of the source BRIRs, subwoofer room impulse responses (RIRs) from the SUBRIR database [2] were combined with sets of BRIRs to create generalised low frequency BRIRs which were integrated back into the BRIRs to extend the frequency ranges and allow for high quality reproduction of low frequencies within the rooms.

Reverberation Reduction

• In some rooms containing excessive reverberation, reverberation times were reduced through windowing and filtering methods in an effort to improve the acoustics of the rooms.

In addition to the improved BRIRs, Headphone Correction Filters (HpCFs) are provided for a wide range of headphones. The filters can be used to equalise the listener’s headphones to the diffuse-field target response. Refer to the HpCF page for more information.

Figure 2: Approximate headphone pinna interaction curve for over-ear and on-ear headphones

Figure 3: Room target curve applied to the BRIRs

References

[1] F. Melchior, D. Marston, C. Pike, D. Satongar and Y. Lam, “The Salford BBC Spatially-sampled Binaural Room Impulse Response dataset“. University of Salford, 26 Feb 2014.

[2] G. Vairetti, N. Kaplanis, E. De Sena, S. H. Jensen, S. Bech, M. Moonen, and T. van Waterschoot, “The subwoofer room impulse response database (SUBRIR),” KU Leuven, Tech. Rep., 2016.