Accurate loudness estimation is essential for audio production, quality control, and loudness compliance, but no established recommendation exists for binaural playback over headphones. This paper investigates the influence of binauralizers and HRTF processing on objective loudness estimation for binauralized Ambisonics content. Two experiments were conducted using 163 Ambisonics clips binauralized with two open-source renderers and three HRTF sets under three HRTF preprocessing conditions. Objective loudness metrics were compared against ground truth loudness data derived from 7.1+4 loudspeaker feeds according to ITU-R BS.1770. Results reveal small to moderate differences in Integrated Loudness and larger differences in the True Peak values between the evaluated binauralizers, and that diffuse-field equalization can effectively eliminate loudness and True Peak differences across binauralizers and across sets of HRTFs. The findings can help to better predict and ensure loudness compliance in binauralized audio consumption in XR and gaming, especially when importing 3rd-party HRTFs is supported.
