  Audio and video are two most common modalities in the mainstream media
platforms, e.g., YouTube. To learn from multimodal videos effectively, in this
work, we propose a novel audio-video recognition approach termed audio video
Transformer, AVT, leveraging the effective spatio-temporal representation by
the video Transformer to improve action recognition accuracy. For multimodal
fusion, simply concatenating multimodal tokens in a cross-modal Transformer
requires large computational and memory resources, instead we reduce the
cross-modality complexity through an audio-video bottleneck Transformer. To
improve the learning efficiency of multimodal Transformer, we integrate
self-supervised objectives, i.e., audio-video contrastive learning, audio-video
matching, and masked audio and video learning, into AVT training, which maps
diverse audio and video representations into a common multimodal representation
space. We further propose a masked audio segment loss to learn semantic audio
activities in AVT. Extensive experiments and ablation studies on three public
datasets and two in-house datasets consistently demonstrate the effectiveness
of the proposed AVT. Specifically, AVT outperforms its previous
state-of-the-art counterparts on Kinetics-Sounds by 8%. AVT also surpasses one
of the previous state-of-the-art video Transformers [25] by 10% on VGGSound by
leveraging the audio signal. Compared to one of the previous state-of-the-art
multimodal methods, MBT [32], AVT is 1.3% more efficient in terms of FLOPs and
improves the accuracy by 3.8% on Epic-Kitchens-100.
