  We present a neural-network-based fast diffuse room impulse response
generator (FAST-RIR) for generating room impulse responses (RIRs) for a given
acoustic environment. Our FAST-RIR takes rectangular room dimensions, listener
and speaker positions, and reverberation time as inputs and generates specular
and diffuse reflections for a given acoustic environment. Our FAST-RIR is
capable of generating RIRs for a given input reverberation time with an average
error of 0.02s. We evaluate our generated RIRs in automatic speech recognition
(ASR) applications using Google Speech API, Microsoft Speech API, and Kaldi
tools. We show that our proposed FAST-RIR with batch size 1 is 400 times faster
than a state-of-the-art diffuse acoustic simulator (DAS) on a CPU and gives
similar performance to DAS in ASR experiments. Our FAST-RIR is 12 times faster
than an existing GPU-based RIR generator (gpuRIR). We show that our FAST-RIR
outperforms gpuRIR by 2.5% in an AMI far-field ASR benchmark.
