Advanced variants of DNNs, such as convolutional neural nets (CNNs) [12] , recurrent neural nets (RNNs) [13] , long short-term memory nets (LSTMs) [14] , time-delay neural nets (TDNNs) [15, 29] , <cite>VGG-nets</cite> <cite>[8]</cite> , have significantly improved recognition performance, bringing them closer to human performance [9] .