The present work assessed the effects of flattening the fundamental frequency (F0) contour and processing by single-channel noise suppression on the intelligibility of low-pass (LP)-filtered (LPF) sentences. The original F0 contour was replaced by an average flat F0 contour or treated by single-channel noise suppression, followed by application of LP filtering to Mandarin sentences. Processed stimuli were presented to normal-hearing listeners to recognize. Flattening the F0 contour significantly affected the understanding of LPF sentences. Noise suppression by existing single-channel algorithms did not improve the intelligibility of LPF sentences.