Abstract: The Speech Intelligibility (SI) can be affected by the ambient noise in monaural hearing. This paper in- vestigates the SI of a speech signal with an intensity of 70 dB under 3 different types of ambient noise with different Signal to Noise Ratio (SNR) in the signal ear and with different level of noise in the contralateral ear. The experimental results show that the contralateral noise has no significant effect on the SI when the SNR in the signal ear is greater than a certain level which depends on noise type; however, when the SNR in the signal ear is low, a proper increase of noise level improves the SI, and the most suitable noise level is 78-82 dB, but the SI decreases when the noise level in the contralateral ear is greater than a certain level. Based on the preliminary psychoacoustic and physiological fact, it is found that it is the muscle contractions in middle ear that suppress the perception of noise under noisy speech environment improves the SI of the communication