This letter proposes a new masking threshold adjustment method to improve the quality for the speech signals in low bit-rate audio coding. The Enhanced aacPlus (EAAC) audio codec increases the masking threshold of all frequency bands to be suitable for the given encoding rate by considering equal loudness noises only, which is a representative way for implementing the adjustment technique. The proposed method, however, dynamically adjusts the masking threshold of each frequency band based on the energy ratio of each band to the average band energy. More quantization noises are added to formant regions that have relatively large energy ratio values, but less distortion is allowed in spectral valley regions, which eventually helps to enhance perceptual quality for speech signals. The proposed idea reflects the spectral weighting criterion in searching optimal excitation codebooks used in many speech coding algorithms. Simulation results confirm that the proposed method implemented on the EAAC coder improves quality for the speech input signals at the same bit-rate while keeping equivalent quality for music contents.