The mobile terminal handsets for the digital cellular phone systems use low bit rate speech CODEC. Although the speech quality of the handsets is strongly related to the speech CODEC performance, the effect of the handset acoustic characteristics on reconstructive speech quality cannot be negligible. This is because the equantizers of the speech CODECs are generally designed based on predetermined speech databases. Moreover, the speech databases are designed by using the ideal acoustic characteristic instruments. However, it is difficult to realize the ideal acoustic characteristics in real handsets. This paper discusses the acoustic characteristics realizable and suitable for low bit rate speech coding. The acoustic characteristics were prescribed in terms of input--output linearity and frequency characteristics. Six types of the acoustic characteristics for the receiver and three types for the transmitter were prepared, and a subjective evaluation test based on MOS was carried out over the speech stimuli created through the above nine types of the acoustic characteristics. The evaluation results showing which acoustic characteristics are suitable for the mobile terminal handset will be presented at the conference. Moreover, noise canceler property is critical to improve the speech quality. This paper also discusses the preferable time-varying property.