Protecting speech privacy in a specific room is an important challenge in room acoustics. However, protecting people's conversation from being overheard by an unintended listener, that is, making them not understandable, is difficult. This paper proposes a method for protecting speech privacy by actively controlling the speech transmission index (STI) in a simulated room containing an unintended listener. In this method, the STI in the simulated room can be controlled by manipulating the parameters of the simulated room impulse response (RIR). We can control the STI by convolving speech with the simulated RIR because the presentation of speech and additive delayed-manipulated speech can be regarded as the convolution of speech with late reverberation in the simulated room. Three experiments (world intelligibility, listening difficulty, and annoyance tests) were conducted to compare the proposed method with two conventional methods (noise masking and reverberation). The results showed that speech privacy can be protected by controlling STI derived by manipulating the simulated RIR. The results also showed that the proposed method can protect the privacy of conversations as effectively as those other methods can by using lower noise levels and shorter reverberation.