The silencer—waveguide isolator (WI) is theoretically, numerically and experimentally investigated. Transmission loss values of more than 15 dB in the 2-octave band are obtained. Numerical comparison of its efficiency with the efficiency of dissipative silencer is carried out. It is shown that in the region of low frequencies (up to 1 kHz) WI has greater efficiency. The advantages of the WI are noted: the possibility of adjusting the design to a given frequency range with the choice of design parameters using a simple calculation methodology, high efficiency with small dimensions, the absence or small amount of sound-absorbing material.