Capacitive Micromachined Ultrasonic Transducers (CMUTs) have become very popular for medical imaging and sonar applications during the last decade. They can compete with piezoelectric transducers in terms of efficiency and bandwidth. Recently, it has been noticed that these devices couple energy into silicon wafer in addition to the immersion medium. By placing the CMUTs on a substrate in an interdigitated configuration, Lamb wave or Rayleigh wave modes can be excited with very high efficiency without a need for any piezoelectric material. In this study, the acoustic power coupled into the silicon substrate as well as the acoustic impedance of the CMUT membrane are calculated by using finite element analysis. Normal mode theory is used to find the distribution of the acoustic power among different Lamb wave modes. For low frequency (1 MHz) devices, the lowest order antisymmetric (A0) mode Lamb wave is the dominant mode in the substrate. For high frequency devices (100 MHz), interdigital CMUTs excite Rayleigh wave with efficiencies comparable to piezoelectric surface acoustic wave (SAW) devices. Experimental results for Lamb wave devices in the vicinity of 1 MHz will also be presented. [Work supported by ONR.]