Transcranial focused ultrasound stimulation (tFUS) is a promising noninvasive neuromodulation tool for targeting brain regions with millimeter-scale spatial resolutions. In conventional tFUS studies, a focused ultrasound beam generated by an external ultrasound transducer is delivered to the neural target. In tFUS, ultrasound should travel through the skull that features large attenuation and different acoustic impedance compared with the soft tissue, thereby resulting in acoustic impedance mismatch and reflections. In this paper, we study the impact of the rat skull on the ultrasound beam profile generated by both focused and unfocused (but with a natural focus) transducers at a wide sonication frequency (fp) range of 0.5-9.5 MHz. At different axial and lateral distances from the transducers, we measured ultrasound intensity profiles of three transducers operating at fps of 0.5 MHz, 1.2 MHz and 9.5 MHz with and without the skull. Our results showed that ultrasound beam profiles were significantly distorted by the skull. The transmission factor due to skull attenuation was measured 0.79,0.34 and 0.03 at fps of 0.5 MHz, 1.2 MHz and 9.5 MHz, respectively, when the skull was close to transducers focal zones.