Sound-speed measurement in various materials is widely used for getting information about their elasticity and density. It has already been used in many researches to evaluate different biological tissue conditions and for distinguising the normal and diseased tissues. For these purposes it is necessary to implement a precise measurement, reducing the error sources. A sound wave produced by a real (finite size) transducer does not propagate completely as a plane wave. Calculation is done for both a planar circular transducer and a concave circular transducer, considering the uniform vibration velocity and burst pulse activation. The effect of nonplanar transmission in every point of the sound field is an amplitude change and a phase change. The phase change will cause a time shift on ultrasound pulses that will cause some errors in the transmission time measurement. The results are shown that, for the same transducer size and frequency, this effect is more serious for a concave transducer than a plane transducer, and this effect should be considered for measuring the sound velocity with precision better than 0.1%.