1. An ultrasound system, comprising: an ultrasound data acquiring unit, including an ultrasound probe, configured to transmit ultrasound signals to a living body in at least one transmission direction and receive ultrasound echo signals from the living body in at least one reception direction to acquire ultrasound data corresponding to a first sample volume and a second sample volume different from the first sample volume; a processor configured to form a first Doppler spectrum image corresponding to the first sample volume and a second Doppler spectrum image corresponding to the second sample volume based on the ultrasound data corresponding to the first sample volume and the second sample volume, the processor being further configured to perform an image process for forming a compound image upon the first Doppler spectrum image and the second Doppler spectrum image to form the compound image; and a display configured to display the compound image, wherein the image process includes at least one of a first image process for adding pixel values corresponding to pixels of same positions on the first Doppler spectrum image and the second Doppler spectrum image, a second image process for subtracting the pixel values corresponding to the pixels of the same positions on the first Doppler spectrum image and the second Doppler spectrum image, a third image process for multiplying the pixel values corresponding to the pixels of the same positions on the first Doppler spectrum image and the second Doppler spectrum image, and a fourth image process for performing a blending process among the pixels of the same position on the first Doppler spectrum image and the second Doppler spectrum image.