The reference wave necessary for recording a hologram of an object irradiated with acoustical waves A. Metherell and H. El-Sum, Appl. Phys. Letters (July 1967)] can be provided externally by appropriately controlling the phase between two coherent signals, the one that derives the sound source, and the other which is used as a reference by summing it electronically with the signal diffracted from the object. The shape of the wavefront of the reference wave and the angle it makes with the object wave are determined by the phase shift between the two waves on the recording plane. This in turn depends on the wavelength, the recording technique (continuous or sampled [A. Metherell, H. El-Sum, J. Dreher, and L. Larmore, Appl. Phys. Letters 10, 277 (1967); J. Acoust. Soc. Am. (Oct. 1967), the type of the sought hologram (narrow or wide angle, Fourier, Fresnel, etc.), and the final optical reconstruction. These and other factors affecting the choice and control of the reference beam will be discussed. Experimental results of acoustical holograms taken with various sound frequencies (9–21 kHz) will be presented.