This paper describes an efficient digital implementation scheme of the phase delay compensation beam-former (PBF) for acoustic imaging using arrays, which is based on direct quadrature demodulation through sampling for narrowband signals. The scheme eliminates the need for analog quadrature demodulators employed in conventional digital PBFs and enables the sampling frequency to be reduced lower than the Nyquist rate. The lower bound of sampling frequency, within which images can be reconstructed without unacceptable degradation, has been experimentally studied by Bmode imaging in air using a microphone array.