1. (1) Leug, P., “Process of silencing sound oscillations”, US Patent No. 2043416 (1936)
2. (2) Peter J. Westervelt, “Parametric Acoustic Array”, Journal of Acoustic Society of America, Vol. 35, No. 4 (1963), pp. 535-537.
3. (3) Berktay, H.O., “Parametric amplification by the use of acoustic non-linearity and some possible application”, Journal of Sound and Vibration, Vol. 2, No. 4 (1965), pp. 462-470.
4. (4) Hobak, H. and Vestrheim, M., “Parametric acoustic arrays formed by diverging sound beams”, Acustica, Vol. 37 (1977), pp. 74-82.
5. (5) Tan, K.S., Gan, W.S., Yang, J., and Er, M.H., “Constant beamwidth beamformer for difference frequency in parametric array”, IEEE International Conference on Acoustics, Speech and Signal Processing, (2003), pp. 361-364.