Frequency response curves of parabolas

The right figure shows the theoretic response of a parabola. The upper left is a response of a real-world reflector made of aluminum. Below is a reflector made of plastic.

This figure is a citation from (2). It was made using data taken from (1).

(1) Little, R.S. (1964): Acoustic properties of parabolic reflectors. Bio-Acoust. Bull. 4.
(2) Tembrock, G. (1982): Tierstimmenforschung. A. Ziemsen Verlag Wittenberg Lutherstadt.