COPYRIGHT AND CREDITS
Copyright exists in the sound recordings and they may not be used for commercial purposes. The copyright holders are: F. Becker, P. Bishop, R. Boycott, H. Braack, M. Burger, A. Channing, M. Cherry, W. Conradie, M. Cunningham, A. de Villiers, L. du Preez, A. Elepfandt, M. Jennions, L. Minter, D. Moyer, N. Passmore, M. Picker, M. Pickersgill, B. Sternstedt, R. Toms, A. Turner and L. Verburgt.

SPECTROGRAMS
By definition, spectrograms are visual images of sounds - they let you 'see' the sound of the frog's call. They are simple graphs that show you the frequency, duration and pulse of a call, and how loud or soft each part of the call is. These parts of the call appear as lines, streaks and dots on the spectrogram. The density of the marks reflects how loudly or softly each part of the call is made. Dense, black marks indicate loud sounds; lighter grey patterns indicate softer sounds. In a way, spectrograms are more precise than words and, with some practice, are easy to read.

The vertical axis measures the frequency of the call. Frequency is determined by the wavelength of the sound and is measured in kilohertz (kHz). The lower the pitch of a call, the closer the call is to the bottom of the graph. Conversely, the higher the pitch, the closer to the top of the graph the marks will appear. Many frog calls are made up of a mixture of high- and low-pitched sounds and their component parts are therefore spread over the height of the spectrogram.

The horizontal axis reflects the sound over time, which is measured in seconds or fractions of a second. The length of each part of the call and the duration of the whole call are shown horizontally across the width of the spectrogram. A long, slow call stretches across the width of the spectrogram, a short, sharp click appears as a spot or narrow vertical strip. A protracted, pure flute-like tone extends unbroken across the spectrogram. A call that is made up of short, closely spaced pulses or vibrations appears as a series of short marks.

The call of the Western Leopard Toad, shown in the spectrogam on the right, can be described as a long drawn-out snore with the pitch ranging from low (0 kHz) to high (6 kHz). At the lower end of the graph, the dense, dark marks indicate the louder part of the call, while the thinner, lighter lines represent softer notes.