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Anatomy of a baby's cryClick to hear the cry. (It will play through twice.)
The sounds of a baby's cry are produced by vibrations in the baby's vocal cords*, which, in turn, cause molecules in the air to vibrate at the same frequency. As the air molecules move back and forth, they create areas of high pressure where molecules are closer together and areas of low pressure where the molecules are further apart. These alternating bands of high and low pressure spread out through the air just like waves in the ocean. When they reach our ears, they are perceived as sound. The diagrams below show two different ways of representing sound that the researchers can use to analyze the physical properties of a baby's cry (or any another other type of sound).
The spectrogram of the cry shows, along the y axis, how the frequency of vibrations changes over the duration of the cry. (Frequency is measured in a unit called hertz, where 1 hertz = 1 vibration per second.) We hear differences in frequency as differences in pitch, and, as the graph shows, the cry at any point in time can be composed of more than one frequency (pitch). (Otherwise the graph would just consist of a line with one frequency noted for each point in time.) The differences in light and dark indicate the amplitude (loudness) of the cry at that point. The dark bands are called formants and indicate the more dominant frequencies.
The bottom graph, a waveform or oscillogram, is another way of representing changes in amplitude only. Changes in amplitude, measured in hertz, is represented on the y-axiz.
The researchers record sound using Sound Designer II on a Mac computer. Sound analysis is done using Praat and Canary.
*These vibrations are actually filtered as they pass throught the mouth.