Entrainment can be defined as “ the tendency for two oscillating bodies to lock into phase so that they vibrate in harmony. It is also defined as a synchronization of two or more rhythmic cycles. The principle of entrainment is universal, appearing in chemistry, pharmacology, biology, medicine, psychology, sociology, astronomy, architecture and more” 21 When we are exposed to periodic signals, such as a sound, a light or electrical signals, our bodies tend to track and match the core frequency and phase of the applied signals. For example, if you look at a light blinking at about 4 Hz, your heart rate and EGG or "brain waves" (electroencephalogram) will tend to match the rate of the light and shift more of their energy toward 4 Hz. Entrainment causes systems to vibrate more in phase or move in synchrony. If the phase of two oscillations match, the most energy will transfer between them. Stephen Birch showed that entrainment of the human EEG occurred during and after swims with free dolphins. The EEG of the human subjects reduced in frequency and increased in power after swimming with free dolphins. 22
T o better understand the sound capability of dolphins, it is useful to review a little of their anatomy and their means of sound production. The drawing below is a tracing of the midplane of a dolphin. Labeled areas: 1. Rostrum or “nose”; 2. External surface of the melon; 3. Blowhole. The skull and upper jaw are black. The airway passes through the bony nares (“nostrils”) anterior to the brain (B) in the skull. The food-way (F) starts in the mouth and passes on each side of the airway (A) at the larynx. (After J. C. Lilly)23
The palatine bones in front of the skull (left of A, above) form a parabolic sound reflector behind the phonators. The naso-pharynges are located at the focus of this parabola. Therefore, sounds from the phonators, reflected from the palatines, go forward as collimated parallel beams. The sounds transit a structure called the melon, an oil-filled sack located at the front of the dolphin’s head. The melon is, in part, an acoustic lens that focuses out-going sounds. Four sound-producing naso-pharynges (also called “phonators”) are arranged in left and right pairs below the blowhole, roughly at point A in the diagram above.