The resulting display has units of time on the horizontal axis and range in metres to potential echoic targets on the vertical axis. The colour of each pixel represents the instantaneous signal-to-noise ratio (SNR) of the received signal. Echograms require some practice to interpret, and the key features are indicated in the schematic representation on the right. Thin diagonal traces in the echogram (A, B) are echoes from small organisms in front of the porpoise. The slope of each echo trace indicates the closing speed (metres per second, a negative slope implies a positive closing speed). These organisms are likely initially moving slowly relative to the porpoise and so the closing speed is largely due to movement of the porpoise towards the targets. The high-intensity outgoing echolocation clicks (C) appear at distance 0 throughout the echogram and act as a reminder that the display represents a moving frame of view anchored to the porpoise. Clicks are produced at a rate of 10–50/s during searching and initial approach but a rapid acceleration in the clicking rate, termed a buzz, occurs during close approaches with prey (D). As per Johnson et al., 2006 and Wisniewska et al., 2012, rapid sequences of clicks are identified as foraging buzzes when the inter-click interval drops below 100 ms for beaked whales and 13 ms for harbour porpoise. In the example here, the buzz is followed by a short pause in clicking (E) before slower search clicking resumes (F). Due to the high speed of clicking during the buzz, outgoing clicks appear multiple times in the echogram (G, H) at spacings equal to the inter-click interval multiplied by one half of the sound speed. One of the echo traces from the organisms evident before the buzz (A) continues into the buzz and provides information about the movement of the organism (most likely a small fish) as it is targeted by the porpoise. The fish reacts to the porpoise’s approach at a distance of about 0.6 m (I) and attempts to escape as evidenced by a sharp change in the slope of the echo trace. Shortly thereafter the porpoise’s clicking rate suddenly begins decreasing (J) after a period of monotonic increase. This results in a V-shape in the repetition curves of the outgoing clicks (G). Although the prey can outswim the porpoise for a brief interval, leading to an increase in range between the porpoise and prey, it is ultimately overtaken and captured by the porpoise (K).