Big brown bats (Eptesicus fuscus) emit trains of brief, wideband frequency-modulated (FM) echolocation sounds and use echoes of these sounds to orient, find insects, and guide flight through vegetation. They are observed to emit sounds that alternate between short and long inter-pulse intervals (IPIs), forming sonar sound groups. The occurrence of these strobe groups has been linked to flight in cluttered acoustic environments, but how exactly bats use sonar sound groups to orient and navigate is still a mystery. Here, the production of sound groups during clutter navigation was examined. Controlled flight experiments were conducted where the proximity of the nearest obstacles was systematically decreased while the extended scene was kept constant. Four bats flew along a corridor of varying widths (100, 70, and 40 cm) bounded by rows of vertically hanging plastic chains while in-flight echolocation calls were recorded. Bats shortened their IPIs for more rapid spatial sampling and also grouped their sounds more tightly when flying in narrower corridors. Bats emitted echolocation calls with progressively shorter IPIs over the course of a flight, and began their flights by emitting shorter starting IPI calls when clutter was denser. The percentage of sound groups containing 3 or more calls increased with increasing clutter proximity. Moreover, IPI sequences having internal structure become more pronounced when corridor width narrows. A novel metric for analyzing the temporal organization of sound sequences was developed, and the results indicate that the time interval between echolocation calls depends heavily on the preceding time interval. The occurrence of specific IPI patterns were dependent upon clutter, which suggests that sonar sound grouping may be an adaptive strategy for coping with pulse-echo ambiguity in cluttered surroundings.

Biosonar guidance in a rapidly changing complex scene was examined by flying big brown bats (Eptesicus fuscus) through a Y-shaped maze composed of rows of strongly reflective vertical plastic chains that presented the bat with left and right corridors for passage. Corridors were 80-100 cm wide and 2-4 m long. Using the two-choice Y-shaped paradigm to compensate for left-right bias and spatial memory, a moveable, weakly reflective thin-net barrier randomly blocked the left or right corridor, interspersed with no-barrier trials. Flight path and beam aim were tracked using an array of 24 microphones surrounding the flight room. Each bat flew on a path centered in the entry corridor (base of Y) and then turned into the left or right passage, to land on the far wall or to turn abruptly, reacting to avoid a collision. Broadcasts were broadly beamed in the direction of flight, smoothly leading into an upcoming turn. Duration of broadcasts decreased slowly from 3 to 2 ms during flights to track the chains' progressively closer ranges. Broadcast features and flight velocity changed abruptly about 1 m from the barrier, indicating that echoes from the net were perceived even though they were 18-35 dB weaker than overlapping echoes from surrounding chains.

The concentration of propane dissolved in water in the presence of propane gas hydrates has been measured at temperatures from (274 to 277) K and pressures from (150 to 350) kPa. Solubility measurements in the absence of hydrate were found to be in agreement with literature values, which predict an increase in solubility as temperature is decreased. In the hydrate formation region, the solubility of propane decreased with decreasing temperature. Therefore, hydrate formation reverses the gas−liquid solubility trend. Results also show that pressure does not have a strong influence on solubility in the presence of gas hydrate.