The efficiencies for detecting low-contrast circular targets in acoustic speckle fields are measured for human observers. Cone-shaped targets of known contrast were scanned in cross section at 5 MHz to generate B-scan images. Target images were paired with uniform speckle fields and presented to observers who were asked to identify which of the two images contained the target. In these two-alternative forced-choice (2AFC) observer experiments, the image data were specified exactly. Signal-to-noise ratios (SNR) defined the average observer performance. SNR values were computed from the fraction of correct observer responses and compared with the corresponding SNR for the ideal observer to estimate detection efficiencies. The average human-observer detection efficiency is approximately 50% for both positive- and negative-contrast targets. These results using commercial imaging systems are in basic agreement with those obtained previously using simulated images, although the added experimental uncertainty, particularly uncertainty about the exact position of the target, marginally decreased relative performance and increased observer variability.