Metal structures can fail because of fatigue crack propagation or because of section loss from corrosion. Regular inspection is required to intercept such failures, and in situ sensors would be a superior technology for that purpose. We have designed and fabricated arrays of MEMS capacitive diaphragm transducers and we report on their performance as pulse-echo detectors in direct contact with solids. Our chip is approximately 1-cm square and features nine detectors in a linear array, each detector containing 180 hexagonal diaphragms. Performance of the detector array was studied by bonding the chip to test specimens and applying an ultrasonic pulse using a commercial ultrasonic transducer. One experiment recreates an on-axis excitation in which the pulse arrives uniformly at all detectors, and another experiment recreates an off-axis excitation in which the pulse arrival is delayed from one detector to the next along the length of the array, permitting accurate localization of the source using phased array signal processing. The results establish that MEMS transducers can function successfully as phased array detectors of ultrasonic signals in solids.