A communication system may communicate by backscattered acoustic signals that propagate through a liquid or solid. In this system, one or more transmitters may transmit acoustic signals that travel to, and are reflected by, an acoustic backscatter node. The backscatter node may modulate the amplitude and/or phase of the reflected acoustic signals, by varying the acoustic reflectance of a piezoelectric transducer onboard the node. The modulated signals that reflect from the backscatter node may travel to a microphone and may be decoded. The backscatter node may include sensors, and the uplink signals may encode sensor readings. The backscatter node may harvest energy from the downlink acoustic signals, enabling the node and the sensors to be battery-free. Multiple backscatter nodes may communicate concurrently at different acoustic frequencies. To achieve this, each node may have a matching circuit with a different resonant frequency.