Abstract: An apparatus for heating a liquid includes an electro-acoustic resonator disposed inside a container. The electro-acoustic resonator includes a concave rigid emitter and a substantially hollow receiver spaced closely apart from and mounted in the emitter. The emitter may be configured in the form of a hemisphere, and the receiver in the form of generally spherical bladder containing a free-moving sold mass in a gas. The container may be in the form of a cylinder. During operation, an AC power source is applied to the emitter and the receiver is wired to ground. The resonator generates acoustical energy, heating a liquid (e.g., water) as it flows through the container.