​Long-range Underwater Acoustic Sound S​ystems, such as those used in ocean acoustic tomography, require low-frequency signals covering a broad frequency band. To meet this requirement, a novel design based on of a tunable narrow-band high-efficiency sound projector has been exploited. The projector transmits a frequency sweep signal by mechanically tuning a resonator tube (or organ pipe) to match the frequency and phase of a reference signal. The resonator tube projector consists of a symmetrical pressure-balanced Tonpilz driver placed between two coaxially mounted tubes. The Tonpilz acoustical driver is composed of two pistons separated by preloaded ceramic stacks. The resonant tube is a simple. efficient, narrow-band, medium-output projector that operates at any ocean depth. Both projector tubes have slots (or vents), which are progressively covered or uncovered by sliding coaxial tubular sleeves. The frequency varies with the sleeves position. A computer-controlled electromechanical actuator moves the cylindrical sleeves along the tubes, keeping the projector in resonance at the instantaneous frequency of a swept frequency signal. The actuator smoothly tunes the frequency of the resonator tube in the bandwidth of 200 to 300 Hz during a 135-s transmission. A computer synthesizes the linear frequency-modulated signal; compares the phase between transmitted and reference signals; and, using a phase-lock loop (PLL) system, keeps the resonator tube frequency in resonance with the driver frequency. The estimated PLL precision is better than 3 phase error. The system was analyzed by means of finite element analysis and electrical equivalent circuit simulation. The projector prototype was first tested at theWoods Hole Oceanographic Institution (WHOI) dock inWoods Hole, MA and later in the Pacific​​