By designing a unimorph suitable for the material properties of (K, Na)NbO3-based lead-free piezoelectric ceramics (KNN), we have developed a piezoelectric sounder with a sound pressure level that is superior to that of similar devices based on Pb(Zr, Ti) [Formula: see text] (PZT) over a wide temperature range. The KNN ceramics used in this work have two disadvantages: their piezoelectric constant, d31 , at room temperature is less than 80% of that of PZT, and their piezoelectric properties deteriorate because of phase transitions to the orthorhombic phase at low temperatures. The former issue was alleviated via design of a thin-layer unimorph that uses the specific mechanical properties of KNN, i.e., a large Young's modulus and low specific gravity when compared with the corresponding properties of PZT; a unimorph was then fabricated with both a vibration velocity and a resonance frequency that are comparable to those of PZT unimorphs. The latter issue was then resolved by reducing the resonance frequency of the unimorph relative to the acoustic resonance frequency of the resonator, which produced a stronger vibration-acoustic coupling effect within the temperature range in which the piezoelectric properties of KNN are lower. The KNN sounder designed in this manner exhibited a sound pressure level that was 1.4 dB greater than that of a PZT sounder at room temperature, and the range of change in the sound pressure level from -20 °C to 90 °C was 2 dB less than that for the PZT sounder.