When two waves have the same frequency and phase and interacted, they will combine and producea stronger wave which process is called constructive interference. Since sound is a wave, this process can also be applied to speakers. The produced sound wave due to constructive interference has a higher amplitude and intensity than the former two waves. And when two sounds are out of phase, they will cancel each other producing lesser waves which process is called destructive interference. The produced sound wave due to destructive interference have lower amplitude and intensity. Constructive and destructive interference is the main principles that LRAD used for creating a directional very loud sound.
LRAD or Long-Range Acoustic Device is a device that American Technology Corporation developed as a substitute and discourages in use of lethal force in US military ships. It is also called an acoustic hailing device (AHD) and sound canon.
LRAD uses piezoelectric transducers to produce sound in a speaker. Many piezoelectric speakers are then aligned in an array and connected to an electrical power source. The power source applies electrical current to the transducers which will then each create sound waves. Identical sound waves, having the same phase and frequency, are then produced in the array of transducers. Due to constructive interference, the sound wave produced has a higher amplitude that helps LRAD have powerful sounds.
Unlike other dispersed speakers’ sounds, LRAD produces a focused beam of sound. It is a directional model which creates a sound that travels farther than other speakers. To have a directional LRAD, the outer speakers’ sound wave must not be in phase with the inner speakers in the array of piezoelectric transducer speakers. When LRAD produces sounds, the sound waves in the outermost will cancel each other out or destructive interference happen. The LRAD sounds produced have less intensity or are less loud in the outermost compared to the innermost sound.
The maximum distance an LRAD can cover depends on the sound level, directionality, and frequency of the acoustic source, the sensitivity and directionality of the receiver, and the transmission channel environment. Aside from a loud painful sound, LRAD can also amplify voices that are loud and clear.
Piezo Source speakers find application for longer distance communication that it is used for mass notification, early warning systems, critical infrastructure protection, wildlife protection, crowd control, and maritime shipping, safety, and communication.
The Long Range Acoustic Device https://newatlas.com/lrad-long-range-acoustic-device/11433/
How LRAD Works? https://science.howstuffworks.com/lrad.html