The Long Range Acoustic Device made by LRAD in San Diego, can beam sound where needed. LRAD’s product portfolio includes public safety, wildlife and asset protection, law enforcement, and military where its products can use sound as a weapon.
Sound from ultrasound uses frequencies from 25 to 50 kHz. LRAD uses ultrasound to create a directed beam of sound. Ultrasound can’t normally be heard, but LRAD uses two different ultrasonic frequencies, with the difference signal varying at an audio rate.
That way intelligible audio instructions, warnings, or even painfully loud sounds can be “beamed” and travel relatively long distances. The ultasonic signal demodulates into baseband audio when the beam hits its target.
When it comes to emergency notifications, cities up and down the Oregon Coast used different ways to alert citizens of pending emergencies such as a tsunami.
In Tillamook County, tsunami sirens will be phased out by the end of the year. While other coastal towns continue to rely on sirens, Tillamook County will depend on planes flying overhead with speakers attached, an ATV on the beach and a reverse 911 system; but no sirens. Save our Sirens is a citizen-movement to keep them operational. Washington Coast uses an All-Hazard Alert Broadcast.
The LRAD 360X broadcasts powerful emergency/warning siren sounds, messages and a variety of predator calls in a 360° pattern over distances up to 2 miles. It can be used for mass notification systems or wildlife protection. The Oregon Tsunami Warning System uses similar sirens.
The concept of heterodyning frequencies is interesting. I’m no engineer, but could two 16 x 16 arrays, centered at 3.5 GHz, be offset by 450 MHz for a similar affect in the RF band? Perhaps the two beams could demodulate into a 450 MHz difference carrier signal that travels though walls when it hits a building.