The LRAD sound is an uncomfortable audible deterrent tone that travels very long distances within a narrow beam of focused soundwaves. The Long Range Acoustic Device (LRAD) was developed by American Technology Corporation (Now Genasys) and is a popular crowd control tool used by government agencies, law enforcement, and marine companies worldwide. The warning tones produced by the LRAD are in the normal audible range of hearing and driven by multiple high-efficiency audio mid-range compression drivers. The focused beam of sound is possible through a unique design that combines folded horn speakers within a large parametric array.
We will get into the design of the LRAD nonlethal weapon system in future articles, but for now, let’s take a look at the annoying sound that it produces. You can hear from the video above that it wouldn’t take long for a crowd of people to disperse if you pointed a speaker at them playing that unbearable sound. Add the directional nature of the design of the device, and you have a very effective tool for maintaining law and order. This unique audio system is considered a non-lethal weapon for this reason. It is a practical and safe alternative to more dangerous methods generally used for crowd control.
Spectrum analysis of the LRAD sound
There is some confusion regarding the source of the LRAD sound and the frequencies at which this device operates. Many websites have mistakenly stated that the LRAD produces ultrasound to transmit its beam of deterrence. If we look at the spectrum analysis of the audio output from the Long Range Acoustic Device, we can see that the frequencies of sound fall well below the ultrasonic range and are, in fact, within the mid-ranges of normal hearing levels. Frequencies above 2000Hz are still directional when combined with good speaker design. The extremely high power of the LRAD transducer’s outputs also adds to the effectiveness of this system.
The Soundlazer is an example of a device that uses ultrasonic audio to transmit a beam of audible sound capable of bouncing off an object like a laser beam. These types of devices are used in museums, kiosks, and retail displays to beam sound to individuals without disturbing others in the same room.
LRAD uses an audible sweep tone
If we examine the spectrum analyzer’s output closely, we can see that there are two underlying tones with a continuous upward sweep from 2200Hz (2.2KHz) to 3200Hz (3.2KHz). The frequency of the sound starts at 2200Hz and sweeps up to 3200Hz in a matter of a few hundred milliseconds. This sweep occurs only in one direction and then starts over at 2200Hz, sweeping in that same upward direction over and over.
I have analyzed many audio and video recordings of this sound cannon’s output and have also seen the audio reproduced in a downward sweep. Both versions seem to have the same effect. No matter which methods produce this annoying tone, I am sure we would all clear the area very quickly.
There isn’t anything special about this set of tones or the sweep. We could use any number of methods to move around various frequencies that would annoy anyone within the path of the audio beam. It’s just that LRAD was the first company to envision this type of device, so the sound it produces has become sort of the trademarked sound for non-lethal weapons and crowd control devices.
Upon further investigation into the source of the LRAD sound, I am fairly certain that the tone is a square wave produced by a simple microcontroller. If we use this really cool online sweep tone generator and enter the various settings, we can hear that it sounds very similar to our LRAD sound. Unfortunately, this app doesn’t repeat continuously, so just keep hitting the “play” button over and over quickly.
Methods we could use for producing annoying sounds
- Sweeping patterns in the audible range
- Random frequencies at various intervals
- Pulsating tones at stepped frequencies
- Police and Emergency Siren noises
Reproducing the Long Range Acoustic Device sound
Now that we know that there are no unique technologies needed to produce the LRAD sound, all we have to do is come up with a way to generate those tones. For now, the most straightforward way to do that would be to create an MP3 recording of the sound. But don’t worry, we have already done the work for you. Just download the file and play it back on your mobile device or computer.
Methods to produce LRAD sounds
- Mp3 Player
- Signal Generator
- Sound Editing Software
- Arduino and other Microcontrollers
This LRAD Mp3 recording can be very annoying and loud, so be careful and never place the speakers next to your ears. If you are wearing headphones, make sure you lower the volume before hitting play. You can also download the LRAD sound HERE.
It’s not just about the frequency of the sounds
Now that we have established the source of the soundwaves, we need to begin exploring the design of the LRAD device itself. The sound source is only a small part of the design, and the volume or pressure of the soundwaves it produces are also part of this unique system.
LRAD 450XL Acoustic Performance
|Maximum Peak Output:||150dB SPL @ 1 meter, C-weighted|
|Maximum Continuous Output:||146db SPL @ 1 meter, A-weighted|
|Sound Projection:||+/- 15° @ 1kHz/-3dB|
|Communication Range:||Up to 1,700 meters|
LRAD devices boast output levels of 140dB or more depending on the model. Pain in the ears begins to occur at 120dB and becomes intolerable over 135dB. Our sound source must go through some high power and efficient speakers to produce those levels at long distances. The good news is that high frequencies do not require that much power to reproduce. In fact, a typical LRAD system only draws around 100-300 watts, which is well within the range of our home audio or car stereo equipment.
DIY LRAD sound
These reasonable power requirements mean that building a low-cost version of the LRAD system should be feasible. If we want to design a system to produce only the warning tone, we won’t have to worry about transmitting the human voice in our design. Voice would require a more complex speaker enclosure and drivers to transmit the wider range of frequencies.
For our first project, we will merely be taking our recorded sounds and testing them with some audio amplifiers and speakers. We will measure their output and see what kind of results we can achieve.
Once we understand how to best reproduce the effect of the LRAD in the real world, we can begin thinking about how to make a low-cost version of this amazing technology.