How does a long range acoustic device (LRAD) work? teardown

I have always been fascinated with amazing audio equipment like the LRAD. These units are very expensive and difficult to get your hands on to see exactly how they work. When the opportunity came up for me to purchase one of these at a reasonable price on eBay, I jumped at the chance.

The LRAD is a narrow beam voice communications system that also produces an audible deterrent tone through 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. In this post, we will tear down the insides of an LRAD model 300X. Let’s see exactly how does a long-range acoustic device work?

The unit I received was missing all the electronic components but I had a suspicion that there wasn’t anything really all that special about how the LRAD was designed. We can see from the outside that it looks very similar to any public address system you can purchase, but with a few unique differences.

The first thing I noticed is how well built this unit appears to be. You could literally drive over the thing and cause very little damage. The entire unit is designed for extreme conditions. We can assume that this unit was for military use in the desert due to its tan color. The fact that sand keeps spilling out of this well used piece of equipment is another great clue. It also comes in grey which is the color commonly seen used by police.

Weight of LRAD 300X main array: 25 pounds (11.3 kgs)

Front View of LRAD 300X

Looking at the front of the LRAD, you can see there is a metal speaker grill to protect the insides and still let sound emit through the front. There are also many stainless steel screws holding it together. There are also a handles on the top and sides that show signs that the unit has been dropped a few times!

Bottom view of LRAD 300X

This particular model has a larger steel post bolted to the bottom. This is usually for mounting to vehicles. It is very likely that this long-range acoustic device was mounted to a military truck or even possibly a tank. Let’s open it to find out what’s inside.

Tearing apart an LRAD

I start by removing all of these screws to see what secrets we can reveal. I particularly like how the metal frame is actually made up of small sections. This is done to save on aluminum during the machining process. Otherwise, they would have to cut the frame out of one large sheet and waste all kinds of material and time during manufacturing.

Inside view of the LRAD

The next thing to learn is exactly how these speaker arrays work. And yes, I do believe that these may be standard speaker coils of some kind. There is a bunch of confusion on the internet about exactly how the LRAD works and that is why I wanted to create this article. If you haven’t read it HERE is the post about the LRAD sound and how it is produced. Unlike the misinformation being claimed by other websites, LRADs do not appear to produce any signals in the ultrasonic range. Their design appears to be optimized for soundwaves in the audible range. In fact, they may transmit audio most efficiently below 10,000 Hz.

So it looks like LRAD models come in eight, six, four, two and single driver units. That large one sure would take a huge amount of audio power from a large audio amplifier. It will be interesting to learn more about these fascinating devices.

LRAD sound driver mounted inside

The drivers are mounted via a metal X-shaped bracket in the rear. The entire tan housing is made of fiberglass with metal reinforcements embedded into the resin. These things appear to be really well designed. I must give credit to Woody Norris for being an amazing inventor. It just goes to show you how a good design can make an otherwise normal product into something spectacular.

The second interesting item to look at is how the driver is mounted to the bottom of the speaker enclosure. There seems to be a spacer and vents on the sides. I assume this is to allow sound to escape through the rear and sides of the driver. We will have to look at that further once we get it removed. For now, let’s examine how sound is generated within the LRAD.

There are only two wires connected to the LRAD sound driver. It uses a waterproof connector and everything else seems to be sealed with silicone. A quick removal of the screws and cut with a knife reveals everything. I bet if I measure the impedance between these two wires it will be between 4-8 ohms. That’s because it’s the standard impedance (resistance) for mostly all audio speaker elements. If this turns out to be the case, any normal Hi-Fi audio amplifier will work with this system.

If we look at the bottom of the sound cavity, we can see a rounded bump. This must be how they focus and reflect the sound backward. In fact, I am starting to think that this is a simple folded horn design.

Closeup of where sound exits the audio driver

You can see from our folded horn diagram that the audio must leave the magnetic coil and smaller cone in order to reflect off of the back surface of the larger part of the horn. This area of the audio cavity further amplifies the soundwaves until they exit out the front. Many folded horn designs bounce the sound off of the horns surfaces multiple times so that the large magnetic drivers sit in the rear of the bullhorn. With the LRAD, the weight of the heavy magnetic driver is in front. Since the enclosure is already built like a tank, it all seems to balance nicely.

Here are all the parts of the audio driver system. If we measure the resistance, it is exactly eight (8) ohms which means this is just an off the shelf speaker element. Although it is an extremely powerful one.

Internal Audio Driver Impeadance: 8 Ohms

Wattage: 100-150 watts per horn (estimated)

Finally, we get to see the other side of the LRAD sound driver. I am very confident now that this is nothing more than a standard 2″ audio compression driver rated at 8 ohms. We already determined that the LRAD sound is between 2200Hz and 3300Hz. Human voice is also most intelligible around that frequency range. That means the drivers are most likely speakers that produce frequencies in the mid-range of audio and not ultrasonic as claimed on the internet.

All we have to do now is find an audio amplifier powerful enough to drive the system. Since our LRAD uses two compression drivers, any powerful stereo amplifier should do the trick. Here is an example of an off the shelf 2″ compression driver I found on eBay that should work exactly the same if we wanted to build our own version of an LRAD.

We will be exploring the LRAD non-lethal weapon system in even more detail in future posts. Maybe we will even make our own version of this amazing piece of technology.