Dave & Gabe - LRAD SHIELD RESEARCH

We’d like to point out up front that if you feel saddened by the misuse of LRADs or the weaponization of sound by the police, then you must also feel a much deeper sadness for the systemic oppression and brutalization of black people in America. This work is rooted in justice for the black community and we recognize that the misuse of LRAD technology by police is only a small example of the broad and unacceptable misuse of power against black people throughout the history of the United States.

LRAD INFO

Genasys is the manufacturer of LRAD systems - the company used to be called American Technology Corporation, then later became LRAD Corporation before landing on Genasys. They trademarked the term Long Range Acoustic Device, and own various associated trademarks.

Quoting the wiki, LRADs are used for long range acoustic hailing, mass notification, and as a communication and crowd control tool, and [are] used for [a] variety of situations, including military communication, pirate deterrent, search-and-rescue, public announcement, and crowd dispersal.

It should be noted these systems are not far off from normal PA / large venue sound systems - they offer the possibility to efficiently communicate over large distances if used properly and safely.

It does not appear that you can easily acquire LRADs - I (Dave) have requested info from Genasys on procurement of their products, have not yet heard back.

Shout out to @curtparr (IG) who shared this link containing cut sheets on Genasys’ products.

@curtparr also shared this tear down of the 300x model.

A common misconception is LRADs employ ultrasonic techniques to achieve tight directivity. This is not true - LRADs use standard speaker drivers and reentrant horn designs, similar to megaphones or bullhorns. In the tear down above, you can see there are 2 drivers used in the design of the 300x model.

Something to look out for in the data sheets is the 1m max sound pressure level (SPL) measurement - this is the loudest the speaker can get at a distance of 1m. For reference, a good rule of thumb is sound dissipates in air by -6dB for every doubling of distance. So in the case of the 100X model, 137dB is it’s max SPL at 1m. That means at 2m the max SPL will be around 131dB, and at 4m 125dB and so on.

The NYPD used a 100x model in 2014 at the BLM / Eric Garner protests, which resulted in a lawsuit around the use of the LRAD. Federal judges ruled in 2018 that the police used excessive force in the 2014 incident.

ANIKA EDREI v City of NY

NY DAILY NEWS Story

NBC NEWS Story

Gideon Orion Oliver and Elena L. Cohen are two lawyers actively involved in litigation around the NYPD’s use of LRADs. We (D&G) are in touch and will update this doc with info as we learn more from them. For now, here’s a press release from 2016 outlining FOIL requests to the NYPD, requesting proof that officers are properly trained and knowledgeable about LRADs (which was not adequately given)

BENN & GEAR YOUTUBE VID

https://www.youtube.com/watch?v=3sqIvak-4Ek

@curtparr (IG) wrote a nice response to this vid, and we share his view that Benn’s video comes from a positive place but is not entirely accurate. Please don’t expect glossy paper to shield you from high SPLs from an LRAD.

It’s worth noting that Benn, similar to us, was testing with an ultrasonic speaker. These speakers use ultrasonic waves to carry audible frequencies with a tight directivity like a flashlight or a laser. It’s a cool technology that you can read more about here.

These types of speakers are commercially available and are used in museums, restaurants, malls, etc. While these speakers can be used in ways that feel uniquely awful for the listener, they cannot reach the amplitudes that LRADs reach.

We don’t doubt that Benn was feeling a reduction in levels with the glossy paper from the speaker producing frequencies at 15kHz and 19kHz. Our interpretation of why is because wavelengths in the 15-19kHz range are on the order of 0.75” - the glossy paper looks more like a rigid mass at frequencies in this higher range and it probably was reflecting / absorbing some amount of the acoustic energy. Benn was also reporting unshielded values in the 81dB range, which we’re sure felt plenty annoying at those frequencies but is nowhere near the danger of levels in the 120+ dB range.

The standard warning chirp that we’ve seen used with LRADs is in the range of 2-3kHz. This makes sense because this is where human hearing is most sensitive and it is also where the LRAD frequency response is strongest. 2-3kHz wavelengths are on the order of 6” - the glossy paper would give very little sound attenuation for a 2-3kHz tone, and the attenuation it would provide pales in comparison to levels upwards of 120+dB.

It remains to be seen whether or not sound produced via an ultrasonic array is more easily isolated than sound produced by standard loudspeakers. My (Dave) gut is it is not - I believe the pressure fronts are the same if produced via an ultrasonic speaker or a regular speaker, but the ultrasonic method simply gives a tighter directivity to the audible frequencies. But this would be interesting to test/research further.

Again, PLEASE do not expect glossy paper to give you protection against LRADs.

D&G LRAD SHIELD

We built our shield with conventional knowledge of sound isolation and noise control techniques. We basically built a portable wall. Acoustic assemblies are typically layers of rigid material on a framed buildout with insulation stuffed in the gap. Similar to a stud wall with insulation. No fancy tricks or acoustic miracles.

Ours is made of ½” acrylic on both sides of a pine batten frame with recycled denim insulation. We used acrylic so we could build a window into the shield, allowing the person holding it to see through. This assembly is in the range of an STC40 rated partition.

Because of the ~50lb weight, we built ours to stand on a monopod style pole. So you can get in position by carrying the entire buildup, and then rest the weight of the panel on the pole.

If you want to build your own, you don’t need to stick to our exact materials. Instead of plexi, you can use ⅝” drywall or ½” plywood. We used pine battens because they’re light and rigid, you can use 2x4s or steel studs if you like. Keep in mind, the more rigidity and mass you have as the surface layers, the more sound you’ll block.

Also a note about panel size - we sized ours based on being able to carry it (after it’s been driven in the vicinity of an LRAD). Ours is 26” wide x 34” tall. The width is approx 4x the size of the wavelength of a 2kHz tone, so we expect most of the sound energy incident on the shield will be blocked. Some will bend around the sides based on diffraction.

The idea is you can transport multiple shields in the vicinity of a handheld or tripod mounted LRAD, and get close to it to block as much energy as you can. Using the shield in the crowd very far away from the LRAD will only protect the person right behind the shield. If you can increase the surface area by using multiple shields together near the LRAD, you will reflect more sound and the sonic shadow behind the shields will be larger.

Also note that you should be wearing earplugs + over-ear cans if you’re going to put yourself in the way of a small LRAD.

Note that if the police are using a larger model LRAD that is mounted on top of a vehicle or on a tower, our method is drastically less effective. If a device is mounted high above the ground you would need a very tall reflector. Additionally, the larger LRAD models produce SPLs in the 150-160dB range at 1m - we wouldn’t recommend going near one of these if you spot it. Refer to the LRAD cut sheets to identify the model police are using.

^ frame of the shield made of 3” thick pine battens - notice holes drilled out on the bottom center in order to receive a ¾” dia. piece of conduit that acts as the monopod

^ the interior is stuffed with recycled denim (you can use any insulating material like rockwool or fiberglass - the kind of insulation you’d put in a residential stud wall will work great). Layers of ½” acrylic are screwed into both sides of the assembly. If you’ve got an adhesive like green glue, using it between the faces and the frame will help.

^ ^ if you have a laser cutter we can share the template for the stencil

^ We tested our assembly in our studio space - which is a large, relatively reverberant build space. We tested with a Holosonics as-24i speaker (the most directional speaker we have) playing a chirp tone in the 2-3kHz frequency range. An iPhone Xs running the RTA module made by AudioTools captured the sound pressure levels 2m away from the source with and without the shield. When the shield was present, the phone was held approx 6” from the back of the shield in between the shield and body holding it.

We recognize this is not a formal lab-conditions test setup, but given it’s a relative measurement, we stand by the reduction of SPL values. Also given the reverberant field in the studio, we imagine there may be more reduction in the high frequency range than our measurement indicates.

D&G BACKGROUND

Dave & Gabe is an interactive installation studio based in Brooklyn, in operation since 2014. We focus on experimental audiovisual projects.

daveandgabe.care

Previous to starting the studio, Dave worked in architectural acoustics for 7 years at Arup Acoustics. He has undergraduate degrees in physics and math from Truman State University and a masters degree in acoustic engineering from the Institute of Sound and Vibration Research (ISVR) at the University of Southampton, UK.

Gabe has a degree in music technology from NYU Steinhardt. Prior to starting the studio, he worked in music production and restored audio for classic films at the Criterion Collection.

Our motivation for the shield came from participating in various NYC BLM protests and seeing police agitation firsthand (in addition to the seemingly unending videos of police violence from around the country on the internet).

Rumors were circulating of the NYPD deploying LRADs around the city and we had the idea to make a semi-portable shield that could help block and reflect the sound during inappropriate use of these devices. We had the idea on a Saturday night (06/06/2020) and built/tested the shield the following Sunday.

Additional testing of the shield in a free field environment (the roof of our studio) using a bullhorn or more standard driver would be the next step. Also testing multiple shields in an array would be illuminating. If anyone has access to an LRAD they want to send our way, please get in touch.

IG: @daveandgabe

email: get@daveandgabe.care