Why dual factor sensors are essential for accurate gunshot detection

Most of the assumptions you’ve made about gunshot detection could be wrong. To those security professionals who stay on top of their homework, this is probably not true, but the ‘you’ in this context is directed to the collective security industry reader that, when looking into indoor gunshot detection products, sees terms like concussive force, percussion, or shockwave and wonders if these are important differentiators – or something the marketing department decided would get a reader’s attention.

The truth is that all these terms, from an application standpoint, mean the same thing. Detractors of gunshot detection might question the validity of the technology since the perpetrator is already in action and shooting is in progress.

Directing emergency personnel

The purpose of gunshot detection is to alert with absolute certainty and as soon as possible that there is a shooting in progress, and very importantly, to identify the location of the incident.

The purpose of gunshot detection is to alert with absolute certainty and as soon as possible

With this information, we can guide victims away from the shooter while also directing emergency personnel to the perpetrator’s location so they can mitigate the threat. If your loved one found themselves in a work or school shooting, which would you prefer – that they hide until law enforcement arrives, or that they know a safe path for immediate evacuation? As a security operator, would you prefer to manually search camera feeds to find the shooter, or have gunshot detection instantly cue cameras to the incident location?

Gunshot detection technologies

When you see the value gunshot detection has to offer, the next step is gaining a deeper understanding of the technology and sifting through marketing claims to find the system that works for your application. Before jumping into the circuit boards and algorithms of modern indoor gunshot detection sensors, it helps to go back in time and learn about the genesis of gunshot detection technologies. Many sources generalise gunshot detection into two classifications: military or civilian/law enforcement application.

Gunshot detection’s history tracks back to systems built in the 1990s for military applications that detected and reported on sniper fire from high calibre weapons. These systems were then made available for civilian applications in the form of city-wide applications that triangulate signals to locate gunfire in urban areas.

Existing outdoor technology

This feeds the collective understanding that all gunshot detection systems operate in one simplistic way

Although these systems differ in how they detect, locate, and report gunfire, at their core is a reliance on acoustics (and therefore microphones) as their single mode of detection. Because these systems were essentially first to market, they are the most publicised and reported on. This feeds the collective understanding that all gunshot detection systems operate in one simplistic way; they ‘listen’ for the sound of gunshots, triangulate a location, and then produce some type of alert.

Detecting a gunshot outdoors is different than detecting one indoors. Early attempts by indoor gunshot detection pioneers to use the existing outdoor technology didn’t work very well. There are several reasons for this, but they can be summarised as follows:

• The acoustic pattern of a gunshot indoors is different than one that is produced outdoors. Walls, furniture, and general building layout will create echoes and reverberations that combine to create a signal that is difficult to interpretate.
• Some outdoor systems were reliant on the shockwave of supersonic rounds outdoors. In the indoor environment, the acoustic properties of that shockwave can break down quickly, rendering the result degraded and therefore unusable as a reliable indicator of gunfire.
• Systems reliant upon triangulation, like the ones used in city applications, also do not work indoors because those echoes and reflections in the acoustic environment interfere with both detection and location accuracy.

Indoor gunshot detection

This compression continues as the bullet moves forward through the air

Indoor gunshot detection requires a different approach. But before diving into the deep end of the technical pool, a few words about what happens when a gun is fired. We are all familiar with the bullet flying out of the gun part, but what else happens?

• When the bullet leaves the muzzle (end) of the gun, air molecules around the bullet get rapidly compressed. Just like a guitar string that has just been plucked, rapid compression or vibration moves the air molecules around it. This compression continues as the bullet moves forward through the air, but the initial, violent change is what we first capture.
• The gasses that are part of the gun powder burned to propel the bullet out of the gun are also released. This causes a ‘flash’ which can been recorded as part of the visible and infrared (IR) spectrum.

Indoor gunshot detection requires a different approach

Sound pressure sensor

The talk of 'rapid vibrating or compression of air molecules’ is where confusion can begin. Why? Because this same property can be measured in different ways. One way is acoustical – a microphone recording the ‘bang’.

Another way is through pressure. But wait a minute – if we remember our high school physics, a sound wave is the same as a pressure wave! Simply stated, a microphone is a pressure sensor - usually tuned to capture very small changes - and a sound pressure sensor is just a clunky microphone. As humans, we forget that sound – like the loud bang of a firearm – is just our way of describing a very distinct change in the vibrations of air molecules around us.

Air pressure sensors

Gunshot detection devices use sensors to detect a change in the compression of air molecules

Gunshot detection devices use sensors to detect a change in the compression of air molecules. Manufacturers can say that they use different types of sensors – microphones, air pressure sensors, concussive force sensors, shockwave sensors, etc. – but these devices are all measuring the same thing.

The primary difference between different sensors is how sensitive to disruption they are, which is usually a function of cost, and what frequency or range of disruption they are designed to look for. Some manufacturers claim to capture the ‘shockwave’ of a bullet. Shockwave is a rather loose term, but the manufacturer is usually referring to a special pressure wave in the air that is created when an object travels faster than the speed of the sound. An object passing through the sound barrier will make a loud noise – or large, fast pressure change – but again, the sensor is simply capturing a change in the compression of air molecules.

Serving particular market

At first, the idea of using a bullet’s speed – and therefore shockwave - as an inexpensive mechanism for capturing and verifying that a gunshot has occurred may seem interesting. But be careful of products that promote shockwave as their principal factor of gunshot detection. Many types of bullets – a 45 calibre pistol for example – do not create a shockwave when fired, meaning these types of gunshots fired in your building will be excluded from detection.

Manufacturers all take different approaches to serving a particular market. In the world of security, one would hope that all products deliver exactly as advertised. Unfortunately, a sensor that is calibrated to one range of sound or pressure can inadvertently register other, unwanted sounds.

Different building configurations

A nail gun or balloon popping will trip a sensor calibrated to listen for the frequency

A nail gun or balloon popping will trip a sensor calibrated to listen for the frequency – or vibrational – range of most gunshots. Sensors can be pre-programmed with a library of gunshot-like sounds, but this requires the sensor to analyse the signal it just captured, creating a delay in alert times.

A study done by Florida Atlantic University’s College of Engineering and Computer Science recently concluded that ‘most gunshot detection systems cannot tell the difference between a gunshot and a plastic bag popping’ – and this points to why consumers believe that gunshot detection systems are unreliable. Also, as we learned above, different building configurations will create different signal characteristics. Is there a library of gunshots signals for all possible building or room configurations?

Single sensor approach

As a way to reduce costs, some manufacturers offer a single measurement sensor for both indoor and outdoor applications, but this approach requires extensive calibration for two different environments, meaning that accuracy inevitability suffers.

As a way to reduce costs, some manufacturers offer a single measurement sensor

Define your expectations accordingly because the single sensor approach is limited. The level of accuracy will be low, and you will experience false alerts, creating a need for an additional layer of verification that further delays notification. In the physics lesson above, we mentioned that there was a second characteristic of a gunshot – the release of light – or more accurately, heat – from the infrared (IR) spectrum. To avoid the problem of concussive force/shockwave/microphone sensors capturing signals that are not gunshots, a second sensor dedicated to measuring the IR signal emitted by a bullet leaving a gun is added.

Generate false alerts

Just like online banking services enforce two-factor authentication to ensure authenticity, gunshot detectors can use two sensors measuring the two different characteristics of a gunshot to deliver the highest level of accuracy. And since the speed of light and the speed of sound are different, programming can be employed to further validate that the signal received was truly a gunshot and not a similar-sounding non-ballistic event, like a nail gun.

From an accuracy standpoint, single measurement sensors perform worse than a dual-sensor in all cases and are significantly more likely to generate false alerts. The only benefit of a single-sensor shot detection system is that they may be cheaper to purchase in terms of upfront costs. But those savings will quickly be erased by the cost of avoidable business disruptions due to a higher false alert rate.

Normal hospital services

Schools are probably the hardest to estimate as intangible costs like staff/student wellbeing

In manufacturing, warehousing, or transportation environment your accounting department can easily calculate how much a one-hour shutdown of work will cost the organisation if a false alert causes an interruption in operations.

In industries like healthcare, calculating the cost of a false alert is a little more complicated, but one study determined that cancelling normal hospital services for just one day can cost approximately $978,418.00. Schools are probably the hardest to estimate as intangible costs like staff/student wellbeing and law enforcement-related activities do not lend themselves to a straightforward financial calculus.

Single measurement sensors

However, when investigating indoor gunshot detection systems, the following top five questions should be asked of each vendor you contact:

• Does the system use Single Factor or Dual Factor sensor verification to reduce false alerts? The definition of Dual Factor, in this case, is that both factors are required to produce a gunshot alert. Remember that merely adding more single measurement sensors to a product will not reduce false alerts.
• Does the manufacturer have an independent, third-party verification of their product? If so, what were the testing parameters? Look for gunshots occluded (facing away) from the sensor, types of noises tested including firearms and false alert tools used, sensor distance from the gunshot, and the validity of the testing agency as a trusted, impartial operation. Real-world testing is also extremely valuable, meaning in a customer’s installed environment, outside of the gun range.
• If the single factor sensor is advertised to work both indoors and outdoors, how does the device technology compensate for these two different environments?
• Does the manufacturer’s solution require a human to validate that the signal received was a gunshot? Systems that require human verification will considerably slow down the speed at which Law Enforcement is contacted and, because of normal human behaviors during periods of stress, can also lead to inaccurate information being communicated to first responders.
• Is the manufacturer an expert in gunshot detection or did they just add a microphone to a product and bundle it with other types of sensors like smoke detection, aggression, vaping, glass breaking, etc.? These all-in-one sensors are currently common as manufacturers attempt to test the market’s appetite for gunshot detection.

False gunshot alert

There are currently no regulatory bodies monitoring gunshot detection manufacturers and their claims

Gunshot detection does not have to be complicated, but when an emerging technology begins to gain traction in the marketplace, it is important to follow the science and keep learning. There are currently no regulatory bodies monitoring gunshot detection manufacturers and their claims, so doing your due diligence is key with this technology.

Talk to your business leaders about the impact a false gunshot alert could have on the organisation from a risk standpoint. Seek to understand the underlying technology of any shot detection system you are considering and look for two factors of gunshot authentication.

Hopefully, this technology primer on gunshot detection helps integrators and end-users understand the underlying technology so they can find the most effective solution for their facilities.

Bill Aheimer, Principal of Security DNA, is co-author of this article.