Research Scientist Walter Reed Army Institute of Research Silver Spring, Maryland
Abstract Text: Service Members (SMs) are exposed to blast overpressure (BOP) during routine training with a myriad of weapon systems currently in use by the United States Armed Forces. The diversity of the overpressure field around the weapon is related to the design of the weapon, its caliber, type of ammunition, and a standoff distance from the source. For propellant-based weapons, such as pistols, rifles, and machine guns, that source is usually located in the proximity of the muzzle exit. The ESiT team at WRAIR developed a series of unique protocols to characterize the occupational overpressure exposure among operators using a wide range of weapons: 1) grenades, 2) sniper rifles (suppressed or unsuppressed), 3) artillery, 4) mortars, 5) high cyclic rate of fire weapons, to name a few categories. Over the years, our research team has acquired expertise in these BOP measurements and acquired know-how on how to execute them efficiently in high-tempo military training environments.
The availability of the small form factor BlackBox Biometrix (B3) Blast Gauges facilitated these studies since these sensors can be deployed in large numbers. We also used high-resolution overpressure sensors as a reference to compare the readings between these “gold standard” sensors and emerging technology devices. A critical evaluation of next-generation pressure transducers is necessary to establish what is an acceptable deviation from the standard. It is particularly true considering that this high-priority task has been shifted toward the end user. While performing these types of comparative measurements, we noted that thanks to particular design choices, these wearable devices have: 1) a lower overpressure threshold of 0.5 psi (~3.5 kPa, or 165 dB), 2) a recording duration of 20 ms, and 3) recharge time of approximately 1 second. These characteristics will result in incomplete overpressure history recording if the duration exceeds 20 ms, there are multiple blast events within 1 second, or their magnitude is below the threshold of the sensor. All of these factors can yield a severely diminished cumulative daily dosage and thus distort the description of the reality of the training with the specific weapon system. In principle, that can also be misleading if a researcher would like to establish a threshold at which the effects of overpressure exposure on performance and health emerge.
Another type of wearable sensor in routine use by our research team is Larson-Davis SoundTrack® LxT. These noise dosimeters can record peak overpressures in the 100-191 dB range 0.0029-10 psi (0.002-70 kPa, 191 dB). We have learned that the dynamic range of this device is much more suitable for the overpressure levels experienced by personnel in training. It records the data continuously and has a designated peak detection module that doesn’t rely on self-triggering. Hence, it records the overpressure history that is superior compared to current devices that depend on self-triggering technological solutions. However, these dosimeters don’t record the raw overpressure waveforms and report only the cumulative dosage metrics such as TWA-8 or LAeq. These measures were developed for continuous noise monitoring and are difficult to transcribe into simple and easy-to-interpret parameters, such as cumulative impulse, commonly used by blast research and medical communities. However, the LZpeak values recorded by noise dosimeters can still be invaluable in calculating the cumulative impulse. The only prerequisite is discovering how the impulse changes with the standoff distance.
As proof of concept, we performed measurements with three weapon systems, an M2A1 machine gun, M107 SASR, and M777 155 mm howitzer. We compared the peak overpressures recorded by pencil probes against LZpeak values recorded by noise dosimeters. The calibration curves correlating LZpeak and impulse were calculated for a wide range of standoff distances. The cumulative impulse values obtained using two different methods1) subject-worn wearable sensors and 2) LZpeak values with calibration curves were then compared and analyzed in detail.
