SSS.3 - Hearing Effects

If you read the introduction to this standard, you’ll notice that we covered peak pressure and duration (re: impulse), but we haven’t yet covered two of the four factors that influence loudness:

• Sound pressure wave oscillations

• Interaction of the human ear with sound pressure

Honestly, it would be great if the above two factors didn’t matter; we could just plot all of our properly measured gunshot sound waveform peak pressures and impulses and call it a day (Fig 14). Unfortunately, that’s not good enough. Here are some harsh realities of firearm noise:

1. It’s not all clean shock waves with almost textbook-like shapes. Therefore, computation of cumulative positive phase impulse can sometimes not tell the whole “momentum transfer story” when comparing certain weapon systems to others.

2. Not all firearms emit sound from one source (semi- and fully-automatic firearms introduce further multiple pressure pulses in multiple time windows), so there may be multiple (numerous) high pressure peaks.

3. Firearm noise has a tendency to have positive-to-negative phase oscillatory events and it turns out that the human inner ear, like any dynamically responding structural system, is influenced by this.

Despite the above known hurdles, peak pressure and impulse are useful, objective metrics. They don’t tell the whole story, but they certainly help.

Pressure-impulse (P-i) diagrams, like the one shown in Fig 14, are tremendously useful tools for the relative comparison of blast waveform characteristics and subsequent damage or hazard to systems (a building, your kitchen window, an armored personnel carrier, your ears, your lungs, or whatever you can model or test). However, one of the inherent characteristics of P-i diagrams is their validity only for waves of similar shape; or at least similar wave-structure interaction. So, if we’re going to play in P-i space, we need multiple spaces and another objective metric to map those spaces.

The Hearing Damage Metric Decision

This Standard uses a tertiary simplified metric; a mature, peer reviewed, and objective metric that considers the biology of the human ear.

The United States Army Research Lab (ARL) at Aberdeen Proving Ground, Maryland developed The Auditory Hazard Assessment Algorithm for Humans (AHAAH), which is an electro-acoustic analytical model of the human ear (Fig 15). Of the mature hearing damage metrics, it is the most accurate and practical analytical model available, when examining its correlation to unprotected human hearing damage from impulsive noise in the amplitude regime of suppressed small arm weapon systems.

AHAAH has been correct in 95% of the tests with protected hearing and 96% of the instances for all tests. MIL STD-1474 has been correct 38% of the time (protected hearing only) and A-weighted energy has been correct 24% of the time for protected hearing and 30% of the time for all tests analyzed.

PEW Science has correlated the previous two simplified metrics of peak pressure [dB] and peak positive phase impulse [dB-ms] to the postulated damage to a shooter’s unprotected ears.

In order to predict the complex interactions of the outer, middle, and inner ears ... and to provide insight in designing experiments, an electro-acoustic model of the ear was developed. The model was developed to conform with the structure of the ear. It could have been simpler, but the goal of modeling is insight. A solid theoretical base, coupled with the restraint imposed by the known anatomical structure of the ear kept the model properly formed and focused.

Many elements of the model had already been developed by others and had appeared in the literature. However, no one had put all the elements together or focused on predicting the effect of intense sounds on the ear. When connected, the conductive path matched closely the measured transfer functions for the external and middle ears. Additional elements had to be created to allow the analysis of the effect of intense sounds on the ear. These included modeling changes in the flow of energy in the conductive path at high intensities as well as the algorithm for calculating loss within the cochlea. The loss calculation was made at 23 locations evenly spaced along the basilar membrane (roughly 1/3 octave apart). At each location, the upward flexes of the basilar membrane were tracked (upward flex puts the sensitive elements in tension -- a common mode for tissue failure), their amplitude in microns was squared and the sum maintained for each location. The units are called auditory hazard units (AHUs).

Keep reading - you’re in the home stretch of this section!

Auditory Risk Units (ARUs)

The Auditory Hazard Unit (AHU), now known as the Auditory Risk Unit (ARU) generated from AHAAH by ARL, is relatively accurate, established, peer reviewed, and present in MIL-STD 1474-E.

There are two types of risk analyses that can be performed to generate ARUs:

• Type 1: Warned: The analysis is considered “Warned” when you know the noise is going to occur. For example, a “Warned” analysis is appropriate when you are analyzing hearing damage risk to the personnel firing the weapon. The inner ear muscles contract during a “Warned” response, lessening the severity of the event.

• Type 2: Unwarned: The analysis is considered “Unwarned” when the shot surprises you. For example, an “Unwarned” analysis is appropriate when you are analyzing hearing damage risk to your hunting buddy who doesn’t know you’re about to merk the wild hog you just spotted through thermal with your AR10 at 0300 with him standing behind the ATV, 1 yard away, getting a beverage out of the cooler.

A total of 500 ARUs is the maximum allowable “dose” for occasional exposures within a 24-h sliding period occurring not more than once per week. Doses greater than 500 ARUs are predicted to produce permanent hearing loss. For daily or near daily occupational exposures, the limit should be reduced to 200 ARUs.

It should be noted that the AHAAH algorithm and the associated ARU metric are not perfect. Over the past decade, further refinements and advancements have been performed by several research teams to address some anomalies observed in the consistency of the ARU metric for certain overpressure regimes. Details regarding the hearing reflex phenomenon delineating Warned vs. Unwarned response is also a subject of debate. Furthermore, there are other more advanced metrics that have built upon AHAAH, based upon cochlear energy rather than only ligament displacement models, and show promise. For the purposes of this Standard, PEW Science deems the ARU metric adequate; at least for now.

In this Standard, primary data is associated with Warned ARUs. An argument can be made for both; and PEW Science recognizes that at first glance, the conservative decision would seem to be to use the Unwarned ARU metric, in accordance with the literature. Both have been calculated by PEW Science for all data, to date. In the interest of practicality and weapon system performance data granularity, the Warned ARU type is maintained for much of the current data presentation.

Head over to SSS.4 - Test Method and Results to see how this all fits together.