SSS.6.184 - Research Supplement: The Quietest 9mm Silencers - MP5 Silencer Detailed Loudness and Tone Comparisons (Article Preview)

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CAT MOB, Dead Air Wolfman, GSL Phoenix, HUXWRX CASH 9K, Otter Creek Labs Lithium, PTR VENT 2, and Resilient RS9 on the HK MP5

As is the case with rifle silencers, there are also a variety of technologies implemented in modern submachine gun silencers. Principally, silencers reduce the severity of the signature produced by a small arm weapon system by modifying the propagation of combustion gasses such that they enter the ambient atmosphere at a lower temperature and pressure than they would otherwise, had a silencer not been installed on the weapon. These parametric differentials, along with the rate of gas propagation, dictate the resulting suppressed small arm weapon system signature.

Unlike suppressed centerfire rifles, suppressed submachine guns may often be operated by the user and around bystanders without hearing protection. This factor may increase operator and bystander risk, despite the gross blast overpressure hazards being reduced when compared to suppressed centerfire automatic rifles.  In-depth examination of the suppressed weapon system signatures is therefore important for personnel safety.  Ejection port blast loads to which the operator is subjected, inherent to semiautomatic and automatic weapon systems, may be significantly exacerbated by the use of silencers.  Furthermore, due to the settings in which suppressed subsonic weapon systems may be used, sound signature characteristics may be of significant interest to weapon operators.

As previously presented, there exist three different primary classes of silencer technologies.  They may be used to categorize rifle silencers as well as pistol caliber silencers used to suppress the 9x19mm NATO cartridge:

  1. Conventional designs, in which shock and blast loads are significantly reflected in early time, rearward toward the muzzle orifice, with later time gas propagation significantly restricted to atmosphere. Examples of such silencers include the Dead Air Wolfman (6.182) and Otter Creek Labs Lithium (6.102), along with pistol silencers such as the Rugged Obsidian 9 (6.172), SilencerCo Omega 9K (6.153), AAC Ti-RANT 9 (6.118), and others.  Conventional pistol silencers may be used to suppress submachine guns, but their performance is often not optimal, due to their lower gross flow rate (higher back pressure).

  2. High Flow Rate designs, in which blast reflections in the proximal expansion chamber are redirected, reduced, or otherwise altered to reduce the influence on reciprocating weapon function, with later time gas propagation expediently venting to atmosphere. Examples of these silencers include the HUXWRX CASH 9K (6.97).  The CASH 9K shares similarities with some HUXWRX rifle silencers, such as the HUXWRX HX-QD 556 (6.54), HX-QD 556K (6.64), FLOW 556K (6.83), FLOW 762 Ti (6.114), and FLOW 556 Ti (6.167) which all use various generations of HUXWRX Flow-Through technology.  The CASH 9K uses flow baffles due to true Flow-Through designs exhibiting lower performance efficiencies with the lower combustion pressure and duration of 9mm.  It is important to note that the CASH 9K may be considered a Hybrid Design (Group 3), due to its departure from pure Flow-Through geometry.

  3. Hybrid designs, in which elements of both Conventional and High Flow Rate silencers are used, coupled with other technology variations and staged elements to provide parametrically varied performance attributes. Such attributes include the ability to reduce internal blast load impulse accumulation by managing early time shock reflections while significantly altering the rate of proximal venting. These silencers may allow for minimal reciprocating weapon functional influence, while at the same time significantly suppressing signature to the operator and bystanders. Hybrid designs span a large range of the market in both brand and performance in different combustion regimes.  These silencers include the CAT MOB (6.148), GSL Phoenix (6.86), PTR VENT 2 (6.131), Dead Air Mojave 9 (6.177), and to some extent, the Resilient Suppressors RS9 (6.90).  Technologies implemented in these types of silencers are varied, including PTR Purposely induced Porosity (PIP), CAT DiVerge, coaxial designs, early-venting designs, and others.

The above three classes of silencers produce varied performance on different weapon systems and with different ammunition types. Of the three classes, the most significant overall performance potentials remain confined to Hybrid designs, in accordance with the PEW Science Silencer Sound Standard public research pedigree, to date.

It is important to note that higher distal gas velocity typically produces low frequency-biased inner ear response. This signature characteristic is interpreted by bystanders and operators as sounding “boomy,” and is usually a characteristic of silencers in the “High Flow Rate” design class, though it is not restricted to only that class.  The “boomy” signature characteristic is often less pronounced from silencers that generate high flow rates through different mechanisms (including those using PTR PIP or CAT DiVerge, coaxial elements, etc).  While “boomy,” “sharp,” and “full” descriptors of sound signatures are subjective in nature, the phenomena responsible for these impressions have physical mechanisms.  These signature characteristics have been examined in-depth by PEW Science and analysis was previously published in Member Research Supplement 6.103 (supersonic 5.56 NATO), Supplement 6.115 (supersonic 7.62 NATO), Supplement 6.124 (subsonic 300 BLK), and Supplement 6.169 (supersonic 5.56 NATO).  This current article is authored to include analysis of subsonic 9mm suppression with pistol caliber carbines (PCCs) or submachine guns (subguns).

As a consequence of both the phenomena above and pressure field shape from distal vent arrays, blast load reflections may also be perceived as more severe to both bystanders and the operator when fielding High Flow Rate silencers. Although the Suppression Rating characterizes personnel risk at the instrumented test locations (MIL-STD muzzle and MIL-STD shooter’s ear), testing and analysis is performed in the true free field away from any reflecting surfaces other than the ground, which is 1.6 m below the weapon system muzzle. Bystanders and operator personnel may be located near berms, vehicles, structures, tree-lines, and other obstacles that act as reflecting surfaces. In these environments, it is likely that Flow-Through silencers and other silencers exhibiting the measured traits of low frequency-biased signatures presented in the above Research Supplements, will present as “louder;” the already more noticeable late-time components of their signatures will be further exacerbated by the later-time wave reflection components of the total signature to which personnel are subjected.  All weapon systems have more severe signatures near reflecting surfaces.  However it is postulated that Flow-Through silencer signatures may be perceived as even more severe in these environments.  Research is ongoing.

This Research Supplement compares a select group of silencers, in the subsonic ammunition combustion regime tested on the PEW Science standard PEW Science HK SP5-A2 test host weapon system.  In general, increasing gross flow rate through a silencer can significantly reduce ejection port blast hazards to the weapon operator on standard subguns and PCCs (even on the roller-delayed blowback HK MP5 operating system).  However, certain designs may significantly outperform others in this regard.  The eight (8) selected silencer configurations analyzed in this study have varying shooter’s ear Suppression Ratings on the standard HK MP5.  There are significant signature differences between the models.  Quantifying these performance differentials is essential for characterizing hazards to the system operator and bystanders.

6.184.1 Quantifying Sound Suppression Performance

The severity of a suppressed small arm weapon signature, to the human inner ear, is quantified by the PEW Science Suppression Rating (Figure 1). As the Suppression Rating is a Damage Risk Criterion (DRC), it only expresses the degree to which a human may experience hearing damage.  The Suppression Rating captures human inner ear damage risk potential from a measured impulsive complex blast overpressure signature during the entire time regime of weapon operation, including combustion, complete blowdown, and all mechanical operation, including the carrier group returning to battery, in the true free field, away from reflecting surfaces other than the ground 1.6 meters below the weapon muzzle. Although complex signature characteristics are included in its computation, the end result is only a DRC; a lower Suppression Rating indicates a higher personnel hazard in the free field - it is not a subjective quantity; it is an objective quantification of hearing damage risk potential. Suppression Rating Rankings can be found in Section 7 of the Standard.

The nature of human inner ear response, and human interpretation or perception of such response, dictates further signature examination if one desires information outside the purposes of DRC use. For example, end users may be interested in “how a silencer sounds,” or if the silencer has “a pleasant tone,” rather than in the hearing damage risk potential of using the silencer on their weapon. These phenomena are not necessarily coupled for all users.

The Suppression Rating DRC allows the end user to group silencers with similar hearing damage risk potential on a variety of weapon systems. After such grouping, further analysis can provide insight into the aforementioned phenomena. To that end, eight silencer configurations evaluated by PEW Science are included in this members-only Research Supplement to examine relative First Round Pop (FRP) and overall sound signature suppression performance characteristics. The silencers in this group were selected due to their use on the HK MP5 submachine gun, and other pistol caliber carbine (PCC) platforms. The eight silencers behave differently, even despite some of them exhibiting similar performance quantified by the PEW Science Suppression Rating.  Their 9mm MP5 suppression performance provides an excellent case study comparing current 9mm MP5 silencer technology (Figure 2).

Fig 1. PEW Science Suppression Rating Scale

The published Sound Signature Review engineering test reports of these eight silencers on the standard HK SP5 are linked below (chronologically, in order of PEW Science laboratory testing and analysis publication). Their technology class and subcategory or designation is noted:

Because the PEW Science Suppression Rating is a DRC, it characterizes the postulated risk of hazard to bystanders, or the weapon operator, from a suppressed small arm weapon system. Despite achieving a similar Suppression Rating, some silencers may have certain signature characteristics that differ from others, and those differences are of interest to users, as noted above. Coaxial designs, Flow Baffles, conventional baffles, DiVerge, and PIP all respond differently to changes in muzzle blast pressure input.  Furthermore, as implemented in the subject silencers, all of these technologies result in different system back pressure which can alter the severity of ejection port blast on semiautomatic and automatic weapon systems.

In addition to flow rate, flash reduction, and back pressure reduction, sound suppression performance of 9mm silencers, particularly during the first shot, is of significant concern to many weapon system operators. Relatively high sound signature amplitude during the first shot from a suppressed weapon system is referred to as First Round Pop (FRP). Prior to the weapon system being fired, a sound suppressor is filled with air containing an oxidizer from the surrounding environment; this air occupies the internal silencer volume and supports ancillary combustion during the first shot. It is this ancillary combustion supported by the presence of an oxidizer that may increase sound signature relative to subsequent shots. The FRP phenomenon is present and measurable in all suppressed systems unless the internal silencer atmosphere is purged such that ancillary combustion is not supported within the silencer.

Fig 2. Suppression Rating Comparisons Of 9mm Silencers Using PEW-SOFT 9×19mm subsonic MP5 Test Data

Typically, silencers possessing a high flow rate also possess lower sound suppression performance with subsonic ammunition and potentially lower bystander suppression performance, in general.  Also typically, silencers possessing a lower flow rate (higher back pressure) can reduce the level of operator protection by increasing ejection port blast.  There are exceptions to these observations, and hybrid designs have the potential to exhibit high suppression performance in multiple combustion regimes while also reducing back pressure.

Although changes to gross gas flow rate (flow restriction, or back pressure) strongly correlate to sound signature suppression, there are other flow dynamics and frequency components of silencer sound signatures that result in varying signature severity to the human inner ear for a given suppressed system. These gas dynamics can significantly influence some signature characteristics. Furthermore, certain personnel may have preexisting hearing damage or other hearing sensitivity characteristics that differ from the 95th-percentile inner ear response with which the PEW Science Suppression Rating correlates. The impact of these differences on the human perception of silencer sound suppression performance has been quantified by PEW Science.

This research supplement is intended to provide more information to PEW Science members with regard to specific sound signature characteristics of the tested configurations in the aforementioned analytical test reports and to help frame objective loudness comparisons between eight 9mm suppressors that use various technologies to suppress 9mm weapon signatures. Both FRP and total sound signature suppression regimes are examined. This supplement is part of ongoing PEW Science small arm weapon system sound signature research. PEW Science thanks you for your support.

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