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

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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.

  • Section 6.184.2 provides an overall sound suppression performance summary at the muzzle and shooter’s ear.

  • Section 6.184.3 provides detailed comparisons of the sound signatures measured at the muzzle.

  • Section 6.184.4 provides detailed comparisons of the sound signature measured at the shooter’s ear.

  • Section 6.184.5 repeats the performance summary.

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.

6.184.2 Overall Sound Suppression Performance Summary

Bystander Perception:

To personnel located 1.0 m left of the weapon muzzle, the PTR VENT 2 is postulated to sound the quietest during the first shot, with the short configuration of the Dead Air Wolfman being the loudest.

During the first shot, bystanders will likely perceive the CASH 9K and short Wolfman to be the loudest, with the short Wolfman possessing a “harsher” tone than the CASH 9K, and the CASH 9K sounding extremely “boomy.” During the first shot, bystanders will likely perceive the VENT 2, MOB, and Phoenix to be quieter than the long Wolfman. The CAT MOB is likely to be perceived as having a “deep tone” relative to the quietest silencers in the group, during the first shot. It is likely that the Otter Creek Labs Lithium will be perceived as similar in loudness to the Resilient RS9 during the first shot, with the RS9 being more “boomy.” It is possible that the Lithium may sound harsher than the RS9 to some users.  Bystanders with high frequency hearing loss may perceive the sound delta between the Lithium and RS9 to be less pronounced, during the first shot.  These same bystanders may perceive the CAT MOB to be “rich” in tone and the Phoenix and VENT 2 to sound almost identical during FRP.

On average, the CAT MOB is postulated to sound the quietest, with the short Wolfman and HUXWRX CASH 9K being the loudest silencers, on average. After FRP, the long Wolfman is much more similar to the GSL Phoenix in maximum severity, but is still louder. The VENT 2 is quieter than the Phoenix, on average. The CASH 9K will be perceived as “boomy” by bystanders, on average.

Shooter Perception:

To personnel firing the weapon, the CAT MOB is the quietest silencer during the first shot. The short Wolfman is the loudest to the shooter, during the first shot, and interestingly, the long Wolfman is not far behind.  Back pressure is playing a significant role in exacerbating ejection port blast that coalesces with enhanced FRP muzzle blast.  The same exaggerated lower frequency inner ear response of the CASH 9K and short Wolfman that was observed in the muzzle signatures, relative to that of some of the other silencers, is observed at the shooter’s ear. This indicates that during FRP, these silencers may still be perceived by the shooter as “boomy” compared to other silencers.

During FRP, it is very likely that most weapon operators will perceive the CAT MOB to be the quietest silencer.  The only exception may be if the shooter has severe high frequency hearing damage.  Volume, tone, and all other subjective determinations of signature severity may be perceived as superior with the CAT MOB during the first shot, to the weapon operator, using the tested configuration.  Both the long and short configurations of the Dead Air Wolfman exhibit severe inner ear response characteristics driven by system back pressure and FRP muzzle blast.  During the first shot, the PTR VENT 2 and CASH 9K exhibit very similar severity, which may be unintuitive to the reader until examining the shooter’s ear Suppression Rating for both silencers.  Shooters with high frequency hearing loss may perceive the GSL Phoenix to sound quieter than the VENT 2 during FRP.

To personnel firing the weapon, on average, the CAT MOB is the quietest to the shooter, with both configurations of the Wolfman again being the loudest to the shooter, on average.  To the weapon operator, on average, the RS9 may result in a more “harsh” signature than the smaller CASH 9K silencer, in the free field.  Late time phenomena from muzzle blast will cause weapon operators to hear “boomy” and “loud” signature components from the CASH 9K, but the operator risk will still be lower in the free field.  Near reflecting surfaces, the RS9 would likely provide more operator protection.  The GSL Phoenix will likely sound harsher to the shooter than the VENT 2. However, it will protect the operator to a significantly greater degree than the Wolfman configurations.

Interestingly, on average, the high(er) back pressure Wolfman long/short and Lithium behave more similarly at the operator’s head.  This is due to ejection port blast starting to dominate the signature risk profile to the operator, after FRP is diminished.

Quite stark is the difference between the CAT MOB and the rest of the group. The frequency response characteristics are lower across the entire range; this is largely due to the technology used in the silencer; ejection port blast is not being exacerbated like with a typical suppressed MP5 system.

Normalized FRP and average performance is compared in Figure 3.  Note that there are significant performance differentials shown.  Below are summaries of some high-level conclusions gleaned from this research.

Here are some muzzle behavior notes (not all-inclusive; please use the chart for more):

  • On average, the CAT MOB signature is approximately 5% less severe than the PTR VENT 2 and GSL Phoenix signatures.

  • The long Dead Air Wolfman signature, on average, is approximately 13% less severe than the Otter Creek Labs Lithium signature. The short Dead Air Wolfman signature, on average, is approximately 30% more severe than the Lithium signature.

  • The Resilient RS9 signature, on average, is approximately 13% more severe than the Lithium signature, and 17% less severe than the short Dead Air Wolfman signature. The RS9 signature is over 50% less severe than the HUXWRX CASH 9K signature, on average.

  • The FRP signatures from the CAT MOB, PTR VENT 2, and GSL Phoenix are relatively close in gross peak severity to bystanders.

  • The FRP signatures of the HUXWRX CASH 9K and the short Dead Air Wolfman are very severe. In the long configuration, the Dead Air Wolfman has an FRP signature that is over 10% higher severity than the quietest silencers tested.

Here are some shooter’s ear behavior notes (not all-inclusive; please use the chart for more):

  • On average, the CAT MOB signature is approximately 24% less severe and 36% less severe than the PTR VENT 2 and GSL Phoenix signatures, respectively.

  • The long Dead Air Wolfman signature, on average, is approximately 11% more severe than the Otter Creek Labs Lithium signature. The short Dead Air Wolfman signature, on average, is approximately 16% more severe than the Lithium signature.

  • The Resilient RS9 signature, on average, is approximately 29% less severe than the Lithium signature, and 45% less severe than the short Dead Air Wolfman signature. The RS9 signature is approximately 3% more severe than the HUXWRX CASH 9K signature, on average.

  • The FRP signatures from the CAT MOB, PTR VENT 2, and GSL Phoenix are not as close in operator severity than they are in bystander severity.  To the weapon operator, during the first shot, the CAT MOB signature is over 10% less severe than the PTR VENT 2 and over 28% less severe than the GSL Phoenix.

  • The FRP signatures of the Dead Air Wolfman in both long and short configurations, to the weapon operator, are very severe. In the long configuration, the Dead Air Wolfman has an FRP signature that is up to 70% higher in severity than the quietest silencers tested.

As noted, the relative the Muzzle and Ear Average and FRP measurements shown in Figure 3 are not comparable to one another. Muzzle numbers should be compared with muzzle, and ear with ear. This is a consequence of the chosen normalization and the hearing damage potential at the muzzle and ear being different (as with any silencer).  Compare like-patterns and like-colors to each other, per the figure legend.

Fig 3. Normalized FRP and Average Performance Summary (9mm MP5)

The data shown in Figure 3 is intended to present the likely human relative perception of FRP magnitude of the silencers to both bystanders and the shooter. It is important to note that the human inner ear responds differently to certain frequencies. On average, it is postulated that the relations in Figure 3 will directly correlate to human inner ear response. However, some users may have hearing sensitivity that is compromised or different than others in certain frequency ranges. Those phenomena are considered in this article.

The following subsections of this Research Supplement present in-depth comparisons of human inner-ear modeling response to the FRP and average sound signatures.  The below will help you understand subjective impressions of “tone” and other descriptors by end users, and how they may objectively occur.

6.184.3 Comparisons of Muzzle Waveforms Measured in the Free Field

It is not always possible to determine relative, objective loudness from only the measured average peak sound pressure amplitude and measured peak sound pressure momentum transfer potential (impulse). Therefore, the Suppression Rating also considers physical ear response to measured sound signatures. The human inner ear responds to different sound pressure frequencies with varying sensitivity. Physically, these frequencies excite different regions of the basilar membrane within the cochlea. The human ear is typically most sensitive to sounds that excite the membrane near a frequency of 4,000 Hz. However, the ear may be exercised, and therefore damaged, at different physical regions. It is postulated that this inner ear response directly correlates to the perceived loudness of suppressed small arms.

PEW Science Research Note: As stated in previous Research Supplements, it is important not to misconstrue the frequency-domain data in this Research Supplement with a simple frequency analysis (Fourier transform) of the time-domain overpressure waveforms presented in the reviews. The data shown in this research supplement is the output from analytical human inner ear modeling with the measured test data used as free field overpressure loading input.

6.184.3.1 FRP Muzzle Comparisons

Figure 4 presents the results of an inner ear analysis performed using measured sound overpressure waveforms from the first shots in the referenced tests. The curves show normalized physical response of the human inner ear as a function of basilar membrane location within the cochlea and corresponding sensitivity frequencies. Fig 4a shows the response on the vertical axis with a linear scale. Fig 4b and Fig 4c show the same data on a logarithmic scale, in the low and high frequency hearing response regimes, respectively. Note that the data is normalized; this lets you see the relative theoretical ear response for the silencers.

To personnel located 1.0 m left of the weapon muzzle, the PTR VENT 2 is postulated to sound the quietest, with the short configuration of the Dead Air Wolfman being the loudest, during the first shot.

Fig 4a. Comparison of FRP Human Inner Ear Response - Subsonic 9mm MP5 at the Muzzle (Linear Scale)

Note that in the very low frequency response regime during the first shot, there is a commonality in inner ear response characteristics from some of the technologies, and some differentiators.

The HUXWRX CASH 9K, short Dead Air Wolfman, Resilient RS9, and Otter Creek Labs Lithium all produce greater low frequency response during the first shot.  The response amplitudes below 2,000 Hz are all exacerbated.  Interestingly, below approximately 1,000 Hz, the short Wolfman and Lithium exhibit more intense response than the RS9.  However, between 1,000 Hz and 2,000 Hz, the RS9 is louder.  Above 2,500 Hz, the RS9 is once again quieter.

The significantly quieter silencers in the group (CAT MOB, long Wolfman, PTR VENT 2, and GSL Phoenix) also induce varied response in the low frequency ranges. Although the MOB, VENT 2, and Phoenix all have similar maximum FRP intensities around 4,000 Hz, the MOB displays exaggerated response below 2,500 Hz, inducing similar response to the long Wolfman in that range.

This analysis indicates that during the first shot, bystanders will likely perceive the CASH 9K and short Wolfman to be the loudest, with the short Wolfman possessing a “harsher” tone than the CASH 9K, and the CASH 9K sounding extremely “boomy.”

The analysis further indicates that during the first shot, bystanders will likely perceive the VENT 2, MOB, and Phoenix to be quieter than the long Wolfman. The CAT MOB is likely to be perceived as having a “deep tone” relative to the quietest silencers in the group, during the first shot.  All analysis is pertinent to the free field (away from reflecting surfaces, other than the ground).

It is likely that the Otter Creek Labs Lithium will be perceived as similar in loudness to the Resilient RS9 during the first shot, with the RS9 being more “boomy.” It is possible that the Lithium may sound harsher than the RS9 to some users, considering both the aforementioned frequency assessment as well as its high frequency response above 5,500 Hz.

The distribution of bystander-perceived severity, during the first shot with these silencers, exhibits a wide spread.

Fig 4b. Comparison of FRP Low Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Muzzle (Log Scale)

Fig 4c. Comparison of FRP High Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Muzzle (Log Scale)

PEW Science Research Note: Bystanders with hearing loss above the 4,000 Hz range may perceive the sound delta between the Lithium and RS9 to be less pronounced, during the first shot.  These same bystanders may perceive the CAT MOB to be “rich” in tone and the Phoenix and VENT 2 to sound almost identical during FRP.

6.184.3.2 Average Muzzle Comparisons

Figure 5 presents the average results of an inner ear analysis performed using measured sound overpressure waveforms from all shots in the referenced tests. Again, the curves show normalized physical response of the human inner ear as a function of basilar membrane location within the cochlea and corresponding sensitivity frequencies. Fig 5a shows the response on the vertical axis with a linear scale. Fig 5b and Fig 5c again show the same data on a logarithmic scale, in the low and high frequency hearing response regimes, respectively. Note that the data is normalized; this lets you see the relative theoretical ear response for the silencers.

To personnel located 1.0 m left of the weapon muzzle, the CAT MOB is postulated to sound the quietest on average, with the short Wolfman and HUXWRX CASH 9K being the loudest silencers, on average.

Fig 5a. Comparison of Average Human Inner Ear Response - Subsonic 9mm MP5 at the Muzzle (Linear Scale)

The average signatures (using all 5 analyzed shots in the tested 6-shot strings in the Standard)  do change the relative hearing damage severity computations from the previously presented FRP analysis.  On average, the HUXWRX CASH 9K produces much more severe inner ear response than the other models, and it does so across the entire frequency spectrum.  This conclusion should be reasonable to the reader, given its lower overall muzzle Suppression Rating. 

Another significant change to the analysis is apparent when looking at the average signatures rather than isolating FRP. Note the inner ear response below approximately 3,000 Hz is more similar in the two previously compared groups. The short Wolfman, RS9, and Lithium exhibit similar low frequency excitation.  The short Wolfman and the RS9, again, become louder than the Lithium in the midrange frequencies and continue to produce a louder signature to the ear throughout the higher frequency range.

The quietest silencers of the group become somewhat more differentiated, in that the CAT MOB becomes the quietest silencer overall, while retaining its exaggerated low frequency behavior (“deep tone”). After FRP, the long Wolfman is now much more similar to the GSL Phoenix in maximum severity, but is still louder. The VENT 2 is quieter than the Phoenix, on average.

Fig 5b. Comparison of Average Low Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Muzzle (Log Scale)

Fig 5c. Comparison of Average High Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Muzzle (Log Scale)

PEW Science Research Note: Note that the logarithmic scales may help you to better understand low frequency response variation between the silencer models. In particular, please examine Figure 5b. Note the shape of the curves below 3,000 Hz; the GSL and PTR silencers exhibit similar shapes, whereas the long Wolfman does not exhibit a dip near 2,000 Hz.  The CAT MOB shifts its low frequency emphasis to below 1,500 Hz.  The CASH 9K shifts its low frequency emphasis even further to the left, with much higher amplitude than the other silencers. This is a very clear quantitative representation of the “boomy” subjective phenomenon described by users when using HUXWRX silencers and silencers with high distal flow rate, in general.

6.184.4 Comparisons of Waveforms Measured near the Shooter’s Ear

The sound signatures measured at the ear during the tests of each silencer are significantly different than those measured at the weapon muzzle and this difference is not only shown in the average peak sound pressure and impulse measurements, but also with inner ear analysis.  Note that ejection port blast now plays a significant role in the measured test data and analysis.

6.184.4.1 FRP Ear Comparisons

Figure 6 presents an inner ear analysis performed using measured sound overpressure waveforms at the shooter’s right ear from the first shots in all tests. Fig 6a shows the response on a linear vertical scale. Fig 6b and Fig 6c show the same data on a logarithmic vertical scale, in the low and high frequency hearing response regimes, respectively.

To personnel firing the weapon, the CAT MOB is the quietest silencer during the first shot. The short Wolfman is the loudest to the shooter, during the first shot, and interestingly, the long Wolfman is not far behind.  Back pressure is playing a significant role to exacerbating ejection port blast that coalesces with enhanced FRP muzzle blast.

The same exaggerated lower frequency inner ear response of the CASH 9K and short Wolfman that was observed in the muzzle signatures, relative to that of some of the other silencers, is observed at the shooter’s ear. This indicates that during FRP, the these silencers may still be perceived by the shooter as “boomy” compared to other silencers.

Fig 6a. Comparison of FRP Human Inner Ear Response - Subsonic 9mm MP5 at the Ear (Linear Scale)

As with the signatures at the muzzle, the data measured at the shooter’s ear may be viewed on the linear scale above or on the logarithmic scales, below.  It is the same set of calculations, from the same test data, just plotted on a different scale.

During FRP, it is very likely that most weapon operators will perceive the CAT MOB to be the quietest silencer.  The only exception may be if the shooter has severe hearing damage above 3,000 Hz.  Volume, tone, and all other subjective determinations of signature severity may be perceived as superior with the CAT MOB during the first shot, to the weapon operator, using the tested configuration.

The GSL Phoenix exhibits exaggerated inner ear excitation above 4,000 Hz, to the weapon operator. This may be due to the silencer’s construction (resonance), coalescing with muzzle blast and ejection port blast. The back pressure of the silencer is not severe.

The RS9, CASH 9K, and VENT 2 possess similar peak severities around 4,000 Hz to the weapon operator, during the first shot, with the Otter Creek Labs Lithium being somewhat similar in severity to the Phoenix, though most likely with less “ringing” or resonance.

Both the long and short configurations of the Dead Air Wolfman exhibit severe inner ear response characteristics driven by system back pressure and FRP muzzle blast.

The different technology classes tend to begin to show similarities at the shooter’s ear, due to flow rate.  The Lithium, though quieter than the Wolfman(s) during FRP, exhibits similar frequency response trend.  This becomes more apparent when examining the average signatures in the next section. 

During the first shot, the PTR VENT 2 and CASH 9K exhibit very similar severity, which may be unintuitive to the reader until examining the shooter’s ear Suppression Rating for both silencers.  As shown in the average shot analysis, the silencers produce very similar hearing damage risk potential to the operator, in the free field, on this weapon system.  Flow rate is exceedingly important for ejection port blast hazard reduction.  Ejection port blast hazard can be significant for the weapon operator.

Fig 6b. Comparison of FRP Low Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Ear (Log Scale)

Fig 6c. Comparison of FRP High Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Ear (Log Scale)

PEW Science Research Note: Shooters with high frequency hearing loss may perceive the GSL Phoenix to sound quieter than the VENT 2 during FRP.

6.184.4.2 Average Ear Comparisons

Figure 7 presents the average results from an inner ear analysis performed using measured sound overpressure waveforms at the shooter’s right ear from all the shots in the referenced tests. Fig 7a shows the response on a linear vertical scale. Fig 7b and Fig 7c show the same data on a logarithmic vertical scale, in the low and high frequency hearing response regimes, respectively.

To personnel firing the weapon, on average, the CAT MOB is the quietest to the shooter, with both configurations of the Wolfman again being the loudest to the shooter, on average.

The silencers form 3 distinct groups:

  1. CAT MOB

  2. Phoenix, CASH 9K, VENT 2, RS9

  3. Lithium, and long/short Wolfman

These groups are clustered due to early-time flow rate and pressure staging technology.  Recall that ejection port blast hazard reduction is paramount to increase the level of protection for the weapon operator.

Fig 7a. Comparison of Average Human Inner Ear Response - Subsonic 9mm MP5 at the Ear (Linear Scale)

It is likely that environmental variables, such as nearby reflecting surfaces, will significantly influence the relative perceptions of the weapon operator when firing this weapon system with these silencers, on average.  This is a function of muzzle and shooter’s ear Suppression Rating divergence, along with how the different technologies produce varying sound field shapes and wave particle velocities.  However, in the free field, the comparisons shown hold.

At frequencies below 1,500 Hz, the VENT 2 and Lithium act somewhat similarly, but in the primary frequencies of human ear response, the Lithium is significantly louder than the VENT 2.  Above 4,000 Hz, the RS9 provides a more intense response than the CASH 9K.  It is for this reason that to the weapon operator, on average, the RS9 may result in a more “harsh” signature than the smaller CASH 9K silencer, in the free field.  Late time phenomena from muzzle blast will cause weapon operators to hear “boomy” and “loud” signature components from the CASH 9K, but the operator risk will still be lower in the free field.  Near reflecting surfaces, the RS9 would likely provide more operator protection.

Again, the GSL Phoenix exhibits some high exaggerated higher frequency response components, and will likely sound harsher to the shooter than the VENT 2. However, it will protect the operator to a significantly greater degree than the Wolfman configurations.  The Wolfman configurations have extremely similar risk profiles, despite being significantly different lengths.  This is a direct quantitative illustration of back pressure varying with distal flow rate but remaining similar in early time, as the blast chambers do not change between the two configurations of the Wolfman silencer.

Interestingly, on average, the high(er) back pressure Wolfman long/short and Lithium behave more similarly at the operator’s head.  This is due to ejection port blast starting to dominate the signature risk profile to the operator, after FRP is diminished.

Quite stark is the difference between the CAT MOB and the rest of the group. The frequency response characteristics are lower across the entire range; this is largely due to the technology used in the silencer; ejection port blast is not being exacerbated like with a typical suppressed MP5 system.

Fig 7b. Comparison of Average Low Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Ear (Log Scale)

Fig 7c. Comparison of Average High Frequency Human Inner Ear Response - Subsonic 9mm MP5 at the Ear (Log Scale)

6.184.5 Research Supplement Summary

This Summary is Repeated from earlier in the Article.

Bystander Perception:

To personnel located 1.0 m left of the weapon muzzle, the PTR VENT 2 is postulated to sound the quietest during the first shot, with the short configuration of the Dead Air Wolfman being the loudest.

During the first shot, bystanders will likely perceive the CASH 9K and short Wolfman to be the loudest, with the short Wolfman possessing a “harsher” tone than the CASH 9K, and the CASH 9K sounding extremely “boomy.” During the first shot, bystanders will likely perceive the VENT 2, MOB, and Phoenix to be quieter than the long Wolfman. The CAT MOB is likely to be perceived as having a “deep tone” relative to the quietest silencers in the group, during the first shot. It is likely that the Otter Creek Labs Lithium will be perceived as similar in loudness to the Resilient RS9 during the first shot, with the RS9 being more “boomy.” It is possible that the Lithium may sound harsher than the RS9 to some users.  Bystanders with high frequency hearing loss may perceive the sound delta between the Lithium and RS9 to be less pronounced, during the first shot.  These same bystanders may perceive the CAT MOB to be “rich” in tone and the Phoenix and VENT 2 to sound almost identical during FRP.

On average, the CAT MOB is postulated to sound the quietest, with the short Wolfman and HUXWRX CASH 9K being the loudest silencers, on average. After FRP, the long Wolfman is much more similar to the GSL Phoenix in maximum severity, but is still louder. The VENT 2 is quieter than the Phoenix, on average. The CASH 9K will be perceived as “boomy” by bystanders, on average.

Shooter Perception:

To personnel firing the weapon, the CAT MOB is the quietest silencer during the first shot. The short Wolfman is the loudest to the shooter, during the first shot, and interestingly, the long Wolfman is not far behind.  Back pressure is playing a significant role in exacerbating ejection port blast that coalesces with enhanced FRP muzzle blast.  The same exaggerated lower frequency inner ear response of the CASH 9K and short Wolfman that was observed in the muzzle signatures, relative to that of some of the other silencers, is observed at the shooter’s ear. This indicates that during FRP, these silencers may still be perceived by the shooter as “boomy” compared to other silencers.

During FRP, it is very likely that most weapon operators will perceive the CAT MOB to be the quietest silencer.  The only exception may be if the shooter has severe high frequency hearing damage.  Volume, tone, and all other subjective determinations of signature severity may be perceived as superior with the CAT MOB during the first shot, to the weapon operator, using the tested configuration.  Both the long and short configurations of the Dead Air Wolfman exhibit severe inner ear response characteristics driven by system back pressure and FRP muzzle blast.  During the first shot, the PTR VENT 2 and CASH 9K exhibit very similar severity, which may be unintuitive to the reader until examining the shooter’s ear Suppression Rating for both silencers.  Shooters with high frequency hearing loss may perceive the GSL Phoenix to sound quieter than the VENT 2 during FRP.

To personnel firing the weapon, on average, the CAT MOB is the quietest to the shooter, with both configurations of the Wolfman again being the loudest to the shooter, on average.  To the weapon operator, on average, the RS9 may result in a more “harsh” signature than the smaller CASH 9K silencer, in the free field.  Late time phenomena from muzzle blast will cause weapon operators to hear “boomy” and “loud” signature components from the CASH 9K, but the operator risk will still be lower in the free field.  Near reflecting surfaces, the RS9 would likely provide more operator protection.  The GSL Phoenix will likely sound harsher to the shooter than the VENT 2. However, it will protect the operator to a significantly greater degree than the Wolfman configurations.

Interestingly, on average, the high(er) back pressure Wolfman long/short and Lithium behave more similarly at the operator’s head.  This is due to ejection port blast starting to dominate the signature risk profile to the operator, after FRP is diminished.

Quite stark is the difference between the CAT MOB and the rest of the group. The frequency response characteristics are lower across the entire range; this is largely due to the technology used in the silencer; ejection port blast is not being exacerbated like with a typical suppressed MP5 system.

This supplement is part of ongoing PEW Science small arm weapon system sound signature research. PEW Science thanks you for your support.