SSS.6.169 - Research Supplement: Low Back Pressure 5.56 Silencer Performance - Flow-Through vs. SURGE BYPASS, PIP, and LBP. Head-to-Head Detailed Comparisons (Members Only)

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HUXWRX FLOW Series vs. Combat Application Technologies CAT/WB/A1/718 vs. SilencerCo Velos LBP vs. PTR VENT 3 on the 5.56 MK18 with 10.3-inch Barrel

There are a variety of technologies implemented in modern rifle 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 signature.

Currently, there exist three different primary classes of rifle silencer technology used to suppress the 5.56x45mm NATO rifle 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. High-performance examples of such silencers include the Maxim Defense DSX (6.110), Otter Creek Labs Polonium (6.75), Aero Precision Lahar-30L (6.119), and others. 

  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 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 use various generations of HUXWRX Flow-Through technology.

  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 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 PTR VENT 3 (6.135), CAT/WB/A1/718 (6.129), SilencerCo Velos LBP (6.134), the Surefire RC2 family, the Surefire SOCOM556-RC3 (6.151), and others.  Technologies implemented in these types of silencers are varied, including PTR Purposely induced Porosity (PIP), elements of Surefire Total Signature Reduction, CAT SURGE BYPASS, 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. There exists a subcategory of this class of silencers, that is shown to demonstrate efficacy in multiple flow regimes. To date, the best example of such a subcategory includes silencers like the Combat Application Technologies CAT/ODB/A1/718 (6.122). These silencers not only combine performance parameters from all three classes, but have been shown to demonstrate so-called pressure agnostic behavior, in which input variation exemplified by both supersonic and subsonic cartridge combustion regimes both result in high signature suppression efficacy.

It is important to note that higher distal gas velocity from Flow-Through designs produces low frequency-biased inner ear response. This signature characteristic is interpreted by bystanders and operators as sounding “boomy,” more so than that of silencers generating high flow rates through different mechanism (including those using PTR PIP or CAT SURGE BYPASS).  While such descriptors of sound signatures are subjective in nature, the phenomenon responsible for this impression has physical mechanism.  This signature characteristic has 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), and Supplement 6.124 (subsonic 300 BLK).  This current article is authored to include analysis of the recently examined HUXWRX FLOW 556 Ti.

As a consequence of both the phenomena above and pressure field shape from the distal vent array, blast load reflections may also be perceived as more severe to both bystanders and the operator when fielding HUXWRX Flow-Through 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 supersonic ammunition combustion regime tested on the PEW Science standard MK18 weapon system.  In general, increasing gross flow rate through a silencer can significantly reduce ejection port blast hazards to the weapon operator on standard untuned AR15 rifles.  However, certain designs may significantly outperform others in this regard.  All six (6) of the selected silencers analyzed in this study have relatively high shooter’s ear Suppression Ratings on the standard untuned MK18.  However, there are significant signature differences between the models.  Quantifying these performance differentials is essential for characterizing hazards to the system operator and bystanders.

6.169.1 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, and it does so 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. Suppression Rating Rankings can be found in Section 7 of the Standard.

The nature of human inner ear response, and human interpretation 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,” rather than in the hearing damage risk potential of using the silencer on their weapon. The two 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, six silencers 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 for their high flow rate characteristics and relatively high operator (shooter’s ear) Suppression Ratings on the untuned MK18 5.56x45mm NATO MK18 weapon system. The six silencers behave differently, even despite some of them exhibiting similar performance quantified by the PEW Science Suppression Rating.  Their 5.56 MK18 suppression performance provides an excellent case study comparing current “low back pressure” rifle silencer technology (Figure 2).

Fig 1. PEW Science Suppression Rating Scale

The published Sound Signature Reviews of these six silencers, on the standard MK18, are linked below (chronologically, in order of PEW Science laboratory testing and analysis publication). Their technology class and subcategory or designation is noted:

*Note that the FLOW 762 Ti is included in this publication, despite being a 30 caliber silencer, due to the relevance of the current generation HUXWRX Flow-Through technology performance comparison with the dedicated bore FLOW 556 Ti analysis recently published.  The 30 caliber CAT/ODB/A1/718 SURGE BYPASS example is omitted for brevity.

Because the 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. Flow-Through, SURGE BYPASS, LBP, and PIP all respond differently to changes in muzzle blast pressure input.

Sound signature suppression is not the only performance attribute of interest to many users. Back pressure reduction is often paramount for reducing gas toxicity and weapon over-function. The higher flow rate models above do not represent an exhaustive list of silencers that may reduce back pressure; there exist other models such as the Surefire SOCOM556-RC3 (6.151). The RC3 is intended to exhibit higher gross flow rate, reduce over-function on reciprocating weapon systems, and reduce gas toxicity to the end user. The silencer uses a higher flow variant of Surefire Total Signature Reduction technology to reduce back pressure. This silencer is not included in this research supplement because it exhibits significantly lower operator hazard reduction performance than the six aforementioned models.

In addition to flow rate, flash reduction, and back pressure reduction, sound suppression performance of centerfire rifle 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 from the surrounding environment; this air occupies the internal silencer volume and supports ancillary combustion during the first shot. It is this ancillary combustion 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 Rifle Silencers Using PEW-SOFT 5.56×45mm NATO MK18 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. However, there are significant exceptions. Hybrid designs have the potential to exhibit high performance in multiple regimes.

Although changes to gross gas flow rate (flow restriction, or back pressure) strongly correlates 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 6.169.2 provides an overall sound suppression performance summary at the muzzle and shooter’s ear.

  • Section 6.169.3 provides detailed comparisons of the sound signatures measured at the muzzle.

  • Section 6.169.4 provides detailed comparisons of the sound signature measured at the shooter’s ear.

  • Section 6.169.5 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 six low back pressure 5.56 rifle silencers that use various technologies to suppress AR15 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.

6.169.2 Overall Sound Suppression Performance Summary

Bystander Perception:

To personnel located 1.0 m left of the weapon muzzle, the HUXWRX FLOW 556 Ti is postulated to sound the quietest, with the SilencerCo Velos LBP being the loudest, during the first shot.

During the first shot, bystanders will most likely perceive many of these low back pressure silencers to be “boomy,” and the CAT WB is expected to sound the least “boomy” with a “higher tone” than the FLOW 556 Ti. All other silencers shown are expected to be “louder” than those two silencers, during the first shot, to bystanders in the free field.  Bystanders with high frequency hearing loss may perceive the sound delta between the CAT WB and FLOW 556 Ti to be greater, during FRP. These same bystanders may perceive the FLOW 556 Ti as more “boomy.”  They may also find the FLOW 556K to be potentially more pleasing than the PTR VENT 3, during the first shot.  The FRP signature environment with high flow rate silencers is exceedingly complex.

On average, the PTR VENT 3 is postulated to sound the quietest to bystanders, with the FLOW 556K and Velos LBP being the loudest silencers.

It is likely that bystanders will perceive both full-size HUXWRX FLOW silencers as more “boomy” than the CAT WB and PTR VENT 3, in the free field, on average. It is likely that the CAT WB will be perceived as being “higher pitched” than the FLOW 556 Ti. All four of those silencers (including the PTR VENT 3) are postulated to sound “better” or “more pleasing” to bystanders than the Velos LBP and Flow 556K, on average.

Shooter Perception:

To personnel firing the weapon, the FLOW 556 Ti is the quietest silencer during the first shot (by a small margin). The Velos LBP is the loudest to the shooter, during the first shot, again by a small margin.

During FRP, the FLOW 556K may still be perceived by the shooter as “boomy” compared to other silencers, but it still produces an objectively less severe inner ear response to the shooter than the Velos LBP.  During FRP, it is likely than many weapon operators will interpret the CAT WB as being quieter and “less boomy” than all of the HUXWRX FLOW silencers on this weapon system, even though the peak inner ear response from the silencers is similar.  The FLOW 762 Ti and FLOW 556 Ti may be indistinguishable to each other by some weapon operators, during FRP, on this weapon system. It is likely that users with high frequency hearing damage will hear them to be almost identical, during the first shot.

On average, to personnel firing the weapon, the FLOW 556 Ti is nominally the quietest to the shooter, with the Velos LBP being the loudest to the shooter.  

It is very likely that the weapon operator will perceive the CAT WB to exhibit a less “boomy” signature with this weapon system than many of the other silencers, on average.  The inner ear response of the CAT WB and FLOW 762 Ti leap frog at three distinct frequency regions.  This is one example of why subjective shooter impressions can vary significantly between individuals.  Even if users have identical weapon systems and environments, any deviation from the 95th percentile human inner ear can skew perception outcomes from the analyzed case.

On average, the compact FLOW 556K and full-size FLOW 556 Ti have very similar peak severities. This is reflected in their similar shooter’s ear Suppression Ratings on this host weapon system. However, it is extremely likely that the weapon operator will perceive the FLOW 556K as more “boomy” than the FLOW 556 Ti, and this is independent of the muzzle Suppression Rating, or any reflecting surfaces. Changing environment from the free field will most likely exacerbate the difference between the two silencers, to the operator.

It is likely that, on average, both the Velos LBP and VENT 3 present as more “boomy” than the CAT WB, to the operator in the free field. High frequency hearing loss may lead an operator to interpret the CAT WB as the quietest silencer on average.

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 for all four of the examined technology designations.

6.169.2.1 SilencerCo VELOS (LBP Technology)

In the technology classes noted above, LBP technology from SilencerCo is shown to experience potentially severe signature suppression performance losses in this configuration, on this host weapon.  SilencerCo LBP technology uses annular venting for significant flow rate increase, but it does not have significantly lengthened flow paths, nor does it possess significant staging in its design.  It is possible that testing with the direct-thread mount, which was intended by PEW Science to offer a reduction in performance comparison variables to the previously evaluated Saker 556, somewhat reduces performance when compared to the Silencer ASR mount with this silencer.  However, it is highly unlikely that use of the ASR mount would close the overall performance cap between the SilencerCo Velos LBP and the other tested silencers shown in Figure 3.

6.169.2.2 PTR VENT 3 (PIP Technology)

An interesting conclusion from this research is the FRP intensity from the PIP technology in the VENT 3 on this weapon system. The VENT 3 does exhibit a more significant relative FRP severity than some of the other silencers, and that is in contrast with its relatively high bystander (muzzle) Suppression Rating. It is important to note that the FRP signatures are included in the Suppression Rating calculations, at both muzzle and ear. So, the FRP signature is weighted equally in the five shot analytical computation. Therefore, if the FRP severity of the PIP-equipped PTR VENT 3 was lower, its overall Suppression Rating metrics would increase.  As it stands, the muzzle Suppression Rating of the VENT 3 is very high as reflected in the figure (its muzzle signature is only 19.2% as severe as the most severe muzzle signature in the study, on average).  The data plots in this report are all normalized to allow this type of quantitative comparison.

6.169.2.3 HUXWRX FLOW Series (Flow-through Technology)

As expected, significant low frequency inner ear response is characteristic in the behavior of all of the HUXWRX FLOW series silencers. This has been shown in previous research supplements; the addition of the newer full-size model FLOW 556 Ti confirms this is a persistent technology performance trait.  In general, it is important for users to note that the Suppression Rating is a metric characterizing human inner ear damage risk potential.  The information in this report is intended to assist users understand how what they hear may differ based upon each technology. HUXWRX FLOW silencers may present as more “boomy” than other models to both operators and bystanders and the hearing damage risk may be lower than other models.  Even with this signature trait, evaluation of the most recent full-size FLOW 556 Ti highlights how over-bore was a performance limitation with the FLOW 762 Ti for this platform, and how caliber-specific optimization of the Flow-Through technology can provide high efficacy with high pressure. Further information regarding performance scaling with input blast pressure differential (e.g. lower pressure on longer barrels) is presented in the recent 14.5-in testing publication.

6.169.2.4 CAT / WB / A1 (SURGE BYPASS Technology)

As shown in the referenced testing and analysis publications, the CAT WB exhibits extremely high performance for its size. Its FRP performance, in general, is very strong through the whole pressure field and the frequency ranges through which the inner ear is excited by its signatures do differ significantly from pure Flow-Through designs.  The SURGE BYPASS hybrid technology does throttle distal flow enough such that particle velocities remain below a critical threshold.  In examining all of the frequency response data presented herein, there are persistent reduced amplitudes in the lower frequency ranges.  This analysis highlights why, most likely, pervasive user experiences indicate the WB sounds “quieter” than Flow-Through silencers when the Flow-Through silencers may still induce less severe overall inner ear response in the free field on an untuned weapon.  Weapon tuning variations notwithstanding, this conclusion is also likely to be pervasive due to lower relative reflection intensities from SURGE BYPASS equipped silencers compared with the HUXWRX models.  Research is ongoing. 

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

  • On average, the VENT 3 signature is approximately 81% less severe than the FLOW 556K signature, to bystanders, and 57% less severe than the Velos LBP signature.  On average, the FLOW 556 Ti signature, to bystanders, is approximately 47% less severe than the Velos LBP signature, and 72% less severe than the FLOW 556K signature.  The FLOW 556 Ti has the quietest FRP to bystanders, with the CAT WB having a FRP that is 2.5% more severe.

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

  • On average, the FLOW 556 Ti signature is approximately 34% less severe than the Velos LBP signature, to to the shooter, and 27% less severe than the VENT 3 signature.  On average, the FLOW 556K, to the shooter, is approximately 1%  more severe than the FLOW 556 Ti signature, but has an FRP that is 61% more severe to the shooter.  The FRP severity of the CAT WB and FLOW 762  Ti, to the shooter, is almost identical in absolute maximum risk.

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 (5.56 NATO MK18 Rifle)

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.

6.169.3 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.

6.169.3.1 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 HUXWRX FLOW 556 Ti is postulated to sound the quietest, with the SilencerCo Velos LBP being the loudest, during the first shot.

Fig 4a. Comparison of FRP Human Inner Ear Response - 5.56 MK18 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 outliers.

The HUXWRX FLOW 556K, PTR VENT 3, and SilencerCo Velos LBP all produce greater low frequency response during the first shot.  The response amplitudes below 2,000 Hz are all exacerbated.  Interestingly, between approximately 1,000 Hz and 2,500 Hz, the CAT WB exhibits less intense response than the FLOW 556 Ti.  It is not until frequencies higher than 2,500 Hz that the response from the CAT WB is more severe, during the first shot.

This analysis indicates that during the first shot, bystanders will most likely perceive many of these low back pressure silencers to be “boomy,” and the CAT WB is expected to sound the least “boomy” with a “higher tone” than the FLOW 556 Ti. All other silencers shown are expected to be “louder” than those two silencers, during the first shot, to bystanders in the free field.

It is likely that the FLOW 556K will be interpreted as more “boomy” than the PTR VENT 3 to some observers. However, as the two silencers induce very similar low frequency response during the first shot, the higher mid-range response amplitude from the VENT 3 may result in it being perceived as louder during the first shot than everything except for the SilencerCo Velos LBP. The Velos produces more severe response throughout the entire frequency range during the first shot, by a significant margin.

Fig 4b. Comparison of FRP Low Frequency Human Inner Ear Response - 5.56 MK18 at the Muzzle (Log Scale)

Fig 4c. Comparison of FRP High Frequency Human Inner Ear Response - 5.56 MK18 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 CAT WB and FLOW 556 Ti to be greater, during FRP. These same bystanders may perceive the FLOW 556 Ti as more “boomy.”  They may also find the FLOW 556K to be potentially more pleasing than the PTR VENT 3, during the first shot.  The FRP signature environment with high flow rate silencers is exceedingly complex.

6.169.3.2 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 PTR VENT 3 is postulated to sound the quietest on average, with the FLOW 556K and Velos LBP being the loudest silencers, on average.

Fig 5a. Comparison of Average Human Inner Ear Response - 5.56 MK18 at the Muzzle (Linear Scale)

The average signatures (using all 5 analyzed shots in the tested 6-shot strings in the Standard) significantly change the relative hearing damage severity computations from the previously presented FRP analysis.  On average, the HUXWRX FLOW 556K 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; both the hybrid technology CAT WB (SURGE BYPASS) and PTR VENT 3 (PIP) induce less inner ear response than the Flow-Through HUXWRX FLOW 762 Ti and FLOW 556 Ti.  The PTR VENT 3 continues this dominance throughout the frequency spectrum (note its higher muzzle Suppression Rating).

The CAT WB maintains suppression superiority to the FLOW 556 Ti until a point slightly about 3,000 Hz in which it produces more severe response, almost reaching that of the FLOW 762 Ti. Based upon the frequency analysis, it is likely that bystanders will perceive both full-size HUXWRX FLOW silencers as more “boomy” than the CAT WB and PTR VENT 3, in the free field, on average. It is likely that the CAT WB will be perceived as being “higher pitched” than the FLOW 556 Ti. All four silencers (including the PTR VENT 3) are postulated to sound “better” or “more pleasing” to bystanders than the Velos LBP and Flow 556K, on average.

Users with high frequency hearing damage may perceive the CAT WB to be quieter than the FLOW 556 Ti, in general.

Fig 5b. Comparison of Average Low Frequency Human Inner Ear Response - 5.56 MK18 at the Muzzle (Log Scale)

Fig 5c. Comparison of Average High Frequency Human Inner Ear Response - 5.56 MK18 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 CAT and PTR silencers exhibit similar shapes, whereas the FLOW silencers have exaggerated response. This is a very clear quantitative representation of the “boomy” subjective phenomenon described by users when using HUXWRX FLOW silencers.  The Velos LBP possesses the same frequency curve features, but it induces such severe response in all other parts of the spectrum that its shape is of less consequence.  Nonetheless, when comparing the Velos LBP to the FLOW 556k, the difference in higher frequency response above 3,500 Hz is postulated to play a significant role in individuals interpreting the FLOW 556k as “louder.”  They could also both be described as “boomy.”

6.169.4 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.

6.169.4.1 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.

This is a relatively unique portion of the analysis, because all 6 silencers exhibit low back pressure and they all significantly reduce ejection port blast. Nonetheless, there are still nuanced differences in how their signatures will likely be interpreted by humans.

To personnel firing the weapon, the FLOW 556 Ti is the quietest silencer during the first shot (by a small margin). The Velos LBP is the loudest to the shooter, during the first shot, again by a small margin.

The same exaggerated lower frequency inner ear response of the FLOW 556K 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 FLOW 556K may still be perceived by the shooter as “boomy” compared to other silencers, but it still produces an objectively less severe inner ear response to the shooter than the Velos LBP.

Fig 6a. Comparison of FRP Human Inner Ear Response - 5.56 MK18 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 data.

During FRP, it is likely than many weapon operators will interpret the CAT WB as being quieter and “less boomy” than all of the HUXWRX FLOW silencers on this weapon system, even though the peak inner ear response from the silencers is similar.  The significant drop in CAT WB signature severity below 3,000 Hz is the reason for this postulation.  The deviation between muzzle signature severity and shooter’s ear signature severity, is a function of sound field shape influenced by particle velocity and flow rate, forming the complete gas momentum transfer mechanisms. Impulse signatures delineating these behaviors may be examined in detail in each respective Sound Signature Review white paper.  Please refer to the specific links at the beginning of this article to ensure you reference the exact test report for the data examined in this analysis.

During the first shot, the PTR VENT 3 shares a much higher amplitude lower frequency response with the full-size HUXWRX FLOW models. This phenomenon also presented during FRP at the muzzle with the VENT 3.

The FLOW 762 Ti and FLOW 556 Ti may be indistinguishable to each other by some weapon operators, during FRP, on this weapon system. It is likely that users with high frequency hearing damage will hear them to be almost identical, during the first shot.

Fig 6b. Comparison of FRP Low Frequency Human Inner Ear Response - 5.56 MK18 at the Ear (Log Scale)

Fig 6c. Comparison of FRP High Frequency Human Inner Ear Response - 5.56 MK18 at the Ear (Log Scale)

PEW Science Research Note: The same shooters with high frequency hearing loss may perceive the FLOW 556K to be louder than the SilencerCo Velos LBP, during the first shot, and considerably more “boomy” than the Velos.

6.169.4.2 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 overall signatures begin to experience less overall divergence.  However, some trends in the analysis continue.  The FLOW 556 Ti is nominally the quietest to the shooter, on average, with the Velos LBP being the loudest to the shooter, on average.

The exaggerated lower frequency inner ear response of the FLOW 556 Ti is somewhat shared with that of the Velos LBP and PTR VENT 3, as shown below approximately 2,000 Hz. This indicates, with a high degree of confidence, that the weapon operator will perceive the CAT WB to exhibit a less “boomy” signature with this weapon system than many of the other silencers, on average.

Fig 7a. Comparison of Average Human Inner Ear Response - 5.56 MK18 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.

At frequencies above 2,500 Hz, operators of the MK18 may start to be subjected to more severe inner ear response with the CAT WB and PTR VENT 3 than they will with the FLOW 556K and FLOW 556 Ti. The FLOW 762 Ti may also induce less inner ear response. However, moving to higher frequencies above 4,000 Hz, the Flow 762 Ti response is higher than that from the CAT WB. This comparison is best viewed on the linear scale above; the FLOW 762 Ti has a high frequency bias when compared to the CAT WB at the shooter’s ear, on average. This conclusion may be counterintuitive to the reader, based upon the more severe lower frequency response of the FLOW 762 Ti below 2,500 Hz. This outlier case is most likely due to over-bore. The inner ear response of the CAT WB and FLOW 762 Ti leap frog at three distinct frequency regions (below 2,500 Hz, between 2,500 Hz and 4,000 Hz, and above 4,000 Hz).  This is one example of why subjective shooter impressions can vary significantly between individuals.  Even if users have identical weapon systems and environments, any deviation from the 95th percentile human inner ear can skew perception outcomes from the analyzed case.

Interestingly, on average, the compact FLOW 556K and full-size FLOW 556 Ti have very similar peak severities. This is reflected in their similar shooter’s ear Suppression Ratings on this host weapon system. However, the low frequency bias of the FLOW 556K still exists; this phenomenon occurs from increased particle velocity and is reflected below 2,500 Hz in the analysis. It is extremely likely that the weapon operator will perceive the FLOW 556K as more “boomy” than the FLOW 556 Ti, and this is independent of the muzzle Suppression Rating, or any reflecting surfaces. Changing environment from the free field will most likely exacerbate the difference between the two silencers, to the operator.  With the combined low frequency excitation of the operator’s ear, and the severe muzzle signature, the HUXWRX FLOW 556K may be perceived by some individuals to be the “loudest” silencer of the group.

It is likely that, on average, both the Velos LBP and VENT 3 present as more “boomy” than the CAT WB, to the operator in the free field. High frequency hearing loss may lead an operator to interpret the CAT WB as the quietest silencer on average.  This is, again, due to the exaggerated low frequency excitation from the full-size flow silencers.

Fig 7b. Comparison of Average Low Frequency Human Inner Ear Response - 5.56 MK18 at the Ear (Log Scale)

Fig 7c. Comparison of Average High Frequency Human Inner Ear Response - 5.56 MK18 at the Ear (Log Scale)

6.169.5 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 HUXWRX FLOW 556 Ti is postulated to sound the quietest, with the SilencerCo Velos LBP being the loudest, during the first shot.

During the first shot, bystanders will most likely perceive many of these low back pressure silencers to be “boomy,” and the CAT WB is expected to sound the least “boomy” with a “higher tone” than the FLOW 556 Ti. All other silencers shown are expected to be “louder” than those two silencers, during the first shot, to bystanders in the free field.  Bystanders with high frequency hearing loss may perceive the sound delta between the CAT WB and FLOW 556 Ti to be greater, during FRP. These same bystanders may perceive the FLOW 556 Ti as more “boomy.”  They may also find the FLOW 556K to be potentially more pleasing than the PTR VENT 3, during the first shot.  The FRP signature environment with high flow rate silencers is exceedingly complex.

On average, the PTR VENT 3 is postulated to sound the quietest to bystanders, with the FLOW 556K and Velos LBP being the loudest silencers.

It is likely that bystanders will perceive both full-size HUXWRX FLOW silencers as more “boomy” than the CAT WB and PTR VENT 3, in the free field, on average. It is likely that the CAT WB will be perceived as being “higher pitched” than the FLOW 556 Ti. All four of those silencers (including the PTR VENT 3) are postulated to sound “better” or “more pleasing” to bystanders than the Velos LBP and Flow 556K, on average.

Shooter Perception:

To personnel firing the weapon, the FLOW 556 Ti is the quietest silencer during the first shot (by a small margin). The Velos LBP is the loudest to the shooter, during the first shot, again by a small margin.

During FRP, the FLOW 556K may still be perceived by the shooter as “boomy” compared to other silencers, but it still produces an objectively less severe inner ear response to the shooter than the Velos LBP.  During FRP, it is likely than many weapon operators will interpret the CAT WB as being quieter and “less boomy” than all of the HUXWRX FLOW silencers on this weapon system, even though the peak inner ear response from the silencers is similar.  The FLOW 762 Ti and FLOW 556 Ti may be indistinguishable to each other by some weapon operators, during FRP, on this weapon system. It is likely that users with high frequency hearing damage will hear them to be almost identical, during the first shot.

On average, to personnel firing the weapon, the FLOW 556 Ti is nominally the quietest to the shooter, with the Velos LBP being the loudest to the shooter.  

It is very likely that the weapon operator will perceive the CAT WB to exhibit a less “boomy” signature with this weapon system than many of the other silencers, on average.  The inner ear response of the CAT WB and FLOW 762 Ti leap frog at three distinct frequency regions.  This is one example of why subjective shooter impressions can vary significantly between individuals.  Even if users have identical weapon systems and environments, any deviation from the 95th percentile human inner ear can skew perception outcomes from the analyzed case.

On average, the compact FLOW 556K and full-size FLOW 556 Ti have very similar peak severities. This is reflected in their similar shooter’s ear Suppression Ratings on this host weapon system. However, it is extremely likely that the weapon operator will perceive the FLOW 556K as more “boomy” than the FLOW 556 Ti, and this is independent of the muzzle Suppression Rating, or any reflecting surfaces. Changing environment from the free field will most likely exacerbate the difference between the two silencers, to the operator.

It is likely that, on average, both the Velos LBP and VENT 3 present as more “boomy” than the CAT WB, to the operator in the free field. High frequency hearing loss may lead an operator to interpret the CAT WB as the quietest silencer on average.

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