SSS.6.42 - Research Supplement: Balancing Back Pressure and Suppression Rating (Members Only)
/Dead Air Sandman-S, CGS Hyperion K, Surefire SOCOM762-RC2, and OSS HX-QD 762 on a .308WIN Bolt Action Rifle with 20-inch Barrel
Four 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 ability to balance sound suppression and flow restriction (back pressure); silencers maximizing their PEW Science Suppression Rating (Figure 1) while minimizing their PEW Science Omega Back Pressure Metric (Figure 2). These silencers all possess characteristics allowing them to keep flow restriction to a minimum while still keeping sound suppression above a minimum threshold.
Fig 1. PEW Science Suppression Rating Scale
This is not an exhaustive list of so-called low back pressure rifle silencers on the current market, but is a relatively varied representation of available current technology in the size and weight regime of interest that also balance sound suppression. The published public Sound Signature Reviews of these four silencers, with supersonic ammunition, are linked below:
Sound suppression and back pressure characteristics of centerfire rifle silencers typically influence their use when fielded on semiautomatic and automatic small arm weapon systems. For many types of autoloading weapons, silencer back pressure performance characteristics are paramount to the user due to the influence of this flow restriction on weapon function. Often times, a host weapon system can be adjusted or tuned for adequate function when additional flow restriction is introduced. However, for some weapon systems, this is either not possible or logistically unacceptable for the operator or armorer.
By their very nature, firearm silencers restrict gas flow to reduce sound signature; the performance trait of sound suppression being a significant performance factor. Though not the most important performance characteristic for all operators, sound suppression performance is desired by many. Purposely and effectively balancing gas flow restriction and sound suppression, by employing a variety of energy dissipating techniques within the silencer body, is relatively uncommon in today’s centerfire rifle silencer market.
Fig 2. PEW Science Silencer Relative Back Pressure Metric, Omega, For 30 Caliber Rifle Silencers
To further complicate this balanced performance requirement, the systems must also be able to exhibit adequate sound signature suppression of the first shot during many engagements. FRP is therefore also a metric of interest. Minimizing a drop in flow velocity to create less flow restriction, while still minimizing the propagation of ancillary FRP combustion, is a challenge.
Section 6.42.1 provides an overall sound suppression performance summary at the muzzle and shooter’s ear.
Section 6.42.2 provides detailed comparisons of the sound signatures measured at the muzzle.
Section 6.42.3 provides detailed comparisons of the sound signature measured at the shooter’s ear.
Section 6.42.4 presents some concluding thoughts.
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 reviews and to help frame objective loudness comparisons between the four low back pressure silencers in the FRP and total sound signature suppression regime. This supplement is part of ongoing PEW Science small arm weapon system sound signature research.
6.42.1 Overall Sound Suppression Performance Summary
The perception of a “lower tone” of the OSS HX-QD 762 sound signature is likely, for both bystanders and the shooter, on this weapon platform. The similarity in sound signature of the OSS and CGS silencers to bystanders is likely, on average, with similarity in sound signature of the Surefire and CGS silencer to shooters being likely, on average. During the first shot, and on average, it is likely that the Sandman-S sounds the loudest to shooters and bystanders, unless they have significant pre-existing high frequency hearing damage.
The OSS silencer is very efficient at managing relative FRP signature. This is postulated to be due to the primary mechanism of suppression being energy loss during flow rather than flow restriction. Ancillary combustion will induce additional flow loss; its suppression mechanism may have higher suppression efficiency with flow rate, in contrast with the flow restriction mechanisms of the other silencers.
The CGS silencer is very efficient at managing sound signature for its size; its Suppression Rating is relatively high for both its size and lack of flow restriction. This combination of performance is atypical.
The Surefire silencer exhibits low flow restriction for its size; silencers of its size typically have relatively high back pressure. It is able to balance Suppression Rating well.
The Dead Air silencer balances flow restriction and sound suppression with a larger orifice; the venting schemes of the other silencers are more advanced and efficient than this method, as highlighted by the more severe sound signature of the Sandman-S.
In the low back pressure silencer flow regime, relative FRP performance does not correlate with overall Suppression Rating. Normalized FRP performance is compared in Figure 3. Note that the maximum human inner ear damage potential of the silencer with the loudest measured FRP (the Sandman-S) is used in the normalization. This allows strict relative comparison of all the silencers shown. Absolute average sound signature and back pressure performance is compared in Figure 4.
The data should be viewed with the following muzzle behavior notes in mind:
The Surefire SOCOM762-RC2 has the highest average muzzle Suppression Rating (27.7) but has louder muzzle FRP than the OSS HX-QD (average muzzle SR = 27.1) and CGS Hyperion K (average muzzle SR = 22.5).
The Dead Air Sandman-S has the loudest muzzle FRP of all the silencers shown, and also the lowest average muzzle Suppression Rating (18.6).
At the Shooter’s Ear, the following notes apply:
The FRP signature of the OSS HX-QD and the CGS Hyperion K are very similar in relative severity; this is despite the Hyperion K having a higher average ear Suppression Rating (37.1) than the HX-QD ( 34.0).
Once again, the Dead Air Sandman-S has the loudest ear FRP of all the silencers shown, and also the lowest average ear Suppression Rating (32.3). The Surefire SOCOM762-RC2 has both the quietest FRP at the shooter’s ear, and the highest average ear Suppression Rating (37.8).
The OSS silencer is very efficient at managing relative FRP signature. This is postulated to be due to the primary mechanism of suppression being energy loss during flow rather than flow restriction. Ancillary combustion will induce additional flow loss; its suppression mechanism that may have higher suppression efficiency with flow rate, in contrast with the flow restriction mechanisms of the other silencers.
The CGS silencer is very efficient at managing sound signature for its size; its Suppression Rating is relatively high for both its size and lack of flow restriction. This combination of performance is atypical.
The Surefire silencer exhibits low flow restriction for its size; silencers of its size typically have relatively high back pressure. It is able to balance Suppression Rating well.
The Dead Air silencer balances flow restriction and sound suppression with a larger orifice; the venting schemes of the other silencers are more advanced and efficient than this method, as highlighted by the more severe sound signature of the Sandman-S.
As noted, the relative the Muzzle and Ear 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 with the Sandman-S at the muzzle and ear being different (as with any silencer). Absolute Suppression Rating can be compared in Figure 4.
Fig 3. Normalized FRP Performance Summary (Supersonic .308 Bolt-Action 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.
Figure 4, below, is reproduced for reference from the OSS HX-QD 762 Sound Signature Review; the latest review publication at the time of this Research Supplement publication. The silencers of interest in this article are highlighted in red.
Fig 4. Omega Back Pressure Metric And Suppression Rating Comparisons Of .308 Rifle Silencers Using PEW-SOFT Data And Analysis
The following subsections of this Research Supplement present in-depth comparisons of human inner-ear modeling response to the FRP and average sound signatures.
6.42.2 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 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.
6.42.2.1 FRP Muzzle Comparisons
Figure 5 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 5a shows the response on the vertical axis with a linear scale. Fig 5b and Fig 5c 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 OSS HX-QD 762 is postulated to sound the quietest, with the Dead Air Sandman-S being the loudest.
Note that In the low frequency response regime, the Surefire SOCOM762-RC2 signature is less intense, while the OSS HX-QD 762 seems to excite the ear with significantly higher response, resulting in a possibly “lower tone” perception to a bystander.
In the mid to high frequency regimes, where the human inner ear exhibits the greatest sensitivity, the HX-QD is the quietest at the muzzle during the first shot. The CGS Hyperion K is measured to be louder than the Sandman-S in the low frequency range during the first shot, but being quieter in the mid-range. Not only is the Sandman-S the loudest silencer during FRP, but it also has high frequency bias compared to all of the other silencers, except for the SOCOM762-RC2 at the highest frequency.
Fig 5a. Comparison of FRP Human Inner Ear Response - .308 at the Muzzle (Linear Scale)
Fig 5b. Comparison of FRP Low Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
Fig 5c. Comparison of FRP High Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
PEW Science Research Note: It is postulated that the first shot with the OSS HX-QD 762 will most likely be perceived to be less severe to bystanders, with a very noticeable deep tone. The quietest FRP muzzle signature, with the highest flow rate, indicates an efficiency in its energy-loss centric design. Bystanders with high frequency hearing loss may perceive the sound delta between the Sandman-S and the other silencers to be less severe during FRP.
Fig 6a. Comparison of Average Human Inner Ear Response - .308 at the Muzzle (Linear Scale)
6.42.2.2 Average Muzzle Comparisons
Figure 6 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 6a shows the response on the vertical axis with a linear scale. Fig 6b and Fig 6c 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 SOCOM762-RC2 is postulated to sound the quietest on average, with the Sandman-S being the loudest being the loudest on average.
Note that In the low frequency response regime, again, the average ear response from the muzzle signature of the HX-QD 762 is exaggerated, with the SOCOM762 signature being less severe. The lower tone of the OSS silencer is again highlighted, this time in average signature data at the muzzle.
In the low frequency hearing response regime, the Hyperion K exhibits higher amplitude than the Sandman-S; but in the mid to high frequency hearing response range, where the human ear is most sensitive, the Sandman-S is again the loudest silencer by at least 25 percent, on average.
Fig 6b. Comparison of Average Low Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
Fig 6c. Comparison of Average High Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
6.42.3 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.
6.42.3.1 FRP Ear Comparisons
Figure 7 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 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, the SOCOM762-RC2 is the quietest silencer during the first shot. The HX-QD 762 exhibits quieter FRP performance at the shooter’s ear than the Hyperion K. The Sandman-S is the loudest to the shooter, by a significant margin.
The OSS silencer has a low-frequency bias at the shooter’s ear when compared to all of the other silencers (similar to how it did at the muzzle) and the Hyperion K exhibits a very similar hearing response severity to the OSS silencer in the regime of the greatest hearing sensitivity. These inner ear frequency response components may indicate that the OSS silencer may sound louder to shooters of the weapon on the first shot, although their inner ear response is less severe or on-par with that in response to the other silencers. Regardless of hearing sensitivity, the Sandman-S is postulated to be the loudest to the shooter during the first shot of the weapon.
Fig 7a. Comparison of FRP Human Inner Ear Response - .308 at the Ear (Linear Scale)
Fig 7b. Comparison of FRP Low Frequency Human Inner Ear Response - .308 at the Ear (Log Scale)
Fig 7c. Comparison of FRP High Frequency Human Inner Ear Response - .308 at the Ear (Log Scale)
PEW Science Research Note: Shooters exhibiting some high frequency hearing loss may perceive the Sandman-S to be closer in sound signature amplitude to the OSS HX-QD than shooter’s with healthy ears; regardless, the hearing damage potential of the Sandman-S is higher. During the first shot, almost all shooters will observe the OSS silencer to have a so-called deep tone. Individual hearing sensitivity differing from 95th percentile human physiology may skew the postulated perception outcomes. Again, the efficient FRP signature of the OSS is highlighted, despite high flow rate.
6.42.3.2 Average Ear Comparisons
Fig 8a. Comparison of Average Human Inner Ear Response - .308 at the Ear (Linear Scale)
Figure 8 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 8a shows the response on a linear vertical scale. Fig 8b and Fig 8c 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 SOCOM762-RC2 is the quietest silencer, on average, with the Hyperion K being similar to the shooter in the region of peak hearing sensitivity.
On average, the OSS silencer still induces the highest low frequency hearing response, and the Sandman-S is still louder overall. However, on average, the hearing response of the shooter from both those silencers is closer than observed during FRP.
Where the ear is most sensitive, the data indicates that the Surefire and CGS silencers will sound very similar to the shooter; due to the higher low frequency hearing excitation of the CGS Hyperion K, some shooters may perceive it has louder than the SOCOM762-RC2, but with a slightly deeper tone.
Fig 8b. Comparison of Average Low Frequency Human Inner Ear Response - .308 at the Ear (Log Scale)
Fig 8c. Comparison of Average High Frequency Human Inner Ear Response - .308 at the Ear (Log Scale)
PEW Science Research Note: Shooters exhibiting some high frequency hearing loss may perceive the Sandman-S, again, to sound more similar to the Hyperion K or even the OSS silencer. However, the majority of shooters, on average, will perceive the Sandman-S to be the loudest silencer.
6.42.4 Research Supplement Summary
The perception of a “lower tone” of the OSS HX-QD 762 sound signature is likely, for both bystanders and the shooter, on this weapon platform. The similarity in sound signature of the OSS and CGS silencers to bystanders is likely, on average, with similarity in sound signature of the Surefire and CGS silencer to shooters being likely, on average. During the first shot, and on average, it is likely that the Sandman-S sounds the loudest to shooters and bystanders, unless they have significant pre-existing high frequency hearing damage.
The OSS silencer is very efficient at managing FRP signature. This is postulated to be due to the primary mechanism of suppression being energy loss during flow rather than flow restriction. Ancillary combustion will induce additional flow loss; its suppression mechanism may have higher suppression efficiency with flow rate, in contrast with the flow restriction mechanisms of the other silencers.
The CGS silencer is very efficient at managing sound signature for its size; its Suppression Rating is relatively high for both its size and lack of flow restriction. This combination of performance is atypical.
The Surefire silencer exhibits low flow restriction for its size; silencers of its size typically have relatively high back pressure. It is able to balance Suppression Rating well.
The Dead Air silencer balances flow restriction and sound suppression with a larger orifice; the venting schemes of the other silencers are more advanced and efficient than this method, as highlighted by the more severe sound signature of the Sandman-S.
This supplement is part of ongoing PEW Science small arm weapon system sound signature research. PEW Science thanks you for your support.
