SSS.6.56 - Research Supplement: The Quietest 308 Rifle Silencers - Detailed Sound Comparison (Members Only)
/Thunder Beast ULTRA 9, Otter Creek Labs PR30L, CGS Hyperion, and Dead Air Nomad-L 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 high sound suppression performance; silencers maximizing their PEW Science Suppression Rating (Figure 1) on a supersonic centerfire bolt action rifle. Silencers with suppression performance in this range typically have relatively high flow restriction, or back pressure, characterized by a relatively high PEW Science Omega Back Pressure Metric (Figure 2). Suppression Rating Rankings can be found in Section 7 of the Standard.
Fig 1. PEW Science Suppression Rating Scale
The below is not an exhaustive list of so-called high performance 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 possess extreme sound signature suppression. The published Sound Signature Reviews of these four silencers, with supersonic ammunition, are linked below:
Sound suppression performance of centerfire rifle silencers, particularly during the first shot, is of significant concern to many weapon system operators. High sound signature amplitude during the first shot from a suppressed weapon system is referred to as First Round Pop, or 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. PEW Science Silencer Relative Back Pressure Metric, Omega, For 30 Caliber Rifle Silencers
Typically, silencers possessing a high Omega Metric exhibit high sound suppression performance, but there are several significant exceptions. Although gas flow rate reduction (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. 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 6.56.1 provides an overall sound suppression performance summary at the muzzle and shooter’s ear.
Section 6.56.2 provides detailed comparisons of the sound signatures measured at the muzzle.
Section 6.56.3 provides detailed comparisons of the sound signature measured at the shooter’s ear.
Section 6.56.4 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 quietest supersonic 308 rifle silencers in the FRP and total sound signature suppression regimes. This supplement is part of ongoing PEW Science small arm weapon system sound signature research.
6.56.1 Overall Sound Suppression Performance Summary
Bystander Perception:
To personnel located 1.0 m left of the weapon muzzle, the CGS Hyperion is postulated to sound the quietest, with the Thunder Beast ULTRA 9 being the loudest.
Bystanders with hearing loss above the 3,000 Hz range may perceive the sound delta between the ULTRA 9 and the PR30L to be less severe during FRP. These same bystanders may also perceive the Nomad-L to be quieter than the Hyperion during FRP.
Bystanders with hearing damage around 4,000 Hz may find the Nomad-L and Hyperion to sound almost identical in overall loudness, on average. On average, the signature of the PR30L is approximately 26% less severe than the ULTRA 9, to bystanders. The Nomad-L and Hyperion signatures are approximately 67% less severe than the ULTRA 9 to bystanders, on average.
Shooter Perception:
It is likely that the Hyperion is perceived as quieter than the Nomad-L to the shooter during FRP, particularly due to the low frequency bias over the Nomad-L and the Nomad-L high frequency bias over the Hyperion. Regardless of hearing sensitivity, the ULTRA 9 is postulated to be the loudest to the shooter during the first shot of the weapon. The ULTRA 9 is shown to sound harsher.
Shooters exhibiting some high frequency hearing loss may perceive the Nomad-L to be closer in sound signature amplitude to the Hyperion than shooters with healthy ears; regardless, the hearing damage potential of the Nomad-L is higher during FRP. During the first shot, almost all shooters will observe the Otter Creek Labs silencer to have a so-called “deep tone” and the Thunder Beast silencer to be louder or “harsher.” Individual hearing sensitivity differing from 95th percentile human physiology may skew the postulated perception outcomes.
Where the ear is most sensitive, the data indicates that the Dead Air and CGS silencers will sound very similar to the shooter, on average. However, the zones of inner ear sensitivity cross at multiple frequencies, and indicate that the Nomad-L has a high frequency bias compared to that of the Hyperion; the Hyperion is postulated to have a more pleasing tone to the majority of shooters, on average.
The majority of shooters, on average, will perceive the Hyperion to be the quietest silencer, the ULTRA 9 to be the loudest silencer, and the PR30L to have a pronounced deep tone.
In the high back pressure silencer flow regime, relative FRP performance correlates strongly with overall composite Suppression Rating. This in contrast with its lack of correlation in the low back pressure silencer flow regime (see previous Research Supplement 6.42). Normalized FRP performance is compared in Figure 3. Despite the composite Suppression Rating correlation, there are outliers in the detailed Suppression Rating correlation with FRP. Note that the maximum human inner ear damage potential of the silencer with the loudest measured FRP (the Thunder Beast ULTRA 9) is used in the normalization. This allows strict relative comparison of all the silencers shown.
The data should be viewed with the following muzzle behavior notes in mind:
The Dead Air Nomad-L has the highest average muzzle Suppression Rating (41.9) but has louder muzzle FRP than the CGS Hyperion (average muzzle SR = 41.7). This is the sole outlier in this data subset.
The Thunder Beast ULTRA 9 has the loudest muzzle FRP of all the silencers shown, and also the lowest average muzzle Suppression Rating (30.7).
At the Shooter’s Ear, the following notes apply:
The FRP signature of the Otter Creek Labs PR30L and the Thunder Beast ULTRA 9 are very similar in relative severity; this is in keeping with the similar average at-ear Suppression Ratings of the two silencers (nominally 45.9).
The FRP signature of the CGS Hyperion is the quietest of the group; this is in keeping with its at-ear Suppression Rating (51.7) compared to that from the Dead Air Nomad-L (51.3).
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 ULTRA 9 at the muzzle and ear being different (as with any silencer).
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.
The following subsections of this Research Supplement present in-depth comparisons of human inner-ear modeling response to the FRP and average sound signatures.
6.56.2 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.
6.56.2.1 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 CGS Hyperion is postulated to sound the quietest, with the Thunder Beast ULTRA 9 being the loudest.
Note that in the very low frequency response regime, the Otter Creek Labs PR30L signature is less intense than that of the ULTRA 9, while the ULTRA 9 seems to excite the ear with higher response, resulting in a possibly “lower tone” perception to a bystander, when comparing the two silencers during FRP. This trend reverses at a hearing sensitivity of 2,400 Hz. When examining the CGS Hyperion and Dead Air Nomad-L low frequency hearing response regime, the Hyperion is shown to result in a possibly lower tone to bystanders than the Nomad-L; the Hyperion excites the inner ear more than the Nomad-L (is louder) in the low frequency range. The PR30L and ULTRA 9 are both noticeably louder than the Hyperion and Nomad-L during FRP, across all frequency sensitivities of the human inner ear.
In the mid to high frequency regimes, where the human inner ear exhibits the greatest sensitivity, the Hyperion is the quietest at the muzzle during the first shot (the Nomad-L and Hyperion change relative loudness at an ear sensitivity of approximately 2,300 Hz). The PR30L and the ULTRA 9 change relative loudness at an ear sensitivity of approximately 2,500 Hz, and again at 3,700 Hz; therefore, the PR30L and ULTRA 9 may sound very similar to bystanders during FRP, but there is a higher frequency inner ear response bias in the ULTRA 9 signature that may be noticeable to some bystanders. When combined with its aforementioned low frequency bias, it is likely the ULTRA 9 will be perceived as the loudest silencer to bystanders, in the group.
Fig 4a. Comparison of FRP Human Inner Ear Response - .308 at the Muzzle (Linear Scale)
Fig 4b. Comparison of FRP Low Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
Fig 4c. Comparison of FRP High Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
PEW Science Research Note: Bystanders with hearing loss above the 3,000 Hz range may perceive the sound delta between the ULTRA 9 and the PR30L to be less severe during FRP. These same bystanders may also perceive the Nomad-L to be quieter than the Hyperion during FRP.
Fig 5a. Comparison of Average Human Inner Ear Response - .308 at the Muzzle (Linear Scale)
6.56.2.2 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 Nomad-L is postulated to sound the quietest on average, with the ULTRA 9 being the loudest on average.
Note that In the midrange response regime, the Hyperion and Nomad-L curves cross twice (at 2,700 Hz and 3,700 Hz). This highlights the similar signatures of the two silencers, to bystanders (on average). Bystanders with hearing damage around 4,000 Hz may find the Nomad-L and Hyperion to sound almost identical in overall loudness.
On average, the PR30L signature is approximately 26% less severe than the ULTRA 9 signature, to bystanders. The Nomad-L and Hyperion signatures are approximately 67% less severe than the ULTRA 9 signature to bystanders, on average.
Fig 5b. Comparison of Average Low Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
Fig 5c. Comparison of Average High Frequency Human Inner Ear Response - .308 at the Muzzle (Log Scale)
6.56.3 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.
6.56.3.1 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 Hyperion is the quietest silencer during the first shot. The ULTRA 9 is the loudest to the shooter.
Between 1,500 Hz and 1,700 Hz, the PR30L silencer begins to excite the human ear more severely; this trend continues until approximately 4,000 Hz. This indicates that during FRP, the PR30L may sound significantly deeper in signature, to the shooter, than the ULTRA 9. The PR30L may also sound deeper than the Hyperion and Nomad-L depending on the hearing health of the shooter.
The Hyperion excites the inner ear more than the other silencers in a narrow range of 1,100 Hz to 1,700 Hz; the region where the Nomad-L is the quietest during FRP. At 2,800 Hz, the Nomad-L gets louder than the Hyperion. These zones of inner ear response frequency overlap represent the complex differences in the perception of the signatures to the shooter. It is likely that the Hyperion is perceived as quieter to the shooter during FRP, particularly due to the low frequency bias over the Nomad-L and the Nomad-L high frequency bias over the Hyperion.
Regardless of hearing sensitivity, the ULTRA 9 is postulated to be the loudest to the shooter during the first shot of the weapon. The ULTRA 9 is shown to sound harsher.
Fig 6a. Comparison of FRP Human Inner Ear Response - .308 at the Ear (Linear Scale)
Fig 6b. Comparison of FRP Low Frequency Human Inner Ear Response - .308 at the Ear (Log Scale)
Fig 6c. 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 Nomad-L to be closer in sound signature amplitude to the Hyperion than shooters with healthy ears; regardless, the hearing damage potential of the Nomad-L is higher. During the first shot, almost all shooters will observe the Otter Creek Labs silencer to have a so-called deep tone and the Thunder Beast silencer to be louder or “harsher.” Individual hearing sensitivity differing from 95th percentile human physiology may skew the postulated perception outcomes.
6.56.3.2 Average Ear Comparisons
Fig 7a. Comparison of Average Human Inner Ear Response - .308 at the Ear (Linear Scale)
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, the Hyperion is again the quietest silencer, on average, with the Nomad-L being similar to the shooter, albeit with a higher frequency (harsher) bias.
On average, the ULTRA 9 silencer still induces the highest hearing response, and the PR30L is still biased to the lower frequencies with a deep tone. 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 Dead Air and CGS silencers will sound very similar to the shooter. However, again, the zones of inner ear sensitivity cross at multiple frequencies, and indicate that the Nomad-L has a high frequency bias compared to that of the Hyperion; the Hyperion is postulated to have a more pleasing tone to the majority of shooters, on average.
Fig 7b. Comparison of Average Low Frequency Human Inner Ear Response - .308 at the Ear (Log Scale)
Fig 7c. 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 Nomad-L, again, to sound more similar to the Hyperion silencer. However, the majority of shooters, on average, will perceive the Hyperion to be the quietest silencer, the ULTRA 9 to be the loudest silencer, and the PR30L to have a pronounced deep tone.
6.56.4 Research Supplement Summary
Bystander Perception:
To personnel located 1.0 m left of the weapon muzzle, the CGS Hyperion is postulated to sound the quietest, with the Thunder Beast ULTRA 9 being the loudest.
Bystanders with hearing loss above the 3,000 Hz range may perceive the sound delta between the ULTRA 9 and the PR30L to be less severe during FRP. These same bystanders may also perceive the Nomad-L to be quieter than the Hyperion during FRP.
Bystanders with hearing damage around 4,000 Hz may find the Nomad-L and Hyperion to sound almost identical in overall loudness, on average. On average, the signature of the PR30L is approximately 26% less severe than the ULTRA 9, to bystanders. The Nomad-L and Hyperion signatures are approximately 67% less severe than the ULTRA 9 to bystanders, on average.
Shooter Perception:
It is likely that the Hyperion is perceived as quieter than the Nomad-L to the shooter during FRP, particularly due to the low frequency bias over the Nomad-L and the Nomad-L high frequency bias over the Hyperion. Regardless of hearing sensitivity, the ULTRA 9 is postulated to be the loudest to the shooter during the first shot of the weapon. The ULTRA 9 is shown to sound harsher.
Shooters exhibiting some high frequency hearing loss may perceive the Nomad-L to be closer in sound signature amplitude to the Hyperion than shooters with healthy ears; regardless, the hearing damage potential of the Nomad-L is higher during FRP. During the first shot, almost all shooters will observe the Otter Creek Labs silencer to have a so-called “deep tone” and the Thunder Beast silencer to be louder or “harsher.” Individual hearing sensitivity differing from 95th percentile human physiology may skew the postulated perception outcomes.
Where the ear is most sensitive, the data indicates that the Dead Air and CGS silencers will sound very similar to the shooter, on average. However, the zones of inner ear sensitivity cross at multiple frequencies, and indicate that the Nomad-L has a high frequency bias compared to that of the Hyperion; the Hyperion is postulated to have a more pleasing tone to the majority of shooters, on average.
The majority of shooters, on average, will perceive the Hyperion to be the quietest silencer, the ULTRA 9 to be the loudest silencer, and the PR30L to have a pronounced deep tone.
This supplement is part of ongoing PEW Science small arm weapon system sound signature research. PEW Science thanks you for your support.
