Moderator Configuration & Application Optimization

So you have an airgun, and you want a moderator for it to muzzle the bark, but you’re not sure which one to choose. How do you optimize for this, how do you get a moderator tuned for your rifle? The purpose of this article is to inform that choice using a formula for your rifle’s “flow factor.” You should then select a moderator as similar as possible to your own rifle’s flow factor.

Calculating Flow Factor

The equation at right is derived from a larger formula based on the ideal gas law. It allows us to relatively easily put a number to what sort of air is being served up by the rifle with every shot. Units must be maintained for this to work, pressures are in BAR and tank volume in cc (cubic centimeters). Shots taken is, of course, the number of shots required to produce the pressure delta above (tank start and end pressure).

We have a form you can use to calculate the Flow Factor by just entering in the necessary variables here.

Click this link to access a simple spreadsheet calculator for flow factor.

But there is one thing missing, an absolutely critical thing, to determining what sort of air is being served by the rifle to the moderator: the shroud. This is where the shroud factor comes into play. If you have an efficient and reasonably large shroud (the FX Crown shroud is the standard here, having excellent flow into the shroud volume before the moderator, and is 28mm OD), your shroud factor is 1. But what if you don’t have a shroud, and are threading the moderator directly onto the barrel? That’d be a shroud factor of 6. Why 6? Well it has to do with the relative volume of the FX Crown shroud and the gas-diode sections of these moderators. (This is backed by testing performed on unshrouded rifles, yes shrouds are that important and work that well.)

What if you have a shroud but it isn’t as good as a Crown’s? If you have a shroud which is small or inefficient (smaller diameter, or air can enter the shroud but preferentially is directed into the moderator) the shroud factor needs to be greater than one, but in most cases less than 3. If you’re not sure and/or your shroud is a little small, go for somewhere between 1.2 and 2. Very small and full of barrel, try somewhere between 2 and 3. Keep in mind even small shrouds can produce substantial improvements, so don’t discount them. Conversely, an even larger and more efficient shroud could potentially result in a flow factor of less than one, such as a design where there is a moderator/baffles already in the shroud (this requires further testing). And those improvements are seen in the final sound attenuation as well by the way; guns with worse shrouds or no shrouds will simply be a lot louder all else being equal. Yes you read that right, this, to a limited extent, is also a predictive model of airgun loudness.

Applied to the three test hosts, the formula produces the following:
Standard Flow - FX Crown .22 producing 32 foot pounds. Factory configuration w/ extended shroud acoustic peak is 288.0. Flow Factor 460.
Moderate Flow - FX Crown .30 producing 80 foot pounds. Factory configuration w/ extended shroud acoustic peak is 637.6. Flow Factor 1,111.
High Flow - FX Dreamline .30 producing 87 foot pounds. Bare muzzle configuration (un-moderated) acoustic peak is 3,668.4. Flow Factor 6,500.

If you have a rifle significantly outside this performance range, you might want to get in touch before picking up a moderator as it may not be suitable for your application.

A Little Background on Flow Factor

While flow factor is superficially a question of what gun the moderator was tuned on, some of it comes down to an internal design ratio; given space constraints, you’re forced to split a design between elements which delay the flow of air and ones which absorb sound. The important difference is that the former acts to reduce peak sound signature while the latter primarily acts to clean up the noise produced after the peak.

And this post peak sound matters, really matters. The analogy I use to explain it is a hammer striking a nail. A hammer blow is measurably an extremely loud event, but its brevity causes the ear to perceive it as not particularly loud. This is a known acoustic phenomena, and has to do with a muscle reflex inside the year. Were the sound of that hammer blow to be played continuously though, it would be physically debilitating. This is why traces, like the ones shown at left, are provided with every peak measurement: so you can get an idea of the total sound profile rather than just nominal peak.

When it comes to higher airflow designs, a greater ratio of their internal space must be utilized to delay that air to keep it from all immediately rushing out the muzzle. That means there will be more post-peak-noise. This is why you really don’t want the higher flow variants unless you actually need them, because they will not outperform their standard flow counterparts in lower flow applications. The other reason is that gas diodes can be tuned to saturate and perform best at different flow levels. While the intricacies of this are too complex to cover here, once again putting a higher flow tuned gas diode in front of a lower flow gun will not result in optimal sound attenuation.

Take the two traces on the left, noting that they are NOT on the same Y-axis scale. Both produce roughly the same peak, one averaging just shy of 80 and the other just over 90. That is not a huge difference. The difference in perception though is stark, and that mostly comes down to the fact that the moderate flow Pilum produces louder and more sustained sound after the peak.

Further Tricks for Sound Attenuation