Noise level and what causes it

[_walter_]

I’m just trying to understand.. is it the velocity that makes the report loud or is it energy?

For example 177 10gr@1000fps vs 22 15gr@800fps if both are the same approx 22ft/lb, which is louder?
Or say 22 16gr @890 vs 22 25gr @700 coming out the same gun/settings

 

[N2 Shooter]

Let's assume you are talking about projectiles that are traveling less than the speed of sound, so we can remove the supersonic crack from the analysis.

It takes more air to move a heavier projectile, and that will add to the noise. Additionally, if using slugs, the contact area of the projectile to the barrel will require more air than a pellet of similar weight in order to achieve the same velocity as the pellet, and therefore, it will be louder.

 

[nervoustrig]

The simplest answer is the residual pressure at the muzzle dictates the report.

A few useful relationships to bear in mind:

• For a given energy level (fpe), the smaller caliber will be louder. It is more difficult to accelerate the smaller caliber because its base has a smaller surface area against which the air pressure can exert its force.
• Likewise, for a given energy level, a tune which uses high pressure and short valve dwell will be quieter. In other words a very short blip (read: small volume) of high pressure air produces a relatively large amount of acceleration early on, and continues to expand and accelerate the projectile down the barrel. By the time the projectile reaches the muzzle, the initial high pressure has decayed quite a bit, and therefore produces less noise. In contrast, an equivalent tune using low pressure + long dwell tends to use more air. Note there are tradeoffs associated with manipulating dwell that I talked about a little here https://www.airgunnation.com/threads/how-do-i-tune-the-fx-impact-to-be-quieter.972959/#post-974139
• This next item is probably intuitively obvious, that a longer barrel is quieter. And the reason is fundamentally because of the previous point...the air has a longer runway upon which to push on the projectile, allowing the pressure to decay to a lower value by the time the projectile emerges from the muzzle.
• Lastly, in the same caliber, the heavier pellet will achieve the same energy with a quieter report. It will of course achieve this equal energy at a lesser muzzle velocity.

Granted, these are all generalizations that assume other variables are held relatively constant. For example, high pressure and short dwell paired with a short barrel will be loud. And as @N2 Shooter said, a slug has a lot more friction in the barrel than a pellet, along with usually also being heavier so it needs more pressure and more valve dwell to get it up to a "useful" velocity. It's all about balancing tradeoffs for whatever goals one is trying to reach. I hope this helps.

 

[Motorhead]

Jason,
Very well stated

Also a point to be made, PCP's that get tuned for minimal " Realistic" dwell at a given power, will struggle to make more power with increases in HST, tho will shoot at reduced power near equally as well.
This is why we see many PCP's coming in a factory state of tune to be biased on the longer less efficient dwell so that a broader adjustable power range is available.

No free lunch .. always a compromise somewhere.

 

[jmohme]

Compressed air released into the atmosphere and instantly expanding 200 or more times its compressed volume.
Thats one.

 

[JimD]

I agree that Jason's explanation is great but doesn't that mean that the current trend to really large plenums mean the affected guns will be louder? Instead of turning up the moderator we are increasing the size of the plenum so we can use a greater quantity of lower pressure air. That seems to suggest the gun will be louder with the big plenum.

 

[bigHUN]

It is funy you have mentioned the larger plenum - makes a louder shot.
To explain: Over the winter I replaced the Impact parts with the "power upgrade" kit for MK2 (powerblock + 720c plenum + heavy HS and VS + tungsten 14 gr weight).
Yesterday only I had a first chance this year to get to my gunclub.
I started with getting on paper followed by a liner indexing.
Right beside me were couple rimfire guys, and everybody was surprised my airgun was the most loudest from all. (25x700 Superior Heavy liner)
The Reg was still at last year tune at 135 bars, I had to lower it to 115 bars to get a same speed at 915 (JSB 34gn MK2) where I zeroed my skope at 50 meters (VS stil at bare Min).
A whole day and night I am tinkering where to start next... I would tune the pellets for 100 meters with in mind for slugs next to 200 meters. This bigger plenum gives me surprising amount of power with a Reg 20 bars lower...
I am speculating only but I must increase the Reg gradually somewhere to 130-140 bars but with tightening the VS in same time. Seem to me way too high power curve from a larger plenum...
Btw, I don't care how loud it is for me, I am shooting in my gunclub only...