PCPs are a blast...

[Phil Russell]

Anyone who has shot, or been close to a pcp being shot without a silencer/moderator will know that the shot is accompanied by a very audible blast. But the loudness of the blast can differ different pcps. I have never fully got to understand why this is but was reminded of it when reading Russ Douglas'review of the Lobo Mustang in AGW in which he warns you that it is LOUD.
But why?
In my simplistic way of thinking, clearly too simplistic, for any calibre and desired mv, the volume of air being expelled depends on the bore and the air released by the valving. Now I can appreciate that the air pressure largely determines the ultimate velocity of the pellet and the length of the barrel influences the velocity as well. So, question: what are the critical factors that determine the loudness of the shot? In a regulated pcp for which we assume the air pressure is, or should be constant for each shot, is the determining factor the barrel length? Is this the same for the 'sweet spot' of an unregulated pcp? If the mv is the same for a regged and an unregged rifle using the same pellets and the barrel length is the same, will the loudness be the same? If so, could the loudness be modified by careful manipulation of both barrel length and air release pressure from the valve?
Any pneumatic airflow experts around who can clarify the clearly complex interactions?

Just wondering over coffee this morning.
Cheers, Phil

[Bellycrawler]

I've only shot a pcp without a sound mod once, an old Daystate Harrier. I forgot my mod when I went ratting, personaly I thought it wasn't that loud, a but like a springer without the twang. I might now be tempted to use an air stripper instead.

[Murphy]

One of the most stupidest things i have ever done with an air rifle was dry fire my brand new UN-silenced AA S410k in the box room of my house with the door shut.

[Phil Russell]

A factor I forgot could be the duration of the released air blast. Does a higher pressure blast but for a shorter time, generate more muzzle noise than a lower pressure for a longer time? And how is this influenced by the 'start pressure' of the pellet in the bore?

Still musing
Cheers, Phil

[Shed tuner]

Valve / hammer bounce is one of the greatest causes of noise / wasted air on PCPs. SSGs on such bouncy guns make them much quieter, as well as increasing shot count.

Heavy hammers designed for FAC, then strangled down to UK power levels with weedy springs are a perfect recipie for this.

Pressure is also a factor in terms of the frequency. This is even noticable on springers - shorter stroke, especially will smaller clylinder diameters, maintain higher pressure, and can give more of a crack. However, due to the lower air volume, they respond better to moderation.

[Shed tuner]

One of the most stupidest things i have ever done with an air rifle was dry fire my brand new UN-silenced AA S410k in the box room of my house with the door shut.

my fac career carbine in .25 nearly lifted the roof off the covered firing area at my club - in my defence I was "asked" to try it

[Frog]

The frequency of the noise is also something to consider. I had a hatsan 55 that I carbined to 13inches. It had a really irritating crack to it - quite loud but also just the right pitch to be annoying to everyone on the firing line - I had to put a mod on it....

[angrybear]

Remember school Physics, the amount of energy that exists is finite, all we can do is make it work by changing it's format, as it works it releases & becomes heat energy, sound energy or light energy. it can also be transferred as kinetic energy.

We take energy in the form of fuel use it to compress gas (air is a gas) we then store that energy inside a cylinder & transfer it to the PCP.
we then make the energy work again by expanding & pushing the pellet, as the air expands (works) it gives up heat energy so cools a bit, much of it transfers to the pellet in the form of kinetic energy which it takes away (12ftlbs of it)
the remaining work energy it gives up is in the form of sound energy.

To accelerate the the pellet the air energy has to work hard to overcome friction & transfer in to the mass of the moving projectile so it has to sacrifice more in the way of sound/heat energy to do that work, and as it's easier to transfer sound than heat it goes for a bigger bang.

The prime requirement of the air rifle is to send the pellet off at speed, so it needs to be pushed right to the end of the barrel and we accept that there will be wasted energy behind it in the form of sound,
the art of tuning is balancing the volume, pressure, timing & transfer to get the most energy in to the pellet for the least waste of air energy.

A loud bang means quite a lot of air is being wasted compared to the efficient transfer to the pellet, often because the rifle is designed for FAC but has been made inefficient to keep the pellet to sub 12.

[Phil Russell]

A loud bang means quite a lot of air is being wasted compared to the efficient transfer to the pellet, often because the rifle is designed for FAC but has been made inefficient to keep the pellet to sub 12.

Yes ... good reasoning.
But: Now i may be wrong but are Steyrs regulated? I thought 'regulation' is meant to only use the minimum air and hence give higher shot count because less air is wasted. yet my experience is that very often a Steyr (don't know the model) is quite loud / very loud.
Does pellet choice influence loudness?
There could be a research project hidden in here somewhere ....

Cheers, Phil

[TonyL]

A loud bang means quite a lot of air is being wasted .

I'm no PCP expert but my thoughts align with this thinking. So a tweaking of the balance between the hammer, spring and any regulator fitted should see an improvement?

[xbow]

Firing a cleaning pellet through a PCP is fun …

[deejayuu]

I assume, probably in error, that for a given amount of pressure/energy being released the barrel length will determine the point on the amplitude of the soundwave when it leaves the barrel.

[Murphy]

Firing a cleaning pellet through a PCP is fun …

They give quite a crack out of a springer.

[robs5230]

Oh Phil . So many random questions and unfortunately, interconnnecting ones at that.

what are the critical factors that determine the loudness of the shot?
The efficiency of air transfer, barrel length, and the combination of the hammer / valve springs, and their specs.

In a regulated pcp for which we assume the air pressure is, or should be constant for each shot, is the determining factor the barrel length?
No. It's one of the factors. All regulated PCP's are not equal

Is this the same for the 'sweet spot' of an unregulated pcp?
No. An unregulated PCP, largely will consume more air per shot, than a regulated PCP. But all regulated PCP's are not equal.

If the mv is the same for a regged and an unregged rifle using the same pellets and the barrel length is the same, will the loudness be the same?
Not necessarily. It depends on the factors in the 1st point.

If so, could the loudness be modified by careful manipulation of both barrel length and air release pressure from the valve?
Not necessarily.

You need to break the questions down further, to get answers which will make more sense.

[robs5230]

Yes ... good reasoning.
But: Now i may be wrong but are Steyrs regulated? I thought 'regulation' is meant to only use the minimum air and hence give higher shot count because less air is wasted. yet my experience is that very often a Steyr (don't know the model) is quite loud / very loud.
Does pellet choice influence loudness?
There could be a research project hidden in here somewhere ....

Cheers, Phil

You're getting there with this one Phil.

Steyrs are indeed regulated, and yes they are loud, but a different sort of loud to some rifles.

I have a couple of "very well set up" regulated Ultras here. These Ultras, shot without a silencer produce a very different sort of "noise" to my Steyr LG110.
The "crack" from the Steyr is higher pitched, cleaner, and of less duration than that of the Ultras.
In this, shot side by side, unsilenced, the Ultra seems noisier.
I'd say the Steyr is actually louder in decibels, but it doesn't seem so.

To put this in perspective, if I shot my Ultra (unslienced) in the field, all the rabbits within about 500 yards would be gone.
The noise from the Steyr, does not have the same effect. As it's higher pitched and "cleaner", it seems to spook the rabbits less. Sort of like they cannot pinpoint where the noise comes from. A bit like a HMR in this respect