Sound level and barrel length.

[bluez]

I have the impression that the shorter the barrel, the louder the blast is? Or is it just that short barrel canons/mortars have a better sound dispersion?

[Artilleryman]

Short barreled, relatively small bore cannon such as a Hughes gun, Williams gun, or Ellsworth seem to have a sharper pitch or crack than large bore long barreled guns such as a 10 pdr Parrott.  I haven't been around the smaller guns that most of the discussion on this board covers in a very long time so I really don't remember exactly what they sounded like.  I do remember a .58 cal. foot long barrel that I made as being very loud.

[bluez]

I discussed the subject again with a very experienced black powder shooter. In his opinion the trick is to ignite all the powder and then let it burn / expand on the outside. If dia on the bore maters or if it's just the amount of powder i still don't know.

[Parrott-Cannon]

There are four contributions to the sound made by cannon:
First is the rapid change in pressure as the gases from the burning of the gunpowder are released to the atmosphere.  Small bore cannons have less powder but releases the pressure in a smaller area and at less pressure than a large bore cannon.  Pressure in large bore cannon can exceed 20000 psi while those in smaller bore cannons rarely exceed 1000 psi.  The bore area of a Napoleon 12 pounder is about 16.9 inches squared while the bore area for a golf ball Parrot Cannon is about 2.2 inches squared.  The shock wave produced in the air as the exit pressure drops to atmospheric is what we sense as noise.  This process is actually more complicated sense the temperatures of the exit gases and the atmosphere also play a role as well as the weight of the gas.
Another cause for noise is when the projectile’s muzzle velocity exceeds the speed of sound.   A Napoleon 12 pounder has a muzzle velocity of about 1400 fps while a golf ball cannon shooting a golf ball has a muzzle velocity of about 650 fps.  The speed of sound is 1,126 ft/s.  Therefore the Napoleon will produce a sonic boom or crack as the shell exit’s the barrel while the golf ball cannon will not.
A third part of the noise equation is dependent on how much of the powder load has been burnt before the projectile exit’s the barrel.  Powder that is not burnt does so as it exit the barrel.  This burning of unused powder causes another pressure wave to form i.e. more noise.  Larger cannons with heavy projectiles and long barrel lengths burn over 90% of the powder load while a golf ball cannon shooting a golf ball only burn about 20% of the powder.
A forth effect is a ringing effect that can is created by the barrel shape and material of construction.  This like a bell and may be because as the projectile bounces down the barrel.