How does a person compute what is subsonic?

[hardcase]

Since I got my .30 cal can a few weeks ago I've been tinkering with sub-sonic loads for my 300 Blackout. I have one of the more expensive infrared chronographs set up in a storage building beside my house about 8-9 ft from the muzzle. I loaded some that clocked at 1,025-1,070 fps. Some of them had a noticeable super-sonic crack. Seems like I heard somewhere where the sound barrier could be computed based on altitude or air density (barometric pressure & temperature) . It would be interesting to know how accurate that formula would be for subsonic rounds. I've loaded some that are in the 940-1,000 fps that are noticeably quieter.

[continuity]

The 1000 ft/sec. point is a practical velocity for subsonic projectiles, relative the nuances of air density that impact a change in actual "sonic velocity" condition... humidity, temp, elevation, being primary actors.

However... a little googlefu for the case of absolutely must know, yielded...


Speed of Sound - Sonic Velocity - in Ideal Gases
Since the acoustic disturbance introduced in a point is very small the heat transfer can be neglected and for gases assumed isentropic. For an isentropic process the ideal gas law can be used and the speed of sound can be expressed as

c = (k p / ρ)1/2

= (k R T)1/2 (3)

where

k = ratio of specific heats (adiabatic index)

p = pressure (Pa, psi)

R = gas constant

T = absolute temperature (oK, oR)

For an ideal gas the speed of sound is proportional to the square root of the absolute temperature.

Example - Speed of Sound in Air
The speed of sound in air at 0 oC and absolute pressure 1 bar can be calculated as

c = (1.4 (287 J/K kg) (273 K))1/2

= 331.2 (m/s)

where

k = 1.4

and

R = 287 (J/K kg)

The speed of sound in air at 20 oC and absolute pressure 1 bar can be calculated as

c = (1.4 (287 J/K kg) (293 K))1/2

= 343.1 (m/s)

[doubloon]

... I've loaded some that are in the 940-1,000 fps that are noticeably quieter.

Excellent answer on the computation I would only add this and say relative to the shooter most of the difference in noise you are hearing is due to residual powder burn and not necessarily bullet flight which is like more of a downrange phenomenon given the proximity of the shooter to the initial expansion of gas.

http://www.silencertalk.com/forum3/view ... 084#p15084

[whiterussian1974]

... I've loaded some that are in the 940-1,000 fps that are noticeably quieter.

http://guns.connect.fi/ is a great resource. Doesn't cover competing baffle designs, but plenty of Theory and Data. Plus Legal/Health data that should be used to challege anti-can laws.

Around 985fps max is a good goal. Sound actually gathers NEAR the SoS. The tip collects and reflects sound waves and high pressure ridge.

Also, Transsonic turbulance becomes a stability issue that affects accuracy.

[Vagabond]

here is something i posted a few months ago on another site talking about the speed of sound. No matter what anyone claims, SoS is ONLY dependent on temperature, and has nothing to do with altitude or pressure. Here is how this is done. 1000 FPS is well under the speed of sound.

for those wondering, here are a few equations and a refresher on calculating speed of sound.

we'll use U.S. Customary Units.

Quote:
Ss = sqrt[k gc R T]
Ss= speed of sound
k= specific heat of air = 1.4
gc= gravity = 32.2 ft-lbm/(lbf-sec^2)
R= specific gas constant for air
T= absolute temperature in Rankine= degrees in F plus 460 (or 459.67 to be exact)


1st, calculate the specific gas constant:

R= R* / MW

R* = universal gas constant in Rankine units = 1545.4 ft-lbf/lbmol-Rankine
MW = molecular weight of air = 28.967 lbm/lbmol

R= 1545.4 / 28.967 = 53.35 ft-lbf/lbm-Rankine

Temperature at my shooting time: 37 deg F = 497 Rankine


Therefore:


Ss = sgrt[ (1.4)(32.2)(53.35)(497)]

Speed of sound on Saturday was = 1093.3 FPS




so to simplify this down, to calculate speed of sound for a given temperature (x in Fahrenheit), use this equation:

Ss = sgrt[ (1.4)(32.2)(53.35) (x+460)]

[hardcase]

The 1000 ft/sec. point is a practical velocity for subsonic projectiles, relative the nuances of air density that impact a change in actual "sonic velocity" condition... humidity, temp, elevation, being primary actors.

However... a little googlefu for the case of absolutely must know, yielded...


Speed of Sound - Sonic Velocity - in Ideal Gases
Since the acoustic disturbance introduced in a point is very small the heat transfer can be neglected and for gases assumed isentropic. For an isentropic process the ideal gas law can be used and the speed of sound can be expressed as

c = (k p / ρ)1/2

= (k R T)1/2 (3)

where

k = ratio of specific heats (adiabatic index)

p = pressure (Pa, psi)

R = gas constant

T = absolute temperature (oK, oR)

For an ideal gas the speed of sound is proportional to the square root of the absolute temperature.

Example - Speed of Sound in Air
The speed of sound in air at 0 oC and absolute pressure 1 bar can be calculated as

c = (1.4 (287 J/K kg) (273 K))1/2

= 331.2 (m/s)

where

k = 1.4

and

R = 287 (J/K kg)

The speed of sound in air at 20 oC and absolute pressure 1 bar can be calculated as

c = (1.4 (287 J/K kg) (293 K))1/2

= 343.1 (m/s)

Yikes.
Looks like I'll have to brush up on my Physics math. I was hoping for more like "A square +B square=C square type of simplicity. The graph gives me a useful guideline.

My reloads in the 900-950 fps range are much quieter, especially if I dose my can up with white lithium grease every few shots.

[continuity]

... No matter what anyone claims, SoS is ONLY dependent on temperature, and has nothing to do with altitude or pressure...

Not to pick nits, but a variation in the temperature of air, changes the air pressure/density. Altitude has an effect on the same.

[whiterussian1974]

I learned this in 7th Grade Earth Science w Ms Hirsch.
https://en.wikipedia.org/wiki/Lapse_rate
"Although the actual atmospheric lapse rate varies, under normal atmospheric conditions the average atmospheric lapse rate results in a temperature decrease of 3.5°F/1,000 ft (6.4°C/km) of altitude.
The measurable lapse rate is affected by the moisture content of the air (humidity). A dry lapse rate of 5.5°F/1,000 ft (10°C/km) is often used to calculate temperature changes in air not at 100% relative humidity. A wet lapse rate of 3°F/1,000 ft (5.5°C/km) is used to calculate the temperature changes in air that is saturated (i.e., air at 100% relative humidity). Although actual lapse rates do not strictly follow these guidelines, they present a model sufficiently accurate to predict temperate changes associated with updrafts and downdrafts."

[doubloon]

... No matter what anyone claims, SoS is ONLY dependent on temperature, and has nothing to do with altitude or pressure...

Not to pick nits, but a variation in the temperature of air, changes the air pressure/density. Altitude has an effect on the same.

I believe this is another of those things that depends on context. The first statement is true (I think) in a laboratory for ideal gases. In a messy medium like breathable air the second is true.

But while I'm back in here ...

@hardcase don't worry about being able to calculate the speed of subsonic just remember for all practical applications of earthbound shooting it hovers around 1050fps so as long as you stay below that mark the primary noises you'll be concerned with are bullet flight, escaping gas, action/hammer noise and potentially brass drop ... there will be no sonic crack.

Time would be better spent calculating powder burn and internal pressures with something like Quickload. Or if you can figure out the speed of dark that might be worthwhile. Some claim it's equal to the speed of light but I hypothesize it's faster and light is slowing it down.

[Schulze]

A quick approximation: 1125fps at 70F, increases/decreases about 1fps per degree F from that point.

[whiterussian1974]

Posted by CThomas near bottom of thread http://www.silencertalk.com/silencer-fo ... =2&t=73936
"Speed of sound changes with temperture.
@-40 d speed of sound is 1004 fps
@32 d it is 1087
@75 d it is 1134
@100d it is 1160
and just in case your in death valley or for anyone who lives in TX or Arizona @141d it is 1202"

I think that he meant d=*F. That's what matches up anyway.

[continuity]

... Or if you can figure out the speed of dark that might be worthwhile. Some claim it's equal to the speed of light but I hypothesize it's faster and light is slowing it down.

Once again I'm appropriately bitch slapped by someone with a deeper grasp of practical observations relative the true concerns of the issue at hand... Love ya doubloon.

Will stand on the seemingly practical position that the most productive expenditure of time and energy, given all the other things that require such things relative developing/obtaining a subsonic round loading that is < 1000 fps, is in the realization that the objective of projecting a subsonic round, in all conditions known on planet earth, has been realized.

No problem with the enquiring mind thing, but IMHO, the technical end of how we "know" such stuff, is tatamount to me understanding the "why and how" of when I mash my cruisers accelerator pedal to the floor, my chariot will (eventually) move me through time and space at +130mph. I'm absolutely aware that the reason it happens has understandings that would elude my comprehension... even if I was told why. The equipment wizard overlords told me that's what would happen, and experience has proven it. In another thought evolution, someone discovered that round things are more easily rolled around, than square things.

IMHO, sometimes it's just more productive to take the technical aspects of some stuff at face value.

[continuity]

BTW... welcome to the forums hardcase.

[doubloon]

... Or if you can figure out the speed of dark that might be worthwhile. Some claim it's equal to the speed of light but I hypothesize it's faster and light is slowing it down.

Once again ...

BTW that was a joke, I say a joke, son and if some didn't get it then they are built to low. The fast ones go over their head. They got a hole in their gloves. I keep pitchin' 'em and they keep missin' 'em.
http://www.youtube.com/watch?v=KTwnwbG9YLE

[Vagabond]

... No matter what anyone claims, SoS is ONLY dependent on temperature, and has nothing to do with altitude or pressure...

Not to pick nits, but a variation in the temperature of air, changes the air pressure/density. Altitude has an effect on the same.

Air is often treated as an ideal gas to simplify all the math involved. Would you rather treat it as a viscous fluid? I dont think anyone wants to get into that equation.



Look at my equation... it treats air like an ideal gas and because the ratio of specific heats is constant for an ideal gas, 1.4 for air, and the molecular weight of air does not change since its lbm per lbmol, that means that the specific gas constant for air does NOT change. It is always the Universal Gas Constant divided by the molecular weight of air.

There is no place to vary altitude or pressure. when temperature is changed, the variances in pressure correspond and therefore that term falls out.

Therefore, the only thing that has an effect on the speed of sound is air TEMPERATURE.


if you want to read up on this, Speed of Sound is covered in the "Civil Engineering Reference Manual, Eleventh Edition" by Micheal R. Lindeburg, PE, Chapter 14 Section 14.

[continuity]

...Therefore, the only thing that has an effect on the speed of sound is air TEMPERATURE...

Since it's in caps, it must be the true element.

...if you want to read up on this, Speed of Sound is covered in the "Civil Engineering Reference Manual, Eleventh Edition" by Micheal R. Lindeburg, PE, Chapter 14 Section 14.

Appreciate your positive assessment of my intellect. You're intimating that I could comprehend the contents...

Find it interesting that such a topic would be covered in a Civil Engineering manual. Seems to fit more within those that would have focus in the Mech. Engineering discipline...

It's all good Vagabond. Welcome to the site. Appreciate your perspective, civility, and sharing of information.

Peace.

[doubloon]

I'm confused. Does my stereo sound better in the winter or the summer?

Do I need to play my music back at the same temperature it was recorded?

[Vagabond]

I'm confused. Does my stereo sound better in the winter or the summer?

Do I need to play my music back at the same temperature it was recorded?

The answer is "magnets".

[AkGun&ammo]

in your original post you stated you were "inside" a building...

noise is "louder" when contained in the same room you be in..

"hey, what's that you said"

Chris

[whiterussian1974]

Since I got my .30 cal can a few weeks ago I've been tinkering with sub-sonic loads for my 300 Blackout. I have one of the more expensive infrared chronographs set up in a storage building beside my house about 8-9 ft from the muzzle. I loaded some that clocked at 1,025-1,070 fps. Some of them had a noticeable super-sonic crack. Seems like I heard somewhere where the sound barrier could be computed based on altitude or air density (barometric pressure & temperature) . It would be interesting to know how accurate that formula would be for subsonic rounds. I've loaded some that are in the 940-1,000 fps that are noticeably quieter.

AkGun&ammo: I cant find that in his OP. He said the CHRONO was in a storage building. The storage bldg may have even had open front and rear doors. No mention of where he did his target shooting.
And your "worse inside bldg" comment is true ONLY if your reference point is also inside. The neighbor would experience LESS wave pressure from a small aperature through a window from shooter's position.
Ref: Use of Structures to limit Firearms' Noise