Very Weird Question about Speed of Sound

[BBKChicago]

I'm still pretty new here so take it easy on me, guys This happened to me about 10 years ago and it bothers the crap out of me that I've never figured out what it was.

Long story short: a bullet (Piney Mountain 22lr tracer) leaves my suppressed rifle barrel pointed almost straight up. Before anyone yells, I was in a wilderness area in VERY remote WV and it had been raining for days.

Anyway when the bullet left the suppressor, it was as quiet as a cat fart. However about a second later as the tracer was very high in the sky, I heard the sonic crack. I was already at a pretty high elevation, it was a humid night I think about 60 degrees temp.

What the heck could possibly cause that??? Even if the air pressure dropped to allow a supersonic crack, wouldn't the bullet have slowed also meaning it would still stay below supersonic?

Does it being a tracer have anything to do with it? Because the other subsonic rounds I fired that night didn't do the same thing. Only the tracers.

Someone that's smarter than me, please give me some kind of clue here. Its driving me nuts.

[mozeppa]

echo?

[cp1969]

Believe it or not, the speed of sound is not a function of pressure. Temperature is the only variable in calculating the speed of sound in air.

I have no idea what might have made the 'crack' that you heard but I'm pretty confident it wasn't a bullet exceeding the speed of sound after it started out sub-sonic.

[mattw]

Struck an wayward aircraft!

[Harter66]

Air density .
But it should have worked the other way around .
In the west with high mountains and deep valleys you get inversion layers . Where the warm and less dense air holds the cold high density air in the valley . An altitude change of just 50 ft will take you out of a +7 to -14 socked in valley to 28-40° in the sunshine . So given that it's possible that a 1100 fps sub could have been still going fast enough to be super in the new air layer .

It may have been the pop from the tracer ignition too . Never shot any tracers .

[derek45]

struck an wayward aircraft!

[John Boy]

... a limb breaking in a tree because, 1050 FPS is muzzle velocity and in the air the bullet would be materially be below the speed of sound
2nd Choice, change your brand of beverage

[BBKChicago]

I'm still open to any answers. I was in Monongahela National Forest at river level shooting straight up and over a mountain. Definitely didn't hit any tree branches or anything. If it wasn't a supersonic crack, it sure as heck sounded like it. Shooting level at the same time made no crack - although the tracers didn't get to burn as long since they extinguish when they hit the target. I suppose its possible tracers make some sort of a crack when they burn...but I could see the tracers still burning even after I heard the crack.

The air pressure thing sounds possible. Also it was very very humid and these were basically clouds I was shooting in to. I'm sure some of y'all wanna chastise me....but it was pretty cool looking.

[Winger Ed.]

If a tracer made a unique noise, it'd have to be a zizzle sort of sound as the compound burned.
I've fired several, even in .22LR, and never heard anything.
After taking a couple apart, the compound in them sort of looks like if you broke open an old school road flare.

I really don't think the sound you heard was the tracer element.
I don't know what it was, but I think I know what it wasn't.

[Bazoo]

Your tracer rounds were probably going supersonic. Did you chronograph them? Were they subsonic at other times firing level? Firing straight up all the powder is against the primer, maybe that caused a bump in pressure just enough to get them supersonic.

You said you were firing into clouds? Maybe it was an atmospheric condition similar to a thunder clap.

https://www.scientificamerican.com/a...auses-thunder/

[rbuck351]

Never used any tracers, but I'm wondering if there is any rocket like propulsion from the tracer material making it exceed the sound barrier if it was already close.

[303Guy]

The speed of sound drops as temperature drops. Shooting up, the boolit may have been subsonic at the muzzle but entered a colder layer of air and gone trans-sonic. Remember that the boolit makes a sonic crack from 1050 fps upwards. Why a boolit fired vertically would retain enough velocity at some distance vertically is a mystery to me. I suppose it would depend on the air temperature at ground level and at a higher level. If this is the case then the colder air level would have been lower than what it sounded like and could in fact have been very close to the muzzle but by the time the crack reaches the ear, the boolit would be nearly double the distance from where the crack originated, making it seem like the crack came from quite high.

I've fired sub-sonics in rainy weather but not too cold and they were quiet. The same sub-sonics on a cold evening were cracking, some more than others so it would have been borderline trans-sonic where velocity differences would be more noticeable. But that was at the muzzle.

[uscra112]

Believe it or not, the speed of sound is not a function of pressure. Temperature is the only variable in calculating the speed of sound in air.

Is this "woke physics"? Fighter pilots are going to need a lot of retraining! Never mind aerodynamicists.

[303Guy]

Actually, cp1969 is correct.

At a constant temperature, the gas pressure has no effect on the speed of sound, since the density will increase, and since pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly.

Two main factors affect the speed of sound: the material that makes up the medium—such as air or water—and the temperature.

[uscra112]

Mach 1 at 40,000 feet is 86% slower than at sea level. So, you're saying this is entirely due to temperature?

[Harter66]

Air density is exactly the point of my example . While air density is affected by temperature I can assure that 59° F at sea level doesn't possess the same air density as 0°F at 25,000 ft . If that were the case then it wouldn't take a centrifugal blower behind an exhaust driven supercharger with a ram air intake to get 60% of normally aspirated sea level HP out of a reciprocating engine . It also wouldn't need 16mm or more insulation dia to keep a 12,000 volt spark impulse inside an ignition cable .

Also this is a consideration of shock wave through air mass with the speed of sound being only a numerical point of reference vs a physical point of reference .
Mach .3 , about 200 knots or 230 statute mph at 35,000 ft , indicated is a ground speed well north of 500 mph . This is why team Breedlove and the Britt's went to the Black rock desert platue at 6,000 instead of Bonneville to try for 710 mph with wheels on the ground . They would be able to achieve supersonic against the clock even if the airspeed indicated was short of that . The perfect situation is of course more drive/thrust available than needed on a hot day under a low pressure front with a downwind run . When you 3 mph to make your name imortal and you catch 10mph tailwind ........that's 10 mph over the chassis/airframe/bullets surface you don't have to overcome . If it's in your face it takes 20 mph of power or thrust to overcome it . That is how physical air density works and why altitude , air layers , and water vaper make a huge difference . The OP may have experienced a situation where the bullet crossed a boundary layer of dense air that when the shock wave crossed through it cracked . I'm sure there's a guy with a physics degree that can better explain it but that's the nuts lay mechanic explaination .

[303Guy]

Mach 1 at 40,000 feet is 86% slower than at sea level. So, you're saying this is entirely due to temperature?

Apparently so. But yes, it's essentially entirely due to temperature. If one runs the numbers with the gas laws, the other factors cancel out leaving only temperature as being significant. I only found out about it after shooting in the cold. I looked up the effects of humidity, temperature and pressure and that's when I found out. I didn't believe it at first so I checked other sources and yup, confirmed. I even doubled checked it again today and that's where I found the quotes above. I've forgotten all the gas law stuff but once upon a time I could have done the calculations. I must admit I am still trying to get my head around it.

[Stephen Cohen]

Has anyone considered he may not have been the only one shooting in that area, and they were not shooting suppressed. As a young pre teen I was chopping wood and I heard something hit the wall beside me, and there in the sand beside me was a shiny .22 projectile and I heard no gun shot. There may be some truth to the story, you don't hear the shot that kills you. Regards Stephen

[LoveND]

You live a very good wonderful life if this is the major thing that bothers you. Not being sarcastic or anything, just a good observation as letting this thing bother you. Most people would trade their problems for your. Ive shot a box of sub Sonics recently and hate them. I like the snap of the faster bullets. If i wanted a b b gun to shoot i would buy one. Must have been layers of different air as someone suggested. Also i believe you heard what you heard. It wasn’t the brand of beer. By the way a beer or two makes me see and hear better. It really does. Look it up in the internet. This is a fun post and I’m happy you have not much to worry about. As to sub Sonics they are boring. I live about as remote as one can get. So the snap of a rifle or hand gun doesn’t bother any one as no one live near me.

[303Guy]

Aye. Subs are as boring as. But to suppress, that's what we have to do. Also, subs tend to be more accurate. Then again, there is something very satisfying about a supersonic crack with no muzzle blast.