How many feet per second is the speed of sound?


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grimjaw
July 2, 2006, 11:42 PM
I'm trying to do some guestimations on rimfire ammunition at longer ranges (150-200 yards). I've heard anecdotal evidence that if a projectile moving a supersonic speed falls to subsonic speed, especially if it the fall is rapid, it has an affect on trajectory. I don't know if that is true, maybe someone will come along and enlighten me.

I read up a little bit on the speed of sound, and I understand that it changes depending on medium pressure (?) and thus decreases with altitude in air. 761 mph is the speed of sound at sea level, but at 11000+ feet, it's 660 mph. I'm using the 761 mph number to do my math.

761 miles = 4018080 feet
1 hour = 3600 seconds

Speed of sound at sea level = ~1116.13 feet per second

Are those correct calculations?

jmm

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KimberTLE.45
July 2, 2006, 11:45 PM
Sounds about right. I know subsonic ammo is right around 1050 FPS so 1100 would seem right.

Stiletto Null
July 2, 2006, 11:45 PM
Sure. :)

Speed of sound is dependent on a bunch of other variables like temperature and humidity, but ~340m/s is a good number for napkin math.

Allowing for .3048 m/ft, that comes out to 1115ft/s.

OEF_VET
July 2, 2006, 11:45 PM
That's pretty close. It's somewhere around 1,100 fps, with minor variations based on altitude changes.

Outlaws
July 2, 2006, 11:46 PM
Thats about what google says.

444
July 2, 2006, 11:59 PM
I recently spent some time working up a subsonic load for the AR15.
I found that last week, with a temperature about 110 degrees, in Las Vegas, I started getting a supersonic crack at around 1140 fps. This isn't a solid number but right now, here, it is very close.

RNB65
July 3, 2006, 12:03 AM
There is no exact answer. The speed of sound is not fixed. It varies based on atmospheric conditions (temp, air pressure, humidity).

Stevie-Ray
July 3, 2006, 12:35 AM
1100 fps is the generally accepted answer.

DoubleTapDrew
July 3, 2006, 12:43 AM
I recently spent some time working up a subsonic load for the AR15.
I found that last week, with a temperature about 110 degrees, in Las Vegas, I started getting a supersonic crack at around 1140 fps. This isn't a solid number but right now, here, it is very close.
Wow! I didn't know you could download a .223 to subsonic velocities considering it's usually smoking at about 3000fps. That's cool to know. How does it shoot with those loads?

trapperjohn
July 3, 2006, 12:52 AM
its around 1100 f/s, it will vary a bit depending on atmospheric conditions like temp and humidut, but its not a wild variation.

the reason you want a supersonic bullet ot stay supersonic is to keep it from passing through the shock wave. a supersonic projectile has a shock wave traveling BEHIND it. if the bullet slows down to subsonic velocities it passes through the shockwave which can alter its course slightly.

the naked prophet
July 3, 2006, 01:02 AM
The speed of sound does NOT vary with pressure. It is independent of pressure. It does vary with temperature. http://en.wikipedia.org/wiki/Speed_of_sound I know this from freshman aerodynamics class five years ago.

The difference between winter and summer, or night and day, will be more than the difference between florida and colorado.

In addition to that, the air passing over the bullet may exceed mach 1 even if the bullet is travelling well under that speed. This means a semiwadcutter can have a supersonic crack at much lower speeds than a streamlined bullet. Keep that in mind when you develop subsonic loads.

grimjaw
July 3, 2006, 01:14 AM
trapperjohn, thanks for the explanation. That is what I was trying to avoid. The ballistics tables I've seen for .22WMR show that for 40gr projectiles you'll be hitting ~1100fps somewhere between 150-200 yards. I don't have a place that I can set up 200 yard paper targets, so it's hard for me to see just what kind of accuracy hit I'll take from that "shock wave", if any.

prophet, thanks for the correction and info.

jmm

Stiletto Null
July 3, 2006, 01:17 AM
In addition to that, the air passing over the bullet may exceed mach 1 even if the bullet is travelling well under that speed. This means a semiwadcutter can have a supersonic crack at much lower speeds than a streamlined bullet. Keep that in mind when you develop subsonic loads.Nitpick. :p

redneckdan
July 3, 2006, 09:54 AM
The problem is the transonic barrier, when the projectile drops from super to subsonic, there is a lot of buffeting on the projectile, not exactly a good thing for accurcay because the buffeting is not uniform, every porjectile is differnt.

Dionysusigma
July 3, 2006, 10:06 AM
What about through other mediums, i.e. liquid and solid? And combinations of them? Say, tomato soup versus chili? :o :p

For that matter, what about different gases? Is it different for hydrogen and xenon?

What exactly causes sonic booms and cracks? Is it something along the lines of supercavitation of the medium itself?

:confused:

Stiletto Null
July 3, 2006, 10:18 AM
Supercavitation? I thought that term's only applicable to liquids.

Anyhoo, shock waves (http://en.wikipedia.org/wiki/Sonic_boom) are the answer. You can get a sonic boom with dramatic changes in subsonic flow patterns too.

TallPine
July 3, 2006, 11:13 AM
Somewhere back in my old computer code I have the formula for calculating the speed of sound (Mach). IIRC it is dependent on several variables including altitude (air pressure). Probably it was a multi-step computation, you first have to compute air pressure based on altitude ASL and then use that as an input.

I could go look it up if someone really really needs to know the exact formula.

Creeping Incrementalism
July 3, 2006, 11:39 AM
The speed of sound does NOT vary with pressure. It is independent of pressure. It does vary with temperature. http://en.wikipedia.org/wiki/Speed_of_sound I know this from freshman aerodynamics class five years ago.

If you want to really get technical, the "only vary with temperature thing" depends on air being an ideal gas, which it isn't. Not that that fact makes a significant difference.

the naked prophet
July 3, 2006, 12:30 PM
For most considerations, air at atmospheric pressure (or even a few atmospheres) is close enough to an ideal gas for most approximations. Considering the accuracy and consistency of our chronographs and loads, +/- 10 fps is way more than the error you get from approximating air as an ideal gas.

The reason that the speed of sound varies with altitude is that the temperature varies with altitude. This (http://www.aerospaceweb.org/question/atmosphere/q0102c.shtml) website explains it pretty well, with a simple, correct equation. Although that wikipedia page is technically correct, it isn't the formula that I learned in school, and any joker can change it at any time. If you really want to be sure, get an aerospace book. I've got a few for sale ;)

BigFatKen
July 3, 2006, 12:39 PM
medium pressure (?)
This may be a typo; likely they meant median or "The median is the middle of a distribution: half the scores are above......"

Mal H
July 3, 2006, 01:50 PM
Ken - I imagine the source grimjaw is referring to used the term 'medium' to mean the substance through which the sound is traveling.

That source would be incorrect as the naked prophet points out since pressure is proportional to density in an ideal gas and that ratio is therefore a constant, not a variable, in determining the SOS.

Although, strictly speaking, that is not true. Considering an ideal gas only, if you take pressure to the extreme, say 0 force units/area, then the SOS is 0 because 0 pressure implies no medium for the sound to travel through. So saying the SOS does not vary with pressure is not entirely accurate. But for the purposes as used here, it is.

Travis McGee
July 3, 2006, 02:40 PM
A lot of .22LR ammo comes out of the barrel very close to the speed of sound, so temps will for sure make the difference one way or the other. If you are firing a suppressed .22, this really comes into play, and you can literally hear the difference on a cold day that warms up. The difference firing a suppressed .22LR that is subsonic and crack-less is quite striking.

444
July 3, 2006, 09:41 PM
"Wow! I didn't know you could download a .223 to subsonic velocities considering it's usually smoking at about 3000fps. That's cool to know. How does it shoot with those loads?"

Why couldn't you ?
It shoots great, out to about 75 yards. The wind really played havoc with them at 100 yards. At 50 you could cover the group with a dime easily.
The discussion of working up this load can be found in the ammo/terminal effects section of the www.silencertests.com forum.
The load is a 77 grain Sierra Match King boatail hollow point and IMR Trail Boss powder loaded to just below the speed of sound.
With the suppressor, the report is Hollywood quiet.
If you go to that forum, you will see that my next project is a 100 grain bullet, subsonic.

Cesiumsponge
July 3, 2006, 10:00 PM
I can also suggest anyone bored enough that wants to learn more can look for books, scientific journal entries, and tech papers dealing with light-gas guns (LGG's) as it is a fundamental of their design (driving force of a dyanamic light atomic gas accelerating a projectile into a hard vacuum environment for velocities upwards of 4-5 miles per second (21000fps+)).

There are some reprints that are available but a tad math intensive. In general discussions like this that don't require designing canard wings or ramjets so I think ideal formulas can be presented, and used to get reasonable approximations as it eliminates the added complexities of taking every little variable and fudge factor into account.

Stiletto Null
July 3, 2006, 10:02 PM
I wish someone would make a FSW + canard + double twin-fin (top and bottom) fighter. That would be sooo sexy. :uhoh:

blackops
September 28, 2009, 02:10 AM
So when a sniper that is engaging a target at 1500yds the target is going to hear the shot for a split second before the bullet makes impact?

R.W.Dale
September 28, 2009, 02:36 AM
So when a sniper that is engaging a target at 1500yds the target is going to hear the shot for a split second before the bullet makes impact?


At 1500 yds with a rifle capable of doing so the sound wave will arrive a lot later than "a split second"

cottonmouth
September 28, 2009, 06:01 AM
1,100 fps........... so how may miles per hour is that!

J.B.

MachIVshooter
September 28, 2009, 07:18 AM
First, it is better to start a new thread with this question than dig up a three year old one. Now:

So when a sniper that is engaging a target at 1500yds the target is going to hear the shot for a split second before the bullet makes impact?

No. Even if the round does go subsonic by the time it gets there, the sound wave will not catch up to it. A good round average number for rifle bullets the likes of which would be employed for hunting, be it people or big game animals, is ~3,000 FPS, which is nearly mach III.

Tommygunn
September 28, 2009, 11:02 AM
1,100 fps........... so how may miles per hour is that!

750 miles per hour

Furncliff
September 28, 2009, 01:06 PM
Nat. Wx Svc Speed of Sound Calculator.

http://www.srh.noaa.gov/elp/wxcalc/speedofsound.shtml

twofifty
September 28, 2009, 09:02 PM
quote: "the reason you want a supersonic bullet ot stay supersonic is to keep it from passing through the shock wave. a supersonic projectile has a shock wave traveling BEHIND it. if the bullet slows down to subsonic velocities it passes through the shockwave which can alter its course slightly."

Interesting.

I wonder if this now sub-sonic bullet isn't actually overtaken by its own supersonic shock wave, which would disrupt it from the rear, introducing yaw?

otoh, a bullet slows down throughout its flight... So the shock wave ought to overtake the bullet, even a varmint round that starts out at Mach 3+. The speed of sound is a condition dependant constant, but the speed of the shock wave's travel through the air is co-related to bullet velocity.

Since all bullets slow down, all shock waves ought to catch up to them, over and over again, no?

Someone help me out here. lol.

Acera
September 28, 2009, 09:30 PM
Result: 1116 ft/s (feet per second)

Some other calculations to amuse you on the speed of sound.

~~ 340.3 m/s (meters per second)
~~ 0.3403 km/s (kilometers per second)
~~ 0.2115 mi/s (miles per second)
~~ 12.69 mi/min (miles per minute)
~~ 761.2 mph (miles per hour)
~~ 20.42 km/min (kilometers per minute)
~~ 29400 km/day (kilometers per day)
~~ 661.5 knots

Slowness from S=1/v: 0.0029 s/m (seconds per meter)



You guys have to learn the value of Wolfram/Alpha. All of this type of stuff is easily calculated by the most basic of inputs. Here is the link for this calculation.

http://www.wolframalpha.com/input/?i=speed+of+sound+in+feet+per+second+


If you don't know about it, check it out and watch some of the introduction videos, it's amazing.

robmkivseries70
September 28, 2009, 09:39 PM
Another constant, roughly speaking: 88 f/s = 60 mph.
Best,
Rob

blackops
September 29, 2009, 03:17 AM
You would think that at 3000yds you would hear it for an instant before it made contact.

Uncle Mike
September 29, 2009, 04:08 AM
1,100 fps........... so how may miles per hour is that!

That's about how fast my old Ford will go...no, wait...I think it's 1,100 SPF!

Don't get too hung up on the transonic barrier thing, true a bullet will experience buffeting transitioning this barrier however this does not mean the projectile becomes permanently unstable or permanently displaced for the remainder of its journey.

The projectile may experience buffeting which may cause longitudinal and or lateral displacement... but if the projectile is uniform and it had departed with the proper stabilizing centrifugal inertia (proper twist rate) then the bullet possesses positive static stability... meaning it will return to its original position after being disturbed...given enough room between the transonic barrier and the target.

Good bullets are said to posses excellent dynamic stability also...meaning that not only does the bullet want to, and makes an effort to, return to its original position after being disturbed(Positive Static Stability)...but it will accomplish this in decreasing cycles (it will return to its original position quickly).

So, just because your 22MAG bullet goes 'slowsonic', does not necessarily mean that your going to become inaccurate all of a sudden.

jakemccoy
September 29, 2009, 04:11 AM
Off topic... The rough speed of sound is a good thing to know during a lightning/thunder storm, to figure out how far the last lightning strike was away.

ScottG1911
September 29, 2009, 04:21 AM
Speed of sound is dependent on a bunch of other variables like temperature and humidity, but ~340m/s is a good number for napkin math.

Exactly correct. in fact I can still remember from my freshman science book, that sound travels at 324M/s at 20 degrees C. at sea level. so you're looking at 1050fps at 62 degrees F. and it will increase speed with increased temp. and decrease with altitude

hammerklavier
September 29, 2009, 09:18 AM
The speed of sound most certainly is dependent on the air pressure, or as NOAA puts it, the density of the air. However, since pressure and temperature in an open system are related, you can make a simplified formula that uses just the temperature. If you were to measure the speed of sound inside a compressed air tank, you'd find it does make a difference apart from temperature.

mec
September 29, 2009, 09:43 AM
Whatever it is the speed of sound can get interesting with certain reloads. Get a load that straddles it and some rounds will go pop while others go crash. I sounds like some seriously inconsiistency taking place but, over the chronograph, the velocities can actually be very close together.

matrem
September 29, 2009, 06:45 PM
Another constant, roughly speaking: 88 f/s = 60 mph.

That's not rough, that's exact.
There are 5280 feet in a mile.
There are 3600 seconds per hour.
Each - divided by 60.

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