# Speed of sound and medium. **URL:** \ **Category:** Factual Questions\ **Created:** [March 11, 2000, 5:54am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511 "2000-03-11T05:54:00Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Lexicon](https://avatars.discourse-cdn.com/v4/letter/l/ac8455/32.png) [@Lexicon](https://boards.straightdope.com/u/Lexicon)\ **Post date:** [March 11, 2000, 5:54am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/1 "2000-03-11T05:54:00Z")
This post by Pluto (bolding mine): > [@](#): > > We had a long thread about this a while back but I can’t make the search engine do anything I want it to so you’re on your own to find it. There is some question as to whether he actually exceeded the **local** speed of sound but he was close enough as makes me no never mind. Reminded me of another question. What is the speed of sound? I mean, I know what it is, but how fast is it? And is it “local”? Is it relative to the medium through which it travels? See, some friends of mine were at my pub. They were talking about the speed of sound and I was listening. One guy was saying that the speed of sound is relative to the medium through which it travels. The other guy was saying that the speed of sound is like 700 mph no matter what, and that that’s the speed to break the sound barrier-in a jet for example. The first guy was saying the second guy was wrong, and vice versa. Who was right? I wasn’t about to open my big mouth cuz I had no idea. But I think that the first guy was right. What’s the straight dope? * * * “Winners never quit and quitters never win, but those who never win and never quit are idiots.” ---
**Author:** ![waterj2](https://avatars.discourse-cdn.com/v4/letter/w/858c86/32.png) [@waterj2](https://boards.straightdope.com/u/waterj2)\ **Post date:** [March 11, 2000, 7:07am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/2 "2000-03-11T07:07:00Z")
The speed of sound is dependant on the medium through which it travels. The speed of sound in water is much higher than the speed of sound in air. The speed of sound in a vacuum is 0. I don’t have anything to cite offhand, but I can present a logical reason behind this. The speed of sound is basically how long it takes for each molecule along the path to bump into its neighbor and pass the sound wave along. The closer the molecules are together, the faster this happens. There are other factors as well, but density is the predominant one. This principle comes into play in a number of places, such as layers of water with different densities distorting sonar signals. ---
**Author:** ![Lexicon](https://avatars.discourse-cdn.com/v4/letter/l/ac8455/32.png) [@Lexicon](https://boards.straightdope.com/u/Lexicon)\ **Post date:** [March 11, 2000, 7:13am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/3 "2000-03-11T07:13:00Z")
> [@](#): > > I don’t have anything to cite offhand, but I can present a logical reason behind this. The speed of sound is basically how long it takes for each molecule along the path to bump into its neighbor and pass the sound wave along. The closer the molecules are together, the faster this happens. There are other factors as well, but density is the predominant one. You sound exactly like guy number one. This is almost word for word what he was saying… I thought he was right. * * * “Winners never quit and quitters never win, but those who never win and never quit are idiots.” ---
**Author:** ![waterj2](https://avatars.discourse-cdn.com/v4/letter/w/858c86/32.png) [@waterj2](https://boards.straightdope.com/u/waterj2)\ **Post date:** [March 11, 2000, 7:30am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/4 "2000-03-11T07:30:00Z")
Unfortunately, I no longer have any fluids textbooks lying around. I could go far more in depth, with all sorts of formulae and estimations for various altitudes (all in bizarre english system units) and so on until I look like a goddam know-it-all… well maybe it’s not so unfortunate. ---
**Author:** ![Alphagene](https://avatars.discourse-cdn.com/v4/letter/a/8e7dd6/32.png) [@Alphagene](https://boards.straightdope.com/u/Alphagene)\ **Post date:** [March 11, 2000, 7:52am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/5 "2000-03-11T07:52:00Z")
Lex, the first guy was right. The speed of sound varies with the medium it travels through, among other things. The speed of sound through air at room temp is about 767 MPH. At zero degrees celcius, it’s about 742 MPH. In pure carbon dioxide at zero degrees celsius it’s 580 MPH. Through steel, sound travels at about 13,300 MPH. [Here](http://library.thinkquest.org/19537/Physics4.html)’s a good link about the speed of sound. * * * **Gypsy** : Tom, I don’t get you. **Tom Servo** : Nobody does. I’m the wind, baby. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [March 11, 2000, 3:22pm UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/6 "2000-03-11T15:22:00Z")
Yes, and inside a neutron star where the density is 10^14 grams per cubic centimeter, the sound speed approaches the speed of light. IIRC, this happens when the neutron star is more than 3 times the mass of our sun. Hence that is how big a neutron star can be before it collapses into a black hole. ---
**Author:** ![Whammo](https://avatars.discourse-cdn.com/v4/letter/w/8dc957/32.png) [@Whammo](https://boards.straightdope.com/u/Whammo)\ **Post date:** [March 11, 2000, 9:07pm UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/7 "2000-03-11T21:07:00Z")
The speed of sound varies with the density of the air, depending on altitude, speed of sound at sea level is different then the speed of sound upper altitude… just dont push me for numbers, do a search for a thread I once posted on a skydiver breaking the sound barrier (this may be what the original question was on… dont know) lots of great sound barrier stuff in that thread. I will see if the search engine is working. and post if it is. ---
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [March 11, 2000, 9:15pm UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/8 "2000-03-11T21:15:00Z")
If you need a simple example next time you’re at the pub, think of the expression “having youur ear to the ground” which I guess derives from American Indians putting their ears on a railroad track to hear a train coming well before the sound wouuld reach their ears through the air. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [March 12, 2000, 3:54am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/9 "2000-03-12T03:54:00Z")
Pub argument, huh? The worst ones are where both sides are right, but aren’t listening to the other side. The second guy was right, \*if[/] his point was that ‘people breaking the sound barrier’ only refers to the speed of sound in air. Somehow I doubt he was insisting that the speed of sound in steel was the same as the speed of sound in air–but if he was, he was wrong. * * * rocks ---
**Author:** ![Alphagene](https://avatars.discourse-cdn.com/v4/letter/a/8e7dd6/32.png) [@Alphagene](https://boards.straightdope.com/u/Alphagene)\ **Post date:** [March 12, 2000, 4:19am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/10 "2000-03-12T04:19:00Z")
The second guy sounds wrong to me. Even if he meant it just in terms of ‘breaking the sound barrier’, a plane would have to go faster though air at 20 degrees to get a nice sonic boom than he would at zero degrees. Therefore “the speed of sound through air” is still not a defined value. Even if you’re talking about Mach numbers, a Mach number is “a number representing the ratio of the speed of a body (as an aircraft) to the speed of sound **in a surrounding medium** (as air)” So “Mach 1” is not a universal value. * * * **Gypsy** : Tom, I don’t get you. **Tom Servo** : Nobody does. I’m the wind, baby. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [March 12, 2000, 12:45pm UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/11 "2000-03-12T12:45:00Z")
Another round for the house! * * * rocks ---
**Author:** ![Lexicon](https://avatars.discourse-cdn.com/v4/letter/l/ac8455/32.png) [@Lexicon](https://boards.straightdope.com/u/Lexicon)\ **Post date:** [March 14, 2000, 4:12am UTC](https://boards.straightdope.com/t/speed-of-sound-and-medium/8511/12 "2000-03-14T04:12:00Z")
You guys kick ass. Thanks. The second guy was way wrong, I just didn’t have enough information to correct him. He was actually saying that the speed of sound (“how fast you have to go to cause a sonic boom in a super sonic jet”) is always the same. The second guy was saying no, a jet flying at sea level would have to go faster than a plane at 60,000 feet would, because the speed of sound is faster at sea level, and the air is denser and it is warmer. So, this is what started all of this in the first place. * * * “Winners never quit and quitters never win, but those who never win and never quit are idiots.”