Correction! You are wrong. The more and more air you take out, the fainter, and fainter and fainter the sound will become. But the speed at which it propagates will remain the same. This is assuming that you would be able to heat the remaining air as you removed air, thus keeping the temperature constant with respect to the dropping pressure.

As far as the sos in a solid, verses a liquid:
Those are different substances. The sos is different in different substances. In steel, the molecules are arranged in actual crystal lattices. These lattices are tugged on by sound waves and sound waves will propagate much more directly because the molecules in the steel are physically coupled to one another. In a gas, the molecules only come into "contact", if you can call it that, when they happen to vibrate into one another. Thus the degree of vibrational energy that the gas molecules posess determines the aggregate "contact rate" for the molecules in the gas. The only way that a compression wave will propagate faster through such a medium is if the molecular "contact" rate is higher. The common name for the level of vibrational energy that the molecules of a substance have is : Temperature.

So if you double the temperature you double the contact rate?

And also if you double the density, you also double the contact rate?

Just another thought. I looked it up. At 25 degrees centigrade the speed of sound in :

Water = 1493 m/s
Mercury=1450 m/s
Pyrex Glass=5640 m/s
Rubber = 1600 m/s
Diamond = 12,000 m/s !!!!

Mercury is over 13 times denser than water. Why is the speed of sound lower in mercury? Pyrex glass, while heavier than water, is not 4 times heavier. Why is the speed of sound in Pyrex Glass that much faster?

The speed of sound in any substance is governed by how well in "contact" the molecules are. In a gas, the molecules are "in contact" at a rate governed by thier temperature.

Think about it this way. Sound propagation in air is a wave. Air molecules at 20 degrees c have a velocity (RMS) of 502 m/s. They are "bouncing around" at this speed. The actual air molecules do not travel with the sound wave. So the sound wave (which is 343 m/s in this temp air) must use this velocity to "bounce" the wave from one molecule to another, on average. The only way to increase the available speed is to increase the RMS velocity of the air molecules by raising the temperature. Adding pressure, and subtracting some heat to keep the temp the same, will raise the amount of medium available to propagate the sound (louder), but will not increase its max speed through the medium because the sound cannot propagate any faster than the air molecules are bouncing around.

.....which all goes to show that.......





...it's Miller Time!

What is the speed of sound in beer anyway?

& as far as your argument....

well I'd agree with your logic, and I'd love to argue, coz its fun.

I feel I'm loosing this one!

But the good news is that you always learn something on this forum.

The ref to the NASA site is interesting too.

So my friend, enjoy your brew!

Actually, there's no SoS existing in a vacuum to be equal to zero. It's better to say there's just no sound. Saying it is equal to zero when density is zero implies that theres a continuous change in SoS with density, which there is not.

Actually, there is no sound even when there is some very low density to the air. Aerodynamics ( and sound) is dependent on the air being a density to achieve a continuum - where the gas behaves as a single substance rather than a bunch of newtonian particles zipping around. THis density is sufficient that the gas molecules can be expected to actually colide and rebound off of each other. By definition, this is where sound begins to exist. Below this density, it's not really even a gas - just molecules you run into occasionally. Above this density, the speed of sound is affected only be temperature.

I don't know exactly what density or altitude at which the continuum of air begins....

Soooo.......the speed of nothing through nothing can be accurately measured with nothing.

As far as the speed of sound through beer, welllll.....
you put your ear on one end of the keg, I'll put my ear on the other where the spigot is. When I open the spigot and start to swallow, then, when you hear me swallow the first time, you time how long it takes me to swallow 50 times. Subtract how long it takes for me to swallow 50 times, and that will the the spigot sound trasit time through the keg. Then it will be your turn. We average the difference between the two readings!

The speed of sound in any substance whatsoever is

a = sqrt(B/rho)

where rho is the density, and B is the "bulk modulus". The definition of B is

B = delta(p) / [ delta(vol)/vol ]

or in English (sort of)... B = isentropic change in pressure per fractional change in volume. For isotropic solids, B is proportional to Young's modulus, so that s.o.s. tends to be largest in light and stiff materials. Like diamond, for instance!

From Thermo 101, we know that for an ideal gas

B = gamma*p , gamma = cp/cv

so that

a = sqrt(gamma*p/rho)

and from the Ideal Gas Law we also have

p/rho = R*T

so that finally

a = sqrt(gamma*R*T)


Both gamma and R depend on the molecular type and weight of the gas. For air, gamma = 1.4, and R = 287 J/kg-K, so that

a = sqrt(1.4*287*T) = 20.04 sqrt(T)


Quiz at 11:00 :-)

Here's what temperature will do to altitude..
Those SAE guys that come to Lancaster from sea level in June find this out quickly..

Bruce:
Just so. In the B-57F we had a combination instrument commonly called the "Barber poles." TAS was the center, the left Barber Pole was stall speed, the right Pole was Mach. The job was to keep the left one to the left, and the right one to the right. And the result of not doing so was airframe destruction more often than a many thousand foot spin recovery. Or so we were told. I never had a departure, and I don't know of anyone else who tested the theory. Can't comment on the U-2 or TR-1, never got in either one.

Make sure there's two props,
. Then pull out the stops.

Bill.

What a can of worms!
So there are two ways of computing. One for generation of the Mach # and one for the s.o.s..
I guess I was kinda on the right track... where it was going I'm not really sure. (I think it was heading for one of my physics books.) Sorry for doubting those in the know.
Oh well. Learn something new every day!

mtthomps, still want to do the experiment with the keg? I'm up for it!

By the way, since I haven't made it to my physics book yet, how does one convert Celcius to Kelvin? Is Hobbes involved?

Jazzy:

Celcius temp + 273.15= Kelvin temp.

Twins are really hot!
. But a single? NOT!

Bill.

Aha! I was right-my path lead me to my physics book.
In which I was able to follow most of Mark's equations.
Very interesting; among other things!

The only thing I would add to the eq.: a=(20.04)sqrt(T), is that this is in meters per second and the temperature, T, is in Kelvin.

Through a little algebra I arrived at: a=(44.89)sqrt(T) giving 'a' in mph.

To convert from Celsius to Kelvin just add 273.15.
To convert from Farenheit to Kelvin: K=5/9(F-32)+273.15.

That being said, the speed of sound in mph:
@32 deg F = 742
@50 deg F = 755
@80 deg F = 777
@90 deg F = 784
@100 deg F = 791.5

Further research as to how altitude and air density affects 'a' on my part is not necesary for me to be satisfied.

Thanks All for your inputs.
Jeff


Thanks Will. My thirst for knowledge found it before I logged back on.

Jazzy.....In simple terms computing the speed of sound @ different altitudes ....hence different temperatures........is the same for computing Mach Number because they are in direct proportion to each other......flying @ Mach .75 is flying @ 3/4 the speed of sound at a particular altitude........In a perfect world without any wind to contend with......@ .75 Mach at 33,000 ft would equal a higher ground speed than .75 Mach @ 41,000 ft........because of the change in temperature( Decreasing).....its much simpler than most would think........Bill......

Right Bill,
The computation of the Mach number, as my physiscs book eluded to, is just a fraction. Mach #=(Current Velocity)/(Sound's Velocity). V/V(s).
I think most people figure that out as children.
Jeff

And as we teach commercial pilots, M=TAS/a

a=local speed of sound (speed of sound at altitude you're flying) which varies only with temp

TAS=true airspeed (actual speed of movement through the air)

Now, who's going to be the first to build a model that can break the sound barrier?

Trapnell:

I've already done it.

Launched from a B-57F at 65,000 feet while running 0.9 Mach.

Of course it was only good for one flight.

Haw.

But to speak true, I don't think anyone could fly such an airplane without a virtual cockpit. Watching and flying it from ground level would approach an impossibility. 250 mph is bad enough.

Fly your twin at Mach one plus -
. When it hits the tower, what a fuss!

Bill.