# The Naked Scientists Forum

### Author Topic: How is the speed of sound affected as a vacuum is produced?  (Read 14218 times)

#### dentstudent

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##### How is the speed of sound affected as a vacuum is produced?
« on: 30/07/2007 11:58:53 »
It is said that "In space, noone can here you scream" because you are in a vacuum. At what point does a vacuum become enough of a vacuum to prevent the passage of sound? If you had, say, Vanessa Feltz (please input your own personality of choice as appropriate) in a great big screw-top jar, and you pumped the air out, when would you not have to hear her anymore (assuming she'd not already passed out). Wouldn't it be the case that there would have to be a full vacuum to halt sound, and so in reality is not possible?

#### neilep

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #1 on: 30/07/2007 13:18:18 »
I wish I knew the answer but in any case I would encourage ,with enthusiasm, the experiment you have suggested.

#### another_someone

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #2 on: 30/07/2007 13:21:40 »
I think it becomes around the time when the mean free path of a molecule of the gas is the same order of distance as the wavelength of the sound you are trying to transmit (in other words, ultra low frequency sounds may be transmitted even in a near vacuum).

#### _Stefan_

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #3 on: 31/07/2007 08:18:40 »
If you went into the vacuum with lungs/mouth full of air and then screamed, you would hear yourself scream through the vibrations of your skull and through the vibrations of the gas you exhale. If you don't have any breath, however, you can't scream :p

#### Heronumber0

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #4 on: 31/07/2007 15:17:34 »
I think it becomes around the time when the mean free path of a molecule of the gas is the same order of distance as the wavelength of the sound you are trying to transmit (in other words, ultra low frequency sounds may be transmitted even in a near vacuum).
I bow to your expertise, but can you hear, for example an exploding spaceship in space knowing that there are gases, like oxygen,  in the ship at point of explosion.  Is it possible for the oxygen and other gases to be thrown clear so that the sound can also be transmitted to places other than the spaceship?

Space movies sometimes puzzle me in this regard because you can clearly hear the engine of a spaceship humming as it travels along...

#### another_someone

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #5 on: 31/07/2007 15:42:45 »
I bow to your expertise, but can you hear, for example an exploding spaceship in space knowing that there are gases, like oxygen,  in the ship at point of explosion.  Is it possible for the oxygen and other gases to be thrown clear so that the sound can also be transmitted to places other than the spaceship?

Space movies sometimes puzzle me in this regard because you can clearly hear the engine of a spaceship humming as it travels along...

The oxygen would not reach you any sooner than the shrapnel from the explosion.

In any case, there are two different issues - there is sound, and there are shock waves (these are travelling faster than sound).  The relatively near vacuum of interplanetary space can still carry shock waves from solar flares, etc.; but cannot carry sound waves.

Ofcourse, the other thing to bear in mind is that space ships tend to be quite far apart, so the explosion of a space ship, unless it was right next to you, will have dissipated much of the energy of the shock wave by the time it reaches you (if it has not, then you are too close to the explosion for your own safety).

#### daveshorts

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #6 on: 01/08/2007 12:00:59 »
In an ideal gas the gas pressure shouldn't change the speed of sound

in a gas the speed of sound is proportional to √(p/ρ) where p is the pressure and ρ is the density.
Now because in gasses p T / V (volume) - where T is the temperature
so p ρ

so if you halve the pressure you also halve the density keeping the speed of sound the same.

So the major effect on the speed of sound is the temperature. In the stratosphere it is very cold, which is why the speed of sound is reduced at high altitudes

#### dentstudent

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #7 on: 01/08/2007 12:09:37 »
Thanks for the post Dave! So as you decrease in temperature the speed of sound reduces. So the speed of sound in the freezer is less than it is in the room in which it stands. How measureable is this? The frequency doesn't change though, is that right? So although the sound is traveling faster, it's pitch doesn't change.

#### daveshorts

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #8 on: 01/08/2007 12:19:55 »
The difference will be the square root of the difference in temperatures from absolute zero so for a freezer at -18°C and a room at 20°C it will be √(255K/293K) = 0.93 of the speed in the room in the freezer.

If you were blowing over two empty glass bottles one out of a freezer and one not, the one from the freezer should be slightly more than a semitone lower than the warm one. hmmm that sounds like a kitchen science experiment to me...

#### dentstudent

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #9 on: 01/08/2007 12:30:49 »
I look forwards to hearing it! Thanks again Dave!

#### another_someone

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #10 on: 01/08/2007 12:58:17 »
In an ideal gas the gas pressure shouldn't change the speed of sound

Isn't the problem there that real gasses are not ideal gasses, and although at STP the difference is slight, in near vacuum conditions, the difference is anything but slight.

Even at STP, when the frequency of sound is extremely high (close to the distance of the mean free path of the molecules), one gets non-ideal behaviour.

http://en.wikipedia.org/wiki/Speed_of_sound#Effect_of_frequency_and_gas_composition
Quote
The limitations of the concept of speed of sound due to extreme attenuation are also of concern. The attenuation which exists at sea level for high frequencies applies to successively lower frequencies as atmospheric pressure decreases, or as the mean free path increases. For this reason, the concept of speed of sound (except for frequencies approaching zero) progressively loses its range of applicability at high altitudes.

http://nvl.nist.gov/pub/nistpubs/sp958-lide/html/069-072.html
Quote
Greenspan [1] described this behavior in the follow-ing way. "Nonuniform effects in gases are best studied at small acoustic amplitudes where relaxation effects can be observed. From kinetic-theoretical considerations in a gas of smooth rigid spheres, the speed of sound depends only on the mean speed of the molecules, provided that the gas is sufficiently dilute so that practically all of the molecular momentum is transferred and that the mean collision rate is very high. Laplace's formula states that the sound speed depends on the molecular mass and the temperature, and these deter-mine the molecular speed. Any dispersion must depend on the ratio of collision rate to sound frequency. A suitable parameter for comparison is one proportional to this ratio (pressure/ frequency). For example, a sound wave at an audio frequency of some kHz in a gas near atmospheric temperature and pressures will have a mean collision rate of order of 10 10 s Ð1 . The medium is very fine grain and the dispersion is negligible. As the sound frequency is steadily increased, the frequency becomes comparable at first to the collision rate of the slower molecules. The collision rates becomes positive corre-lated with the molecular speed. The slower molecules can not transfer the acoustic momentum coherently, and this burden shifts to the faster molecules. Accordingly, the speed of sound steadily increases with frequency. The effect is negligible unless the frequency is very high."

This work was extended to the measurement of viscosity effects for the diatomic gases nitrogen and oxygen and to dry air. Moe compared his theoretical predictions [5] to the attenuation and propagation constant of the diatomic gases as measured. The behav-ior at higher pressures follows the classic theory. As the mean free path approaches the ultrasonic wave length, Greenspan's constructed theory fitted at the intermedi-ate pressures. For lower pressures significant deviations were found [7]

His work on propagation of sound in rarefied gases is a classic example of how to examine a complex system for relaxations resulting in a measured dispersion. He was able to show that the Stokes-Navier equation gave a surprisingly good quantitative account of attenuation and dispersion of sound in monatomic gases down to wavelengths approaching the mean free path. Moreover, he succeeded in making measurements to much lower pressures where the mean free path was significantly greater than the wavelength, and found substantial deviations from the theories. New theoretical many-body results are now judged by their agreement with these data. For diatomic and polyatomic gases, where molecular relaxation processes associated with vibra-tional and rotational modes occur in addition to the translation relaxation, he was able to demonstrate experimentally and theoretically how they combine to affect acoustic dispersion and attenuation.

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##### How is the speed of sound affected as a vacuum is produced?
« Reply #10 on: 01/08/2007 12:58:17 »