I am the only one who thinks that this column made no sense? Just what is the difference between pitch and frequency? How can helium, which presumbably would affect all frequencies by the same factor, change the pattern of frequencies? (Not the frequencies themselves, but their pattern?)
I got the two “pitch” samples from http://www.phys.unsw.edu.au/PHYSICS_!/SPEECH_HELIUM/speech.html and pasted them together. I ran the pasted sample through CoolEdit using its spectrum display. A 550K jpeg of this is available on my site.
The frequency scale on the right of the screen is labeled linearly - a given distance on it indicates the same frequency difference regardless of location on the scale.
Here are my observations on the portion of the samples where “ah” is sung.
“ah” in air (0-2.5 secs): The fundamental is about 120 Hz. 8 pronounced overtones appear next. The first is at about 240 Hz. The rest appear to be spaced at roughly 120 Hz intervals. Each of the first 3 overtones shows a narrower frequency spread than each of the last 5. There are additional overtones around 2600-3000 Hz.
“ah” in helium (6.5-9 secs): There are now 12 pronounced overtones. Again, the first is at about 240 Hz and the rest are spaced at about 120 Hz. Each of the first 5 overtones shows a narrower frequency spread than each of the last 7. There is no sign of the overtones which would be shifted from the 2600-3000 Hz overtones in the air wave. They may have shifted beyond the frequency limit of the sample, which is shown on CoolEdit’s frequency scale as 5500 Hz.
I don’t trust this completely. The overtones should show some signs of logarithmic spacing, but they appear to be linear.
This isn’t right. If you reduce the density of a gas while keeping the temperature constant, the pressure drops and the spacing of the molecules increases, but the speed of sound doesn’t change (for an ideal gas, anyway).
Ranma’s comment that “the sound barrier’s speed is inversely proportional to the altitude” is probably due to the temperature dropping, not the pressure.
Hmm, ZenBeam, I’m willing to admit that I’m wrong, but could I have a cite/calculation/formula for that?
Stepping in for a second, Alphagene states above that:
Speed of sound = SQR(Bulk modulus/density)
The definition of bulk modulus is:
Bulk Mod = (change in pressure)/((change in volume)/volume) = (change in pressure)/((change in density)/density)
Subbing the last equation into the first, you’ll note that the density cancels out, leaving
Speed of sound = SQR((change in pressure)/(change in density))
For an ideal gas, dP/d(density) should be constant, right? (Meaning, I just reasoned this through on my own, and it seems to make sense, but I’m willing to be wrong).
Do’oh! Thanks, zut. I stand corrected.
Ok, first off, let me make sure I have the basic premise correct:
- In a gas, the lower the molecular density, the faster sound will travel; conversely, the higher the molecular density, the slower sound will travel.
Put in layman’s terms: the lighter the gas, the faster the sound; the heavier the gas, the slower the sound.
Is that right?
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Next, how does gas pressure affect sound transmission? Will a gas under pressure transmit sound at any different speed than the same gas in a partial vacuum? (I realize Chronos and Zut went over this, but I’m still a bit confused.)
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And what about the speed of sound in solids? Is the same true as in a gas (i.e., heavy molecules transmit slower than light molecules)? Referencing the experiment I described above, will the steel rod transmit the kinetic energy faster than the aluminum rod? If so, does the same hold true for sound in a solid?
STARK, here’s a link which might help. If by “molecular density” you mean density in terms of numbers of molecules per a given volume, then no, the density of the gas doesn’t matter. If by “molecular density” you mean a gas which is lighter, but has the same number of molecules, then yes, it does matter, since this implies the individual molecules are lighter. It’s the molecular mass, and temperature which affect typical particle speed, which in turn determines speed of sound.
To quote the very bottom of the link, “The speed of sound in a gas depends on the temperature, molecular weight, and molecular structure, but not on the pressure of the gas.”
The way I visualize it is to imagine, say, half of the particles in a gas simply disappearing. The remaining particles are moving just as fast, so the speed of sound won’t change, but the pressure will be lower.
No, there are completely different mechanisms at work. For a gas, it’s the moving particles which transmit the sound (although, there are a lot of collisions going on). In a solid, it’s the bonds between adjacent atoms which transmit the force, hence the sound. The atoms vibrate, but don’t actually move. If you think back to high school physics (if you took it) and vibration of a string (say on a guitar) heavier masses (thicker strings) will vibrate more slowly than lighter masses, but the tension also determines the vibration (you tighten the strings to increase the pitch). For equal bond strength, the vibration will be faster for aluminum than steel, but the bond strengths probably aren’t equal. I am not a metalurgist (I’m not even sure how to spell it) so I don’t know what the case is here.