Question Board -- Other Questions

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Questions about temperature and heat
Questions about temperature and heat
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The questions (Click on a line to jump to the entry)
Other Questions:
Why does a dome amplify sound?
Can sound energy be converted to heat?
What is the effect of the tube diameter on a siphon?
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My Question: I am using Star Lab, which is a dome-shaped, portable planetarium. It has great acoustics! How does sound travel in a dome which gives it (the sound) an amplified effect?

Joe's explanation I believe the Secret of the Star Lab is that the dome is somewhat rigid, and it isn't very big. Sound reflects from the walls, so you get to hear the same sound three or four times as it bounces around inside the dome. This would not be so good in a big space, because sound doesn't travel very fast, and if the reflecting surface is more than 10 m away, the reflected sound is heard as a separate echo instead of a part of the same sound -- it's confusing instead of helpful (you have probably noticed that announcements in a large echo-ey room are hard to understand).

Sound doesn't travel in straight lines the way light does, but thinking about sound in optical terms is a good place to start. The dome is like a curved mirror, which reflects and focuses the sound. There's probably a magic point where everything you say is focussed back on you -- this will give you the illusion that the acoustics are good, because you can hear yourself great, even if no one else can. But there can be "sound shadows" too, and the dome may be reflecting the sound so that the people in the back of the dome are not in the shadow of the people in front. You can get an idea of how this works if you put a single light bulb where the speaker stands. The lighting inside the dome is a lot better than it would be without the dome, due to the reflections from the surfaces.

Another feature is that sounds that are outside of the dome can't get in. They get reflected, too. It is a part of our modern world that silence is a nonexistent commodity, and the information stream that we are trying to produce when we talk is competing with sounds coming from elsewhere. Cutting out the background sound makes it possible to talk in a whisper.

This last idea leads to a long digression:
The science of building acoustics was practically invented by a professor at Harvard. They had just built a large auditorium, and the acoustics were awful. He studied this problem and eventually solved it. One of the things he did during his study was to determine the range of loudnesses that people can hear. It turns out that if you increase the power of a sound by a factor of 3, it sounds just barely louder; and between the softest sound you can hear and the sound that is so loud that it hurts you can put about 20 of these just-barely-louder steps. So the possible power levels are 1,3,10,30,100,300,1000,3000,10000,30000,100000,300000, ... ...10,000,000,000 (where 1 is the softest power level, and I have rounded off 3x3=10). On this scale, a quiet room with normal background sound (the hum of a computer monitor, air handling system...) has a power level of 10,000! Finding an environment where you could listen to the softest possible sound is really hard! This still gives you a number of loudness levels, but some of them require you to shout as loudly as you can.
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My Question: Can sound energy be converted to heat? I taped a thermometer to one of the speakers on my computer and then played a song really loud to see if the sound energy could be converted to heat, but the temperature didn't go up enough to detect. Is this because the thermometer was not sensitive enough, or is none of the sound being converted to heat?

Joe's discussion
Sound can be converted into heat, but it is going to be hard to design an experiment to show this. The ear is a very sensitive detection system: the softest sound you can hear (well, some people, anyway) is 0.000,000,000,001 Watt per square meter, and a really loud sound is still less than 1 Watt per square meter. This is a lot less energy than sunlight (1000 W/m2). You also need to provide a mechanism for absorbing the energy: I suspect that most of the energy blew by the thermometer. Perhaps putting a dry sponge right over the speaker aperture would work. I'll note that most of the energy is coming out the high frequency speakers, even though the low frequency speakers are doing the most jumping up and down.

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My Question: What effect does the diameter of a tube have on the flow rate of a siphon?

Joe's comment
A fellow called Poiseuille achieve undying fame for working out the answer to that! A wider tube will give a larger flow rate, for two different reasons: there is more water in the tube to move, and the friction is less. This is an experiment that we can do easily -- does anyone have some data to share?
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