# comparing smooth surfaces

I have to build a non-electric parabollic satellite and because smooth surfaces are better at directing soundwaves, I need to find a material
that is smoother that paper, but just as easily or similarly bendable and/or cuttable. If anyone can help me on this, I would really appreciate it.
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snipped-for-privacy@gmail.com wrote:

You are overgeneralizng the role of "smoothness" to perhaps absurdity.
The wavelength of speech sound, about 300 HZ, is about 3 feet in ordinary air. Call it 30 inches for rough calculation purposes.
You are aware of the relation that the frequency times the wevelength equals the speed of (sound) the wave.
"Smooth" would be somewhere between the wavelength divided by something like 10 or even 100.
So, "Smooth" for an audio reflection surface would be somethng like 3 inches or .3 inches.
YOur hands cupped to your ears are quite good sound reflectors, even though your hands are not "Smooth as paper", because they are "Smooth enough" for sound wave reflections.
I suggest that a better experiment would be to make reflection cones from sandpaper of the coarsest to the finest grades, and then the smoothest paper possible.
You could even imagine use of a small microphone and an amplifier feecing a voltmeter to make measurements.
Physiological impressions of loudness are quite unreliable, because "BELIEF" is quite a good placebo of human responses.
The scientist approach would be to effectively cleanly determine the effect of roughness.
The engineer approach would be to instead find the best "design" given the available resources.
The homework assignment appears to be engineer driven ....
The geometry of the device, and the size, are likely to be the dominant factors in the engineering of the device.
Just because "Smooth is 'good'" doesn't mean that "smoothness increases are always better".
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jbuch wrote:

...(big clip)...
jbuch is quite right. I wrote a message directly to Toni giving some suggestions on how to create a parabolic surface, because obviously paper or cardboard cannot be bent into even an approximation of such a shape without "crinkling" deformation or lots and lots of snipping and taping. (And because my news reader wouldn't allow me to post last night for some reason.) But the way Toni described his project made me wonder whether he's got a grasp on how a parabolic reflector works.
Just whispering into an opening in the end of a pretty good parabolic reflector isn't likely to achieve much. Unless the origin of the sound is at the focus of the parabola the geometry will not do much to direct and concentrate the sound in a tight beam. Same for listening at the butt end of a parabolic reflector. In both cases one would do better to just use a megaphone (a cone). To use a parabolic reflector effectively one would have to whisper into the dish from the focal spot. If the focal spot is far enough in front of the dish, one could stand in front of it. Otherwise one would need to place a second smaller reflector facing inward at the focus and whisper into THAT from a hole in the butt end of the larger reflector. Same with the listening end -- again, unless the focal point is out in front of the dish.
Mark 'Sporky' Stapleton Watermark Design, LLC www.h2omarkdesign.com
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