MadSci Network: Physics

Re: what is a parobalic sound collecting dish how does it work and collect wave

Date: Mon Apr 5 00:23:44 1999
Posted By: Abtin Spantman, , Electrical Engineering, L. S. Research, Inc.
Area of science: Physics
ID: 919016128.Ph

What a parabolic dish is
A parabolic dish is nothing more than a concave surface - it looks like the 
inside of a bowl. This bowl is made in such a way that if a light was 
shined into the bowl, all of the light rays would reflect off of the inside 
of the bowl, and would focus on a point in the center of the bowl - 
probably suspended in the middle of the air in the bowl.  The parabolic 
dish also reflects sound the same way, and focuses it in the middle where 
the vertex (focus point) is.  The microphone is usually placed at this 
vertex, pointing into the bowl to catch all the reflected waves.
A car's head lights are parabolic with the light bulb at the vertex, as 
well as the reflector of a flash light (usually silver in color).  
Yes, you can take the reflector of a flash light, measure where the light 
bulb filament (coiled wire) sits inside the reflector, and mount your 
microphone on that same spot. Remember to point the microphone inside the 
reflector.  If nothing else, a plastic bowl will do just fine.

You can make your own parabolic reflector dish if you use the equation 
(y)^2 = (4)(v)(x)     where y^2 is y squared (y times itself)        
where x and y would be numbers like 1,2,3, and v would be the distance you 
need to mount the microphone from the center of the dish.

I calculated some of the numbers here as an example, assuming the 
microphone would be 10 inches (or centimeters) from the center bottom of 
the dish.  Then the rest of the dish must follow the curvature on the 
		v=	10
X	Y^2	Y	
0	0	0	
1	40	6.325	
2	80	8.944	
3	120	10.95	
4	160	12.65	
5	200	14.14	
6	240	15.49	
7	280	16.73	
8	320	17.89	
9	360	18.97	
10	400	20	
11	440	20.98	
12	480	21.91	
13	520	22.8	
14	560	23.66	
15	600	24.49	
16	640	25.3	
17	680	26.08	
18	720	26.83	
19	760	27.57	
20	800	28.28	
21	840	28.98	
22	880	29.66	

As far as construction material, try to use substances that are hard or 
have a flat surface. Avoid constructing with foam or paper as they tend to 
absorb sound instead of reflecting it.  
Also, since the microphone is placed at X=10, you don't need to go beyond 
10 when constructing the dish.
And remember to face the microphone inside the bowl.
This will be a tuff but fun project.  Don't give up.
Let me know if you need more guidence with the microphone or the 
construction techniques.

Abtin Spantman

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