great idea, bad result

G

garf

New member
I know you have all been thinking about doing this, so I will save you some time by posting my experiences. You know that satellite dish you picked up at the garbage sale? After watching all that football, and wondering how those big clear parabolic dishes managed to give such a directed sound of the players smashing into each other, the idea dawned that maybe you could get the effect of close micing from a distance by aiming the satellite dish at the sound source, and then putting the mic at the focal point of the parabola.

Well guess again. I tried this using 2 mic stands, one for the satellite dish, and 1 for the mic. There was a definite increase in the gain, but the sound was muddy, Tried it in about 4 different locations varying from 3 to 10 feet away.

Once again, I have damaged my credibility with the wiff...

Maybe a deep dish parabola would work, but mostly it seems like a good idea gone bad...
 
What kind of mic did you use and which way was it pointing in relation to the dish ........ just out of curiosity????

-mike
 
Design of the dish has a lot to do with it, but I would imagine that in any case you'll see a pretty severe roll-off of high frequencies over distance, as it's more likely do get completely absorbed or blocked by small particles in the air and such on the way. It also usually carries less energy to begin with than LF sound and you notice this when transmitting through air at distance.
My guess is that the special dishes they use to do this are specifically tuned to give optimal response and that the mics or circuitry may be doing some compensation EQ. I wonder what polar pattern is best? Perhaps omni so you don't get a muddying proximity effect of having the mic so close to the dish.
I'd imagine dish construction has a lot to do with it... you don't want the dish to be resonating sympathetically, and it should probably be smooth rather than roughly finished so the high-end doesn't get diffused.

I'm obviously no expert though, so I'd be interested in hearing what others have to say.
 
At Science World in Vancouver they have something like this, where you have two dishes about 15 m apart facing one another. You can talk in one and the person at the other can hear you amazingly well.
 
Heh... I was going to mention that about Science World actually, but figured no one would have any idea what I was talking about.
 
I tried it with a cardoid - a Blue Blueberry which is the only large capsule mic that I have. I faced the open end towards the parabola, and also tried covering the other end with both my hand and foam, but made no difference. Interesting comment about the texture of the satellite dish, because it is rough which could easily take out the high frequency. I have seen aluminum search light dishes on ebay, they have the advantage of being smooth & considerably deeper, unfortunately expensive.

One wierd result, is that there was no definite focal point, there was a pretty broad area where I saw the increased gain (used a set of monitoring headphones to locate best point). I am guessing that indicates it was picking up sound that was reflected from the walls (tried this in my living room).
 
There are commercially available dishes for this purpose, I've seen 'em used with shotgun mics although I've never done it myself. They are transparent to look less obtrusive.
 
garf said:
...There was a definite increase in the gain, but the sound was muddy...
Click to expand...
The relative frequencies reflected by the parabolic dish depend on the size of the dish, and to some degree its other physical properties such as material type and thickness, focal length, surface smoothness, etc.

A dish needs to be several times larger than the longest wavelength it will reflect: If a 100 HZ wave is about 10 feet long, then the reflector needs to be 20 or more feet in diameter to get significant reflection. To reflect 40 Hz, it would need to be 50 or more feet in diameter.

The reflector may also resonate at certain frequencies, giving an irregular frequency response.

Read more about it here:
http://home.att.net/~rsl/APPR1.htm
 
CrazyDoc, that was a great reference. I now understand why there did not seem to be any true focal point, and why it sounds somewhat muddy (variability of reflection in the acoustic spectrum even in the portion that it can reflect).
 
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