Satellite TV Dish    Parabolic microphone: When we first moved to our present home I had high hopes for the DIRECTV satellite dish that previous owners had left in the back yard—or I suppose the company abandoned it when the customer discontinued service. That must be common practice, as unused TV satellite dishes can be found all over.

The photo shows only the front of the dish. On the back was an adjustable azimuth-elevation mount, which by itself was sure to be useful. Maybe the whole thing could be repurposed as a ham radio microwave antenna, or a receive antenna for the NOAA GOES satellite. If not, perhaps a different dish could be attached to the adjustable mount.
1.9 GHz antenna
Time passed. Eventually I stowed the satellite TV dish in the garage and bolted a different antenna to the mount (photo right). That antenna has nothing to do with the present project, however. For that matter, it is only incidental to the current project that I used the DIRECTV dish as a sound reflector—Others have used trash can lids or stripped out patio umbrellas, basically any big curved thing that does not absorb sound.

    The plan literally fell into place. As I was retrieving the dish from the garage floor, a nearby piece of aluminum bar stock fell over. It appeared to be the same width as some LM386-based amplifier boards that were on-hand (1/2 inch), and it could be easily cut and bent. A piece of Plexiglas could be glued to the underside of an amplifier board to insulate it from the bar. Then to make the microphone’s position adjustable, the insulated amplifier could be secured to the bar with rubber bands. Although the plan was simple and seemed foolproof, I was not entirely confident it would work. There’s usually a catch.

Microphone internals and biasing

    Electret microphone: The small condenser microphone is actually a circuit. The ‘button’ shaped device has an internal JFET for preamplification. If the JFET is not biased, the microphone’s output will be insufficient to drive an LM386. Another consideration is coupling a biased microphone to the op amp input pin. The pre-made circuit board did not have a capacitor between the input header and pin 3 of the LM386—just a trimmer potentiometer for level adjustment. Thus the microphone adapter would need to include a coupling capacitor.
Microphone ‘daughterboard’

I planned to power the LM386 with a 9-volt battery, so used a 10K resistor for bias. The idea was for the adapter to plug into the LM386 board. Actualization of this idea (right) was rather messier than the Fritzing diagram (above). The resistor and capacitor are on the underside of the small add-on board (opposite side from the microphone). I used a 4-wire shielded cable for power and audio output. Two male header pins, for through-connecting Vcc and ground to the LM386, are difficult to see in the photo. The terminal block at the other end of the assembly is the output.


Enclosure for wiring

The original DIRECTV dish included a heavy-duty arm to which the LNA/antenna box was attached. (I don’t know the proper terms for these parts.) For distributing power and audio output, I printed an enclosure the same width as this gray steel arm (2 inches). The enclosure top is a cut piece of Plexiglas, same as used to insulate the amplifier PCB. (It is not weatherproof.) I accidentally interchanged the power and line out jack hole diameters in FreeCAD, but decided not to reprint the box.

Test tone    Positioning the microphone: Somehow I had expected a sharp focus, like the solution to an algebra problem would be. To find this hypothetical point I placed a tone oscillator at the other side of the garage from the dish, making the volume as low as possible. Although the tone could still be heard by the unaided ear, snug-fitting headphones blocked the sound. Holding the microphone (and attached amplifier) in my hand and slowly waving it about through the space in front of the dish identified a focus zone that was much larger than I had thought it should be. It was clearly a zone of reflected sound, as positioning the microphone outside the region attenuated tone volume in the headphones. But it was not a small or exact location, as had been imagined. Another surprise was the level of background sound that was being reflected into the microphone. I could almost hear myself breathe, and became aware of jingling keys in my pocket.
Field Test
Having identified approximately where the microphone should be positioned, I cut and bent the aluminum slat and put the rest of the assembly together. Later I moved the whole thing to the back porch in order to listen to outside sounds, which in the daytime consist mainly of passing cars! My wife assisted in a field test of sorts by walking toward the back of the lot and speaking at a conversational level. With amplification it was possible to make out what she was saying. Such subjective impressions are not trustworthy. Nevertheless we both agreed that the setup was collecting more sound than could be heard or would be noticed without acoustic amplification.

The dish does not exhibit much directionality with respect to sound sources. I guess its apparent original purpose of detecting weak RF energy from a point in the sky led me to imagine that aiming it toward a distant bird would pick up that bird’s song more prominently than background sounds. That was an erroneous expectation! So far, the most interesting outside sounds are heard at dusk. Unfortunately that is also the time when mosquitos swarm, or whatever it is they do.


Speaker amplifierSpeakerAn obvious antidote for the mosquito problem was to place a speaker inside the house. To that end I set a small amplifier on the outside window ledge and ran a short speaker wire under the window to the adjoining room. (This was not to be a permanent installation!) What can I say?! It was nice to hear birds chirping and other outdoor sounds along with the high-pitched whine of the 3D printer and the chug-a-chug of the washing machine. Later, in the quiet of the evening—oops, there is no quiet in the evening, or night—Anthropogenic noise intrudes at all hours of day and night.

Although out-of-doors is quieter in early morning, the background hum in the recording is an electric motor
that never stops running—It is about 100 meters from the microphone.


    Recording: Morning










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