for a 10-32 screw that holds the board to the coupling nut
at the center of the reflector. The smaller hole is for 6-32
threaded rod. Since the parabolic dish we used has a
three inch focal distance, we used a three inch 6-32 screw
instead of threaded rod.
Adjusting Microphone Position
If you know the focal length of your dish, then use a
ruler to position the mic at the focal point. If you don’t know
the focal length, there are two empirical ways to find it.
If your dish is shiny and reflects light, expose it to a
bright light and use a piece of paper to find the focal
point. Move the paper towards the bottom of the dish and
find the point where the spot of light on the paper
reaches its smallest diameter. That’s the focal point. (If you
use sun light, be careful not to burn a hole in the paper.)
If you can’t use light, use sound. You’ll need
something that generates a steady audio tone at a
frequency around 1 kHz. Point the dish towards the sound
and adjust the position of the mic while you listen to the
output of the amplifier. The sound will be loudest at the
focal point.
Headphones
Inexpensive headphones usually
have ear pieces that sit on the ears
letting some sound leak out. You can
get feedback if the mic picks up the
leaked sound. Better headphones
have ear pieces that fit over the ears
and don’t leak sound. However,
better headphones often use a 1/4
inch plug. You can get a 1/4 inch to
1/8 inch adapter at RadioShack (part
number 274-875). Since 3. 5 mm is
slightly larger than 1/8 inch, the
adapter will fit. Or, if you plan to use
only those headphones, you can
replace the 3. 5 mm jack with a 1/4
inch jack.
Wrap-up
Well, that’s about it. Go out and
discover all the neat sounds you’ve
been missing with your new Big Ear. NV
A complete kit for this project
can be purchased from the
Nuts & Volts Webstore
@ www.nutsvolts.com
or call our order desk,
800 783-4624.
December 2008 39