A snag I'm finding with joining this list is that it's getting me off
my 70 year old butt doing things I only talked about. I've tried out
one idea I've had for years and never quite got around to it. It's an
alternative to using a parabolic reflector - a horn microphone. I used
a cardboard box and a mono miniature lapel mic.
Get a large cardboard box with sides measuring at least a foot. Cut
down three edges leaving a sort of right angled pyramid. If the lid is
serviceable, make another one. (see below) You can round off the
pointy bits as they don't do much. What you've made is a horn. OK I
said before that cardboard is not ideal, but whatever.
Cut a small hole in the apex, seal up any gaps in the box with gaffer
tape and put a miniature mic into the hole. The acoustics theory I
learnt all those decades ago (but have forgotten the equations) said
that a horn with sides wider than 45 degrees won't resonate (like a
tuba) but will amplify.
You should find a healthy front/back ratio from this arrangement and
the bigger the box, the lower the directional frequency response. I've
been getting about 14dBs front to back with no back lobes and that's
with buildings and a bank surrounding me. Next thing to try is to tape
two of these boxes together using one mini mic in each box for stereo,
but I haven't done that yet.
The cutoff when the box masks the mic is quite rapid, and it may be
necessary with a stereo pair to angle in the two boxes to give more
centre overlap. I tried offsetting the mic a couple of inches which
would work with a bowl reflector but as the theory would predict (if I
could remember it) that there would be a bit of phase distortion.
There is and it is bad. I had hoped to put my Tascam mp3 recorder with
built in mics inside two boxes close together, but the mics need to be
at the apexes.
With the boxes held with their bottoms level, the peak sensitivity is
obviously 45 deg upwards and outwards which is good for birds. I was
testing with a car inflator which plugs into the cigar lighter and
which gives a good hiss spectrum.
I established the general principle but I want to try a pair of larger
boxes and also I haven't got a matched pair of mini mics. I'll buy
some budget ones to match the expendidure on the "hemicubic horns".
Also I want to see if I can space the boxes to use the built in mics
on my Tascam DR1. This would also help the centre image.
If it gives a stereo image, remember it's called a Brinihorn. :-) If
it doesn't - leave me out of it. I do anticipate a problem which is
picking up people laughing at such an arrangement. It will need to be
glammed up a bit but could be covered with a wind sock.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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